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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 always successful.
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.
(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 starving. 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 ftp://ftp.netbsd.org/pub/NetBSD/security/advisories/NetBSD-SA2001-006.txt.asc
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.
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