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Linux IP Networking: A Guide to the Implementation and Modification of the Linux Protocol Stack


Linux IP Networking
A Guide to the Implementation and Modification of the Linux Protocol Stack

Glenn Herrin

TR 00-04

Department of Computer Science
University of New Hampshire

May 31, 2000

Abstract

Этот документ описывает реализацию сетевых протоколов на примере версии 2.2.14. В основном внимание фокусируется на протоколе Internet Protocol (IP). Здесь вы найдете исходные коды примеров. В первой части рассматриваются код,структуры данных и функциональность сети: инициализация и коннект сокетов,получение и отправка пакетов. Во второй части описаны инструкции для модификации исходников ядра и инсталляции новых модулей.

Contents

1  Introduction
    1.1  Background
    1.2  Document Conventions
    1.3  Sample Network Example
    1.4  Copyright, License, and Disclaimer
    1.5  Acknowledgements
2  Message Traffic Overview
    2.1  The Network Traffic Path
    2.2  The Protocol Stack
    2.3  Packet Structure
    2.4  Internet Routing
3  Network Initialization
    3.1  Overview
    3.2  Startup
        3.2.1  The Network Initialization Script
        3.2.2  ifconfig
        3.2.3  route
        3.2.4  Dynamic Routing Programs
    3.3  Examples
        3.3.1  Home Computer
        3.3.2  Host Computer on a LAN
        3.3.3  Network Routing Computer
    3.4  Linux and Network Program Functions
        3.4.1  ifconfig
        3.4.2  route
4  Connections
    4.1  Overview
    4.2  Socket Structures
    4.3  Sockets and Routing
    4.4  Connection Processes
        4.4.1  Establishing Connections
        4.4.2  Socket Call Walk-Through
        4.4.3  Connect Call Walk-Through
        4.4.4  Closing Connections
        4.4.5  Close Walk-Through
    4.5  Linux Functions
5  Sending Messages
    5.1  Overview
    5.2  Sending Walk-Through
        5.2.1  Writing to a Socket
        5.2.2  Creating a Packet with UDP
        5.2.3  Creating a Packet with TCP
        5.2.4  Wrapping a Packet in IP
        5.2.5  Transmitting a Packet
    5.3  Linux Functions
6  Receiving Messages
    6.1  Overview
    6.2  Receiving Walk-Through
        6.2.1  Reading from a Socket (Part I)
        6.2.2  Receiving a Packet
        6.2.3  Running the Network ``Bottom Half''
        6.2.4  Unwrapping a Packet in IP
        6.2.5  Accepting a Packet in UDP
        6.2.6  Accepting a Packet in TCP
        6.2.7  Reading from a Socket (Part II)
    6.3  Linux Functions
7  IP Forwarding
    7.1  Overview
    7.2  IP Forward Walk-Through
        7.2.1  Receiving a Packet
        7.2.2  Running the Network ``Bottom Half''
        7.2.3  Examining a Packet in IP
        7.2.4  Forwarding a Packet in IP
        7.2.5  Transmitting a Packet
    7.3  Linux Functions
8  Basic Internet Protocol Routing
    8.1  Overview
    8.2  Routing Tables
        8.2.1  The Neighbor Table
        8.2.2  The Forwarding Information Base
        8.2.3  The Routing Cache
        8.2.4  Updating Routing Information
    8.3  Linux Functions
9  Dynamic Routing with routed
    9.1  Overview
    9.2  How routed Works
        9.2.1  Data Structures
        9.2.2  Initialization
        9.2.3  Normal Operations
    9.3  routed Functions
10  Editing Linux Source Code
    10.1  The Linux Source Tree
    10.2  Using EMACS Tags
        10.2.1  Referencing with TAGS
        10.2.2  Constructing TAGS files
    10.3  Using vi tags
    10.4  Rebuilding the Kernel
    10.5  Patching the Kernel Source
11  Linux Modules
    11.1  Overview
    11.2  Writing, Installing, and Removing Modules
        11.2.1  Writing Modules
        11.2.2  Installing and Removing Modules
    11.3  Example
12  The proc File System
    12.1  Overview
    12.2  Network proc Files
    12.3  Registering proc Files
        12.3.1  Formatting a Function to Provide Information
        12.3.2  Building a proc Entry
        12.3.3  Registering a proc Entry
        12.3.4  Unregistering a proc Entry
    12.4  Example
13  Example - Packet Dropper
    13.1  Overview
    13.2  Considerations
    13.3  Experimental Systems and Benchmarks
    13.4  Results and Preliminary Analysis
        13.4.1  Standard Kernel
        13.4.2  Modified Kernel Dropping Packets
        13.4.3  Preliminary Analysis
    13.5  Code
        13.5.1  Kernel
        13.5.2  Module
14  Additional Resources
    14.1  Internet Sites
    14.2  Books
15  Acronyms

Chapter 1
Introduction

This is version 1.0 of this document, dated May 31, 2000, referencing the Linux kernel version 2.2.14.

1.1  Background

Linux в качестве альтернативной операционной системы становится все более популярной. Поскольку все части операционной системы являются открытыми, тысячи программистов постоянно работают над кодом и реализуют новые фичи.

В этом документе сделана попытка обьединить воедино обзор сетевого протокола ядра Linux. Материал разбит на 4 уровня : общий обзор,наиболее актуальные темы,конкретное рассмотрение функционала и рассмотрение конкретных структур данных и кода. Обзор построен с использованием редхата 6.1

Здесь вы найдете информацию о компиляции ядра, написанию и инсталляции модулей, работы с файловой системой proc. Представлен пример программы,которая отлавливаеет сетевые пакеты и анализирует их.

Этот проект был начат в университете New Hampshire в целях эксперимента с алгоритмами роутинга.Довольно быстро прояснилось,что модификация ядра- не самая безопасная вещь:-) В каком-то смысле этот документ является своеобразным назиданием для программистов ядра.

1.2  Document Conventions

It is assumed that the reader understands the C programming language and is acquainted with common network protocols. This is not vital for the more general information but the details within this document are intended for experienced programmers and may be incomprehensible to casual Linux users.

Almost all of the code presented requires superuser access to implement. Some of the examples can create security holes where none previously existed; programmers should be careful to restore their systems to a normal state after experimenting with the kernel.

File references and program names are written in a slanted font.

Code, command line entries, and machine names are written in a typewriter font.

Generic entries or variables (such as an output filename) and comments are written in an italic font.

1.3  Sample Network Example

На следующем рисунке представлен пример сетевой структуры:

i_example.gif

Figure 1.1: Sample network structure.

Она представляет из себя компьютерную систему университета. В сетке имеется роутер chrysler,подключенный к внешнему интернету. Он законнекчен через машину jeep с первой локальной под-сеткой университета - u.edu, состоящей из машин : (dodge, eagle, etc.). Также имеется 2-я под-сеть,состоящая cs.u.edu, состоящая из : (stealth, neon, etc.). Она приконнекчена к 1-й через dodge/viper. Обе подсети - u.edu и cs.u.edu - используют Ethernet-протокол.

IP-шники взяты из зарезервированного диапазона класса В,что не гарантирует уникальность. Пример достаточно реальный и иллюстрирует Linux-имплементацию взаимодействия между хостами,подсетями и сетями.

1.4  Copyright, License, and Disclaimer

Copyright (c) 2000 by Glenn Herrin. This document may be freely reproduced in whole or in part provided credit is given to the author with a line similar to the following:

Copied from Linux IP Networking, available at http://www.cs.unh.edu/cnrg/gherrin.
(The visibility of the credit should be proportional to the amount of the document reproduced!) Commercial redistribution is permitted and encouraged. All modifications of this document, including translations, anthologies, and partial documents, must meet the following requirements:
  1. Modified versions must be labeled as such.
  2. The person making the modifications must be identified.
  3. Acknowledgement of the original author must be retained.
  4. The location of the original unmodified document be identified.
  5. The original author's name may not be used to assert or imply endorsement of the resulting document without the original author's permission.

Please note any modifications including deletions.

This is a variation (changes are intentional) of the Linux Documentaion Project (LDP) License available at:

http://www.linuxdoc.org/COPYRIGHT.html
This document is not currently part of the LDP, but it may be submitted in the future.

This document is distributed in the hope that it will be useful but (of course)without any given or implied warranty of fitness for any purpose whatsoever. Use it at your own risk.

1.5  Acknowledgements

I wrote this document as part of my Master's project for the Computer Science Department of the University of New Hampshire. I would like to thank Professor Pilar de la Torre for setting up the project and Professor Radim Bartos for being both a sponsor and my advisor - giving me numerous pointers, much encouragement, and a set of computers on which to experiment. I would also like to credit the United States Army, which has been my home for 11 years and paid for my attendance at UNH.

Glenn Herrin
Major, United States Army
Primary Documenter and Researcher, Version 1.0
gherrin@cs.unh.edu

Chapter 2
Message Traffic Overview

This chapter presents an overview of the entire Linux messaging system. It provides a discussion of configurations, introduces the data structures involved, and describes the basics of IP routing.

2.1  Network Traffic Path

Internet Protocol (IP) - сердцевина коммуникационной системы в Linux. IP - протокол,через который протекают почти все пакеты. На уровне IP сделана реализация роутинга и форвардинга. Диаграмма  2.1 показывает,как пакеты проходят через ядро:

o_path.gif

Figure 2.1: Abstraction of the Linux message traffic path.

Когда приложение генерит трафик,оно посылает пакеты через сокеты в транспортный слой (TCP или UDP) и уже потом в сетевой слой(IP). Ядро ищет маршрут либо в кеше либо в Forwarding Information Base (FIB). Если пакет для другого компьютера,ядро передает пакет сетевой карте - Ethernet device - которая отправляет его дальше.

Когда пакет приходит на сетевой интерфейс,IP-слой анализирует его адрес назначения. Если пакет предназначен для другого компьютера, IP-слой отсылает пакет обратно на сетевой интерфейс.

2.2   Protocol Stack

Сетевые устройства формируют нижний слой стека протокола. Они используют обычно протокол Ethernet для коннекта с другими устройствами. Интерфейсы копируют пакеты из сети,выполняют проверку, и при необходимости делают форвардинг сетевому слою.

IP - стандартный сетевой протокол. Он проверяет входящий пакет и определяет,нужно ли делать его форвард. Он может дефрагментировать этот пакет. Он имеет свою базу данных маршрутов для высылаемых пакетов.

TCP и UDP - основные транспортные протоколы. UDP включает в себя простой фреймворк для отсылки пакетов, в то время как TCP делает дополнительный коннект, включая восстановительный механизм для потерянных пакетов. Обмен пакетами осуществляется между user и kernel space.

IP Specific INET Sockets хранят в себе данные. Они включают очереди и код,который выполняет такие операции,как чтение,запись и коннект.

Generic BSD Sockets - более абстракная базовая структура, которая включает в себя INET sockets. Приложения пишут и читают в BSD sockets; BSD sockets транслируют операции уже в операции уровня INET socket. See Chapter 4 for more on sockets.

Приложения,выполняемые в user space, формируют верхний уровень стека; это может быть обычный коннект two-way или более сложный Routing Information Protocol (RIP - see Chapter 9).

2.3  Packet Structure

Структура пакета - socket buffer, или sk_buff - представлена ниже : Figure 2.2). Несмотря на обилие вызывающих функций и транспортных протоколов, копирование данных происходит лишь дважды: от пользователя ядру и от ядра в сеть.

o_skbuff.gif

Figure 2.2: Packet (sk_buff) structure.

Структура включает в себя указатели на информацию о пакете - на сокет,на устройство,на маршрут и т.д. Транспортные протоколы создают эти структуры для output buffers, в то время как драйвера создают их для входящих данных. Все протоколы - транспортный (TCP/UDP), интернет (IP), и Ethernet - используют один и тот же socket buffer.

2.4  Internet Routing

IP слой управляет маршрутизацией между компьютерами. Он хранит 2 структуры: Forwarding Information Base (FIB) , который хранит детали для всех известных маршрутов, и быстрый кеш маршрутизации. (Имеется еще и 3-я структура - neighbor table - которая хранит пути к локальным машинам)

Вначале идет ссылка на FIB. Она включает 32 зоны (по одной для каждого бита в IPадресе) и всех остальных известных маршрутов. Каждая зона включает в себя наборы всех возможных хостов или сетей, которые уникально идентифицируются определенным номером бита - например сеть с маской 255.0.0.0 имеет 8 значащих бит и будет в зоне 8, в то время как сеть с маской 255.255.255.0 имеет 24 значащих бита и будет в зоне 24. Когда IP нуждается в маршруте, он начинает с наиболее специфичных зон и ищет в таблице , пока не найдет хотя бы одно соответствие. Файл /proc/net/route включает в себя содержимое FIB.

Кеш - хеш-таблица,которую IP использует для текущей маршрутизации пакетов. Он включает последние 256 маршрутов. Когда нужно послать пакет,IP смотрит в этот кеш. Если он пуст,ищется в FIB и новое значение вставляется в кеш. Если позже это значение в кеше не будет использовано,оно будет из него удалено. Файл /proc/net/rt_cache описывает этот кеш.

Эти таблицы выполняют всю работу по маршрутизации. Такие протоколы, как RIP , используют все то же; они модифицируют внутренние таблицы ядра с помощью функции ioctl(). See Chapter 8 for routing details.

Chapter 3
Network Initialization

Здесь описана стартовая инициализация сети. Показано,что происходит при загрузке линукса, как работают утилиты ifconfig и route, показана разница между несколькими вариантами конфигурации.

3.1  Overview

Инициализация таблиц маршрутизации при старте происходит тогда, когда сконфигурирована сеть(даже в случае с loopback device). После загрузки запускаются системные утилиты для установки сетевых параметров. Сюда входит настройка его собственного адреса, инициализацияs Ethernet, маршрут до ближайшего роутера. Если компьютер сам является роутером,он может динамически менять собственную таблицу маршрутизации.

Может быть использован Dynamic Hardware Configuration Protocol (DHCP) для определения информации об адресах,маршрутах и DNS-сервере.

Эта статья сделана с использованием Red Hat Linux 6.1 и статического сетевого окружения.

3.2  Startup

Линукс при старте загружает ядро в память,распаковывает его, инициализирует файловую систему,управление памятью и т.д. После чего запускается init, который читает конфиги (/etc/inittab) , запускается стартовый скрипт ( /etc/rc.d). Запускается сетевой скрипт (/etc/rc.d/init.d/network).

3.2.1  The Network Initialization Script

Скрипт инициализации сети устанавливает переменные окружения. Он включает либо выключает IP forwarding и IP fragmentation. Инициализируются роутер и сетевое устройство. Затем запускаются утилиты ifconfig и route. (В динамическом окружении будет сконфигурирован DHCP)

При старт-апе запускается большое количество различных скриптов. В Red Hat скрипт /etc/rc.d/init.d/network запускает другие скрипты типа interfaces_boot , /etc/sysconfig/network-scripts/ifup. Ключевыми скриптами для инициализации являются /etc/sysconfig/network и /etc/sysconfig/network-scripts/ifcfg-eth0), где можно прописать имена и ip-шники.

По завершени работы сетевых скриптоа, FIB включает в себя маршруты для хоста и сети , и кеш маршрутизации. С появлением трафика ядро начинает делать апдэйт кеша.

3.2.2  ifconfig

Утилита ifconfig конфигурирует сетевые устройства. Она не является частью ядра. Каждому сетевому устройству присваивается свой собственный IP,маска,и широковещательные адреса. Для каждого устройства запускается функция инициализации,которая регистрирует в ядре прерывания. Формат вызова ifconfig :
ifconfig ${DEVICE} ${IPADDR} netmask ${NMASK} broadcast ${BCAST}

ifconfig выдает информацию о всех сконфигурированных сетевых устройствах- для этого есть опция -a: Всегда есть по крайней мере 2 устройства- сетевая карта и виртуальное loopback device.

eth0  Link encap:Ethernet  HWaddr 00:C1:4E:7D:9E:25
       inet addr:172.16.1.1  Bcast:172.16.1.255  Mask:255.255.255.0
       UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
       RX packets:389016 errors:16534 dropped:0 overruns:0 frame:24522
       TX packets:400845 errors:0 dropped:0 overruns:0 carrier:0
       collisions:0 txqueuelen:100
       Interrupt:11 Base address:0xcc00
 
Администратор может использовать ifconfig для изменения сетевых установок из командной строки:
ifconfig interface [aftype] options | address ...
... несколько примеров :
ifconfig eth0 down - shut down eth0
ifconfig eth1 up - activate eth1
ifconfig eth0 arp - enable ARP on eth0
ifconfig eth0 -arp - disable ARP on eth0
ifconfig eth0 netmask 255.255.255.0 - set the eth0 netmask
ifconfig lo mtu 2000 - set the loopback maximum transfer unit
ifconfig eth1 172.16.0.7 - set the eth1 IP address
Изменение конфигурации может косвенным образом затронуть таблицы маршрутизации. Например,изменение маски может привести к тому,что ядро начнет ломать таблицы маршрутизации.

3.2.3  route

Утилита route добавляет маршруты в Forwarding Information Base (FIB). Она также не является частью ядра,вызов которой из командной строки может выглядеть так:
route add -net ${NETWORK} netmask ${NMASK} dev ${DEVICE} -or-
route add -host ${IPADDR} ${DEVICE}

route может удалять маршруты (опция del )

Если запустить утилиту без опций, она выведет FIB :

Kernel IP routing table
 Destination   Gateway        Genmask         Flags Metric Ref Use Iface
 172.16.1.4    *              255.255.255.255 UH    0      0     0 eth0
 172.16.1.0    *              255.255.255.0   U     0      0     0 eth0
 127.0.0.0     *              255.0.0.0       U     0      0     0 lo
 default       viper.u.edu    0.0.0.0         UG    0      0     0 eth0
 
Примеры вызова :
route add [-net|-host] target [option arg]
route del [-net|-host] target [option arg]
route add -host 127.16.1.0 eth1 - adds a route to a host
route add -net 172.16.1.0 netmask 255.255.255.0 eth0 - adds a network
route add default gw jeep - sets the default route through jeep
(Note that a route to jeep must already be set up)
route del -host 172.16.1.16 - deletes entry for host 172.16.1.16

3.2.4  Dynamic Routing Programs

Если компьютер является роутером, будет запущена утилита routed или gated.

3.3  Examples

Следующие 3 примера создают 3 различных конфигурации.

3.3.1  Домашний компьютер

Речь идет о компьютере,который может быть даже не подключен к локальной сети, но имеет модем и ppp - доступ.

Первый файл читает конфигурацию и устанавливает переменные. Первые 2 переменные заставляют компьютер выполнять сетевые программы(даже при отсутствии локалки), но не форвардить пакеты-ибо некуда.

/etc/sysconfig/network

NETWORKING=yes
FORWARD_IPV4=false
HOSTNAME=localhost.localdomain
GATEWAY=
После этого настает очередь конфигурации сетевого устройства. Следующий скрипт настраивает loopback device. Назначаются IP address,network mask, broadcast address. (Флаг ONBOOT указывает на то,что устройство должно быть сконфигурировано при загрузке). loopback device необходим для внутренних коммуникаций.

/etc/sysconfig/network-scripts/ifcfg-lo

DEVICE=lo
IPADDR=127.0.0.1
NMASK=255.0.0.0
NETWORK=127.0.0.0
BCAST=127.255.255.255
ONBOOT=yes
NAME=loopback
BOOTPROTO=none
После этого будет запущен ifconfig. Когда ppp будет коннектиться в интернет, будет установлено устройство ppp.

3.3.2  Host Computer on a LAN

Этот компьютер находится в локальной сети; у него одна Ethernet-карта. У компьютера имя stealth.

Первый скрипт аналогичен предыдущему примеру: работаем с сетью,но не делаем форвард. Последние 4 переменные идентифицируют компьютер и его связь с интернетом.

/etc/sysconfig/network

NETWORKING=yes
FORWARD_IPV4=false
HOSTNAME=stealth.cs.u.edu
DOMAINNAME=cs.u.edu
GATEWAY=172.16.1.1
GATEWAYDEV=eth0
Будут сконфигурированы сетевые устройства. Все аналогично предыдущему варианту.

/etc/sysconfig/network-scripts/ifcfg-eth0

DEVICE=eth0
IPADDR=172.16.1.4
NMASK=255.255.255.0
NETWORK=172.16.1.0
BCAST=172.16.1.255
ONBOOT=yes
BOOTPROTO=none

Будет запущена утилита ifconfig, затем утилита route добавит GATEWAY и другие маршруты, которые можно будет найти в /etc/sysconfig/static-routes file.

3.3.3  Network Routing Computer

Данный компьютер служит роутером между 2-мя сетями; у него две Ethernet cards, по одной для каждой сетки. Одна карта находится во внешней сети (WAN) , приконнекченной к интернету еще через один роутер , другая карта смотрит в локалку. Речь идет о компьютере dodge/viper (смотрите схему выше).

Первые 2 строки инициализируют сеть и форвардинг. Для форварда нужно прописать единицу в /proc/net/ipv4/ip_forward.

/etc/sysconfig/network

NETWORKING=yes
FORWARD_IPV4=true
HOSTNAME=dodge.u.edu
DOMAINNAME=u.edu
GATEWAY=172.16.0.1
GATEWAYDEV=eth1
Будут сконфигурированы сетевые устройства.

/etc/sysconfig/network-scripts/ifcfg-eth0

DEVICE=eth0
IPADDR=172.16.1.1
NMASK=255.255.255.0
NETWORK=172.16.1.0
BCAST=172.16.1.255
ONBOOT=yes
BOOTPROTO=static

/etc/sysconfig/network-scripts/ifcfg-eth1

DEVICE=eth1
IPADDR=172.16.0.7
NMASK=255.255.0.0
NETWORK=172.16.0.0
BCAST=172.16.255.255
ONBOOT=yes
Будет запущена ifconfig, route добавит GATEWAY и другие маршруты в /etc/sysconfig/static-routes file.

3.4  Linux and Network Program Functions

The following are alphabetic lists of the Linux kernel and network program functions that are most important to initialization, where they are in the source code, and what they do. The SOURCES directory shown represents the directory that contains the source code for the given network file. The executable files should come with any Linux distrbution, but the source code probably does not.

These sources are available as a package separate from the kernel source (Red Hat Linux uses the rpm package manager). The code below is from the net-tools-1.53-1 source code package, 29 August 1999. The packages are available from the www.redhat.com/apps/download web page. Once downloaded, root can install the package with the following commands (starting from the directory with the package):

rpm -i net-tools-1.53-1.src.rpm
cd /usr/src/redhat/SOURCES
tar xzf net-tools-1.53.tar.gz
This creates a /usr/src/redhat/SOURCES/net-tools-1.53 directory and fills it with the source code for the ifconfig and route programs (among others). This process should be similar (but is undoubtably not exactly the same) for other Linux distributions.

3.4.1  ifconfig

devinet_ioctl() - net/ipv4/devinet.c (398)
   creates an info request (ifreq) structure and copies data from
       user to kernel space
   if it is an INET level request or action, executes it
   if it is a device request or action, calls a device function
   copies ifreq back into user memory
   returns 0 for success
 
 >>> ifconfig main() - SOURCES/ifconfig.c (478)
   opens a socket (only for use with ioctl function)
   searches command line arguments for options
   calls if_print() if there were no arguments or the only argument
       is an interface name
   loops through remaining arguments, setting or clearing flags or
       calling ioctl() to set variables for the interface
 
 if_fetch() - SOURCES/lib/interface.c (338)
   fills in an interface structure with multiple calls to ioctl() for
       flags, hardware address, metric, MTU, map, and address information
 
 if_print() - SOURCES/ifconfig.c (121)
   calls ife_print() for given (or all) interface(s)
       (calls if_readlist() to fill structure list if necessary and
       then displays information about each interface)
 
 if_readlist() - SOURCES/lib/interface.c (261)
   opens /proc/net/dev and parses data into interface structures
   calls add_interface() for each device to put structures into a list
 
 inet_ioctl() - net/ipv4/af_inet.c (855)
   executes a switch based on the command passed
       [for ifconfig, calls devinet_ioctl()]
 
 ioctl() -
   jumps to appropriate handler routine [= inet_ioctl()]
 

3.4.2  route

INET_rinput() - SOURCES/lib/inet_sr.c (305)
   checks for errors (cannot flush table or modify routing cache)
   calls INET_setroute()
 
 INET_rprint() - SOURCES/lib/inet_gr.c (442)
   if the FIB flag is set, calls rprint_fib()
       (reads, parses, and displays contents of /proc/net/route)
   if the CACHE flag is set, calls rprint_cache()
       (reads, parses, and displays contents of /proc/net/rt_cache)
 
 INET_setroute() - SOURCE/lib/inet_sr.c (57)
   establishes whether route is to a network or a host
   checks to see if address is legal
   loops through arguments, filling in rtentry structure
   checks for netmask conflicts
   creates a temporary socket
   calls ioctl() with rtentry to add or delete route
   closes socket and returns 0
 
 ioctl() - 
   jumps to appropriate handler routine [= ip_rt_ioctl()]
 
 ip_rt_ioctl() - net/ipv4/fib_frontend.c (246)
   converts passed parameters to routing table entry (struct rtentry)
   if deleting a route:
     calls fib_get_table() to find the appropriate table
     calls the table->tb_delete() function to remove it
   if adding a route
     calls fib_net_table() to find an entry point
     calls the table->tb_insert() function to add the entry
   returns 0 for success
 
 >>> route main() - SOURCES/route.c (106)
   calls initialization routines that set print and edit functions
   gets and parses the command line options (acts on some options
       directly by setting flags or displaying information)
   checks the options (prints a usage message if there is an error)
   if there are no options, calls route_info()
   if the option is to add, delete, or flush routes,
       calls route_edit() with the passed parameters
   if the option is invalid, prints a usage message
   returns result of
 
 route_edit() - SOURCES/lib/setroute.c (69)
   calls get_aftype() to translate address family from text to a pointer
   checks for errors (unsupported or nonexistent family)
   calls the address family rinput() function [= INET_rinput()]
 
 route_info() - SOURCES/lib/getroute.c (72)
   calls get_aftype() to translate address family from text to a pointer
   checks for errors (unsupported or nonexistent family)
   calls the address family rprint() function [= INET_rprint()]
 

Chapter 4
Connections

This chapter presents the connection process. It provides an overview of the connection process, a description of the socket data structures, an introduction to the routing system, and summarizes the implementation code within the kernel.

4.1  Overview

The simplest form of networking is a connection between two hosts. On each end, an application gets a socket, makes the transport layer connection, and then sends or receives packets. In Linux, a socket is actually composed of two socket structures (one that contains the other). When an application creates a socket, it is initialized but empty. When the socket makes a connection (whether or not this involves traffic with the other end) the IP layer determines the route to the distant host and stores that information in the socket. From that point on, all traffic using that connection uses that route - sent packets will travel through the correct device and the proper routers to the distant host, and received packets will appear in the socket's queue.

4.2  Socket Structures

There are two main socket structures in Linux: general BSD sockets and IP specific INET sockets. They are strongly interrelated; a BSD socket has an INET socket as a data member and an INET socket has a BSD socket as its owner.

BSD sockets are of type struct socket as defined in include/linux/socket.h. BSD socket variables are usually named sock or some variation thereof. This structure has only a few entries, the most important of which are described below.

  • struct proto_ops *ops - this structure contains pointers to protocol specific functions for implementing general socket behavior. For example, ops- > sendmsg points to the inet_sendmsg() function.
  • struct inode *inode - this structure points to the file inode that is associated with this socket.
  • struct sock *sk - this is the INET socket that is associated with this socket.

INET sockets are of type struct sock as defined in include/net/sock.h. INET socket variables are usually named sk or some variation thereof. This structure has many entries related to a wide variety of uses; there are many hacks and configuration dependent fields. The most important data members are described below:

  • struct sock *next, *pprev - all sockets are linked by various protocols, so these pointers allow the protocols to traverse them.
  • struct dst_entry *dst_cache - this is a pointer to the route to the socket's other side (the destination for sent packets).
  • struct sk_buff_head receive_queue - this is the head of the receive queue.
  • struct sk_buff_head write_queue - this is the head of the send queue.
  • __u32 saddr - the (Internet) source address for this socket.
  • struct sk_buff_head back_log,error_queue - extra queues for a backlog of packets (not to be confused with the main backlog queue) and erroneous packets for this socket.
  • struct proto *prot - this structure contains pointers to transport layer protocol specific functions. For example, prot- > recvmsg may point to the tcp_v4_recvmsg() function.
  • union struct tcp_op af_tcp; tp_pinfo - TCP options for this socket.
  • struct socket *sock - the parent BSD socket.
  • Note that there are many more fields within this structure; these are only the most critical and non-obvious. The rest are either not very important or have self-explanatory names (e.g., ip_ttl is the IP Time-To-Live counter).

4.3  Sockets and Routing

Sockets only go through the routing lookup process once for each destination (at connection time). Because Linux sockets are so closely related to IP, they contain routes to the other end of a connection (in the sock- > sk- > dst_cache variable). The transport protocols call the ip_route_connect() function to determine the route from host to host during the connection process; after that, the route is presumed not to change (though the path pointed to by the dst_cache may indeed change). The socket does not need to do continuous routing table look-ups for each packet it sends or receives; it only tries again if something unexpected happens (such as a neighboring computer going down). This is the benefit of using connections.

4.4  Connection Processes

4.4.1  Establishing Connections

Application programs establish sockets with a series of system calls that look up the distant address, establish a socket, and then connect to the machine on the other end.
    /* look up host */
     server = gethostbyname(SERVER_NAME);
     /* get socket */
     sockfd = socket(AF_INET, SOCK_STREAM, 0);
     /* set up address */
     address.sin_family = AF_INET;
     address.sin_port = htons(PORT_NUM);
     memcpy(&address.sin_addr,server->h_addr,server->h_length);
     /* connect to server */
     connect(sockfd, &address, sizeof(address));
 
The gethostbyname() function simply looks up a host (such as ``viper.cs.u.edu'') and returns a structure that contains an Internet (IP) address. This has very little to do with routing (only inasmuch as the host may have to query the network to look up an address) and is simply a translation from a human readable form (text) to a computer compatible one (an unsigned 4 byte integer).

The socket() call is more interesting. It creates a socket object, with the appropriate data type (a sock for INET sockets) and initializes it. The socket contains inode information and protocol specific pointers for various network functions. It also establishes defaults for queues (incoming, outgoing, error, and backlog), a dummy header info for TCP sockets, and various state information.

Finally, the connect() call goes to the protocol dependent connection routine (e.g., tcp_v4_connect() or udp_connect()). UDP simply establishes a route to the destination (since there is no virtual connection). TCP establishes the route and then begins the TCP connection process, sending a packet with appropriate connection and window flags set.

4.4.2  Socket Call Walk-Through

  • Check for errors in call
  • Create (allocate memory for) socket object
  • Put socket into INODE list
  • Establish pointers to protocol functions (INET)
  • Store values for socket type and protocol family
  • Set socket state to closed
  • Initialize packet queues

4.4.3  Connect Call Walk-Through

  • Check for errors
  • Determine route to destination:
    • Check routing table for existing entry (return that if one exists)
    • Look up destination in FIB
    • Build new routing table entry
    • Put entry in routing table and return it

  • Store pointer to routing entry in socket
  • Call protocol specific connection function (e.g., send a TCP connection packet)
  • Set socket state to established

4.4.4  Closing Connections

Closing a socket is fairly straightforward. An application calls close() on a socket, which becomes a sock_close() function call. This changes the socket state to disconnecting and calls the data member's (INET socket's) release function. The INET socket in turn cleans up its queues and calls the transport protocol's close function, tcp_v4_close() or udp_close(). These perform any necessary actions (the TCP functions may send out packets to end the TCP connection) and then clean up any data structures they have remaining. Note that no changes are made for routing; the (now-empty) socket no longer has a reference to the destination and the entry in the routing cache will remain until it is freed for lack of use.

4.4.5  Close Walk-Through

  • Check for errors (does the socket exist?)
  • Change the socket state to disconnecting to prevent further use
  • Do any protocol closing actions (e.g., send a TCP packet with the FIN bit set)
  • Free memory for socket data structures (TCP/UDP and INET)
  • Remove socket from INODE list

4.5  Linux Functions

The following is an alphabetic list of the Linux kernel functions that are most important to connections, where they are in the source code, and what they do. To follow function calls for creating a socket, begin with sock_create(). To follow function calls for closing a socket, begin with sock_close().
destroy_sock - net/ipv4/af_inet.c (195)
   deletes any timers
   calls any protocols specific destroy functions
   frees the socket's queues
   frees the socket structure itself
 
 fib_lookup() - include/net/ip_fib.h (153)
   calls tb_lookup() [= fn_hash_lookup()] on local and main tables
   returns route or unreachable error
 
 fn_hash_lookup() - net/ipv4/fib_hash.c (261)
   looks up and returns route to an address
 
 inet_create() - net/ipv4/af_inet.c (326)
   calls sk_alloc() to get memory for sock
   initializes sock structure:
     sets proto structure to appropriate values for TCP or UDP
     calls sock_init_data()
     sets family,protocol,etc. variables
   calls the protocol init function (if any)
 
 inet_release() - net/ipv4/af_inet.c (463)
   changes socket state to disconnecting
   calls ip_mc_drop_socket to leave multicast group (if necessary)
   sets owning socket's data member to NULL
   calls sk->prot->close() [=TCP/UDP_close()]
 
 ip_route_connect() - include/net/route.h (140)
   calls ip_route_output() to get a destination address
   returns if the call works or generates an error
   otherwise clears the route pointer and try again
 
 ip_route_output() - net/ipv4/route.c (1664)
   calculates hash value for address
   runs through table (starting at hash) to match addresses and TOS
   if there is a match, updates stats and return route entry
   else calls ip_route_output_slow()
 
 ip_route_output_slow() - net/ipv4/route.c (1421)
   if source address is known, looks up output device
   if destination address is unknown, sets up loopback
   calls fib_lookup() to find route in FIB
   allocates memory new routing table entry
   initializes table entry with source, destination, TOS, output device,
       flags
   calls rt_set_nexthop() to find next destination
   returns rt_intern_hash(), which installs route in routing table
 
 rt_intern_hash() - net/ipv4/route.c (526)
   loops through rt_hash_table (starting at hash value)
   if keys match, put rtable entry in front bucket
   else put rtable entry into hash table at hash
 
 >>> sock_close() - net/socket.c (476)
   checks if socket exists (could be null)
   calls sock_fasync() to remove socket from async list
   calls sock_release()
 
 >>> sock_create() - net/socket.c (571)
   checks parameters
   calls sock_alloc() to get an available inode for the socket and
       initialize it
   sets socket->type (to SOCK_STREAM, SOCK_DGRAM...)
   calls net_family->create() [= inet_create()] to build sock structure
   returns established socket
 
 sock_init_data() - net/core/sock.c (1018)
   initializes all generic sock values
 
 sock_release() - net/socket.c (309)
   changes state to disconnecting
   calls sock->ops->release() [= inet_release()]
   calls iput() to remove socket from inode list
 
 sys_socket() - net/socket.c (639)
   calls sock_create() to get and initialize socket
   calls get_fd() to assign an fd to the socket
   sets socket->file to fcheck() (pointer to file)
   calls sock_release() if anything fails
 
 tcp_close() - net/ipv4/tcp.c (1502)
   check for errors
   pops and discards all packets off incoming queue
   sends messages to destination to close connection (if required)
 
 tcp_connect() - net/ipv4/tcp_output.c (910)
   completes connection packet with appropriate bits and window sizes set
   puts packet on socket output queue
   calls tcp_transmit_skb() to send packet, initiating TCP connection
 
 tcp_v4_connect() - net/ipv4/tcp_ipv4.c (571)
   checks for errors
   calls ip_route_connect() to find route to destination
   creates connection packet  
   calls tcp_connect() to send packet
 
 udp_close() - net/ipv4/udp.c (954)
   calls udp_v4_unhash() to remove socket from socket list
   calls destroy_sock()
 
 udp_connect() - net/ipv4/udp.c (900)
   calls ip_route_connect() to find route to destination
   updates socket with source and destination addresses and ports
   changes socket state to established
   saves the destination route in sock->dst_cache
 

Chapter 5
Sending Messages

This chapter presents the sending side of message trafficking. It provides an overview of the process, examines the layers packets travel through, details the actions of each layer, and summarizes the implementation code within the kernel.

5.1  Overview

s_tx.gif

Figure 5.1: Message transmission.

An outgoing message begins with an application system call to write data to a socket. The socket examines its own connection type and calls the appropriate send routine (typically INET). The send function verifies the status of the socket, examines its protocol type, and sends the data on to the transport layer routine (such as TCP or UDP). This protocol creates a new buffer for the outgoing packet (a socket buffer, or struct sk_buff skb), copies the data from the application buffer, and fills in its header information (such as port number, options, and checksum) before passing the new buffer to the network layer (usually IP). The IP send functions fill in more of the buffer with its own protocol headers (such as the IP address, options, and checksum). It may also fragment the packet if required. Next the IP layer passes the packet to the link layer function, which moves the packet onto the sending device's xmit queue and makes sure the device knows that it has traffic to send. Finally, the device (such as a network card) tells the bus to send the packet.

5.2  Sending Walk-Through

5.2.1  Writing to a Socket

  • Write data to a socket (application)
  • Fill in message header with location of data (socket)
  • Check for basic errors - is socket bound to a port? can the socket send messages? is there something wrong with the socket?
  • Pass the message header to appropriate transport protocol (INET socket)

5.2.2  Creating a Packet with UDP

  • Check for errors - is the data too big? is it a UDP connection?
  • Make sure there is a route to the destination (call the IP routing routines if the route is not already established; fail if there is no route)
  • Create a UDP header (for the packet)
  • Call the IP build and transmit function

5.2.3  Creating a Packet with TCP

  • Check connection - is it established? is it open? is the socket working?
  • Check for and combine data with partial packets if possible
  • Create a packet buffer
  • Copy the payload from user space
  • Add the packet to the outbound queue
  • Build current TCP header into packet (with ACKs, SYN, etc.)
  • Call the IP transmit function

5.2.4  Wrapping a Packet in IP

  • Create a packet buffer (if necessary - UDP)
  • Look up route to destination (if necessary - TCP)
  • Fill in the packet IP header
  • Copy the transport header and the payload from user space
  • Send the packet to the destination route's device output funtion

5.2.5  Transmitting a Packet

  • Put the packet on the device output queue
  • Wake up the device
  • Wait for the scheduler to run the device driver
  • Test the medium (device)
  • Send the link header
  • Tell the bus to transmit the packet over the medium

5.3  Linux Functions

The following is an alphabetic list of the Linux kernel functions that are most important to message traffic, where they are in the source code, and what they do. To follow function calls, begin with sock_write().
dev_queue_xmit() - net/core/dev.c (579)
   calls start_bh_atomic()
   if device has a queue
     calls enqueue() to add packet to queue
     calls qdisc_wakeup() [= qdisc_restart()] to wake device
   else calls hard_start_xmit()
   calls end_bh_atomic()
 
 DEVICE->hard_start_xmit() - device dependent, drivers/net/DEVICE.c
   tests to see if medium is open
   sends header
   tells bus to send packet
   updates status
 
 inet_sendmsg() - net/ipv4/af_inet.c (786)
   extracts pointer to socket sock
   checks socket to make sure it is working
   verifies protocol pointer
   returns sk->prot[tcp/udp]->sendmsg()
 
 ip_build_xmit - net/ipv4/ip_output.c (604)
   calls sock_alloc_send_skb() to establish memory for skb
   sets up skb header
   calls getfrag() [= udp_getfrag()]  to copy buffer from user space
   returns rt->u.dst.output() [= dev_queue_xmit()]
 
 ip_queue_xmit() - net/ipv4/ip_output.c (234)
   looks up route
   builds IP header
   fragments if required
   adds IP checksum
   calls skb->dst->output() [= dev_queue_xmit()]
 
 qdisc_restart() - net/sched/sch_generic.c (50)
   pops packet off queue
   calls dev->hard_start_xmit()
   updates status
   if there was an error, requeues packet
 
 sock_sendmsg() - net/socket.c (325)
   calls scm_sendmsg() [socket control message]
   calls sock->ops[inet]->sendmsg() and destroys scm
 
 >>> sock_write() - net/socket.c (399)
   calls socki_lookup() to associate socket with fd/file inode
   creates and fills in message header with data size/addresses
   returns sock_sendmsg()
 
 tcp_do_sendmsg() - net/ipv4/tcp.c  (755)
   waits for connection, if necessary
   calls skb_tailroom() and adds data to waiting packet if possible
   checks window status
   calls sock_wmalloc() to get memory for skb
   calls csum_and_copy_from_user() to copy packet and do checksum
   calls tcp_send_skb()
 
 tcp_send_skb() - net/ipv4/tcp_output.c (160)
   calls __skb_queue_tail() to add packet to queue
   calls tcp_transmit_skb() if possible
 
 tcp_transmit_skb() - net/ipv4/tcp_output.c (77)
   builds TCP header and adds checksum
   calls tcp_build_and_update_options()
   checks ACKs,SYN
   calls tp->af_specific[ip]->queue_xmit()
 
 tcp_v4_sendmsg() - net/ipv4/tcp_ipv4.c (668)
   checks for IP address type, opens connection, port addresses
   returns tcp_do_sendmsg()
 
 udp_getfrag() - net/ipv4/udp.c (516)
   copies and checksums a buffer from user space
 
 udp_sendmsg() - net/ipv4/udp.c (559)
   checks length, flags, protocol
   sets up UDP header and address info
   checks multicast
   fills in route
   fills in remainder of header
   calls ip_build_xmit()
   updates UDP status
   returns err
 

Chapter 6
Receiving Messages

This chapter presents the receiving side of message trafficking. It provides an overview of the process, examines the layers packets travel through, details the actions of each layer, and summarizes the implementation code within the kernel.

6.1  Overview

r_rx.gif

Figure 6.1: Receiving messages.

An incoming message begins with an interrupt when the system notifies the device that a message is ready. The device allocates storage space and tells the bus to put the message into that space. It then passes the packet to the link layer, which puts it on the backlog queue, and marks the network flag for the next ``bottom-half'' run.

The bottom-half is a Linux system that minimizes the amount of work done during an interrupt. Doing a lot of processing during an interrupt is not good precisely because it interrupts a running process; instead, interrupt handlers have a ``top-half'' and a ``bottom-half''. When the interrupt arrives, the top-half runs and takes care of any critical operations, such as moving data from a device queue into kernel memory. It then marks a flag that tells the kernel that there is more work to do - when the processor has time - and returns control to the current process. The next time the process scheduler runs, it sees the flag, does the extra work, and only then schedules any normal processes.

When the process scheduler sees that there are networking tasks to do it runs the network bottom-half. This function pops packets off of the backlog queue, matches them to a known protocol (typically IP), and passes them to that protocol's receive function. The IP layer examines the packet for errors and routes it; the packet will go into an outgoing queue (if it is for another host) or up to the transport layer (such as TCP or UDP). This layer again checks for errors, looks up the socket associated with the port specified in the packet, and puts the packet at the end of that socket's receive queue.

Once the packet is in the socket's queue, the socket will wake up the application process that owns it (if necessary). That process may then make or return from a read system call that copies the data from the packet in the queue into its own buffer. (The process may also do nothing for the time being if it was not waiting for the packet, and get the data off the queue when it needs it.)

6.2  Receiving Walk-Through

6.2.1  Reading from a Socket (Part I)

  • Try to read data from a socket (application)
  • Fill in message header with location of buffer (socket)
  • Check for basic errors - is the socket bound to a port? can the socket accept messages? is there something wrong with the socket?
  • Pass the message header with to the appropriate transport protocol (INET socket)
  • Sleep until there is enough data to read from the socket (TCP/UDP)

6.2.2  Receiving a Packet

  • Wake up the receiving device (interrupt)
  • Test the medium (device)
  • Receive the link header
  • Allocate space for the packet
  • Tell the bus to put the packet into the buffer
  • Put the packet on the backlog queue
  • Set the flag to run the network bottom half when possible
  • Return control to the current process

6.2.3  Running the Network ``Bottom Half''

  • Run the network bottom half (scheduler)
  • Send any packets that are waiting to prevent interrupts (bottom half)
  • Loop through all packets in the backlog queue and pass the packet up to its Internet reception protocol - IP
  • Flush the sending queue again
  • Exit the bottom half

6.2.4  Unwrapping a Packet in IP

  • Check packet for errors - too short? too long? invalid version? checksum error?
  • Defragment the packet if necessary
  • Get the route for the packet (could be for this host or could need to be forwarded)
  • Send the packet to its destination handling routine (TCP or UDP reception, or possibly retransmission to another host)

6.2.5  Accepting a Packet in UDP

  • Check UDP header for errors
  • Match destination to socket
  • Send an error message back if there is no such socket
  • Put packet into appropriate socket receive queue
  • Wake up any processes waiting for data from that socket

6.2.6  Accepting a Packet in TCP

  • Check sequence and flags; store packet in correct space if possible
  • If already received, send immediate ACK and drop packet
  • Determine which socket packet belongs to
  • Put packet into appropriate socket receive queue
  • Wake up and processes waiting for data from that socket

6.2.7  Reading from a Socket (Part II)

  • Wake up when data is ready (socket)
  • Call transport layer receive function
  • Move data from receive queue to user buffer (TCP/UDP)
  • Return data and control to application (socket)

6.3  Linux Functions

The following is an alphabetic list of the Linux kernel functions that are most important to receiving traffic, where they are in the source code, and what they do. To follow functions calls from the network up, start with DEVICE_rx(). To follow functions calls from the application down, start with sock_read().
>>> DEVICE_rx() - device dependent, drivers/net/DEVICE.c
   (gets control from interrupt)
   performs status checks to make sure it should be receiving
   calls dev_alloc_skb() to reserve space for packet
   gets packet off of system bus
   calls eth_type_trans() to determine protocol type
   calls netif_rx()
   updates card status
   (returns from interrupt)
 
 inet_recvmsg() - net/ipv4/af_inet.c (764)
   extracts pointer to socket sock
   checks socket to make sure it is accepting
   verifies protocol pointer
   returns sk->prot[tcp/udp]->recvmsg()
 
 ip_rcv() - net/ipv4/ip_input.c (395)
   examines packet for errors:
     invalid length (too short or too long)
     incorrect version (not 4)
     invalid checksum
   calls __skb_trim() to remove padding
   defrags packet if necessary
   calls ip_route_input() to route packet
   examines and handle IP options
   returns skb->dst->input() [= tcp_rcv,udp_rcv()]
 
 net_bh() - net/core/dev.c (835)
   (run by scheduler)
   if there are packets waiting to go out, calls qdisc_run_queues()
       (see sending section)
   while the backlog queue is not empty
     let other bottom halves run
     call skb_dequeue() to get next packet
     if the packet is for someone else (FASTROUTED) put onto send queue
     loop through protocol lists (taps and main) to match protocol type
     call pt_prev->func() [= ip_rcv()] to pass packet to appropriate
         protocol
   call qdisc_run_queues() to flush output (if necessary)
 
 netif_rx() - net/core/dev.c (757)
   puts time in skb->stamp
   if backlog queue is too full, drops packet
   else
     calls skb_queue_tail() to put packet into backlog queue
     marks bottom half for later execution
 
 sock_def_readable() - net/core/sock.c (989)
   calls wake_up_interruptible() to put waiting process on run queue
   calls sock_wake_async() to send SIGIO to socket process
 
 sock_queue_rcv_skb() - include/net/sock.h (857)
   calls skb_queue_tail() to put packet in socket receive queue
   calls sk->data_ready() [= sock_def_readable()]
 
 >>> sock_read() - net/socket.c (366)
   sets up message headers
   returns sock_recvmsg() with result of read
 
 sock_recvmsg() - net/socket.c (338)
   reads socket management packet (scm) or packet by
       calling sock->ops[inet]->recvmsg()
 
 tcp_data() - net/ipv4/tcp_input.c (1507)
   shrinks receive queue if necessary
   calls tcp_data_queue() to queue packet
   calls sk->data_ready() to wake socket
 
 tcp_data_queue() - net/ipv4/tcp_input.c (1394)
   if packet is out of sequence:
     if old, discards immediately
     else calculates appropriate storage location
   calls __skb_queue_tail() to put packet in socket receive queue
   updates connection state
 
 tcp_rcv_established() - net/ipv4/tcp_input.c (1795)
   if fast path
     checks all flags and header info
     sends ACK
     calls _skb_queue_tail() to put packet in socket receive queue
   else (slow path)
     if out of sequence, sends ACK and drops packet
     check for FIN, SYN, RST, ACK
     calls tcp_data() to queue packet
     sends ACK
 
 tcp_recvmsg() - net/ipv4/tcp.c (1149)
   checks for errors
   wait until there is at least one packet available
   cleans up socket if connection closed
   calls memcpy_toiovec() to copy payload from the socket buffer into
       the user space
   calls cleanup_rbuf() to release memory and send ACK if necessary
   calls remove_wait_queue() to wake process (if necessary)
 
 udp_queue_rcv_skb() - net/ipv4/udp.c (963)
   calls sock_queue_rcv_skb()
   updates UDP status (frees skb if queue failed)
 
 udp_rcv() - net/ipv4/udp.c (1062)
   gets UDP header, trims packet, verifies checksum (if required)
   checks multicast
   calls udp_v4_lookup() to match packet to socket
   if no socket found, send ICMP message back, free skb, and stop
   calls udp_deliver() [= udp_queue_rcv_skb()]
 
 udp_recvmsg() - net/ipv4/udp.c (794)
   calls skb_recv_datagram() to get packet from queue
   calls skb_copy_datagram_iovec() to move the payload from the socket buffer
       into the user space
   updates the socket timestamp
   fills in the source information in the message header
   frees the packet memory
 

Chapter 7
IP Forwarding

This chapter presents the pure routing side (by IP forwarding) of message traffic. It provides an overview of the process, examines the layers packets travel through, details the actions of each layer, and summarizes the implementation code within the kernel.

7.1  Overview

See Figure 7.1 for an abstract diagram of the the forwarding process. (It is essentially a combination of the receiving and sending processes.)

f_fwd.gif

Figure 7.1: IP forwarding.

A forwarded packet arrives with an interrupt when the system notifies the device that a message is ready. The device allocates storage space and tells the bus to put the message into that space. It then passes the packet to the link layer, which puts it on the backlog queue, marks the network flag for the next ``bottom-half'' run, and returns control to the current process.

When the process scheduler next runs, it sees that there are networking tasks to do and runs the network ``bottom-half''. This function pops packets off of the backlog queue, matches them to IP, and passes them to the receive function. The IP layer examines the packet for errors and routes it; the packet will go up to the transport layer (such as TCP or UDP if it is for this host) or sideways to the IP forwarding function. Within the forwarding function, IP checks the packet and sends an ICMP message back to the sender if anything is wrong. It then copies the packet into a new buffer and fragments it if necessary.

Finally the IP layer passes the packet to the link layer function, which moves the packet onto the sending device's xmit queue and makes sure the device knows that it has traffic to send. Finally, the device (such as a network card) tells the bus to send the packet.

7.2  IP Forward Walk-Through

7.2.1  Receiving a Packet

  • Wake up the receiving device (interrupt)
  • Test the medium (device)
  • Receive the link header
  • Allocate space for the packet
  • Tell the bus to put the packet into the buffer
  • Put the packet on the backlog queue
  • Set the flag to run the network bottom half when possible
  • Return control to the current process

7.2.2  Running the Network ``Bottom Half''

  • Run the network bottom half (scheduler)
  • Send any packets that are waiting to prevent interrupts (net_bh)
  • Loop through all packets in the backlog queue and pass the packet up to its Internet reception protocol - IP
  • Flush the sending queue again
  • Exit the bottom half

7.2.3  Examining a Packet in IP

  • Check packet for errors - too short? too long? invalid version? checksum error?
  • Defragment the packet if necessary
  • Get the route for the packet (could be for this host or could need to be forwarded)
  • Send the packet to its destination handling routine (retransmission to another host in this case)

7.2.4  Forwarding a Packet in IP

  • Check TTL field (and decrement it)
  • Check packet for improper (undesired) routing
  • Send ICMP back to sender if there are any problems
  • Copy packet into new buffer and free old one
  • Set any IP options
  • Fragment packet if it is too big for new destination
  • Send the packet to the destination route's device output function

7.2.5  Transmitting a Packet

  • Put the packet on the device output queue
  • Wake up the device
  • Wait for the scheduler to run the device driver
  • Test the medium (device)
  • Send the link header
  • Tell the bus to transmit the packet over the medium

7.3  Linux Functions

The following is an alphabetic list of the Linux kernel functions that are most important to IP forwarding, where they are in the source code, and what they do. To follow the functions calls, start with DEVICE_rx().
dev_queue_xmit() - net/core/dev.c (579)
   calls start_bh_atomic()
   if device has a queue
     calls enqueue() to add packet to queue
     calls qdisc_wakeup() [= qdisc_restart()] to wake device
   else calls hard_start_xmit()
   calls end_bh_atomic()
 
 DEVICE->hard_start_xmit() - device dependent, drivers/net/DEVICE.c
   tests to see if medium is open
   sends header
   tells bus to send packet
   updates status
 
 >>> DEVICE_rx() - device dependent, drivers/net/DEVICE.c
   (gets control from interrupt)
   performs status checks to make sure it should be receiving
   calls dev_alloc_skb() to reserve space for packet
   gets packet off of system bus
   calls eth_type_trans() to determine protocol type
   calls netif_rx()
   updates card status
   (returns from interrupt)
 
 ip_finish_output() - include/net/ip.h (140)
   sets sending device to output device for given route
   calls output function for destination [= dev_queue_xmit()]
 
 ip_forward() - net/ipv4/ip_forward.c (72)
   checks for router alert
   if packet is not meant for any host, drops it
   if TTL has expired, drops packet and sends ICMP message back
   if strict route cannot be followed, drops packet and sends ICMP
       message back to sender
   if necessary, sends ICMP message telling sender packet is redirected
   copies and releases old packet
   decrements TTL
   if there are options, calls ip_forward_options() to set them
   calls ip_send()
 
 ip_rcv() - net/ipv4/ip_input.c (395)
   examines packet for errors:
     invalid length (too short or too long)
     incorrect version (not 4)
     invalid checksum
   calls __skb_trim() to remove padding
   defrags packet if necessary
   calls ip_route_input() to route packet
   examines and handle IP options
   returns skb->dst->input() [= ip_forward()]
 
 ip_route_input() - net/ipv4/route.c (1366)
   calls rt_hash_code() to get index for routing table
   loops through routing table (starting at hash) to find match for packet
   if it finds match:
     updates stats for route (time and usage)
     sets packet destination to routing table entry
     returns success
   else 
     checks for multicasting addresses
     returns result of ip_route_input_slow() (attempted routing)
 
 ip_route_output_slow() - net/ipv4/route.c (1421)
   if source address is known, looks up output device
   if destination address is unknown, set up loopback
   calls fib_lookup() to find route
   allocates memory new routing table entry
   initializes table entry with source, destination, TOS, output device,
       flags
   calls rt_set_nexthop() to find next destination
   returns rt_intern_hash(), which installs route in routing table
 
 ip_send() - include/net/ip.h (162)
   calls ip_fragment() if packet is too big for device
   calls ip_finish_output()
 
 net_bh() - net/core/dev.c (835)
   (run by scheduler)
   if there are packets waiting to go out, calls qdisc_run_queues()
       (see sending section)
   while the backlog queue is not empty
     let other bottom halves run
     call skb_dequeue() to get next packet
     if the packet is for someone else (FASTROUTED) put onto send queue
     loop through protocol lists (taps and main) to match protocol type
     call pt_prev->func() [= ip_rcv()] to pass packet to appropriate
         protocol
   call qdisc_run_queues() to flush output (if necessary)
 
 netif_rx() - net/core/dev.c (757)
   puts time in skb->stamp
   if backlog queue is too full, drops packet
   else
     calls skb_queue_tail() to put packet into backlog queue
     marks bottom half for later execution
 
 qdisc_restart() - net/sched/sch_generic.c (50)
   pops packet off queue
   calls dev->hard_start_xmit()
   updates status
   if there was an error, requeues packet
 
 rt_intern_hash() - net/ipv4/route.c (526)
   puts new route in routing table
 

Chapter 8
Basic Internet Protocol Routing

This chapter presents the basics of IP Routing. It provides an overview of how routing works, examines how routing tables are established and updated, and summarizes the implementation code within the kernel.

8.1  Overview

Linux maintains three sets of routing data - one for computers that are directly connected to the host (via a LAN, for example) and two for computers that are only indirectly connected (via IP networking). Examine Figure 8.1 to see how entries for a computer in the general example might look.

r_overview.gif

Figure 8.1: General routing table example.

The neighbor table contains address information for computers that are physically connected to the host (hence the name ``neighbor''). It includes information on which device connects to which neighbor and what protocols to use in exchanging data. Linux uses the Address Resolution Protocol (ARP) to maintain and update this table; it is dynamic in that entries are added when needed but eventually disappear if not used again within a certain time. (However, administrators can set up entries to be permanent if doing so makes sense.)

Linux uses two complex sets of routing tables to maintain IP addresses: an all-purpose Forwarding Information Base (FIB) with directions to every possible address, and a smaller (and faster) routing cache with data on frequently used routes. When an IP packet needs to go to a distant host, the IP layer first checks the routing cache for an entry with the appropriate source, destination, and type of service. If there is such an entry, IP uses it. If not, IP requests the routing information from the more complete (but slower) FIB, builds a new cache entry with that data, and then uses the new entry. While the FIB entries are semi-permanent (they usually change only when routers come up or go down) the cache entries remain only until they become obsolete (they are unused for a ``long'' period).

8.2  Routing Tables

Note: within these tables, there are references to variables of types such as u32 (host byte order) and __u32 (network byte order). On the Intel architecture they are both equivalent to unsigned ints and in point of fact they are translated (using the ntohl function) anyway; the type merely gives an indication of the order in which the value it contains is stored.

8.2.1  The Neighbor Table

The Neighbor Table (whose structure is shown in Figure 8.2) contains information about computers that are physically linked with the host computer. (Note that the source code uses the European spelling, ``neighbour''.) Entries are not (usually) persistent; this table may contain no entries (if the computer has not passed any network traffic recently) or may contain as many entries as there are computers physically connected to its network (if it has communicated with all of them recently). Entries in the table are actually other table structures which contain addressing, device, protocol, and statistical information.

r_neigh_struct.gif

Figure 8.2: Neighbor Table data structure relationships.

struct neigh_table *neigh_tables - this global variable is a pointer to a list of neighbor tables; each table contains a set of general functions and data and a hash table of specific information about a set of neighbors. This is a very detailed, low level table containing specific information such as the approximate transit time for messages, queue sizes, device pointers, and pointers to device functions.

Neighbor Table (struct neigh_table) Structure - this structure (a list element) contains common neighbor information and table of neighbor data and pneigh data. All computers connected through a single type of connection (such as a single Ethernet card) will be in the same table.

  • struct neigh_table *next - pointer to the next table in the list.
  • struct neigh_parms parms - structure containing message travel time, queue length, and statistical information; this is actually the head of a list.
  • struct neigh_parms *parms_list - pointer to a list of information structures.
  • struct neighbour *hash_buckets[] - hash table of neighbors associated with this table; there are NEIGH_HASHMASK+1 (32) buckets.
  • struct pneigh_entry *phash_buckets[] - hash table of structures containing device pointers and keys; there are PNEIGH_HASHMASK+1 (16) buckets.
  • Other fields include timer information, function pointers, locks, and statistics.

Neighbor Data (struct neighbour) Structure - these structures contain the specific information about each neighbor.

  • struct device *dev - pointer to the device that is connected to this neighbor.
  • __u8 nud_state - status flags; values can be incomplete, reachable, stale, etc.; also contains state information for permanence and ARP use.
  • struct hh_cache *hh - pointer to cached hardware header for transmissions to this neighbor.
  • struct sk_buff_head arp_queue - pointer to ARP packets for this neighbor.
  • Other fields include list pointers, function (table) pointers, and statistical information.

8.2.2  The Forwarding Information Base

r_fib_gen.gif

Figure 8.3: Forwarding Information Base (FIB) conceptual organization.

The Forwarding Information Base (FIB) is the most important routing structure in the kernel; it is a complex structure that contains the routing information needed to reach any valid IP address by its network mask. Essentially it is a large table with general address information at the top and very specific information at the bottom. The IP layer enters the table with the destination address of a packet and compares it to the most specific netmask to see if they match. If they do not, IP goes on to the next most general netmask and again compares the two. When it finally finds a match, IP copies the ``directions'' to the distant host into the routing cache and sends the packet on its way. See Figures 8.3 and 8.4 for the organization and data structures used in the FIB - note that Figure 8.3 shows some different FIB capabilities, like two sets of network information for a single zone, and so does not follow the general example.)

struct fib_table *local_table, *main_table - these global variables are the access points to the FIB tables; they point to table structures that point to hash tables that point to zones. The contents of the main_table variable are in /proc/net/route.

FIB Table fib_table Structure - include/net/ip_fib.h - these structures contain function jump tables and each points to a hash table containing zone information. There are usually only one or two of these.

  • int (*tb_functions)() - pointers to table functions (lookup, delete, insert, etc.) that are set during initialization to fn_hash_function().
  • unsigned char tb_data[0] - pointer to the associated FIB hash table (despite its declaration as a character array).
  • unsigned char tb_id - table identifier; 255 for local_table, 254 for main_table.
  • unsigned tb_stamp

Netmask Table fn_hash Structure - net/ipv4/fib_hash.c - these structures contain pointers to the individual zones, organized by netmask. (Each zone corresponds to a uniquely specific network mask.) There is one of these for each FIB table (unless two tables point to the same hash table).

  • struct fn_zone *fn_zones[33] - pointers to zone entries (one zone for each bit in the mask; fn_zone[0] points to the zone for netmask 0x0000, fn_zone[1] points to the zone for 0x8000, and fn_zone[32] points to the zone for 0xFFFF.
  • struct fn_zone *fn_zone_list - pointer to first (most specific) non-empty zone in the list; if there is an entry for netmask 0xFFFF it will point to that zone, otherwise it may point to zone 0xFFF0 or 0xFF00 or 0xF000 etc.

Network Zone fn_zone Structure - net/ipv4/fib_hash.c - these structures contain some hashing information and pointers to hash tables of nodes. There is one of these for each known netmask.

  • struct fn_zone *fz_next - pointer to the next non-empty zone in the hash structure (the next most general netmask; e.g., fn_hash- > fn_zone[28]- > fz_next might point to fn_hash- > fn_zone[27]).
  • struct fib_node **fz_hash - pointer to a hash table of nodes for this zone.
  • int fz_nent - the number of entries (nodes) in this zone.
  • int fx_divisor - the number of buckets in the hash table associated with this zone; there are 16 buckets in the table for most zones (except the first zone - 0000 - the loopback device).
  • u32 fz_hashmask - a mask for entering the hash table of nodes; 15 (0x0F) for most zones, 0 for zone 0).
  • int fz_order - the index of this zone in the parent fn_hash structure (0 to 32).
  • u32 fz_mask - the zone netmask defined as ~((1<<(32-fz_order))-1); for example, the first (zero) element is 1 shifted left 32 minus 0 times (0x10000), minus 1 (0xFFFF), and complemented (0x0000). The second element has a netmask of 0x8000, the next 0xC000, the next 0xE000, 0xF000, 0xF800, and so on to the last (32d) element whose mask is 0xFFFF.

Network Node Information fib_node Structure - net/ipv4/fib_hash.c - these structures contain the information unique to each set of addresses and a pointer to information about common features (such as device and protocols); there is one for each known network (unique source/destination/TOS combination).

  • struct fib_node *fn_next - pointer to the next node.
  • struct fib_info *fn_info - pointer to more information about this node (that is shared by many nodes).
  • fn_key_t fn_key - hash table key - the least significant 8 bits of the destination address (or 0 for the loopback device).
  • Other fields include specific information about this node (like fn_tos and fn_state).

Network Protocol Information (fib_info) Structure - include/net/ip_fib.h - these structures contain protocol and hardware information that are specific to an interface and therefore common to many potential zones; several networks may be addressable through the same interface (like the one that leads to the rest of the Internet). There is one of these for each interface.

  • fib_protocol - index to a network protocol (e.g., IP) used for this route.
  • struct fib_nh fib_nh[0] - contains a pointer to the device used for sending or receiving traffic for this route.
  • Other fields include list pointers and statistical and reference data (like fib_refcnt and fib_flags.

r_fib_struct.gif

Figure 8.4: Forwarding Information Base (FIB) data relationships.

FIB Traversal Example:

  1. ip_route_output_slow() (called because the route is not in the routing cache) sets up an rt_key structure with a source address of 172.16.0.7, a destination address of 172.16.0.34, and a TOS of 2.
  2. ip_route_output_slow() calls fib_lookup() and passes it the key to search for.
  3. fib_lookup() calls local_table- > tb_lookup() (which is a reference to the fn_hash_lookup function) to make the local table find the key.
  4. fn_hash_lookup() searches the local table's hash table, starting in the most specific zone - 24 (netmask 255.255.255.0 dotted decimal) (pointed to by the fn_zone_list variable).
  5. fz_key() builds a test key by ANDing the destination address with the zone netmask, resulting in a key value 172.16.0.0.
  6. fz_chain() performs the hash function (see fn_hash()) and ANDs this value with the zone's fz_hashmask (15) to get an index (6) into the zone's hash table of nodes. Unfortunately, this node is empty; there are no possible matches in this zone.
  7. fn_hash_lookup() moves to the next non-empty zone - 16 (netmask 255.255.0.0 dotted decimal) (pointed to by the current zone's fz_next variable).
  8. fz_key() builds a new test key by ANDing the destination address with this zone's netmask, resulting in a key value of 172.16.0.0.
  9. fz_chain() performs the hash function and ANDs this value with the zone's fz_hashmask (15) to get an index (10) into the zone's hash table of nodes. There is a node in that slot.
  10. fn_hash_lookup() compares its search key to the node's key. They do not match, but the search key value is less than that of the node key, so it moves on to the next node.
  11. fn_hash_lookup() compares its search key to the new node's key. These do match, so it does some error checking and tests for an exact match with the node and its associated info variable.
  12. Since everything matches, fn_hash_lookup() fills in a fib_result structure with all the information about this route. (Otherwise it would continue checking more nodes and more zones until it finds a match or fails completely.)
  13. ip_route_output_slow() takes the fib_result structure and, assuming everything is in order, creates a new routing cache entry from it.

8.2.3  The Routing Cache

r_cache_gen.gif

Figure 8.5: Routing Cache conceptual organization.

The routing cache is the fastest method Linux has to find a route; it keeps every route that is currently in use or has been used recently in a hash table. When IP needs a route, it goes to the appropriate hash bucket and searches the chain of cached routes until finds a match, then sends the packet along that path. (See Section 8.2.2 for what happens when the route is not yet in the cache.) Routes are chained in order, most frequently used first, and have timers and counters that remove them from the table when they are no longer in use. See Figure 8.5 for an abstract overview and Figures 8.6 and 8.7 for detailed diagrams of the data structures.

struct rtable *rt_hash_table[RT_HASH_DIVISOR] - this global variable contains 256 buckets of (pointers to) chains of routing cache (rtable) entries; the hash function combines the source address, destination address, and TOS to get an entry point to the table (between 0 and 255). The contents of this table are listed in /proc/net/rt_cache.

Routing Table Entry (rtable) Structure - include/net/route.h - these structures contain the destination cache entries and identification information specific to each route.

  • union < struct dst_entry dst; struct rtable* rt_next) > u - this is an entry in the table; the union structure allows quick access to the next entry in the table by overusing the rtable's next field to point to the next cache entry if required.
  • __u32 rt_dst - the destination address.
  • __u32 rt_src - the source address.
  • rt_int iif - the input interface.
  • __u32 rt_gateway - the address of the neighbor to route through to get to a destination.
  • struct rt_key key - a structure containing the cache lookup key (with src, dst, iif, oif, tos, and scope fields)
  • Other fields contain flags, type, and other miscellaneous information.

Destination Cache (dst_entry) Structure - include/net/dst.h - these structures contain pointers to specific input and output functions and data for a route.

  • struct device *dev - the input/output device for this route.
  • unsigned pmtu - the maximum packet size for this route.
  • struct neighbor *neighbor - a pointer to the neighbor (next link) for this route.
  • struct hh_cache *hh - a pointer to the hardware header cache; since this is the same for every outgoing packet on a physical link, it is kept for quick access and reuse.
  • int (*input)(struct sk_buff*) - a pointer to the input function for this route (typically tcp_recv()).
  • int (*output)(struct sk_buff*) - a pointer to the output function for this route (typically dev_queue_xmit()).
  • struct dst_ops *ops - a pointer to a structure containing the family, protocol, and check, reroute, and destroy functions for this route.
  • Other fields hold statistical and state information and links to other routing table entries.

Neighbor Link (neighbor) Structure - include/net/neighbor.h - these structures, one for each host that is exactly one hop away, contain pointers to their access functions and information.

  • struct device *dev - a pointer to device that is physically connected to this neighbor.
  • struct hh_cache *hh - a pointer to the hardware header that always precedes traffic sent to this neighbor.
  • int (*output)(struct sk_buff*) - a pointer to the output function for this neighbor (typically dev_queue_xmit()?).
  • struct sk_buff_head arp_queue - the first element in the ARP queue for traffic concerning this neighbor - incoming or outgoing?
  • struct neigh_ops *ops - a pointer to a structure that containing family data and and output functions for this link.
  • Other fields hold statistical and state information and references to other neighbors.

r_cache_struct.gif

Figure 8.6: Routing Cache data structure relationships.

r_dst_struct.gif

Figure 8.7: Destination Cache data structure relationships.

Routing Cache Traversal Example:

  1. ip_route_output() (called to find a route) calls rt_hash_code() with a source address of 172.16.1.1, a destination address of 172.16.1.6, and a TOS of 2.
  2. rt_hash_code() performs a hash function on the source, destination, and TOS and ANDs the result with 255 to get an entry into the hash table (5).
  3. ip_route_output() enters the hash table at index 5. There is an entry there, but the destination addresses do not match.
  4. ip_route_output() moves to the next entry (pointed to by the u.rt_next field of the last entry). This one matches in every case - destination address, source address, iif of 0, matching oif, and acceptable TOS.
  5. ip_route_output() updates the statistics in the newfound dst_cache structure of the table entry, sets a pointer for the calling function to refer to the route, and returns a 0 indicating success.

8.2.4  Updating Routing Information

Linux only updates routing information when necessary, but the tables change in different manners. The routing cache is the most volatile, while the FIB usually does not change at all.

The neighbor table changes as network traffic is exchanged. If a host needs to send something to an address that is on the local subnet but not already in the neighbor table, it simply broadcasts an ARP request and adds a new entry in the neighbor table when it gets a reply. Periodically entries time out and disappear; this cycle continues indefinitely (unless a route has been specifically marked as ARP permanent). The kernel handles most changes automatically.

The FIB on most hosts and even routers remains static; it is filled in during initialization with every possible zone to route through all connected routers and never changes unless one of the routers goes down. (See Chapter 9 for details on IP routing daemons). Changes have to come through external ioctl() calls to add or delete zones.

The routing cache changes frequently depending on message traffic. If a host needs to send packets to a remote address, it looks up the address in the routing cache (and FIB if necessary) and sends the packet off through the appropriate router. On a host connected to a LAN with one router to the Internet, every entry will point to either a neighbor or the router, but there may be many entries that point to the router (one for each distant address). The entries are created as connections are made and time out quickly when traffic to that address stops flowing. Everything is done with IP level calls to create routes and kernel timers to delete them.

8.3  Linux Functions

The following is an alphabetic list of the Linux kernel functions that are most important to routing, where they are in the source code, and what they do.
arp_rcv() - net/ipv4/arp.c (542)
   checks for errors (non-ARP device, no device, packet not for host,
       device type does not match, etc.)
   check for operation - only understands REPLY and REQUEST
   extracts data from packet
   check for bad requests - loopback or multicast addresses
   checks for duplicate address detection packet (sends reply if necessary)
   if the message is a request and ip_route_input() is true:
     if the packet is a local one:
       calls neigh_event_ns() to look up and update neighbor that sent packet
       checks for hidden device (does not reply if hidden)
       sends reply with the device address
     otherwise:
       calls neigh_event_ns() to look up and update neighbor that sent packet
       calls neigh_release()
       if necessary, calls arp_send() with the address
       otherwise calls pneigh_enqueue() and returns 0
   if the message is a reply:
     calls __neigh_lookup()
     checks to see if multiple ARP replies have come in; keeps only the
         fastest (first) one
     calls neigh_update() to update ARP entry
 	calls neigh_release()
   frees the skbuffer and returns 0
 
 arp_send() - net/ipv4/arp.c (434)
   checks to make sure device supports ARP
   allocates an skbuffer
   fills in buffer header information
   fills in the ARP information
   calls dev_queue_xmit() with the finished packet
 
 arp_req_get() - net/ipv4/arp.c (848)
   calls __neigh_lookup() to find entry for given address
   copies data from neighbor entry to arpreq entry
   returns 0 if found or ENXIO if address not in ARP table
 
 fib_get_procinfo() - net/ipv4/fib_frontend.c (109)
   prints header and results of main_table->fn_hash_get_info() for proc FS
 
 fib_lookup() - include/net/ip_fib.h (153)
   calls tb_lookup() [= fn_hash_lookup()] on local_table and main_table
   if either one has an entry, it fills in fib_result and returns 0
   else returns unreachable error
 
 fib_node_get_info() - net/ipv4/fib_semantics.c (971)
   prints fib_node and fib_info contents for proc FS
 
 fib_validate_source() - net/ipv4/fib_frontend.c (191)
   tests incoming packet's device and address
   returns error code if something is wrong
   returns 0 if packet seems legal
 
 fn_hash() - net/ipv4/fib_hash.c (108)
   performs a hash function on a destination address:
     u32 h = ntohl(daddr)>>(32 - fib_zone->fz_order);
     h ^= (h>>20);
     h ^= (h>>10);
     h ^= (h>>5);
     h &= FZ_HASHMASK(fz);   // FZ_HASHMASK is 15 for almost all zones
 
 fn_hash_get_info() - net/ipv4/fib_hash.c (723)
   loops through zones in a FIB table printing fib_node_get_info() for
       proc FS
 
 fn_hash_lookup() - net/ipv4/fib_hash.c (261)
   loops through the zones in the given table
     loops through the nodes in each zone (starting at the hash entry)
       if the netmasks (node and destination) match
           checks the TOS and node status
           calls fib_semantic_match() to check packet type
           fills in fib_result with success data and returns 0
   returns 1 if nothing matched
 
 fn_new_zone() - net/ipv4/fib_hash.c (220)
   allocates memory (in kernel) for new zone
   allocates space for 16 node buckets for zone (except first zone -
       0.0.0.0 [loopback] - which only gets one)
   stores netmask (leftmost n bits on, where n is the position of the
       zone in the table)
   searches for more specific zone in parent table
   inserts zone into zone list (most specific zone is head)
   installs new zone into parent table
   returns new zone
 
 fz_chain() - net/ipv4/fib_hash.c (133)
   calls fn_hash() to get a hash value
   returns the fib_node in the fib_zone at the hash index
 
 ip_dev_find() - net/ipv4/fib_frontend.c (147)
   looks up and returns the device with a given address in the local table
 
 ip_route_connect() - include/net/route.h (140)
   calls ip_route_output() to get a destination address
   returns if the call works or generates an error
   otherwise clears the route pointer and try again
 
 ip_route_input() - net/ipv4/route.c (1366)
   calculates hash value for address
   runs through table (starting at hash) to find connection match
       (source, destination, TOS, and IIF/OIF)
   if there is a match, updates stats and returns routing entry
   else calls ip_route_input_slow()
 
 ip_route_input_slow() - net/ipv4/route.c (1097)
   creates a routing table cache key
   checks for special addresses (like loopback, broadcast, or errors)
   calls fib_lookup() to find route
   allocates memory for new routing table entry
   initializes table entry with source, destination, TOS, output device,
       flags
   calls fib_validate_source() to test packet source
   printks message and returns error if source is bad
   calls rt_set_nexthop() to find next destination (neighbor)
   returns rt_intern_hash(), which installs route in routing table
 
 ip_route_output() - net/ipv4/route.c (1664)
   calculates hash value for address
   runs through table (starting at hash) to find connection match
       (source, destination, TOS, and IIF/OIF)
   if there is a match, updates stats and returns routing entry
   else calls ip_route_output_slow()
 
 ip_route_output_slow() - net/ipv4/route.c (1421)
   creates a routing table cache key
   if source address is known, calls ip_dev_find to determine output device
   if destination address is unknown, set up loopback
   calls fib_lookup() to find route
   allocates memory for new routing table entry
   initializes table entry with source, destination, TOS, output device,
       flags
   calls rt_set_nexthop() to find next destination (neighbor)
   returns rt_intern_hash(), which installs route in routing table
 
 ip_rt_ioctl() - net/ipv4/fib_frontend.c (250)
   switches on SIOCADDRT or SIOCDELRT (returns EINVAL otherwise)
   verifies permission and copies argument to kernel space
   converts copied argument to an rtentry structure
   if deleting a route, calls fib_get_table() and table->delete()
   else calls fib_new_table() and table->insert()
   frees argument space and returns 0 for success
 
 neigh_event_ns() - net/core/neighbour.c (760)
   calls __neigh_lookup() to find up address in neighbor table
   calls neigh_update()
   returns pointer to designated neighbor
 
 neigh_update() - net/core/neighbour.c (668)
   checks permissions to modify table
   checks neighbor status if this is not a new entry
   compares given address to cached one:
     if null or device has no address, uses current address
     if different, check override flag
   calls neigh_sync() to verify neighbor is still up
   updates neighbor contact time
   if old entry was valid and new one does not change address, returns 0
   if new address is different from old, replaces old with new
   if new and old states match, returns 0
   calls neigh_connect() or neigh_suspect() to make/check connection
   if old state was invalid:
     goes through packets in ARP queue, calling the neighbor output()
         function on each
     purges the ARP queue
   returns 0
 
 rt_cache_get_info() - net/ipv4/route.c (191)
   prints header and all elements of rt_hash_table for proc FS
 
 rt_hash_code() - net/ipv4/route.c (18)
   uses source address, destination address, and type of service to
       determine (and return) a hash value:
     hash = ((daddr&0xF0F0F0F0)>>4)|((daddr&0x0F0F0F0F)<<4);
     hash = hash^saddr^tos;
     hash = hash^(hash>>16);
     hash = (hash^(hash>>8)) & 0xFF;
 
 rt_intern_hash() - net/ipv4/route.c (526)
   puts new route in routing table
 

Chapter 9
Dynamic Routing with routed

This chapter presents dynamic routing as performed by a router (as opposed to an end host computer). It provides an overview of how the routed program implements routing protocols under Linux, examines how it modifies the kernel routing tables, and summarizes the implementation code.

9.1  Overview

A normal host computer has very limited options for routing packets; a message is either for itself or another computer, and if it is for another computer there are a very limited number of options for sending it on. Usually such a host needs only to put a packet out on a LAN for a ``gateway'' computer (router) to pick up and send on its way. Linux usually does not maintain any metric (distance) information about routes, even though there are variables for storing it in the FIB. For simple end-host routing, the only important question is ``can I get there from here'', not ``which way is best?''

However, a router must make decisions on where to send traffic. There may be several routes to a destination, and the router must select one (based on distance, measured in hops or some other metric such as the nebulous quality of service). The Routing Information Protocol (RIP) is a simple protocol that allows routing computers to track the distance to various destinations and to share this information amongst themselves.

Using RIP, each node maintains a table that contains the distance from itself to other networks and the route along which it will send packets to that destination. Periodically the routers update each other; when shorter routes becomes apparent, the node updates its table. Updates are simply RIP messages with the destination address and metric components of this table. See Figure 9.1 for a diagram of an RIP routing table and an RIP packet.

d_rip.gif

Figure 9.1: Routing Information Protocol packet and table.

9.2  How routed Works

routed is a widely available program for implementing RIP via UDP messages on POSIX computers. It is essentially a stand-alone program which uses ioctl() calls to get information from and update routing tables on the host machine.

9.2.1  Data Structures

routed maintains two identical data tables - one for hosts and one for networks. Each is a hash table with ROUTEHASHSIZ (32) buckets of chains of routing entries. The entries contain the RIP information (but can also line up with a struct rtentry so that routed can pass them to the kernel through ioctl() calls). Along with the basic destination, router, and metric information the entries store flags, state, and timer information.

9.2.2  Initialization

When routed begins, it performs various initialization actions and calls ioctl() to get interface information from the kernel. Next it sends out a RIP/UDP message requesting routing information from all neighboring routers. Finally it enters an infinite loop in which it waits for traffic or timers to make it do something.

9.2.3  Normal Operations

When RIP messages arrive (via a UDP socket), routed parses them and either modifies its table (for response messages) or sends information back to the requesting router (for requests). Sending information is simply a matter of looking up a destination in its own table, putting that information into a RIP packet, and sending it out through a UDP socket. Updating its table may have no impact (if there is no change or the change makes no difference) or it may result in a routing change. If the update reveals a shorter route to a destination, routed will update its own table and then call ioctl() to update the kernel's routing tables (the FIB).

When the update timer expires, every TIMER_RATE seconds, routed goes through every entry in both tables and updates their timers. Entries which are out of date are set to a distance of infinity (HOPCNT_INFINITY) and entries which are too old are deleted (only from the RIP table, not from the kernel's FIB). Finally, it sends an update to its neighboring routers. This update contains the new table information (response messages) for any entries which have changed since the last update.

routed leaves the actual routing to the normal kernel routing mechanisms; all it does is update the kernel's tables based on information from other routers and pass on its own routing information. The updates then change how the kernel routes packets, but routed itself does not actually do any routing.

9.3  routed Functions

The following is an alphabetic list of the routed program functions that are most important to routing, where they are in the source code, and what they do. The SOURCES directory shown represents the directory that contains the source code for the given network file.

The routed source is available as a package separate from the kernel source (Red Hat Linux uses the rpm package manager). The code below is from the netkit-routed-0.10 source code package, 8 March 1997. This package is available from the www.redhat.com/apps/download web page; specifically this came from www.redhat.com/swt/src/netkit-routed-0.10.src.html. Once downloaded, root can install the package with the following commands (starting from the directory with the package):

rpm -i netkit-routed-0.10.src.rpm
cd /usr/src/redhat/SOURCES
tar xzf netkit-routed-0.10.tar.gz
This creates a /usr/src/redhat/SOURCES/netkit-routed-0.10 directory and fills it with the source code for the routed program. This process should be similar (but is undoubtably not exactly the same) for other Linux distributions.

ifinit() - SOURCES/routed/startup.c (88)
   opens a UDP socket
   calls ioctl(SIOCFIGCONF) to get interface configuration
   loops through interfaces:
     calls ioctl() to get flags, broadcast address, metric, and netmask
     creates a new interface structure
     copies info into interface structure
     calls addrouteforif() to add routing entry for interface
   sets supplier variable if necessary
   closes socket
 
 process() - SOURCES/routed/main.c (298)
   starts a continuous loop:
     receives a packet (waits)
     verifies that packet is correct size
     calls rip_input() to handle (RIP) packet
 
 rip_input() - SOURCES/routed/input.c (60)
   traces input if necessary
   checks packet to make sure protocol and address are supported
   checks for RIP version (cannot be 0)
   switch based on packet content - 
     if packet is a request:
       checks request for validity
       if request is for all entries, calls supply()
       else looks up requested address, builds and sends response packet
     if packet is a trace on or off:
       verifies request came from a valid port
       if all is in order, sets trace to on or off
     if packet is a response:
       verifies response came from a router
       updates timer for interface
       loops through each entry in received packet:
         parses route information
         validates address family, host, and metric information
         updates hop count (adds metric in message to hop count to router
             that send message, subject to HOPCNT_INFINITY maximum)
         calls rtlookup() to find address in routing table 
         if this seems to be a new route:
            calls rtfind() to look for an equivalent route
            if it really is new, calls rtadd() and returns
         calls rtchange() to modify route if necessary (new route or
            hopcount change)
         updates route timers
   if there were changes:
     sends an update if neccessary
     updates general update timer information
 
 >>> routed main() - SOURCES/routed/main.c (78)
   opens routed log file
   calls getservbyname() to get UDP router
   sets up a UDP socket for RIP message traffic
   runs through command line arguments to set flags
   if not debugging, forks and runs program in new session (parent dies)
   calls rtinit() to initialize data tables
   calls ifinit() to fill in interface information
   calls toall() to request info from all other routers
   installs signal handlers for ALRM,HUP,TERM,INT,USR1,and USR2
   starts a continuous loop:
     if in need of update, sets up timer variables
     calls select() to wait for traffic
     if select() returns an error (other than EINTR), logs it
     if select() times out (time for update)
         calls toall() to broadcast update
         resets timer variables
     if there is traffic waiting on the socket, calls process()
 
 rtadd() - SOURCES/routed/tables.c (138)
   verifies address family is in proper range
   calls family af_rtflags() function to set routing flags
   determines hash value for appropriate table (host or net)
   creates and fills in new rt_entry structure
   calls insque() to add entry to table
   calls rtioctl() to add entry to kernel table
   if call fails:
     if route should work, calls family af_format() to add destination
         and gateway to kernel tables
     if host is unreachable, removes and frees entry
 
 rtchange() - SOURCES/routed/tables.c (207)
   determines if change necessitates adding or deleting gateways
   calls rtioctl() to add and/or delete routes
 
 rtfind() - SOURCES/routed/tables.c (100)
   determines hash value for host table
   loops through table; returns entry if addresses are equal
   determines hash value for net table
   goes back to loop through table, this time returning entry if a call
       to family af_netmatch() function returns true
   returns null (0) if no match
 
 rtinit() - SOURCES/routed/tables.c (336)
   loops through the net hash table, setting forward and back pointers
   loops through the host hash table, setting forward and back pointers
 
 rtioctl() - SOURCES/routed/tables.c (346)
   fills in rtentry structure from parameters
   outputs trace actions if necessary
   calls ioctl(SIOCADDRT or SIOCDELRT) to update kernel table
   returns result of ioctl() call (or -1 for erroneous parameter)
 
 rtlookup() - SOURCES/routed/tables.c (65)
   determines hash value for address
   runs through host table looking for match
   if unsuccessful at first, tries again with net table
   returns pointer to entry or null (0)
 
 sndmsg() - SOURCES/routed/output.c (77)
   calls the appropriate family output function
   traces the packet if necessary
 
 supply() - SOURCES/routed/ouput.c (91)
   creates an RIP response message
   loops through the routing host table
     loops through the routing entries
       checks to see if routing host needs the entry
       if so, puts routing info into packet and sends it
   goes back and does it again with the routing net table
 
 timer() - SOURCES/routed/timer.c (56)
   updates timer variables
   loops through the host table
     updates timer information for each entry
     deletes entry if it is too old
     changes metric to infinity if it is getting old
   goes back and does it again with net table
   calls toall() if update is due
 
 toall() - SOURCES/routed/output.c (55)
   loops through interfaces:
     sets destination address to broadcast or specific address
     calls passed function [sndmsg() or supply()] with address
 

Chapter 10
Editing Linux Source Code

10.1  The Linux Source Tree

Linux source code is usually in the /usr/src directory (if installed). There may be many versions in different directory trees (such as linux-2.2.5 or linux-2.2.14). There should be one soft link (linux) to the most current version of the code (i.e. linux linux-2.2.14).

This is an overview of the Linux source directory structure (not all branches are shown:

/usr/src/linux/

  • arch - architecture specific code, by processor
    • i386 - code for Intel processors (including 486 and Pentium lines)
      • boot - location of newly compiled kernels

  • drivers - code for drivers of all sorts
    • block - block device drivers (e.g., hard drives)
    • cdrom - CD ROM device drivers
    • net - network device drivers
    • pci - PCI bus drivers

  • fs - code for different file systems (EXT2, MS-DOS, etc.)
  • include - header files used throughout the code
    • asm asm-i386 - processor dependent headers
    • config - general configuration headers
    • linux - common headers
    • net - networking headers

  • kernel - code for the kernel specific routines
  • lib - code for errors, strings, and printf
  • mm - code for memory management
  • modules - object files and references for the kernel to load as required
  • net - code for networking
    • core - protocol independent code
    • ipv4 - code specific to IPv4
    • packet - protocol independent packet code
    • sched - code for scheduling network actions

10.2  Using EMACS Tags

The Linux source is obviously very large and spread throughout many files. A TAGS file allows you to quickly maneuver to a specific file in search of a reference.

10.2.1  Referencing with TAGS

Inside a file, move the cursor to a keyword you would like to look up (e.g., ``sock''). Press ``ESC'' ``.'' - EMACS will prompt for the tag to find (defaulting to the word your cursor is on); hit ``ENTER''. The first time you use it, you will have to specify which TAGS file to use (e.g., /usr/src/TAGS). Next EMACS will automatically open the appropriate file (e.g., /usr/src/linux/include/linux/sock.h) in a new buffer and put the cursor on the definition of that struct, #define, or function. If the definition it brings up is not the one you were looking for, press ``CTRL-U'' ``ESC'' ``.'' to bring up alternate references.

These tags work even as you make changes to the source files, though they will run slower as more and more changes are made. EMACS stores the tags in a file (defaulted to TAGS) with each reference, filename, and line number. If the tag is not at the stored line number, EMACS will search the file to find the new location.

10.2.2  Constructing TAGS files

If you need to start from scratch, follow the steps below.

The command to make a tags file is:

etags filename
The command to append new information onto a tags file is:
etags -a filename
These put the new tags into the file TAGS in the current directory. Filenames are stored as given, so absolute references will always refer to the same files while relative references depend on the position of the TAGS file. (Read the man page for etags for more information).

For example, to create a tags file for the ipv4 source files, enter:

etags /usr/src/linux/net/ipv4/*.c
To add the header files, enter:
etags -a /usr/src/include/net/*.h
The TAGS file will now contain quick references to all the C source code and header information in those directories.

10.3  Using vi tags

The vi editor also supports the use of tags files (and creates them with the gctags command, which works almost exactly like the etags command shown above).

10.4  Rebuilding the Kernel

(See the Linux-kernel-HOWTO for more detailed instructions.)

This is a quick step-by-step guide to recompiling and installing a kernel from scratch.

  1. Go to the top of the source directory (/usr/src/linux). If there is not already a historical copy of a working .config file (such as the current one), MAKE ONE. Until you have enough experience that you no longer need this guide, do not overwrite anything until you have made sure there is a copy to which you can revert. (On the other hand, once you have a stable kernel version, there is no reason to keep old ones around. Even a development system should probably only have an original working version, a last known stable version, and a current version.

  2. Run make xconfig (make config and make menuconfig also work, but xconfig is by far the user-friendliest). Configure the system as desired; there is help available for most options. The config file should default to the current settings, so you should only have to change the things you want to add or take out. As a general rule, select ``Y'' for essential or frequently used features (like the ext2 file system), ``M'' for things that are sometimes useful (like sound drivers), and ``N'' for things that do not apply (like amateur radio support). If in doubt, consult the help text or include something as a module.

  3. Run make dep to make sure the options you heve selected will compile properly. This make take a few minutes as the computer checks all of the dependencies. If all goes well, the make program will simply exit; if there is a problem, it will display error messages and stop.

  4. Run make clean to remove old object files IF you want to recompile everything. This obviously will make the compilation process take longer.

  5. Run make bzImage to build the new kernel. (make zImage and make boot also build kernel images, but the bzImage will compile into the most compact file. If you are using one of these two methods for some reason, you may get a ``kernel too big'' error when you run lilo - try again with a bzImage.) This will take quite some time, depending on available memory.

  6. Run make modules to build any modules (not included in the main kernel image).

  7. Rename the old modules if necessary:
    mv /lib/modules/2.2.xx /lib/modules/2.2.xx-old
    (Note that you will not have to do this if you are compiling a completely new version; the old ones will still be in /lib/modules/2.2.xx when you build version 2.2.yy.)

  8. Run make modules_install to install the new modules. You must do this even if you built a monolithic kernel (one with no modules). (Note that there may be a Red Hat module-info text file or link in the boot directory; it is not terribly important and this does not update it.)

  9. Copy the new kernel to the /boot directory and change the kernel link (usually vmlinuz):
    cp arch/i386/boot/bzImage /boot/vmlinuz-2.2.xx
    ln -sf /boot/vmlinuz-2.2.xx /boot/vmlinuz

  10. Copy the new System.map file to the /boot directory and change the map link:
    cp System.map /boot/System.map-2.2.xx
    ln -sf /boot/System.map-2.2.xx /boot/System.map

  11. Create a new initrd file if there are any SCSI devices on the computer:
    /sbin/mkinitrd /boot/initrd-2.2.xx.img 2.2.xx

  12. Edit the file /etc/lilo.conf to install the new kernel; copy the block for the old kernel (image=vmlinuz) and change the existing one to keep it as an option. For example, rename the image to vmlinuz-2.2.xx-old and change the label to stable. This way you can always reboot to the current (presumably stable) kernel if your changes cause problems.

  13. Run /sbin/lilo to install the new kernel as a boot option.

  14. Reboot the computer with the new kernel.

  15. If the new kernel does not work properly, boot the old kernel and reconfigure the system before trying again.

10.5  Patching the Kernel Source

Linux is a constantly changing operating system; updates can be released every few months. There are two ways to install a new kernel version: downloading the new source in its entirety or downloading patches and applying them.

Downloading the entire source may be preferable to guarantee everything works properly. To do so, download the latest kernel source and install (untar) it. Note that this will (probably) be a complete distribution, not a machine-specific one, and will contain a lot of extra code. Much of this can be deleted, but the configuration Makefiles rely on some for information. If space is an issue, delete the *.c and *.h files in the non-i386 arch/ and include/asm-* directories, but tread lightly.

Downloading patches may be quicker to do, but is somewhat harder. Because of distribution variations, changes you have made, or other modifications the patches may not quite work properly. You must apply patch files in order (to go from 2.2.12 to 2.2.14, first apply patch 2.2.13 then apply 2.2.14). Nevertheless, patches may be preferable because they work on an existing directory tree.

Once you have downloaded a patch (and unzipped it, if necessary), simply put it in the directory above linux (e.g., /usr/src/) and run the patch program to install it:

patch -Np0 -verbose -r rejfile < patch-2.2.xx (where xx is the patch version)
The -N option ignores patches that are already applied, and the -p0 assumes the patch wants to apply itself to a source in a linux directory. The -r rejfile option puts all the patch rejects into one file (rejfile) - which may or may not be what you want to do. If you have not kept the entire source distribution, you will have to skip many changes (for different processor architectures) by simply hitting ``ENTER'' at the ``patch which file'' and ``ignore patch'' prompts. Once you are comfortable with the process, run it without the -verbose and -r rejfile options.

Once you have a new kernel version, follow the instructions on rebuilding the kernel to actually start using it. You probably will not have to change any of the configurations options, but you will almost definitely want to run make clean to remove any old object files.

Chapter 11
Linux Modules

This chapter presents the Linux module system. It provides an overview of how modules work, describes how to install and remove them, and presents an example program.

11.1  Overview

Linux kernels more recent than 2.0 can be (and usually) are modularized. There is a portion of the kernel that remains in memory constantly (the most frequently used processes, such as the scheduler) but other processes are only loaded when needed. An MS-DOS file system for reading disks, for example, might be loaded only on mounting such a disk and then unloaded when no longer needed. This keeps the space the kernel requires at any one time small while allowing it to do more and more. It is still possible to put everything into one ``monolithic'' kernel that will not need modules, but that is usually done only for special purpose machines (where all the required processes are known in advance).

Another advantage of modules is that the kernel can load and unload them dynamically (and automatically with the kerneld daemon). This means that a (super) user can load a module, test it, unload it, and debug it repeatedly without having to reboot the computer. This document assumes that the user has superuser access (you must be root to install and remove modules) and the kernel is configured for modules. (With a monolithic kernel, it is possible to set configuration options not to even allow modules.)

11.2  Writing, Installing, and Removing Modules

11.2.1  Writing Modules

Modules are just like any other programs except that they run in kernel space. As such, they must define MODULE and include module.h and any other kernel header files that define functions or variables they use. Modules can be quite simple (as the example shows) but they can also be quite complex, such as device drivers and entire file systems.

This is the general module format:

#define MODULE
 #include <linux/module.h>
 /* ... other required header files ...  */
 
 /*
  *  ... module declarations and functions ...
  */
 
 int init_module() {
   /* code kernel will call when installing module */
 }
 
 void cleanup_module() {
   /* code kernel will call when removing module */
 }
 

Modules that use the kernel source must be compiled with gcc with the option -I/usr/src/linux/include; this ensures that the files included will be from the proper source tree.

Note that not all kernel variables are exported for modules to use, even if the code declares them to be extern. The /proc/ksyms file or ksyms program display the exported symbols (not many of which are useful for networking). Recent Linux kernels export both the symbol and its version number using the EXPORT_SYMBOL(x) macro. For user created variables, use the EXPORT_SYMBOL_NOVERS(x) macro instead or the linker will not retain the variable in the kernel symbol table. Module writers may also want to use the EXPORT_NO_SYMBOLS macro; modules export all of their variables by default.

11.2.2  Installing and Removing Modules

Installing and removing modules is as simple as calling a program with the name of the compiled module. (You must be a superuser to install or remove a module.)

The insmod program installs a module; it first links the module with the kernel's exported symbol table to resolve references and then installs the code in kernel space.

/sbin/insmod module_name

The rmmod program removes an installed module and any references that it has exported.

/sbin/rmmod module_name

The lsmod program lists all the currently installed modules:

    /sbin/lsmod
     Module       Size  Used by
     cdrom       13368   0 (autoclean) [ide-cd]
     3c59x       19112   1 (autoclean)
 

11.3  Example

This is a complete example of a very simple module.

simple_module.c

/* simple_module.c
  *
  * This program provides an example of how to install a trivial module
  *   into the Linux kernel.  All the module does is put a message into
  *   the log file when it is installed and removed.
  *
  */
 
 #define MODULE
 #include <linux/module.h>
 /* kernel.h contains the printk function */
 #include <linux/kernel.h>
 
 /*************************************************************** init_module
  * the kernel calls this function when it loads the module */
 int init_module() {
   printk("<1>The simple module installed itself properly.\n");
   return 0;
 }  /* init_module */
 
 /************************************************************ cleanup_module
  * the kernel calls this function when it removes the module */
 void cleanup_module() {
   printk("<1>The simple module is now uninstalled.\n");
 }  /* cleanup_module */
 

This is the Makefile:

# Makefile for simple_module
 
 CC = gcc -I/usr/src/linux/include/config
 
 CFLAGS = -O2 -D__KERNEL__ -Wall
 
 simple_module.o: simple_module.c
 
 install:
 	/sbin/insmod simple_module
 
 remove:
 	/sbin/rmmod simple_module
 

To use (must be root):

root# make
 root# make install
 root# make remove
 root# tail /var/log/messages
 ... kernel: The simple module installed itself properly.
 ... kernel: The simple module is now uninstalled.
 

Chapter 12
The proc File System

This chapter presents the virtual proc file system. It provides an overview of how the file system works, shows how the existing network code uses the system, and details how to write and use proc entries from programs.

12.1  Overview

The proc file system is so named because it is found in the /proc directory on most Linux machines. NOT including the proc FS is a configuration option, but the system is a powerful tool of which many programs make frequent use. While designed to appear as a file system with directory structures and inodes, it is in fact a construct of registered functions which provide information about important variables.

The proc directory has many subdirectories - one for each running process and others for subsystems such as file systems, interfaces, terminals, and networking (/proc/net). There are also many files in the main /proc directory itself - interrupts, ioports, loadavg, and version to name a few. Within each process subdirectory (named for the process number) are files that describe the process' command line, current working directory, status, and so on.

The kernel traps proc file access and instead of executing ``normal'' file operations on them calls special (individually registered) functions instead. When a file in the /proc directory is ``created'', it is registered with a set of functions that tell the kernel what to do when the file is read from or written to. Most entries only allow reads and they simply print out the state of certain system variables for use by other programs or for perusal by knowledgeable users.

The only tricky thing about using proc files is that the kernel calls the information generation function each and every time the file is read; subsequent reads of a changing file without copying and buffering the results may yield very different results. The best way to use a proc file is to read it into a PAGE_SIZE-byte buffer. This will read the entire entry at once and the buffer will then allow consistent random accesses.

12.2  Network proc Files

This is a list of the most important files in the /proc/net/ directory, what they contain, and a reference to the function and file that creates them. Note that there are many other interesting proc entries, such as the /proc/sys files, /proc/ksyms, and /proc/modules to name only a few.
arp
displays the neighbor table (arp_tbl); the IP and hardware addresses, hardware type, device, and flags. (arp_get_info() : net/ipv4/arp.c 988)
dev
displays reception and transmission statistics for each registered interface
dev_stat
displays number of received packets dropped and throttle and FASTROUTE statistics (dev_proc_stats() : net/core/dev.c 1228)
netstat
displays sync cookie, pruning, and ICMP statistics (netstat_get_info() : net/ipv4/proc.c 355)
raw
displays address, queue, and timeout information for each open RAW socket from struct proto raw_prot (get__netinfo() : net/ipv4/proc.c 165)
route
displays the FIB table (main_table); the interface, address, gateway, flags, and usage information. (fib_get_procinfo()) : net/ipv4/fib_frontend.c 109)
rt_cache
displays the routing cache (rt_hash_table); the interface, address, gateway, usage, source, and other information. (rt_cache_get_info() : net/ipv4/route.c 191)
sockstat
displays number of sockets that have been used and some statistics on how many were TCP, UDP, and RAW (afinet_get_info() : net/ipv4/proc.c 244)
tcp
displays address, queue, and timeout information for each open TCP socket from struct proto tcp_prot (get__netinfo() : net/ipv4/proc.c 165)
udp
displays address, queue, and timeout information for each open UDP socket from struct proto udp_prot (get__netinfo() : net/ipv4/proc.c 165)

12.3  Registering proc Files

This section describes the simplest method for registering a read-only proc ``file'' entry (available only in Linux 2.0 and later releases). It is possible to create a more fully functional entry by defining file_operations and inode_operations structures. However, that method is significantly more complicated than the one presented here; look in the source code for details on implementing fully functional entry. The method described below - defining a function and then registering and unregistering the function - provides most of the functionality required for testing and tracking system resources. Only the kernel can register a proc file; users can do so by building and installing kernel modules (though only root can install and remove modules). These procedures assume that the Linux source is installed and the kernel is compiled to use modules.

12.3.1  Formatting a Function to Provide Information

static int read_proc_function(char *buf,char **start,off_t offset,int len,int unused)

This is the function that the Linux kernel will call whenever it tries to read from the newly created proc ``file''. The only parameter that is usually significant is buf - a pointer to the buffer the kernel makes available for storing information. The others normally will not change. (read_proc_function is of course the name of the new function.)

Typically this function prints out a header, iterates through a list or table printing its contents (using the normal sprintf routine), and returns the length of the resulting string. The only limitation is that the buffer (buf) is at most PAGE_SIZE bytes (this is at least 4KB).

For an example of this kind of function, look at the fib_get_procinfo() function beginning on line 109 of net/ipv4/fib_frontend.c. This function displays the contents of the main FIB table.

12.3.2  Building a proc Entry

Because this is part of the file system, the entry needs an inode. This is easily constructed using a struct proc_dir_entry:
#include <linux/proc_fs.h>
 struct proc_dir_entry new_proc_entry = {
    0,                     // low_ino - inode number (0 for dynamic)
    5,                     // namelen  - length of entry name
    "entry",               // name
    S_IFREG | S_IRUGO,     // mode
    1,                     // nlinks
    0,                     // uid - owner
    0,                     // gid - group
    0,                     // size - not used
    NULL,                  // ops - inode operations (use default)
    &read_proc_function    // read_proc - address of read function
                           // leave rest blank!
 }
 
The contents of this block can be used as shown by simply replacing the namelen, name, and read_proc_function fields with the desired values. Note that many of the kernel defined entries have predefined inode numbers (like PROC_NET_ROUTE, part of an enumeration defined in include/linux/proc_fs.h.

For an example of this kind of entry, look at the __init_func() function beginning on line 607 of net/ipv4/fib_frontend.c. This functions calls proc_net_register() (described below) with a newly created proc_dir_entry structure.

12.3.3  Registering a proc Entry

Once the read function and the inode entry are ready, all that remains is to register the new ``file'' with the proc system.

int proc_register(struct proc_dir_entry *dir, struct proc_dir_entry *entry)
int proc_net_register(struct proc_dir_entry *entry)

dir is a pointer to the directory in which the entry belongs - &proc_root and proc_net (defined in include/proc_fs.h) are probably the most useful. entry is a pointer to the entry itself, as created above. These two functions are identical except that proc_net_register automatically uses the /proc/net directory. They return either 0 (success) or EAGAIN (if there are no available inodes).

12.3.4  Unregistering a proc Entry

When an entry is no longer needed, it should be deleted by unregistering it.

int proc_unregister(struct proc_dir_entry *dir,int inode)
int proc_net_unregister(int inode)

dir is the proc directory in which the file resides, and inode is the inode number of the file. (The inode is available in the entry's struct proc_dir_entry.low_ino field if it is not a constant.) Again, these functions are identical except that proc_net_unregister automatically uses the /proc/net directory. They return either 0 (success) or EINVAL (if there is no such entry).

12.4  Example

This is a complete example of a module that installs a simple proc entry.

simple_entry.c

/* simple_entry.c
  *
  * This program provides an example of how to install an entry into the
  *   /proc File System.  All this entry does is display some statistical
  *   information about IP.
  */
 
 #define MODULE
 #include <linux/module.h>
 /* proc_fs.h contains proc_dir_entry and register/unregister prototypes */
 #include <linux/proc_fs.h>
 /* ip.h contains the ip_statistics variable */
 #include <net/ip.h>
 
 
 /************************************************************ show_ip_stats
  * this function is what the /proc FS will call when anything tries to read
  *   from the file /proc/simple_entry - it puts some of the kernel global
  *   variable ip_statistics's contents into the return buffer */
 int show_ip_stats(char *buf,char **start,off_t offset,int len,int unused) {
   len = sprintf(buf,"Some IP Statistics:\nIP Forwarding is ");
   if (ip_statistics.IpForwarding)
     len += sprintf(buf+len,"on\n");
   else
     len += sprintf(buf+len,"off\n");
   len += sprintf(buf+len,"Default TTL:  %lu\n",ip_statistics.IpDefaultTTL);
   len += sprintf(buf+len,"Frag Creates: %lu\n",ip_statistics.IpFragCreates);
   /* this could show more.... */
   return len;
 }  /* show_ip_stats */
 
 /**************************************************************** test_entry
  * this structure is a sort of registration form for the /proc FS; it tells
  *   the FS to allocate a dynamic inode, gives the "file" a name, and gives
  *   the address of a function to call when the file is read  */
 struct proc_dir_entry test_entry = {
   0,                     /* low_ino - inode number (0 for dynamic)  */
   12,                    /* namelen - length of entry name          */
   "simple_entry",        /* name                                    */
   S_IFREG | S_IRUGO,     /* mode                                    */
   1,                     /* nlinks                                  */
   0,                     /* uid - owner                             */
   0,                     /* gid - group                             */
   0,                     /* size - not used                         */
   NULL,                  /* ops - inode operations (use default)    */
   &show_ip_stats         /* read_proc - address of read function    */
                          /* leave rest blank!                       */
 };
 
 /*************************************************************** init_module
  * this function installs the module; it simply registers a directory entry
  *   with the /proc FS  */
 int init_module() {
   /* register the function with the proc FS */
   int err = proc_register(&proc_root,&test_entry);
   /* put the registration results in the log */
   if (!err)
     printk("<1> simple_entry: registered with inode %d.\n",
           test_entry.low_ino);
   else
     printk("<1> simple_entry: registration error, code %d.\n",err);
   return err;
 }  /* init_module */
 
 /************************************************************ cleanup_module
  * this function removes the module; it simply unregisters the directory
  *   entry from the /proc FS  */
 void cleanup_module() {
   /* unregister the function from the proc FS */
   int err = proc_unregister(&proc_root,test_entry.low_ino);
   /* put the unregistration results in the log */
   if (!err)
     printk("<1> simple_entry: unregistered inode %d.\n",
           test_entry.low_ino);
   else
     printk("<1> simple_entry: unregistration error, code %d.\n",err);
 }  /* cleanup_module */
 

This is the Makefile:

# Makefile for simple_entry
 
 CC = gcc -I/usr/src/linux/include
 
 CFLAGS = -O2 -D__KERNEL__ -Wall
 
 simple_entry.o: simple_entry.c
 
 install:
 	/sbin/insmod simple_entry
 
 remove:
 	/sbin/rmmod simple_entry
 

To use (must be root):

root# make
 root# make install
 root# cat /proc/simple_entry
 Some IP Statistics:
 IP Forwarding is on
 Default TTL:  64
 Frag Creates: 0
 root# make remove
 root# tail /var/log/messages
 ... kernel: simple_entry: registered with inode 4365.
 ... kernel: simple_entry: unregistered inode 4365.
 

Chapter 13
Example - Packet Dropper

This sample experiment inserts a routine into the kernel that selectively drops packets to a given host. It discusses the placement of the code, outlines the data from an actual trial, presents a lightweight analysis of the results, and includes the code itself.

13.1  Overview

This program is implemented as a module that, while installed, compares each outgoing packet's destination address to a given target. If they match, it randomly drops a percentage of those packets. It does this for all IP traffic, no matter where it was generated and what transport protocol it uses. Implementing this requires a modification to the kernel (to allow a module access to the transmission functions) and a module that takes advantage of that modification.

13.2  Considerations

Code Placement
This code could be built directly into the kernel or it could be designed as a module:
  • Kernel - this is conceptually much simpler; simply adding some code to the kernel is a fairly easy matter. However, it makes semi-permanent changes and takes a long time to debug, since the entire kernel must be recompiled, installed, and rebooted for every change.
  • Module - this is much safer and easier since the (super) user can install, remove, and debug modules quite painlessly. However, it requires access to the kernel that is not always available - even from a module. The kernel does not always export the variables that a module may need to access. (See the discussion on the ksyms program in Chapter 11.)
  • Both - this is the best method; by performing a few minor modifications to the kernel code to export necessary variables and make use of a module only if it is loaded, a user can recompile the kernel once and then perform tests and experiments with modules. This still has the disadvantage of opening potential security holes on a system, but since only the experimenter knows how they are implemented, this is a minimal risk.

Protocol Level
This code could be implemented at many levels:
  • Device Driver - this is a possibility since all traffic comes through the device. However, this breaks the layering protocols and requires hacking a (presumably) stable hardware driver.
  • Generic Device Functions - this is the best choice, since this is the lowest level through which all traffic travels (specifically the dev_queue_xmit() and netif_rx() functions). It still violates the protocol layering, but all of the modifications can be made in one section of code.
  • IP Protocol - this is conceptually the right place to insert a special function, either in the input, routing, or output routines. However, this is unsuitable precisely because there are three different routines in the implementation that a packet might go through - ip_forward() (forwarded packets), ip_queue_xmit() (TCP packets), or ip_build_xmit() (UDP packets). See the coding sections in Chapters 5 and 7 to see how these routines interact. These functions would be a good choice for inserting a special-purpose dropper, but not one that affects all traffic.
  • Transport Protocol - these routines would be appropriate for affecting specific traffic types (such as UDP only) but are not useful for this example.

13.3  Experimental Systems and Benchmarks

This example was implemented on two computers that are connected through a single router as shown in Figure 13.1; the router runs the modifed kernel and packet dropper module. In the general example, this represents traffic flowing between neon and eagle, with dodge/viper dropping packets for eagle.

x_setup.gif

Figure 13.1: Experimental system setup.

The switch is a Cisco Catalyst 2900 set up with Virtual LANs (VLANs) for each ``subnetwork'' (one for the source computer and one for the destination computer, with the routing computer acting as the router between the two. The switch operates entirely on the link level and is essentially invisible for routing purposes.

The routing computer (dodge/viper) is a Dell Optiplex GX1 with a Pentium II/350 processor and 128M of RAM. It has three 3Com 3c59x Ethernet cards with 10Mbps connections to the switch.

One host computer (neon) is an AST Premmia GX with a Pentium/100 processor and 32M of RAM. It has an AMD Lance Ethernet card with a 10Mbps connection to the switch.

The other host computer (eagle) is a Dell Optiplex XL590 with a Pentium/90 processor and 32M of RAM. It has a 3Com 3c509 Ethernet card with a 10Mbps connection to the switch.

All computers have the Red Hat 6.1 distribution of Linux; the source and destination computers have standard recompiled version 2.2.14 kernels, while the router uses either a standard (2.2.14) kernel or a slightly modified one as indicated.

The first benchmark is a ``ping-pong'' test that establishes a TCP connection and then repeatedly sends packets back and forth. It returns a total transmission time (from start to finish, not including making and closing the connection); dividing the time by the number of iterations yields an average Round Trip Time (RTT). This test was run with 20,000 iterations of 5 byte packets and 5,000 iterations of 500 byte packets.

The second benchmark is a ``blast'' test that establishes a TCP connection and then sends data from a source to a destination. It returns a total transmission time (from start to finish, not including making and closing the connection); multiplying the number of packets by the size of the packets and dividing by the time yields the throughput. This test was run with 50,000 5 byte packets, 5,000 500 byte packets, and 1,000 1500 byte packets.

The benchmarks were run on both machines (i.e., from neon to eagle and from eagle to neon), but in both cases only packets to eagle were dropped. In each trial the blast test was run once with default settings (100 packets of 1 byte each) before running the performance tests ``for record'' to ensure that the routing cache and any protocol tables were in a normalized state. The complete suite was run ten times to capture variations between trials (the averages are presented here). None of the machines (including the router) were running any other user programs beyond a login shell and the appropriate module, client, or server programs (not even X Windows).

13.4  Results and Preliminary Analysis

13.4.1  Standard Kernel

These are the reference standards; these routines were run with the two computers directly connected (NOT routed) and while the router had an unmodified Linux 2.2.14 kernel. The error rate on such a direct connection is near zero.

ping-pong

                           Mean Time (sec)     Average RTT (millisec)
                Drop Rate   20K@5    5K@500         20K@5    5K@500
 Direct -
  neon to eagle:   ---      17.24     28.98          0.86     5.80
  eagle to neon:   ---      17.20     28.99          0.86     5.80
 Routed -
  neon to eagle:  (0.0%)    24.53     48.59          1.23     9.72
  eagle to neon:  (0.0%)    24.36     48.46          1.22     9.69
 

blast

                               Mean Time (sec)       Throughput (Mbits/sec)
                Drop Rate   50K*5  10K*500  1K*1500   50K*5  10K*500  1K*1500
 Direct -
  neon to eagle:   ---       0.56    3.19     1.89     3.55    6.26    6.36
  eagle to neon:   ---       0.78    3.03     1.77     2.58    6.61    6.76
 Routed -
  neon to eagle:  (0.0%)     0.56    3.19     1.92     3.60    6.27    6.26
  eagle to neon:  (0.0%)     0.77    3.19     1.93     2.60    6.27    6.23
 

13.4.2  Modified Kernel Dropping Packets

These are the experimental results. The drop rate of 0.0% provides a reference for measuring the overhead of calling the test and random functions without dropping any packets.

ping-pong

                           Mean Time (sec)     Average RTT (millisec)
                Drop Rate   20K@5    5K@500         20K@5    5K@500
 neon to eagle:    0.0%     25.55     49.12          1.28     9.82
                   0.1%     29.87     51.11          1.49    10.22
                   0.5%     44.78     58.07          2.24    11.61
                   1.0%     65.37     68.77          3.27    13.75
                   5.0%    245.51    160.09         12.28    32.02
                  10.0%    506.03    290.77         25.30    58.15
 eagle to neon:    0.0%     25.53     49.21          1.28     9.84
                   0.1%     29.08     50.92          1.45    10.18
                   0.5%     45.87     59.21          2.29    11.84
                   1.0%     66.19     68.66          3.31    13.73
                   5.0%    235.68    156.94         11.78    31.39
                  10.0%    519.61    297.02         25.98    59.40
 

blast

                               Mean Time (sec)       Throughput (Mbits/sec)
                Drop Rate   50K*5  10K*500  1K*1500   50K*5  10K*500  1K*1500
 neon to eagle:    0.0%      0.55    3.19     1.91     3.64    6.26    6.27
                   0.1%      0.55    3.07     1.93     3.62    6.51    6.21
                   0.5%      0.55    2.95     1.76     3.64    6.77    6.82
                   1.0%      0.55    2.87     1.75     3.65    6.96    6.87
                   2.5%      0.59    3.36     2.04     3.38    5.59    5.90
                   5.0%      0.63    4.63     2.71     3.19    4.31    4.43
                  10.0%      1.06    7.08     5.11     1.89    2.83    2.35
                  20.0%      3.43   30.35    18.55     0.58    0.66    0.65
 eagle to neon:    0.0%      0.79    3.21     1.93     2.53    6.23    6.23
                   0.1%      0.77    3.22     1.89     2.59    6.20    6.35
                   0.5%      0.80    3.24     1.88     2.51    6.17    6.39
                   1.0%      0.77    3.24     1.91     2.60    6.17    6.27
                   2.5%      0.79    3.17     1.90     2.53    6.31    6.33
                   5.0%      0.78    3.17     1.91     2.57    6.31    6.29
                  10.0%      0.81    3.85     2.51     2.48    5.20    4.78
                  20.0%      2.02    4.06     2.51     0.99    4.92    4.78
 

13.4.3  Preliminary Analysis

What follows is an elementary examination of the results. It is NOT intended as an exhaustive analysis, and indeed the experiment was not extensive enough to provide hard data from which to draw definite conclusions. However, this does demonstrate the multitude of factors involved and the effects that a few lines of code can have on a network. Further analysis, if desired, is left as an exercise for the reader.

x_pong.gif

Figure 13.2: Ping-pong benchmark results.

x_blast.gif

Figure 13.3: Blast benchmark results.

The kernel modifications and module insertion had a small but measurable impact on a TCP connection (measured by the increased RTT). For very small packets, this difference was approximately 0.05 msec; for large packets it was 0.10 msec. Why should there be a difference? Note that the direction of travel and packet size made a large difference on the throughput. This is an indication that processor speed and layering overhead are affecting the RTT; for a 1500 byte packet, 66 bytes of wrappers (20 for TCP, 20 for IP, and at least 26 for Ethernet) are not very significant - but for a 5 byte packet, that overhead is very large. Assume that the actual ``cost'' of inserting the module the delay for the larger packets, 0.10 msec.

Dropping packets from a TCP connection resulted in a fairly linear drop in performance on the ping-pong test; see the graph in Figure 13.2. This is as expected; when either a packet or acknowledgement is lost, the sender pauses and then sends again. The RTT is also very close (certainly within the expected experimental error) no matter which machine is the ``source''; again this is because the benchmark tests the behavior of both machines at the same time.

At low packet sizes, the throughput was very different depending on which way data was sent. This is because one machine (eagle) was slower than the other. For a large number of very small packets, the chokepoint in the network is not the medium or the interface, but the speed at which the processor can build and send packets. However, for larger packet sizes, the throughput (for low error rate) for both sources is similar; in this case the network is the limiting factor, not the processor.

The most surprising result is the apparent peak in throughput when the loss rate is approximately 1% - better even than no loss at all (for blasted data; loss of ACKs sent from the receiver to the source had little impact). This is a very counter-intuitive finding; why should losing packets speed up the throughput? A 1% error might be just enough to prevent a TCP exponential back-off algorithm from slowing the traffic rate. The immediate ACK that the receiver sends when an out-of-sequence packet arrives might include window size information that keeps the sender from pausing. Interrupts caused by out-of-sequence packets might result in the scheduler running the benchmark process more frequently, emptying the buffer window and again keeping the sender from pausing. There are many potential causes; determining the real one would take much more study - but would be very interesting.

13.5  Code

13.5.1  Kernel

The following code adds a trapdoor to the kernel. It creates a function that will be called (if it exists) from within the dev_queue_xmit() function and exports it so that modules will be able to use it. These lines are added directly to the source code; the kernel then has to be recompiled. installed, and booted. Note that the kernel still functions normally (albeit with one extra comparison) while no test module is installed.

net/core/dev.c (after line 579)

...
 int *test_function(struct sk_buff *)=0;                  /* new */
 
 int dev_queue_xmit(struct sk_buff *skb)...
 
     ...struct Qdisc  *q;
 
     if (test_function && (*test_function)(skb)) {        /* new */
         kfree_skb(skb);                                  /* new */
         return 0;                                        /* new */
     }                                                    /* new */
 
 #ifdef CONFIG_NET_PROFILE...
 

net/netsyms.c (after line 544)

...
 extern int (*test_function)(struct sk_buff *);           /* new */
 EXPORT_SYMBOL_NOVERS(test_function);                     /* new */
 EXPORT_SYMBOL(register_gifconf);...
 

13.5.2  Module

The following is the code for the packet dropping module itself. On installation, it calculates a percentage cut-off and puts an address into the function pointer defined above. From then on, any packets sent through dev_queue_xmit() will also pass through the packet_dropper function, which compares the destination address to a hard coded one. If they match and a random number comes up below the calculated cut-off, it drops the packet; otherwise the packets pass through untouched. When the module is removed, it resets the function pointer to 0 (null) again. (Note that this not very robust code depends on two byte short integers for simplicity. The function get_random_bytes() is only accessible to the kernel - or modules, of course - and provides random numbers that are ``merely cryptographically strong''.)

packet_dropper.c

/* packet_dropper.c
  *
  * This program provides an example of how to install a module into a
  *   slightly modified kernel that will randomly drop packets for a specific
  *   (hard-coded) host.
  *
  * See linux/drivers/char/random.c for details of get_random_bytes().
  *
  * Usage (must be root to use):
  *   /sbin/insmod packet_dropper
  *   /sbin/rmmod packet_dropper
  */
 
 #define MODULE
 #define MAX_UNSIGNED_SHORT 65535
 
 #include <linux/module.h>
 #include <linux/skbuff.h>  /* for struct sk_buff */
 #include <linux/ip.h>      /* for struct iphdr */
 
 extern int (*test_function)(struct sk_buff *);       /* calling function */
 extern void get_random_bytes(void *buf, int nbytes); /* random function */
 unsigned short cutoff;                               /* drop cutoff */
 float rate   = 0.050;                                /* drop percentage */
 __u32 target = 0x220010AC;                           /* 172.16.0.34 */
 
 /************************************************************ packet_dropper
  * this is what dev_queue_xmit will call while this module is installed */
 int packet_dropper(struct sk_buff *skb) {
   unsigned short t;
   if (skb->nh.iph->daddr == target) {
     get_random_bytes(&t,2);
     if (t <= cutoff) return 1;    /* drop this packet */
   }
   return 0;                       /* continue with normal routine */
 }  /* packet_dropper */
 
 /*************************************************************** init_module
  * this function replaces the null pointer with a real one */
 int init_module() {
   EXPORT_NO_SYMBOLS;
   cutoff = rate * MAX_UNSIGNED_SHORT;
   test_function = packet_dropper;
   printk("<1> packet_dropper: now dropping packets\n");
   return 0;
 }  /* init_module */
 
 /************************************************************ cleanup_module
  * this function resets the function pointer back to null */
 void cleanup_module() {
   test_function = 0;
   printk("<1> packet_dropper: uninstalled\n");
 }  /* cleanup_module */
 

Chapter 14
Additional Resources

14.1  Internet Sites

Linux Documentation Project
http://metalab.unc.edu/mdw/index.html
Linux Headquarters
http://www.linuxhq.com
Linux HOWTOs
ftp://metalab.unc.edu/pub/Linux/docs/HOWTO
Linux Kernel Hackers' Guide
http://metalab.unc.edu/mdw/LDP/khg/HyperNews/get/khg.html
Linux Router Project
http://www.linuxrouter.org
New TTCP
http://users.leo.org/~bartel
Red Hat Software
http://www.redhat.com
Requests for Comment
http://www.rfc-editor.org/isi.html

14.2  Books

Computer Networks
Tanenbaum, Andrew, Prentice-Hall Inc., Upper Saddle River, NJ, 1996.
High Speed Networks
Stallings, William, Prentice-Hall Inc., Upper Saddle River, NJ, 1998.
Linux Core Kernel Commentary
Maxwell, Scott, CoriolisOpen Press, Scottsdale, AZ, 1999.
Linux Device Drivers
Rubini, Alessandro, O'Reilly & Associates, Inc., Sebastopol, CA, 1998.
Linux Kernel Internals
Beck, Michael, et al., Addison-Wesley, Harlow, England, 1997.
Running Linux
Welsh, Matt, Dalheimer, Matthias, and Kaufman, Lar, O'Reilly & Associates, Inc., Sebastopol, CA, 1999.
Unix Network Programming, Vol. 1 (2d Ed.)
Stevens, W. Richard, Prentice-Hall Inc., Upper Saddle River, NJ, 1998.

Chapter 15
Acronyms

ARP
Address Resolution Protocol
ATM
Asynchronous Transfer Mode (a protocol)
BSD
Berkeley Software Distribution
DHCP
Dynamic Hardware Configuration Protocol
DNS
Domain Name Server
FIB
Forwarding Information Base
GUI
Graphical User Interface
ICMP
Internet Control Message Protocol
INET
Internet
IP
Internet Protocol
ISP
Internet Service Provider
LAN
Local Area Network
LDP
Linux Documentation Project
lo
Loopback (device or interface)
MTU
Maximum Transfer Unit
PPP
Point-to-Point Protocol
RARP
Reverse Address Resolution Protocol
RIP
Routing Information Protocol
RTT
Round Trip Time
TCP
Transmission Control Protocol
UDP
User Datagram Protocol
UNH
University of New Hampshire
VLAN
Virtual Local Area Network
WAN
Wide Area Network

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