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From: [e-mail] (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 <[e-mail]>

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 <[e-mail]> 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 [e-mail]

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 <[e-mail]> 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 <[e-mail]>
        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 <[e-mail]> 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 <[e-mail]> (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 <[e-mail]> , Lamont
            Granquist <[e-mail]> , and route
            <[e-mail]> 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 <[e-mail]>
      W. Richard Stevens <[e-mail]>
      John W. Temples (III) <[e-mail]>
      Michael Masino <[e-mail]>
      Lamont Granquist <[e-mail]>
      Michael T. Stolarchuk <[e-mail]>
      Mike Borella <[e-mail]>
      route <[e-mail]>
      Derrick J Brashear <[e-mail]>