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 (192.168.4.2)
behind the router (192.168.4.3 to 192.168.1.161) 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
192.168.4.3 192.168.4.2 192.168.4.2 0 1
0 eth0
192.168.1.161 206.191.32.163 192.168.1.2 0 0
0 eth1
# route -Cn
### ping 25x
# route -Cn
Kernel IP routing cache
Source Destination Gateway Flags Metric Ref
Use Iface
192.168.1.161 192.168.1.1 192.168.1.1 0 0
24 eth1
192.168.4.2 192.168.1.1 192.168.1.1 i 0 25
24 eth1
192.168.1.1 192.168.4.2 192.168.4.2 i 0 25
24 eth0
192.168.4.3 192.168.4.2 192.168.4.2 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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