From: "Bosko Radivojevic" <bosko.radivojevic@xxxxxxxxx>
Date: Mon, 15 Sep 2008 00:49:15 +0200
netdev@xxxxxxxxxxxxxxx is where networking developers are, added
to the CC:
Also, most of us are at the kernel summit and the plumbers
conference over the next few days, so don't expect any response
of substance for at least a week. Likely it will take longer,
as after returning most of us will have a huge backlog to process.
> Hi all
>
> Ten days ago I asked if anyone has some info regarding Cisco's GRE
> keepalive support. Later I've realized that idea behing that feature
> is quite simple and nice. As it says on Cisco site:
>
> > Router constructs the inner IP header and GRE header with a Protocol
> > Type (PT) of 0. It then sends that packet out its tunnel interface, which
> > results in the encapsulation of the packet with the outer IP header and a
> > GRE header with PT = IP
>
> Using the same idea, I've written small userspace app that do almost
> the same - instead of constructing "inner" packet with GRE header, I'm
> constructing UDP packet (it is easier to receive it in user space).
> After all, that "keepalive" packet looks like - IP|GRE|IP|UDP. When
> other end decapsulate it, inner IP|UDP packet should be sent back to
> the originating end. So, my primary task to support GRE keepalives on
> Linux side in Cisco-to-Linux "network" is accomplished. It's working
> fine.
>
> Problem is the other situation, when Cisco is sending keepalives, or
> if a GRE tunnel is created between two Linux boxes and my app started
> on any of them. Linux is not sending back the "inner" packet (when
> "keepalive" packet is decapsulated on the other end, inner IP|UDP or
> IP|GRE packet should be sent back to the originating end). Nor Cisco's
> generated, nor generated by my app.
>
> At the beginning I taught it has something with rp_filters or
> something "adjustable" in /proc/sys/net/... But I was wrong. What
> could be a reason for this behavior? I spent some time trying to debug
> this, and it looks like inner packet never leave ip_gre code. It looks
> like the journey of the packet ends in ipgre_ecn_decapsulate() at the
> very beginning: if (INET_ECN_is_ce(iph->tos)) {
>
> Does anyone have some hint for me? :)
>
> Thanks!
>
> PS. More info about GRE keepalive -
> http://www.cisco.com/en/US/tech/tk827/tk369/technologies_tech_note09186a008048cffc.shtml#t6
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