~+'''A Guide to the Implementation and Modification of the Linux Protocol Stack'''+~ [[BR]][[BR]]
~+'''Glenn Herrin'''+~ [[BR]][[BR]]
~+'''TR 00-04'''+~ [[BR]][[BR]]
'''''May 31, 2000''''' [[BR]][[BR]]

~+'''Abstract'''+~ [[BR]][[BR]]
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.

[[BR]][[BR]]

~+'''Contents'''+~
[[TableOfContents(5)]]

[[Anchor(chapter1)]]
~+'''Chapter 1'''+~

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

=== Background ===
Linux is becoming more and more popular as an alternative operating system. Since it is freely available to everyone as part of the open source movement, literally thousands of programmers are constantly working on the code to implement new features, improve existing ones, and fix bugs and inefficiencies in the code. There are many sources for learning more about Linux, from the source code itself (downloadable from the Internet) to books to "HOW-TOs" and message boards maintained on many different subjects.

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 This guide was written specifically about the Linux 2.2.14 kernel (which has already been superseded by 2.2.15) and many of the examples come from the Red Hat 6.1 distribution; hopefully the information provided is general enough that it will still apply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.

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

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

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

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

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

=== Sample Network Example ===
There are numerous examples in this document that help clarify the presented material. For the sake of consistency and familiarity, most of them reference the sample network shown in Figure 1.1.

FIXME: Insert Image here 

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. 

=== 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://original.source/location''. 

(The visibility of the credit should be proportional to the amount of the document reproduced!) Commercial redistribution is permitted and encouraged. All modifications of this document, including translations, anthologies, and partial documents, must meet the following requirements:

   1. Modified versions must be labeled as such.
   2. The person making the modifications must be identified.
   3. Acknowledgement of the original author must be retained.
   4. The location of the original unmodified document be identified.
   5. The original author's name may not be used to assert or imply endorsement of the resulting document without the original author's permission. 

Please note any modifications including deletions.

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

    ''http://www.linuxdoc.org/COPYRIGHT.html'' 

This document is not currently part of the LDP, but it may be submitted in the future.

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

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


Glenn Herrin [[BR]]
Major, United States Army [[BR]]
Primary Documenter and Researcher, Version 1.0 [[BR]]
[[MailTo(gherrin@cs.unh.edu)]]


[[BR]][[BR]]

[[Anchor(chapter2)]]
~+'''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.

=== The Network Traffic Path ===
The Internet Protocol (IP) is the heart of the Linux messaging system. While Linux (more or less) strictly adheres to the layering concept - and it is possible to use a different protocol (like ATM) - IP is almost always the nexus through which packets flow. The IP implementation of the network layer performs routing and forwarding as well as encapsulating data. See Figure 2.1 for a simplified diagram of how network packets move through the Linux kernel.

FIXME: Insert Image here 

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 [#chapter3 Chapter 3] for details of the initialization process. 

=== The Protocol Stack ===
Network devices form the bottom layer of the protocol stack; they use a link layer protocol (usually Ethernet) to communicate with other devices to send and receive traffic. Input interfaces copy packets from a medium, perform some error checks, and then forward them to the network layer. Output interfaces receive packets from the network layer, perform some error checks, and then send them out over the medium.

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 [#chapter4 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 [#chapter9 Chapter 9]). 

=== Packet Structure ===
The key to maintaining the strict layering of protocols without wasting time copying parameters and payloads back and forth is the common packet data structure (a socket buffer, or `sk_buff` - Figure 2.2). Throughout all of the various function calls as the data makes it way through the protocols, the payload data is copied only twice; once from user to kernel space and once from kernel space to output medium (for an outbound packet).

FIXME: Insert Image here 

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.

=== Internet Routing ===
The IP layer handles routing between computers. It keeps two data structures; a Forwarding Information Base (FIB) that keeps track of all of the details for every known route, and a faster routing cache for destinations that are currently in use. (There is also a third structure - the neighbor table - that keeps track of computers that are physically connected to a host.)

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 [#chapter8 Chapter 8] for routing details. 

[[BR]][[BR]]

[[Anchor(chapter3)]]
~+'''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.

=== Overview ===
Linux initializes routing tables on startup only if a computer is configured for networking. (Almost all Linux machines do implement networking, even stand-alone machines, if only to use the loopback device.) When the kernel finishes loading itself, it runs a set of common but system specific utility programs and reads configuration files, several of which establish the computer's networking capabilities. These determine its own address, initialize its interfaces (such as Ethernet cards), and add critical and known static routes (such as one to a router that connects it with the rest of the Internet). If the computer is itself a router, it may also execute a program that allows it to update its routing tables dynamically (but this is NOT run on most hosts).

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. 

=== Startup ===
When Linux boots as an operating system, it loads its image from the disk into memory, unpacks it, and establishes itself by installing the file systems and memory management and other key systems. As the kernel's last (initialization) task, it executes the ''init'' program. This program reads a configuration file (''/etc/inittab'') which directs it to execute a startup script (found in ''/etc/rc.d'' on Red Hat distributions). This in turn executes more scripts, eventually including the network script (''/etc/rc.d/init.d/network''). (See [#sec3.3 Section 3.3] for examples of the script and file interactions.)

==== The Network Initialization Script ====
The network initialization script sets environment variables to identify the host computer and establish whether or not the computer will use a network. Depending on the values given, the network script turns on (or off) IP forwarding and IP fragmentation. It also establishes the default router for all network traffic and the device to use to send such traffic. Finally, it brings up any network devices using the ''ifconfig'' and ''route'' programs. (In a dynamic environment, it would query the DHCP server for its network information instead of reading its own files.)

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 ====
The ''ifconfig'' program configures interface devices for use. (This program, while very widely used, is not part of the kernel.) It provides each device with its (IP) address, netmask, and broadcast address. The device in turn will run its own initialization functions (to set any static variables) and register its interrupts and service routines with the kernel. The ''ifconfig'' commands in the network script look like this:

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

(where the variables are either written directly in the script or are defined in other scripts).

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 
}}}

This provides all the information available about each working interface; addresses, status, packet statistics, and operating system specifics. Usually there will be at least two interfaces - a network card and the loopback device. The information for each interface looks like this (this is the `viper` interface):

{{{
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
}}}

A superuser can use ''ifconfig'' to change interface settings from the command line; here is the syntax:

    `ifconfig` ''interface [aftype] options | address ...''

...and some of the more useful calls:

{{{
    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 
}}}

Note that modifying an interface configuration can indirectly change the routing tables. For example, changing the netmask may make some routes moot (including the default or even the route to the host itself) and the kernel will delete them.
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