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Current issue : #57 | Release date : 2001-08-11 | Editor : Phrack Staff
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Title : ICMP based OS fingerprinting
Author : Ofir Arkin & Fyodor Yarochkin
                             ==Phrack Inc.==

               Volume 0x0b, Issue 0x39, Phile #0x07 of 0x12

|=---=[ ICMP based remote OS TCP/IP stack fingerprinting techniques ]=---=|
|=---------------=[ Ofir Arkin & Fyodor Yarochkin ]=---------------------=|

--[ICMP based fingerprinting approach]--

    TCP based remote OS fingerprinting is quite old(*1) and well-known
    these days, here we would like to introduce an alternative method to
    determine an OS remotely based on ICMP responses which are received
    from the host. Certain accuracy level has been achieved with
    different platforms, which, with some systems or or classes of
    platforms (i.g. Win*), is significally more precise than
    demonstrated with TCP based fingerprinting methods.

    As mentioned above TCP based method, ICMP fingerprinting utilizes
    several tests to perform remote OS TCP/IP stack probe, but unlike
    TCP fingerprinting, a number of tests required to identify an OS
    could vary from 1 to 4 (as of current development stage).
    ICMP fingerprinting method is based on certain discoveries on
    differencies of ICMP replies from various operating systems (mostly
    due to incorrect, or inconsistant implementation), which were found
    by Ofir Arkin during his "ICMP Usage in Scanning" research project.
    Later these discoveries were summarised into a logical desicions
    tree which Ofir entitled "X project" and practically implemented in
    'Xprobe' tool.

--[Information/Noise ratio with ICMP fingerprints]--

    As it's been noted, the number of datagrams we need to send and
    receive in order to remotely fingerprint a targeted machine with
    ICMP based probes is small. Very small. In fact we can send one
    datagram and receive one reply and this will help us identify up to
    eight different operating systems (or classes of operating systems).
    The maximum datagrams which our tool will use at the current stage
    of development, is four. This is the same number of replies we will
    need to analyse. This makes ICMP based fingerprinting very

    ICMP based probes could be crafted to be very stealthy. As on the
    moment, no maliformed/broken/corrupted datagrams are used to
    identify remote OS type, unlike the common fingerprinting methods.
    Current core analysis targets validation of received ICMP responses
    on valid packets, rather than crafting invalid packets themselves.
    Heaps of such packets appear in an average network on daily basis
    and very few IDS systems are tuned to detect such traffic (and those
    which are, presumably are very noisy and badly configured).
--[Why it still works?]--

    Inheritable mess among various TCP/IP stack implementations with
    ICMP handling implementations which implement different RFC
    standards (original RFC 792, additional RFC 1122, etc), partial or
    incomplete ICMP support (various ICMP requests are not supported
    everywhere), low significance of ICMP Error messages data (who
    verifies all the fields of the original datagram?!), mistakes and
    misunderstanding in ICMP protocol implementation made our method

--[What do we fingerprint:]-- 

    Several OS-specific differencies are being utilized in ICMP based
    fingerprinting to identify remote operating system type:

    IP fields of an 'offending' datagram to be examined:

    * IP total length field
     Some operating systems (i.g. BSD family) will add 20 bytes
     (sizeof(ipheader)) to the original IP total length field (which
     occures due to internal processing mistakes of the datagram, please
     note when the same packet is read from SOCK_RAW the same behaviour
     is seen: returned packet ip_len fiend is off by 20 bytes).

     Some other operating systems will decrease 20 bytes from the
     original IP total lenth field value of the offending packet.

     Third group of systems will echo this field correctly.

    * IP ID
     some systems are seen not to echo this field correctly. (bit order
     of the field is changed).

    * 3 bits flags and offset

     some systems are seen not to echo this field correctly. (bit order
     of the field is changed).

    * IP header checksum

    Some operating systems will miscalculate this field, others just
    zero it out. Third group of the systems echoes this field correctly.

    * UDP header checksum (in case of UDP datagram)
    The same thing could happen with UDP checksum header. 

    IP headers of responded ICMP packet:
    * Precedence bits
     Each IP Datagram has an 8-bit field called the 'TOS Byte', which
     represents the IP support for prioritization and Type-of-Service
    The 'TOS Byte' consists of three fields.
    The 'Precedence field'\cite{rfc791}, which is 3-bit long, is intended to
    prioritize the IP Datagram. It has eight levels of prioritization.
    Higher priority traffic should be sent before lower priority traffic.
    The second field, 4 bits long, is the 'Type-of-Service' field. It is
    intended to describe how the network should make tradeoffs between
    throughput, delay, reliability, and cost in routing an IP Datagram.
    The last field, the 'MBZ' (must be zero), is unused and must be zero.
    Routers and hosts ignore this last field. This field is 1 bit long.
    The TOS Bits and MBZ fields are being replaced by the DiffServ
    mechanism for QoS.
    RFC 1812 Requires following for IP Version 4 Routers:
    " TOS and Precedence
    ICMP Source Quench error messages, if sent at all, MUST have their
    IP Precedence field set to the same value as the IP Precedence field
    in the packet that provoked the sending of the ICMP Source Quench
    message. All other ICMP error messages (Destination Unreachable,
    Redirect, Time Exceeded, and Parameter Problem) SHOULD have their
    precedence value set to 6 (INTERNETWORK CONTROL) or 7 (NETWORK
    CONTROL). The IP Precedence value for these error messages MAY be

    Linux Kernel 2.0.x, 2.2.x, 2.4.x will act as routers and will set
    their Precedence bits field value to 0xc0 with ICMP error messages.
    Networking devices that will act the same will be Cisco routers
    based on IOS 11.x-12.x and Foundry Networks switches.

    * DF bits echoing   
    Some TCP/IP stacks will echo DF bit with ICMP Error datagrams,
    others (like linux) will copy the whole octet completely, zeroing
    certain bits, others will ignore this field and set their own.
    * IP ID filend (linux 2.4.0 - 2.4.4 kernels)

    Linux machines based on Kernel 2.4.0-2.4.4 will set the IP
    Identification field value with their ICMP query request and reply
    messages to a value of zero.
    This was later fixed with Linux Kernels 2.4.5 and up.

    * IP ttl field (ttl distance to the target has to be precalculated to
    guarantee accuracy).

    "The sender sets the time to live field to a value that represents
    the maximum time the datagram is allowed to travel on the Internet".
    The field value is decreased at each point that the IP header is
    being processed. RFC 791 states that this field decreasement reflects
    the time spent processing the datagram. The field value is measured
    in units of seconds. The RFC also states that the maximum time to
    live value can be set to 255 seconds, which equals to 4.25 minutes.
    The datagram must be discarded if this field value equals zero -
    before reaching its destination.
    Relating to this field as a measure to assess time is a bit
    misleading. Some routers may process the datagram faster than a
    second, and some may process the datagram longer than a second.
    The real intention is to have an upper bound to the datagram
    lifetime, so infinite loops of undelivered datagrams will not jam the
    Having a bound to the datagram lifetime help us to prevent old
    duplicates to arrive after a certain time elapsed. So when we
    retransmit a piece of information which was not previously delivered
    we can be assured that the older duplicate is already discarded and
    will not interfere with the process.
    The IP TTL field value with ICMP has two separate values, one for
    ICMP query messages and one for ICMP query replies.  

    The IP TTL field value helps us identify certain operating systems
    and groups of operating systems. It also provides us with the
    simplest means to add another check criterion when we are querying
    other host(s) or listening to traffic (sniffing).

    TTL-based fingeprinting requires a TTL distance to the done to be
    precalculated in advance (unless a fingerprinting of a local network
    based system is performed system).
    The ICMP Error messages will use values used by ICMP query request
    A good statistics of ttl dependancy on OS type has been gathered at:
    (Research paper on default ttl values)

    * TOS field

    RFC 1349 defines the usage of the Type-of-Service field with the
    ICMP messages. It distinguishes between ICMP error messages
    (Destination Unreachable, Source Quench, Redirect, Time Exceeded,
    and Parameter Problem), ICMP query messages (Echo, Router
    Solicitation, Timestamp, Information request, Address Mask request)
    and ICMP reply messages (Echo reply, Router Advertisement, Timestamp
    reply, Information reply, Address Mask reply).
   Simple rules are defined:
     * An ICMP error message is always sent with the default TOS (0x0000)
     * An ICMP request message may be sent with any value in the TOS
     field. "A mechanism to allow the user to specify the TOS value to
     be used would be a useful feature in many applications that
     generate ICMP request messages".
     The RFC further specify that although ICMP request messages are
     normally sent with the default TOS, there are sometimes good
     reasons why they would be sent with some other TOS value.
     * An ICMP reply message is sent with the same value in the TOS
     field as was used in the corresponding ICMP request message.
    Some operating systems will ignore RFC 1349 when sending ICMP echo
    reply messages, and will not send the same value in the TOS field as
    was used in the corresponding ICMP request message.
    ICMP headers of responded ICMP packet:

    * ICMP Error Message Quoting Size:
    All ICMP error messages consist of an IP header, an ICMP header
    and certain amount of data of the original datagram, which triggered
    the error (aka offending datagram).
    According to RFC 792 only 64 bits (8 octets) of original datagram
    are supposed to be included in the ICMP error message. However RFC
    1122 (issued later) recommends up to 576 octets to be quoted.
    Most of "older" TCP stack implementations will include 8 octets into
    ICMP Errror message. Linux/HPUX 11.x, Solaris, MacOS and others will
    include more.

    Noticiably interesting is the fact that Solaris engineers probably
    couldn't not read RFC properly (since instead of 64 bits Solaris
    2.x includes 64 octets (512 bits) of the original datagram.

    * ICMP error Message echoing integrity

    Another artifact which has been noticed is that some stack
    implementations, when sending back an ICMP error message, may alter
    the offending packet's IP header and the underlying protocol data,
    which is echoed back with the ICMP error message.

    Since mistakes, made by TCP/IP stack programmers are different and
    specific to an operating system, an analysis of these mistakes could
    give a potential attacker a a possibilty to make assumptions about
    the target operating system type.
    Additional tweaks and twists:
    * Using difererent from zero code fields in ICMP echo requests
    When an ICMP code field value different than zero (0) is sent with
    an ICMP Echo request message (type 8), operating systems that will
    answer our query with an ICMP Echo reply message that are based on
    one of the Microsoft based operating systems will send back an ICMP
    code field value of zero with their ICMP Echo Reply. Other operating
    systems (and networking devices) will echo back the ICMP code field
    value we were using with the ICMP Echo Request.
    The Microsoft based operating systems acts in contrast to RFC
    792 guidelines which instruct the answering operating systems to
    only change the ICMP type to Echo reply (type 0), recalculate the
    checksums and send the ICMP Echo reply away.
    * Using DF bit echoing with ICMP query messages
    As in case of ICMP Error messages, some tcp stacks will respond
    these queries, while the others: will not. 

    * Other ICMP messages:
        * ICMP timestamp request
        * ICMP Information request
        * ICMP Address mask request
    Some TCP/IP stacks support these messages and respond to some of
    these requests.    

--[Xprobe implementation]--
    Currently Xprobe deploys hardcoded logic tree, developed by Ofir
    Arkin in 'Project X'. Initially a UDP datagram is being sent to a
    closed port in order to trigger ICMP Error message: ICMP
    unreachable/port unreach.  (this sets up a limitation of having at
    least one port not filtered on target system  with no service
    running, generically speaking other methods of triggering ICMP
    unreach packet could be used, this will be discussed further).
    Moreover, a few tests (icmp unreach content, DF bits, TOS ...) could
    be combined within a single query, since they do not affect results
    of each other.
    Upon the receipt of ICMP unreachable datagram, contents of the
    received datagram is examined and a diagnostics decision is made, if
    any further tests are required, according to the logic tree, further
    queries are sent.

--[ Logic tree]---

    Quickly recapping the logic tree organization:

    Initially all TCP/IP stack implementations are split into 2 groups,
    those which echo precedence bits back, and those which do not. Those
    which do echo precendence bits (linux 2.0.x, 2.2.x, 2.4.x, cisco IOS
    11.x-12.x, Extreme Network Switches etc), being differentiated
    further based on ICMP error quoting size. (Linux sticks with RFC
    1122 here and echoes up to 576 octets, while others in this subgroup
    echo only 64 bits (8 octets)). Further echo integrity checks are
    used to differentiate cisco routers from Extreme Network switches.

    Time-to-live and IP ID fields of ICMP echo reply  are being used to
    recognize version of linux kernel.

    The same approach is being used to recognize other TCP/IP stacks.
    Data echoing validation (amounts of octets of original datagram
    echoed, checksum validation, etc). If additional information is
    needed to differ two 'similar' IP stacks, additional query is being
    sent. (please refer to the diagram at
    http://www.sys-security.com/html/projects/X.html for more detailed
    explanation/graphical representation of the logic tree).
    One of the serious problems with the logic tree, is that adding new
    operating system types to it becomes extremely painful. At times
    part of the whole logic tree has to be reworked to 'fit' a single
    description. Therefore a singature based fingerprinting method took
    our closer attention.
--[Sinature based approach]--

    Singature based approach is what we are currently focusing on and
    which we believe will be further, more stable, reliable and flexible
    method of remote ICMP based fingerprints.

    Signature-based method is currently based on five different tests,
    which optionally could be included in each operating system
    fingerprint. Initally the systems with lesser amount of tests are
    being examined (normally starting with ICMP unreach test).
    If no single OS stack found matching received signature, those
    stacks which match a part, being grouped again, and another test
    (based on lesser amounts of tests issued principle) is choosen and
    executed. This verification is repeated until an OS stack,
    completely matching the signature is found, or we run out of tests.

    Currently following tests are being deployed:

    * ICMP unreachable test (udp closed port based, host unreachable,
    network unreachable (for systems which are believed to be gateways)
    * ICMP echo request/reply test
    * ICMP timestamp request
    * ICMP information request
    * ICMP address mask request

--[future implementations/development]--

    Following issues are planned to be deployed (we always welcome
    discussions/suggestions though):
    * Fingerprints database (currently being tested)
    * Dynamic, AI based logic (long-term project :))
    * Tests would heavily dependent on network topology (pre-test
    network mapping  will take place).
    * Path-to-target test (to calculate hops distance to the target)
     filtering devices probes.
    * Future implementations will be using packets with
    actual application data to dismiss chances of being detected.
    * other network mapping capabilities shall be included (
     network role identification, search for closed UDP port, reachability
     tests, etc).

--[code for kids]--

  Currently implemented code and further documentation is available at
  following locations: 



Ofir Arkin <ofir@sys-security.com>
Fyodor Yarochkin <fygrave@tigerteam.net>

|=[ EOF ]=---------------------------------------------------------------=|

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