Issue with point to point VPNs behind NAT and asymmetric traffic

Wed Jun 12 21:44:18 UTC 2019

Hello everyone,

Trying to get my head around a certain unexpected behaviour.

I am running two site to site VPNs (wireguard now, OpenVPN earlier) between
my home and a remote server over two different WAN links. Both WAN links
are just consumer connections - one with public IP and one with CGNATed IP.
The redundancy here is taken care of by the OSPF running via FRR on both
ends.

The unexpected behaviour I get is that if I set OSPF cost to prefer say
link1 between home -> server and prefer link 2 between server -> home then
connectivity completely breaks between the routed pools. The point to point
IPs stay reachable (which is over expected links i.e symmetric via both
ends). As long as both ends prefer link1 or link2, it works fine. At first,
I thought it had to do something with NAT but still can't understand how.
Since VPN tunnels have a keep-alive timer (for 10 seconds), the tunnel is
always up. Any idea why asymmetric packets are being dropped here?
This exact behaviour was in case of earlier OpenVPN + bird + iBGP and is
still the same when I moved everything to Wireguard for VPN + FRR for
routing + OSPF.

Thanks.

-- 

Anurag Bhatia
anuragbhatia.com
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