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From: shadows@whitefang.com (Thamer Al-Herbish)
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Subject: Raw IP Networking FAQ
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 Raw IP Networking FAQ 
 --------------------- 
Version 1.3 
 Last Modified on: Thu Nov 11 18:18:19 PST 1999 
 The master copy of this FAQ is currently kept at 
 http://www.whitefang.com/rin/ 
 The webpage also contains material that supplements this FAQ, along 
 with a very spiffy html version. 
 If you wish to mirror it officially, please contact me for details. 
Copyright 
--------- 
I, Thamer Al-Herbish reserve a collective copyright on this FAQ. 
Individual contributions made to this FAQ are the intellectual 
property of the contributor. 
I am responsible for the validity of all information found in this 
FAQ. 
This FAQ may contain errors, or inaccurate material. Use it at your 
own risk. Although an effort is made to keep all the material 
presented here accurate, the contributors and maintainer of this FAQ 
will not be held responsible for any damage -- direct or indirect -- 
which may result from inaccuracies. 
You may redistribute this document as long as you keep it in its 
current form, without any modifications. Please keep it updated if 
you decide to place it on a publicly accessible server. 
Introduction 
------------ 
The following FAQ attempts to answer questions regarding raw IP or 
low level IP networking, including raw sockets, and network 
monitoring APIs such as BPF and DLPI. 
Additions and Contributions 
--------------------------- 
If you find anything you can add, have some corrections for me or 
would like a question answered, please send email to: 
Thamer Al-Herbish <shadows@whitefang.com> 
Please remember to include whether or not you want your email address 
reproduced on the FAQ (if you're contributing). Also remember that 
you may want to post your question to Usenet, instead of sending it 
to me. If you get a response which is not found on this FAQ, and you 
feel is relevant, mail me both copies and I'll attempt to include it. 
Also a word on raw socket bugs. I get approximately a couple of 
emails a month about them, and sometimes I just can't verify if the 
bug exists on a said system. Before mailing in the report, double 
check with my example source code. If it looks like it's a definite 
bug, then mail it in. 
Special thanks to John W. Temples <john@whitefang.com> for his 
constant healthy criticism and editing of the FAQ. 
Credit is given to the contributor as his/her contribution appears in 
the FAQ, along with a list of all contributors at the end of this 
document. 
A final note, a Raw IP Networking mailing list is up. You can join by 
sending an empty message to rawip-subscribe@whitefang.com 
Caveat 
------ 
This FAQ covers only information relevant to the UNIX environment. 
Table of Contents 
----------------- 
 1) General Questions: 
 1.1) What tools/sniffers can I use to monitor my network? 
 1.2) What packet capturing facilities are available? 
 1.3) Is there a portable API I can use to capture packets? 
 1.4) How does a packet capturing facility work? 
 1.5) How do I limit packet loss when sniffing a network? 
 1.6) What is packet capturing usually used for? 
 1.7) Will I have to replace any packets captured off the network? 
 1.8) Is there a portable API to send raw packets into a network? 
 1.9) Are there any high level language APIs (Not C) for raw IP 
 access? 
 2) RAW socket questions: 
 2.1) What is a RAW socket? 
 2.2) How do I use a raw socket? 
 2.2.1) How do I send a TCP/IP packet through a raw socket? 
 2.2.2) How do I build a TCP/IP packet? 
 2.2.3) How can I listen for packets with a raw socket? 
 2.3) What bugs should I look out for when using a raw socket? 
 2.3.1) IP header length/offset host/network byte order 
 (feature/bug?) 
 2.3.2) Unwanted packet processing on some systems. 
 2.4) What are raw sockets commonly used for? 
 3) libpcap (A Portable Packet Capturing Library) 
 3.1) Why should I use libpcap, instead of using the native API on 
 my operating system for packet capturing? 
 3.2) Does libpcap have any disadvantages which I should be aware 
 of? 
 3.3) Where can I find example libpcap source code? 
 4) List of contributors 
 1) General Questions: 
 --------------------- 
 1.1) What tools/sniffers can I use to monitor my network? 
 --------------------------------------------------------- 
 Depending on your operating system, the following is an 
 incomplete list of available tools: 
 tcpdump: Found out-of-the-box on most BSD variants, and 
 also available separately from 
 ftp://ftp.ee.lbl.gov/tcpdump.tar.Z along with 
 libpcap (see below) and various other tools. This 
 tool, in particular, has been ported to multiple 
 platforms thanks to libpcap. 
 ipgrab Compatible with many systems. ipgrab displays 
 link level, transport level, and network level 
 information on packets captured verbosely. 
 http://www.xnet.com/~cathmike/MSB/Software/ 
 Ethereal (GUI) A network packet analyzer (uses GTK+). 
 Supports many systems. Available at: 
 http://ethereal.zing.org/ 
 tcptrace: 
 http://jarok.cs.ohiou.edu/software/tcptrace/tcptrace.html
 Not an actual sniffer, but can read from the logs 
 produced by many other well known sniffers to 
 produce output in different formats and in 
 adjustable details (includes diagnostics). 
 tcpflow 
 http://www.circlemud.org/~jelson/software/tcpflow/
 tcpflow is a program that captures data 
 transmitted as part of TCP connections (flows), 
 and stores the data in a way that is convenient 
 for protocol analysis or debugging. 
 snoop: Solaris, IRIX. 
 etherfind: SunOS. 
 Packetman: SunOS, DEC-MIPS, SGI, DEC-Alpha, and Solaris. 
 Available at 
 ftp://ftp.cs.curtin.edu.au:/pub/netman/ 
 nettl/ntfmt: HP/UX 
 1.2) What packet capturing facilities are available? 
 ---------------------------------------------------- 
 Depending on your operating system (different versions may 
 vary): 
 BPF: Berkeley Packet Filter. Commonly found on BSD 
 variants. 
 DLPI: Data Link Provider Interface. Solaris, HP-UX, SCO 
 Openserver. 
 NIT: Network Interface Tap. SunOS 3. 
 SNOOP: (???). IRIX. 
 SNIT: STREAMS Network Interface Tap. SunOS 4. 
 SOCK_PACKET: Linux. 
 LSF: Linux Socket Filter. Is available on Linux 2.1.75 
 onwards. 
 drain: Used to snoop packets dropped by the OS. IRIX. 
 1.3) Is there a portable API I can use to capture packets? 
 ---------------------------------------------------------- 
 Yes. libpcap from ftp://ftp.ee.lbl.gov/libpcap.tar.Z attempts 
 to provide a single API that interfaces with different 
 OS-dependent packet capturing APIs. It's always best, of 
 course, to learn the underlying APIs in case this library 
 might hide some interesting features. It's important to warn 
 the reader that I have seen different versions of libpcap 
 break backward compatibility. 
 1.4) How does a packet capturing facility work? 
 ----------------------------------------------- 
 The exact details are dependent on the operating system. 
 However, the following will attempt to illustrate the usual 
 technique used in various implementations: 
 The user process opens a device or issues a system call which 
 gives it a descriptor with which it can read packets off the 
 wire. The kernel then passes the packets straight to the 
 process. 
 However, this wouldn't work too well on a busy network or a 
 slow machine. The user process has to read the packets as 
 fast as they appear on the network. That's where buffering 
 and packet filtering come in. 
 The kernel will buffer up to X bytes of packet data, and pass 
 the packets one by one at the user's request. If the amount 
 exceeds a certain limit (resources are finite), the packets 
 are dropped and are not placed in the buffer. 
 Packet filters allow a process to dictate which packets it's 
 interested in. The usual way is to have a set of opcodes for 
 routines to perform on the packet, reading values off it, and 
 deciding whether or not it's wanted. These opcodes usually 
 perform very simple operations, allowing powerful filters to 
 be constructed. 
 BPF filters and then buffers; this is optimal since the 
 buffer only contains packets that are interesting to the 
 process. It's hoped that the filter cuts down the amount of 
 packets buffered to stop overflowing the buffer, which leads 
 to packet loss. 
 NIT, unfortunately, does not do this; it applies the filter 
 after buffering, when the user process starts to read from 
 the buffered data. 
 According to route <route@infonexus.com> Linux' SOCK_PACKET 
 does not do any buffering and has no kernel filtering. 
 Your mileage may vary with other packet capturing facilities. 
 1.5) How do I limit packet loss when sniffing a network? 
 -------------------------------------------------------- 
 If you're experiencing a lot of packet loss, you may want to 
 limit the scope of the packets read by using filters. This 
 will only work if the filtering is done before any buffering. 
 If this still doesn't work because your packet capturing 
 facility is broken like NIT, you'll have to read the packets 
 faster in a user process and send them to another process -- 
 basically attempt to do additional buffering in user space. 
 Another way of improving performance, is by using a larger 
 buffer. On Irix using SNOOP, the man page recommends using 
 SO_RCVBUF. On BSD with BPF one can use the BIOCSBLEN ioctl 
 call to increase the buffer size. On Solaris bufmod and pfmod 
 can be used for altering buffer size and filters 
 respectively. 
 Remember, the longer your process is busy and not attending 
 the incoming packets, the quicker they'll be dropped by the 
 kernel. 
 1.6) What is packet capturing usually used for? 
 ----------------------------------------------- 
 (Question suggested by Michael T. Stolarchuk <mts@rare.net> 
 along with some suggestions for the answer.) 
 Network diagnostics such as the verification of a 
 network's setup, examples are tools like arp, that report 
 the ARP messages sent from hosts. 
 Reconstruction of end to end sessions. tcpshow attempts 
 to do this, but more sophisticated examples are the array 
 of security tools which try to keep tabs on network 
 connections. 
 Monitoring network load. Probably one of the most 
 practical uses, a lot of commercial products usually use 
 specialized hardware to accomplish this. 
 1.7) Will I have to replace any packets captured off the 
 network? 
 
 --------------------------------------------------------------
 
 No, the packet capturing facilities mentioned make copies of 
 the packets, and do not remove them from the system's TCP/IP 
 stack. If you wish to prevent packets from reaching the 
 TCP/IP stack you need to use a firewall, (which should be 
 able to do packet filtering). Don't confuse the packet 
 filtering done by packet capturing facilities with those done 
 by firewalls. They serve different purposes. 
 1.8) Is there a portable API to send raw packets into a 
 network? 
 
 --------------------------------------------------------------
 
 Yes, route <route@infonexus.com> maintains Libnet, a library 
 that provides an API for low level packet writing and 
 handling. It serves as a good compliment for libpcap, if you 
 wish to read and write packets. The project's webpage can be 
 found at: 
 http://www.packetfactory.net/libnet/ 
 1.9) Are there any high level language APIs (Not C) for raw 
 IP access? 
 
 --------------------------------------------------------------
 
 A PERL module that gives access to raw sockets is available 
 at: 
 http://quake.skif.net/RawIP/ 
 A Python library "py-libpap" can be found at: 
 ftp://ftp.python.org/pub/python/contrib/Network/ 
 2) RAW socket questions: 
 ------------------------ 
 2.1) What is a RAW socket? 
 -------------------------- 
 The BSD socket API allows one to open a raw socket and bypass 
 layers in the TCP/IP stack. Be warned that if an OS doesn't 
 support correct BSD semantics (correct is used loosely here), 
 you're going to have a hard time making it work. Below, an 
 attempt is made to address some of the bugs or surprises 
 you're in store for. On almost all sane systems only root 
 (superuser) can open a raw socket. 
 2.2) How do I use a raw socket? 
 ------------------------------- 
 2.2.1) How do I send a TCP/IP packet through a raw 
 socket? 
 
 ----------------------------------------------------------
 
 Depending on what you want to send, you initially open a 
 socket and give it its type. 
 sockd = socket(AF_INET,SOCK_RAW,<protocol>); 
 You can choose from any protocol including IPPROTO_RAW. 
 The protocol number goes into the IP header verbatim. 
 IPPROTO_RAW places 0 in the IP header. 
 Most systems have a socket option IP_HDRINCL which allows 
 you to include your own IP header along with the rest of 
 the packet. If your system doesn't have this option, you 
 may or may not be able to include your own IP header. If 
 it is available, you should use it as such: 
 char on = 1; 
 setsockopt(sockd,IPPROTO_IP,IP_HDRINCL,&on,sizeof(on)); 
 Of course, if you don't want to include an IP header, you 
 can always specify a protocol in the creation of the 
 socket and slip your transport level header under it. 
 You then build the packet and use a normal sendto(). 
 2.2.2) How do I build a TCP/IP packet? 
 -------------------------------------- 
 Examples can be found at http://www.whitefang.com/rin/ 
 which attempt to illustrate the details involved. They 
 also illustrate some of the bugs mentioned below. 
 Briefly, you need to actually write the packet out in 
 memory and hand it over to the socket where it will 
 hopefully fire it away and await more packets. 
 2.2.3) How can I listen for packets with a raw socket? 
 ------------------------------------------------------ 
 Traditionally the BSD socket API did not allow you to 
 listen to just any incoming packet via a raw socket. 
 Although Linux (2.0.30 was the last version I had a look 
 at), did allow this, it has to do with their own 
 implementation of the TCP/IP stack. Correct BSD semantics 
 allow you to get some packets which match a certain 
 category (see below). 
 There's a logical reason behind this; for example TCP 
 packets are always handled by the kernel. If the port is 
 open, send a SYN-ACK and establish the connection, or 
 send back a RST. On the other hand, some types of ICMP (I 
 compiled a small list below), the kernel can't handle. 
 Like an ICMP echo reply, is passed to a matching raw 
 socket, since it was meant for a user program to receive 
 it. 
 The solution is to firewall that particular port if it 
 was a UDP or TCP packet, and sniff it with a packet 
 capturing API (a list is mentioned above). This prevents 
 the TCP/IP stack from handling the packet, thus it will 
 be ignored and you can handle it yourself without 
 intervention. 
 If you don't firewall it, and reply yourself you'll wind 
 up having additional responses from your operating 
 system! 
 Here's a concise explanation of the semantics of a raw 
 BSD socket, taken from a Usenet post by W. Richard 
 Stevens 
 From <rstevens@kohala.com> (Sun Jul 6 12:07:07 1997) : 
 "The semantics of BSD raw sockets are: 
 - TCP and UDP: no one other than the kernel gets these. 
 - ICMP: a copy of each ICMP gets passed to each matching raw 
 socket, except for a few that the kernel generates the reply 
 for: ICMP echo request, timestamp request, and mask request. 
 - IGMP: all of these get passed to all matching raw sockets. 
 - all other protocols that the kernel doesn't deal with (OSPF, 
 etc.): these all get passed to all matching raw sockets." 
 After looking at the icmp_input() routine from the 
 4.4BSD's TCP/IP stack, it seems the following ICMP types 
 will be passed to matching raw sockets: 
 Echo Reply: (0) 
 Router Advertisement (9) 
 Time Stamp Reply (13) 
 Mask Reply (18) 
 2.3) What bugs should I look out for when using a raw socket? 
 ------------------------------------------------------------- 
 2.3.1) IP header length/offset host/network byte 
 (feature/bug?) 
 
 ----------------------------------------------------------
 
 Systems derived from 4.4BSD have a bug in which the 
 ip_len and ip_off members of the ip header have to be set 
 in host byte order rather than network byte order. Some 
 systems may have fixed this. I've confirmed this bug has 
 been fixed on OpenBSD 2.1. 
 2.3.2) Unwanted packet processing on some systems. 
 -------------------------------------------------- 
 Thanks to Michael Masino <mmasino@mitre.org> , Lamont 
 Granquist <lamontg@hitl.washington.edu> , and route 
 <route@infonexus.com> for the submission of bug reports. 
 Some systems will process some of the fields in the IP 
 and transport headers. I've attempted to verify the 
 reports I've received here's what I can verify for sure. 
 Solaris (at least 2.5/2.6) and changes the IP ID field, 
 and adds a Do Not Fragment flag to the IP header (IP_DF). 
 It also expects the checksum to contain the length of the 
 transport level header, and the data. 
 Further reports which I cannot verify (can't reproduce), 
 consist of claims that Solaris 2.x and Irix 6.x will 
 change the sequence and acknowledgment numbers. Irix 6.x 
 is also believed to have the problem mentioned in the 
 previous paragraph. If you experience these problems, 
 double check with the example source code. 
 You'll save yourself a lot of trouble by just getting 
 Libnet http://www.packetfactory.net/libnet/ 
 2.4) What are raw sockets commonly used for? 
 -------------------------------------------- 
 Various UNIX utilities use raw sockets, among them are: 
 traceroute, ping, arp. Also, a lot of Internet security tools 
 make use of raw sockets. However in the long run, raw sockets 
 have proven bug ridden, unportable and limited in use. 
 3) libpcap (A Portable Packet Capturing Library) 
 ------------------------------------------------ 
 3.1) Why should I use libpcap, instead of using the native 
 API on my operating system for packet capturing? 
 
 --------------------------------------------------------------
 
 libpcap was written so that applications could do packet 
 capturing portably. Since it's system independent and 
 supports numerous operating systems, your packet capturing 
 application becomes more portable to various other systems. 
 3.2) Does libpcap have any disadvantages, which I should be 
 aware of? 
 
 --------------------------------------------------------------
 
 Yes, libpcap will only use in-kernel packet filtering when 
 using BPF, which is found on BSD derived systems. This means 
 any packet filters used on other operating systems which 
 don't use BPF will be done in user space, thus losing out on 
 a lot of speed and efficiency. This is not what you want, 
 because packet loss can increase when sniffing a busy 
 network. 
 DEC OSF/1 has an API which has been extended to support 
 BPF-style filters; libpcap does utilize this. 
 In the future, libpcap may translate BPF style filters to 
 other packet capturing facilities, but this has not been 
 implemented yet as of version 0.3 
 Refer to question 1.4 to see how packet filters help in 
 reliably monitoring your network. 
 3.3) Where can I find example libpcap source code? 
 -------------------------------------------------- 
 A lot of the source code found at LBNL's ftp archive 
 ftp://ftp.ee.lbl.gov/ uses libpcap. More specifically, 
 ftp://ftp.ee.lbl.gov/tcpdump.tar.Z probably demonstrates 
 libpcap to a large extent. 
 4) List of contributors. 
 ------------------------ 
 Thamer Al-Herbish <shadows@whitefang.com> 
 W. Richard Stevens <rstevens@kohala.com> 
 John W. Temples (III) <john@whitefang.com> 
 Michael Masino <mmasino@mitre.org> 
 Lamont Granquist <lamontg@hitl.washington.edu> 
 Michael T. Stolarchuk <mts@rare.net> 
 Mike Borella <Mike_Borella@mw.3com.com> 
 route <route@infonexus.com> 
 Derrick J Brashear <shadow@dementia.org>