SSRT5954 rev.0 - HP-UX TCP/IP Remote Denial of Service (DoS)

From: Security Alert (
Date: 04/25/05

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    SSRT5954 rev.0 - HP-UX TCP/IP Remote Denial of Service (DoS)

    There are no restrictions for distribution of this Security
    Bulletin provided that it remains complete and intact.

    The information in this Security Bulletin should be acted upon
    as soon as possible.

    24 April 2005

    Remote Denial of Service (DoS)

    Hewlett-Packard Company
    HP Software Security Response Team

    A potential security vulnerability has been identified with HP-UX
    running TCP/IP (IPv4). This vulnerability could be remotely
    exploited by an unauthorized user to cause a Denial of Service


    SUPPORTED SOFTWARE VERSIONS*: ONLY impacted versions are listed.
    HP-UX B.11.00, B.11.04, B.11.11, B.11.22, B.11.23
    running TCP/IP (IPv4).


         HP-UX B.11.23
         HP-UX B.11.22
         HP-UX B.11.11
         HP-UX B.11.04
         HP-UX B.11.00
         action: set ip_pmtu_strategy = 0 or 3


    Certain network traffic can result in a Denial of Service (DoS)
    for HP-UX systems running TCP/IP (IPv4). Receiving a certain
    packet on any open TCP/IP connection can result in a Denial of
    Service (DoS) condition which can only be corrected by a reboot of
    the affected system.

    Until patches are available to resolve the issue the Denial of
    Service (DoS) can be avoided by setting the ip_pmtu_strategy
    parameter to either 0 or 3.

     Discussion of ip_pmtu_strategy

    The default value for ip_pmtu_strategy is 1. This allows for PMTU
    discovery. Once the issue of this Security Bulletin has been
    resolved via patches the ip_pmtu_strategy value of 1 will again be
    the preferred setting for most situations.

    The ip_pmtu_strategy values of 0 and 3 set the PMTU to a fixed
    size for destinations which are not on the local network.

    The ip_pmtu_strategy value of 0 sets the PMTU to 576 bytes.
    Routers are required to handle packets of at least this size.

    The ip_pmtu_strategy value of 3 sets the PMTU to 1500 bytes. This
    will generally result in more efficient transmission than the 576
    byte PMTU. If it is known that the routers involved can handle a
    1500 byte MTU the ip_pmtu_strategy value of 3 is preferred.

     Patch requirements for setting ip_pmtu_strategy to 3

    All affected HP-UX versions allow setting ip_pmtu_strategy to 0.

    B.11.22 and B.11.23 have no patch requirements for
    setting ip_pmtu_strategy to 3.

    B.11.11, B.11.04, and B.11.00 require the following patches in
    order to set ip_pmtu_strategy to 3:

       B.11.11 - PHNE_25642 or subsequent
       B.11.04 - PHNE_26101 or subsequent
       B.11.00 - PHNE_25423 or subsequent

     Reboot requirement for setting ip_pmtu_strategy

    The ip_pmtu_strategy parameter can be set via the ndd(1M) command
    or by changing the nddconf file and rebooting the system. The
    ndd(1M) command only affects new connections. Existing
    connections will remain vulnerable until they terminate or until
    the system is rebooted and the nddconf value of ip_pmtu_strategy
    takes effect.

    It has been recommended in several places including HP-UX
    Bastille, System Health Check, and the Security Bulletin
    HPSBUX0001-110 that ip_pmtu_strategy be set to 1. This
    recommendation was made to avoid issues with which
    occurred when ip_pmtu_strategy was set to 2. The value 1
    was recommended because it is the system default and
    for most situations results in the best performance. However,
    the values 0 and 3 also avoid the issues which occurred when
    ip_pmtu_strategy was set to 2. As mentioned above once the
    issue of this Security Bulletin has been resolved via patches the
    ip_pmtu_strategy setting of 1 will again be the preferred setting
    for most situations.

    There is a defect that will cause "ndd -c" to fail if there are
    more than 10 directives in /etc/rc.config.d/nddconf. That defect
    is fixed in the following patches:

       B.11.11 - PHNE_25644 or subsequent
       B.11.04 - PHNE_26076 or subsequent
       B.11.00 - PHNE_26125 or subsequent

    Until patches are available the Denial of Service (DoS) can be
    avoided by setting the ip_pmtu_strategy parameter to 0 or 3.

    If the ip_pmtu_strategy value of 3 is desired for B.11.11, B11.04,
    or B.11.00 apply the patches listed in the Background section.

    Edit /etc/rc.config.d/nddconf to add the following:




    where 'n' is the next available index value as described in the
    nddconf comments.

    As discussed in the Background section the system must be rebooted
    for the new ip_pmtu_strategy to take effect.

    Until the system can be rebooted the ip_pmtu_strategy value for
    new connections can be set with the ndd(1M) command:

      /usr/bin/ndd -set /dev/ip ip_pmtu_strategy 0


      /usr/bin/ndd -set /dev/ip ip_pmtu_strategy 3

    MANUAL ACTIONS: Yes - NonUpdate
    set ip_pmtu_strategy = 0 or 3

    Revision 0: 24 April 2005
      Initial release

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    Version: PGP 8.1

    -----END PGP SIGNATURE-----

    Yours truly,
    HP S/W Security Team
    WTEC Cupertino, California

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