[TriLUG] RedHat 8.0 / IPTables / GRE packets?

Phillip Rhodes mindcrime at cpphacker.co.uk
Tue Dec 10 23:23:59 EST 2002

> My guess is that you need something compiled into your kernel in
> for it to handle GRE properly (or even at all).  I've always had
> problems with Red Hat and GRE so I stopped using Red Hat on any
> that needed GRE or IPSec.  I use Mandrake Linux or OpenBSD and am a
> happier camper.

Further exploration has revealed that this actually works, when I'm
making the PPTP
connection from another machine that's on the local LAN with the pptpd
server. In
that case, the incoming connection is on eth1 instead of eth0, and it
accepts the GRE packets, and establishes the connection, no problem.

Despite my best efforts however, I cannot get eth0 to behave likewise.
I've tried
all sorts of permutations of iptables rules that *should* allow
anything on eth0,
but no dice....

Given this additional info, does anybody have any explanation for why
this might be?
Why would one interface accept GRE packets while the other one
doesn't? Again,
I know the GRE packets are making it to the NIC, and being received,
I can see them with tcpdump... but netfilter seems to refuse to
acknowledge that
they exist, when they're on that interface....  <pulling hair out in



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