NAT stops working (more)

Taylor Grant gtaylor at
Tue Apr 26 04:00:33 CEST 2005

Daniel Wittenberg wrote:
> The only ideas people came up with was the conntrack table, but I know
> that's not a problem (I see no errors at all, plus manually checking it
> is fine).  So now I'm wondering how I can debug netfilter itself?
> kernel debugger?  I can see the packets come into the host using
> tcpdump/ethereal, but they don't go out the internal interface, so not
> sure how to "track" the packet within the kernel.  Ideas?
> Thanks,
> Dan

Dan, silly question, but are you sure that your firewall is not somehow interfering with the traffic?  Could you do an iptables dump of the filter, mangle, and nat tables the next time that the traffic exhibits this failure?  If you could post that (scrubbed of sensitive IPs if needed) as well as some of your network config (interfaces and IP addresses, upstream gateways, etc) we might be able to do more for you.  I know that any time that I have had any thing just not work it has usually been a firewall issue.  You say that you are playing with your routing cache, so we might need an output of your routing tables as well, route -n should do the trick unless you are doing any advanced routing.

Grant. . . .

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