[PATCH 3/4] log dropped ICMP redirects

[Philip Craig]

On 05/25/2006 02:26 AM, Patrick McHardy wrote:
> There was a patch called "dropped table" once that hooked in kfree_skb.
> I'm also not completely satisfied with the current situation .. but
> I don't think adding excessive packet logging is the best solution,
> often drops just occur because of failed validity checks which can
> just as well happen outside of the netfilter code.

I'm not sure what checks occur outside of netfilter code, but I assume
they are mostly things that would be dropped by any router, not just
a firewall?

For connection tracking, the goal should be to never drop anything,
and instead mark it as invalid.  My first version of this patch
just removed this test completely, so then I could log it with an
iptables rule before dropping it.  But I didn't think that was suitable
for general use.

For other validity checks within netfilter, this is a possible use for
the unclean match.   This would allow you to use rules to do the
checks, log the packet, and drop it.  And if this was done in the raw
table, then it could cover some of the connection tracking checks too.
What was the problem with the unclean match, did it simply check too much?

> In this case I'm actually having trouble understanding what the code
> is supposed to do, why would we receive an ICMP redirect for a
> unhashed connection (unhashed == first packet hasn't even left the
> box yet)?

I don't understand the reason for the code either.