GRE tunnel stops traffic when idle

[Marlon Dutra]

Hello,

I've set up a GRE tunnel between two Linux boxes and it's working well,
with or without IPSEC (under GRE). The problem is that when I have no
traffic for some minutes, side A cannot communicate to side B any more,
unless side B tries to communidate to side A. The same thing happens in
the other direction.

For example, side A pings side B. No reply. Keep pinging.

Side B pings side A. Reply ok. Side A starts getting reply from side B
too.

It's like the kernel suspends the tunnel for inactivity.

That happens even in a non NAT environment. NAT isn't a problem because
I'm using NAT-T in those environments, and NAT-T itself has a keep alive
method that prevents the NAT routers to drop the mapping.

Is there a "keep alive method" for GRE tunnels. I know that if I keep a
ping once a minute, that would fix the problem, but I'm looking for
something that isn't a band aid.

Thanks.