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Archive-name: internet/tcp-ip/raw-ip-faq
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URL: http://www.whitefang.com/rin/
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>
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