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AUSCERT External Security Bulletin Redistribution
ESB-2001.223 -- NetBSD Security Advisory 2001-006
Denial of service using bogus fragmented IPv4 packets
31 May 2001
AusCERT Security Bulletin Summary
Operating System: NetBSD-current prior to April 17, 2001
NetBSD 1.5.x pror to April 24, 2001
Impact: Denial of Service
Access Required: Remote
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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
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
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
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.
James Thomas for bringing this problem to our attention, and
Jun-ichiro Hagino for providing a fix for the problem.
2001-05-29 - Initial Release
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 $
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