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FreeBSD: 'syncache/syncookies' Denial of Service Print E-mail
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Posted by Team   
FreeBSD Two related problems with syncache were triggered when syncookies wereimplemented. Legitimate TCP/IP traffic may cause the machine to crash.

FreeBSD-SA-02:20                                            Security Advisory
                                                                FreeBSD, Inc.

Topic:          syncache/syncookies denial of service

Category:       core
Module:         net
Announced:      2002-04-16
Credits:        Alan Judge <>
Dima Ruban <>
Affects:        FreeBSD 4.5-RELEASE
                FreeBSD 4.4-STABLE after 2001-12-14 19:53:01 UTC
                FreeBSD 4.5-STABLE prior to the correction date
Corrected:      2002-02-20 16:48:49 UTC (RELENG_4)
                2002-02-21 16:38:39 UTC (RELENG_4_5, 4.5-RELEASE-p1)
FreeBSD only:   YES

I.   Background

The SYN cache ("syncache") and SYN cookie mechanism ("syncookie") are
features of the TCP/IP stack intended to improve resistance to a class
of denial of service attacks known as SYN floods.

II.  Problem Description

Two related problems with syncache were triggered when syncookies were

1) When a SYN was accepted via a syncookie, it used an uninitialized
pointer to find the TCP options for the new socket.  This pointer may
be a null pointer, which will cause the machine to crash.

2) A syncache entry is created when a SYN arrives on a listen socket.
If the application which created the listen socket was killed and
restarted --- and therefore recreated the listen socket with a
different inpcb --- an ACK (or duplicate SYN) which later arrived and
matched the existing syncache entry would cause a reference to the old
inpcb pointer.  Depending on the pointer's contents, this might result
in a system crash.

Because syncache/syncookies support was added prior to the release of
FreeBSD 4.5-RELEASE, no other releases are affected.

III. Impact

Legitimate TCP/IP traffic may cause the machine to crash.

IV.  Workaround

The first issue described may be worked around by disabling syncookies
using sysctl.  Issue the following command as root:

  # sysctl -w net.inet.tcp.syncookies=0

However, there is no workaround for the second issue.

V.   Solution

1) Upgrade your vulnerable system to 4.5-STABLE or the RELENG_4_5
security branch dated after the respective correction dates.

2) To patch your present system: download the relevant patch from the
below location, and execute the following commands as root:

# fetch
# fetch

This patch has been verified to apply to 4.5-RELEASE only.

Verify the detached PGP signature using your PGP utility.

Execute the following commands as root:

# cd /usr/src
# patch -p < /path/to/patch

Recompile your kernel as described in and reboot the

VI.  Correction details

The following list contains the revision numbers of each file that was
corrected in the FreeBSD ports collection.

Path                                                             Revision

VII. References>

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