Possible NetFilter Bug (2.4.27 and 2.6.20 kernel)

From: Glenn Henshaw
Date: Fri May 30 2008 - 13:52:09 EST

I'm running into a problem with uCLinux 3.2 (2.4.17 kernel) router. I can also reproduce the problem on 2.6.20 kernel (Debian 4.0). I suspect that it still exists in more recent versions, but can't create the environment to test this easily.

The router is running a NAT (MASQUERADE) firewall. After a few days of operation with a number of clients , the ip_dst_cache fills and overflows, stopping the router. This is most noticeable with a large number of hosts behind the router doing UDP transactions (although it appears that ICMP and TCP connections produce the same effect).

The culprit seems to be traffic passing through the NAT connections in the router. Each new packet creates a new entry in the rt_cache, or increases the ref count. These entries are never cleared away. Eventually, no more entries can be allocated as the entries in the cache are locked (refcount>0). I traced the issue back to the packets returning to the router from the outside host, where the destination of the packet is changed. This appears to be a netfilter bug?

Reproducing the problem is simple. Given a host ( behind the router ( to and a host on the other side of the router, ping between the hosts. The (edited) results of the route -Cn command on the router are shown below.

### starting state
# route -Cn
Kernel IP routing cache
Source Destination Gateway Flags Metric Ref Use Iface 0 1 0 eth0 0 0 0 eth1
# route -Cn

### ping 25x

# route -Cn
Kernel IP routing cache
Source Destination Gateway Flags Metric Ref Use Iface 0 0 24 eth1 i 0 25 24 eth1 i 0 25 24 eth0 0 1 0 eth0

I've searched most of the lists, and added some patches (which corrected lost ip_dst_cache entries). I'm looking for pointers on how to resolve this. I must fix this in the 2.4 stream as this is an embedded application and a kernel upgrade means recertifying with al of our customers. Is there a better list to post to? Is there a bug tracker somewhere to log this into?

Glenn Henshaw Logical Outcome Ltd.
e: thraxisp@xxxxxxxxxxxxxxxxx w: www.logicaloutcome.ca

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