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HP SECURITY BULLETIN
HPSBUX01137 REVISION: 0
SSRT5954 rev.0 - HP-UX TCP/IP Remote Denial of Service (DoS)
NOTICE: 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.
INITIAL RELEASE: 24 April 2005
POTENTIAL SECURITY IMPACT: Remote Denial of Service (DoS)
SOURCE: Hewlett-Packard Company HP Software Security Response Team
VULNERABILITY SUMMARY: 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 (DoS).
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).
BACKGROUND: AFFECTED VERSIONS
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 END AFFECTED VERSIONS
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.
Note: 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.
Note: 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
RESOLUTION: 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:
TRANSPORT_NAME[n]=ip NDD_NAME[n]=ip_pmtu_strategy NDD_VALUE[n]=0
TRANSPORT_NAME[n]=ip NDD_NAME[n]=ip_pmtu_strategy NDD_VALUE[n]=3
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
BULLETIN REVISION HISTORY: Revision 0: 24 April 2005 Initial release
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