GRE over two NATs

Daniel Lopes lopsch at
Tue Apr 5 01:36:49 CEST 2005

Mariusz Kruk schrieb:
> I have a router with 4 interfaces on which I prepared a "IMQ without IMQ
> setup". Long story short - every packet that traverses the router is
> first pushed through a tunnel set up between and
> Therefore I can easily shape the trafic for all interfaces.
> It looks like this (one interface only for simplicity):
> localnet --- eth3 --- localend <---------> remoteend --- eth0 --- world
> Where localnet is of course my local network, eth3 is a local interface,
> localend is a "local" end of the tunnel, and so on.
> I use NAT and therefore I have to NAT every packet twice, since... I
> don't exactly remember what was the exact reason but it had something to
> do with connection tracking. Anyway, my local addresses
> are first mapped 1-to-1 to addresses and finally all are
> mapped to my public IP.
> Everything runs well except GRE.
> My users complained that they can't connect to their VPNs over PPTP.
> PPTP uses GRE, so I started to log packets in various tables.
> I found that for a GRE packet generated in localnet it goes like this:
> eth3, mangle/prerouting, mangle/forward, filter/forward, mangle/postrouting,
> nat/postrouting, localend.
> Then the packet gets in the box again from the remoteend end of the
> tunnel and, surprisingly to me, gets logged at mangle/prerouting and
> nat/prerouting(!).
> Why is it so?
> I won't be attaching my setup here since it is generated from a script
> and has quite a few hundred rules (yes, I know it's not veryeffective,
> but I wanted to have a vanilla kernel, and it's kind of a
> proof-of-concept), but there is no rule that should filter such packets.
> There is no rule in nat/prerouting table that applies to those packets.
> I have completely no idea. :-(

I don´t really understand your setup so far but for GRE to work behind 
NAT you need connection tracking modules because afaik there´s nothing 
like NAT-T for GRE like for IPSec. I hope I´m not totally telling you 
nonsense. For PPTP and GRE there are conntrack modules in POM-NG.

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