NetBSD: fragmented IPv4 packets vulnerability
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NetBSD Malicious parties may be able to prevent a NetBSD node from communicating with other nodes by transmitting a lot of bogus fragmented IPv4 packets.

                 NetBSD Security Advisory 2001-006

Topic:          Denial of service using bogus fragmented IPv4 packets
Version:        NetBSD 1.4, 1.5, -current
Severity:       Network-connected systems can be crashed remotely
Fixed:          NetBSD-current:    April 17, 2001 (1.5U)
                NetBSD-1.5 branch: April 24, 2001 (1.5.1 will include the fix)
                NetBSD-1.4 branch: (not yet available)


Malicious parties may be able to prevent a NetBSD node from
communicating with other nodes by transmitting a lot of bogus
fragmented IPv4 packets.

For the attack to be effective, the attacker needs to have good
network connectivity to the victim node (like logged onto the victim
machine itself, connected by a fat LAN, or whatever).

There are exploits for this problem available on the Internet.
However, the attack is timing dependent and the attack is not
always successful.

Technical Details

In the IPv4 input path (sys/netinet/ip_input.c), there's code to
reassemble fragmented IPv4 datagrams.  Datagram fragments destined to
the node will be queued for 30 seconds, to allow fragmented datagrams to
be reassembled.

Until recently, there was no upper limit in the number of reassembly
queues.  Therefore, a malicious party may be able to transmit a
lot of bogus fragmented packets (with different IPv4 identification
field - ip_id), and may be able to put the target machine into mbuf
starvation state.

Recently we introduced a new sysctl(3) - net.inet.ip.maxfragpackets.
With this, you can configure an upper limit to the number of reassembly
queues.  If you want the old behavior (no limit), you can set the
value to a negative value.

Solutions and Workarounds

  (1) Upgrade the system from newer sources or binaries:

        Compile and install a kernel which has the sysctl(3) variable
        net.inet.ip.maxfragpackets in the sysctl MIB.  With this
        variable, you can limit the number of IPv4 fragment reassembly
        queues kept on the system.  The value needs to be picked
        carefully, considering the role of the node (i.e. if the
        node is a busy web server, you may want to set the value
        higher).  Note that, however, even with the configuration
        knob, it is possible for attackers to transmit a lot of
        bogus IPv4 fragmented packets, and prevent other fragmented
        IPv4 traffic from getting reassembled.  Unfragmented IPv4
        communication will be kept safe by the variable.

        Systems running NetBSD-current dated from before April 17,
        2001 should be upgraded to NetBSD-current dated April 17,
        2001 or later.

        Systems running NetBSD 1.5.x systems dated from before
        April 24, 2001 should be upgraded to NetBSD 1.5.x dated
        April 24, 2001 or later.
        NetBSD 1.5.1 will ship with the fix.

        There is no fix to 1.4.x available at this time.

  (2) Increase the kernel option NMBCLUSTERS

        Use an appropriate value for NMBCLUSTERS for the node.
        Normally, it is the cluster mbufs which go into a starvation
        state with this attack.  By setting NMBCLUSTERS to a higher
        value, you may be able to prevent the mbuf memory pool from

        Note that a couple of NetBSD device drivers pre-allocate
        cluster mbufs within the driver, for performance reasons
        and DMA management reasons.  For example, the fxp driver
        pre-allocates 64 cluster mbufs per interface.  If you
        are using such network cards, you will want to raise
        NMBCLUSTERS even more.

Thanks To

James Thomas for bringing this problem to our attention, and
Jun-ichiro Hagino for providing a fix for the problem.

Revision History

        2001-05-29 - Initial Release

More Information

An up-to-date PGP signed copy of this release will be maintained at

Information about NetBSD and NetBSD security can be found at 
http://www.NetBSD.ORG/ and  http://www.NetBSD.ORG/Security/.

Copyright 2001, The NetBSD Foundation, Inc.  All Rights Reserved.

$NetBSD: NetBSD-SA2001-006.txt,v 1.7 2001/05/29 05:58:41 lukem Exp $