

Linux IP Networking

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

Glenn Herrin

May 31, 2000

Abstract

This document is a guide to understanding how the Linux kernel (version 2.2.14 specifically) implements networking protocols, focused primarily on the Internet Protocol (IP). It is intended as a complete reference for experimenters with overviews, walk-throughs, source code explanations, and examples. The first part contains an in-depth examination of the code, data structures, and functionality involved with networking. There are chapters on initialization, connections and sockets, and receiving, transmitting, and forwarding packets. The second part contains detailed instructions for modifiying the kernel source code and installing new modules. There are chapters on kernel installation, modules, the proc file system, and a complete example.

Contents

Chapter 1

Introduction

This document is an effort to bring together many of these sources into one coherent reference on and guide to modifying the networking code within the Linux kernel. It presents the internal workings on four levels: a general overview, more specific examinations of network activities, detailed function walk-throughs, and references to the actual code and data structures. It is designed to provide as much or as little detail as the reader desires. This guide was written specifically about the Linux 2.2.14 kernel (which has already been superseded by 2.2.15) and many of the examples come from the Red Hat 6.1 distribution; hopefully the information provided is general enough that it will still apply across distributions and new kernels. It also focuses almost exclusively on TCP/UDP, IP, and Ethernet - which are the most common but by no means the only networking protocols available for Linux platforms.

As a reference for kernel programmers, this document includes information and pointers on editing and recompiling the kernel, writing and installing modules, and working with the /proc file system. It also presents an example of a program that drops packets for a selected host, along with analysis of the results. Between the descriptions and the examples, this should answer most questions about how Linux performs networking operations and how you can modify it to suit your own purposes.

This project began in a Computer Science Department networking lab at the University of New Hampshire as an effort to institute changes in the Linux kernel to experiment with different routing algorithms. It quickly became apparent that blindly hacking the kernel was not a good idea, so this document was born as a research record and a reference for future programmers. Finally it became large enough (and hopefully useful enough) that we decided to generalize it, formalize it, and release it for public consumption.

As a final note, Linux is an ever-changing system and truly mastering it, if such a thing is even possible, would take far more time than has been spent putting this reference together. If you notice any misstatements, omissions, glaring errors, or even typos (!) within this document, please contact the person who is currently maintaining it. The goal of this project has been to create a freely available and useful reference for Linux programmers.

1.2 Document Conventions

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





Figure 1.1: Sample network structure.

This network represents the computer system at a fictional unnamed University (U!). It has a router connected to the Internet at large ( chrysler ). That machine is connected (through the jeep interface) to the campus-wide network, u.edu , consisting of computers named for Chrysler owned car companies ( dodge , eagle , etc.). There is also a LAN subnet for the computer science department, cs.u.edu , whose hosts are named after Dodge vehicle models ( stealth , neon , etc.). They are connected to the campus network by the dodge/viper computer. Both the u.edu and cs.u.edu networks use Ethernet hardware and protocols.

This is obviously not a real network. The IP addresses are all taken from the block reserved for class B private networks (that are not guaranteed to be unique). Most real class B networks would have many more computers, and a network with only eight computers would probably not have a subnet. The connection to the Internet (through chrysler ) would usually be via a T1 or T3 line, and that router would probably be a ``real'' router (i.e. a Cisco Systems hardware router) rather than a computer with two network cards. However, this example is realistic enough to serve its purpose: to illustrate the the Linux network implementation and the interactions between hosts, subnets, and networks.

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 .

Modified versions must be labeled as such. The person making the modifications must be identified. Acknowledgement of the original author must be retained. The location of the original unmodified document be identified. 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:

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.

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 The Network Traffic Path





Figure 2.1: Abstraction of the Linux message traffic path.

When an application generates traffic, it sends packets through sockets to a transport layer (TCP or UDP) and then on to the network layer (IP). In the IP layer, the kernel looks up the route to the host in either the routing cache or its Forwarding Information Base (FIB). If the packet is for another computer, the kernel addresses it and then sends it to a link layer output interface (typically an Ethernet device) which ultimately sends the packet out over the physical medium.

When a packet arrives over the medium, the input interface receives it and checks to see if the packet is indeed for the host computer. If so, it sends the packet up to the IP layer, which looks up the route to the packet's destination. If the packet has to be forwarded to another computer, the IP layer sends it back down to an output interface. If the packet is for an application, it sends it up through the transport layer and sockets for the application to read when it is ready.

Along the way, each socket and protocol performs various checks and formatting functions, detailed in later chapters. The entire process is implemented with references and jump tables that isolate each protocol, most of which are set up during initialization when the computer boots. See Chapter 3 for details of the initialization process.

2.2 The Protocol Stack

IP is the standard network layer protocol. It checks incoming packets to see if they are for the host computer or if they need to be forwarded. It defragments packets if necessary and delivers them to the transport protocols. It maintains a database of routes for outgoing packets; it addresses and fragments them if necessary before sending them down to the link layer.

TCP and UDP are the most common transport layer protocols. UDP simply provides a framework for addressing packets to ports within a computer, while TCP allows more complex connection based operations, including recovery mechanisms for packet loss and traffic management implementations. Either one copies the packet's payload between user and kernel space. However, both are just part of the intermediate layer between the applications and the network.

IP Specific INET Sockets are the data elements and implementations of generic sockets. They have associated queues and code that executes socket operations such as reading, writing, and making connections. They act as the intermediary between an application's generic socket and the transport layer protocol.

Generic BSD Sockets are more abstract structures that contain INET sockets. Applications read from and write to BSD sockets; the BSD sockets translate the operations into INET socket operations. See Chapter 4 for more on sockets.

Applications, run in user space, form the top level of the protocol stack; they can be as simple as two-way chat connection or as complex as the Routing Information Protocol (RIP - see Chapter 9).

2.3 Packet Structure

sk_buff





Figure 2.2: Packet ( sk_buff ) structure.

This structure contains pointers to all of the information about a packet - its socket, device, route, data locations, etc. Transport protocols create these packet structures from output buffers, while device drivers create them for incoming data. Each layer then fills in the information that it needs as it processes the packet. All of the protocols - transport (TCP/UDP), internet (IP), and link level (Ethernet) - use the same socket buffer.

2.4 Internet Routing

The FIB is the primary routing reference; it contains up to 32 zones (one for each bit in an IP address) and entries for every known destination. Each zone contains entries for networks or hosts that can be uniquely identified by a certain number of bits - a network with a netmask of 255.0.0.0 has 8 significant bits and would be in zone 8, while a network with a netmask of 255.255.255.0 has 24 significant bits and would be in zone 24. When IP needs a route, it begins with the most specific zones and searches the entire table until it finds a match (there should always be at least one default entry). The file /proc/net/route has the contents of the FIB.

The routing cache is a hash table that IP uses to actually route packets. It contains up to 256 chains of current routing entries, with each entry's position determined by a hash function. When a host needs to send a packet, IP looks for an entry in the routing cache. If there is none, it finds the appropriate route in the FIB and inserts a new entry into the cache. (This entry is what the various protocols use to route, not the FIB entry.) The entries remain in the cache as long as they are being used; if there is no traffic for a destination, the entry times out and IP deletes it. The file /proc/net/rt_cache has the contents of the routing cache.

These tables perform all the routing on a normal system. Even other protocols (such as RIP) use the same structures; they just modify the existing tables within the kernel using the ioctl() function. See Chapter 8 for routing details.

Chapter 3

Network Initialization

This chapter presents network initialization on startup. It provides an overview of what happens when the Linux operating system boots, shows how the kernel and supporting programs ifconfig and route establish network links, shows the differences between several example configurations, and summarizes the implementation code within the kernel and network programs.

The entire configuration process can be static or dynamic. If addresses and names never (or infrequently) change, the system administrator must define options and variables in files when setting up the system. In a more mutable environment, a host will use a protocol like the Dynamic Hardware Configuration Protocol (DHCP) to ask for an address, router, and DNS server information with which to configure itself when it boots. (In fact, in either case, the administrator will almost always use a GUI interface - like Red Hat's Control Panel - which automatically writes the configuration files shown below.)

An important point to note is that while most computers running Linux start up the same way, the programs and their locations are not by any means standardized; they may vary widely depending on distribution, security concerns, or whim of the system administrator. This chapter presents as generic a description as possible but assumes a Red Hat Linux 6.1 distribution and a generally static network environment.

3.2.1 The Network Initialization Script

The script(s) involved in establishing networking can be very straightforward; it is entirely possible to have one big script that simply executes a series of commands that will set up a single machine properly. However, most Linux distributions come with a large number of generic scripts that work for a wide variety of machine setups. This leaves a lot of indirection and conditional execution in the scripts, but actually makes setting up any one machine much easier. For example, on Red Hat distributions, the /etc/rc.d/init.d/network script runs several other scripts and sets up variables like interfaces_boot to keep track of which /etc/sysconfig/network-scripts/ifup scripts to run. Tracing the process manually is very complicated, but simple modifications of only two configuration files (putting the proper names and IP addresses in the /etc/sysconfig/network and /etc/sysconfig/network-scripts/ifcfg-eth0 files) sets up the entire system properly (and a GUI makes the process even simpler).

When the network script finishes, the FIB contains the specified routes to given hosts or networks and the routing cache and neighbor tables are empty. When traffic begins to flow, the kernel will update the neighbor table and routing cache as part of the normal network operations. (Network traffic may begin during initialization if a host is dynamically configured or consults a network clock, for example.)

ifconfig ${DEVICE} ${IPADDR} netmask ${NMASK} broadcast ${BCAST}

The ifconfig program can also provide information about currently configured network devices (calling with no arguments displays all the active interfaces; calling with the -a option displays all interfaces, active or not):

ifconfig

viper

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

ifconfig interface [aftype] options | address ...

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

route add -net ${NETWORK} netmask ${NMASK} dev ${DEVICE} -or-

route add -host ${IPADDR} ${DEVICE}



The route program can also delete routes (if run with the del option) or provide information about the routes that are currently defined (if run with no options):

route

stealth

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

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

3.3.1 Home Computer

ppp

This is the first file the network script will read; it sets several environment variables. The first two variables set the computer to run networking programs (even though it is not on a network) but not to forward packets (since it has nowhere to send them). The last two variables are generic entries.

/etc/sysconfig/network

NETWORKING=yes

FORWARD_IPV4=false

HOSTNAME=localhost.localdomain

GATEWAY=

/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

ppp

ppp

3.3.2 Host Computer on a LAN

stealth

This is the first file the network script will read; again the first variables simply determine that the computer will do networking but that it will not forward packets. The last four variables identify the computer and its link to the rest of the Internet (everything that is not on the LAN).

/etc/sysconfig/network

NETWORKING=yes

FORWARD_IPV4=false

HOSTNAME=stealth.cs.u.edu

DOMAINNAME=cs.u.edu

GATEWAY=172.16.1.1

GATEWAYDEV=eth0



/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



After setting these variables, the network script will run the ifconfig program to start the device. Finally, the script will run the route program to add the default route ( GATEWAY ) and any other specified routes (found in the /etc/sysconfig/static-routes file, if any). In this case only the default route is specified, since all traffic either stays on the LAN (where the computer will use ARP to find other hosts) or goes through the router to get to the outside world.

3.3.3 Network Routing Computer

dodge/viper

This is the first file the network script will read; it sets several environment variables. The first two simply determine that the computer will do networking (since it is on a network) and that this one will forward packets (from one network to the other). IP Forwarding is built into most kernels, but it is not active unless there is a 1 ``written'' to the /proc/net/ipv4/ip_forward file. (One of the network scripts performs an echo 1 > /proc/net/ipv4/ip_forward if FORWARD_IPV4 is true.) The last four variables identify the computer and its link to the rest of the Internet (everything that is not on one of its own networks).

/etc/sysconfig/network

NETWORKING=yes

FORWARD_IPV4=true

HOSTNAME=dodge.u.edu

DOMAINNAME=u.edu

GATEWAY=172.16.0.1

GATEWAYDEV=eth1



/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

GATEWAY

3.4 Linux and Network Program Functions

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

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()]

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.

4.2 Socket Structures

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.

- this structure contains pointers to protocol specific functions for implementing general socket behavior. For example, points to the 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.

- 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).

- 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.

- this is the head of the receive queue. struct sk_buff_head write_queue - this is the head of the send queue.

- this is the head of the send queue. __u32 saddr - the (Internet) source address for this socket.

- 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.

- 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.

- this structure contains pointers to transport layer protocol specific functions. For example, may point to the function. union struct tcp_op af_tcp; tp_pinfo - TCP options for this socket.

- TCP options for this socket. struct socket *sock - the parent BSD socket.

- 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

sock- > sk- > dst_cache

ip_route_connect()

dst_cache

4.4 Connection Processes

4.4.1 Establishing Connections

/* 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));

gethostbyname()

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

close()

sock_close()

tcp_v4_close()

udp_close()

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

sock_create()

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.





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.

5.2.1 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

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.





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.)

6.2.1 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

DEVICE_rx()

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.

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





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

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.





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

u32

__u32

unsigned int

ntohl

8.2.1 The Neighbor Table





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.

- 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.

- 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.

- 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.

- hash table of neighbors associated with this table; there are (32) buckets. struct pneigh_entry *phash_buckets[] - hash table of structures containing device pointers and keys; there are PNEIGH_HASHMASK+1 (16) buckets.

- hash table of structures containing device pointers and keys; there are (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.

- 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.

- 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.

- pointer to cached hardware header for transmissions to this neighbor. struct sk_buff_head arp_queue - pointer to ARP packets for this neighbor.

- 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





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 () .

- pointers to table functions (lookup, delete, insert, etc.) that are set during initialization to . unsigned char tb_data[0] - pointer to the associated FIB hash table (despite its declaration as a character array).

- 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 .

- table identifier; 255 for , 254 for . 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.

- pointers to zone entries (one zone for each bit in the mask; points to the zone for netmask 0x0000, points to the zone for 0x8000, and 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] ).

- pointer to the next non-empty zone in the hash structure (the next most general netmask; e.g., might point to ). struct fib_node **fz_hash - pointer to a hash table of nodes for this zone.

- pointer to a hash table of nodes for this zone. int fz_nent - the number of entries (nodes) in this zone.

- 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).

- 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).

- 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).

- the index of this zone in the parent 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.

- pointer to the next node. struct fib_info *fn_info - pointer to more information about this node (that is shared by many nodes).

- 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).

- 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.

- 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.

- 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 .





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

FIB Traversal Example:

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. ip_route_output_slow() calls fib_lookup() and passes it the key to search for. 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. 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). fz_key() builds a test key by ANDing the destination address with the zone netmask, resulting in a key value 172.16.0.0. 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. 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). 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. 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. 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. 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. 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.) 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





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.

- this is an entry in the table; the union structure allows quick access to the next entry in the table by overusing the 's next field to point to the next cache entry if required. __u32 rt_dst - the destination address.

- the destination address. __u32 rt_src - the source address.

- the source address. rt_int iif - the input interface.

- the input interface. __u32 rt_gateway - the address of the neighbor to route through to get to a destination.

- 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)

- 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.

- the input/output device for this route. unsigned pmtu - the maximum packet size for this route.

- the maximum packet size for this route. struct neighbor *neighbor - a pointer to the neighbor (next link) for this route.

- 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.

- 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() ).

- a pointer to the input function for this route (typically ). int (*output)(struct sk_buff*) - a pointer to the output function for this route (typically dev_queue_xmit() ).

- a pointer to the output function for this route (typically ). struct dst_ops *ops - a pointer to a structure containing the family, protocol, and check, reroute, and destroy functions for this route.

- 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.

- 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.

- 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() ?).

- a pointer to the output function for this neighbor (typically ?). struct sk_buff_head arp_queue - the first element in the ARP queue for traffic concerning this neighbor - incoming or outgoing?

- 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.

- 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.





Figure 8.6: Routing Cache data structure relationships.





Figure 8.7: Destination Cache data structure relationships.

Routing Cache Traversal Example:

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. 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). ip_route_output() enters the hash table at index 5. There is an entry there, but the destination addresses do not match. 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. 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

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

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.

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.





Figure 9.1: Routing Information Protocol packet and table.

9.2 How routed Works

ioctl()

9.2.1 Data Structures

ROUTEHASHSIZ

struct rtentry

ioctl()

ioctl()

9.2.3 Normal Operations

ioctl()

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 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

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

®

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

- architecture specific code, by processor 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

- code for drivers of all sorts fs - code for different file systems (EXT2, MS-DOS, etc.)

- 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

- header files used throughout the code kernel - code for the kernel specific routines

- code for the kernel specific routines lib - code for errors, strings, and printf

- code for errors, strings, and printf mm - code for memory management

- code for memory management modules - object files and references for the kernel to load as required

- 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

- code for networking

10.2 Using EMACS Tags

10.2.1 Referencing with TAGS

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

The command to make a tags file is:

etags filename

etags -a filename

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

etags /usr/src/linux/net/ipv4/*.c

etags -a /usr/src/include/net/*.h

10.3 Using vi tags

10.4 Rebuilding the Kernel

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

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. 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. 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. Run make clean to remove old object files IF you want to recompile everything. This obviously will make the compilation process take longer. 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. Run make modules to build any modules (not included in the main kernel image). 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.) 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.) 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 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 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 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. Run /sbin/lilo to install the new kernel as a boot option. Reboot the computer with the new kernel. 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)

-N

-p0

-r rejfile

-verbose

-r rejfile

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.

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

MODULE

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

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)

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.

"); 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.

"); } /* 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 presen