> > Are you using tcpdump to determine that the packets are going out over
> > the wrong interface?
> 
> For the last test I only checked with lsof and conntrack:
> 
> # lsof -i -n | grep openvpn
> openvpn   21972     nobody    4u  IPv4 129321      0t0  TCP
> 115.146.92.84:51520->172.26.10.100:openvpn (ESTABLISHED)
> 
> # conntrack -d 172.26.8.100 -L
> tcp      6 431995 ESTABLISHED src=115.146.92.84 dst=172.26.10.100
> sport=51520 dport=1194 src=172.26.8.100 dst=115.146.92.84 sport=1194
> dport=51520 [ASSURED] mark=0 use=2
> conntrack v1.0.0 (conntrack-tools): 1 flow entries have been shown.
> 
> But now using tcpdump, I see that packets ARE leaving via eth1. But
> there is only outgoing packets, no incoming packets at all. Whereas on
> the default gateway device, packets are only incoming, no outgoing
> packets. The openvpn tunnel seems fine with that. So I need to change
> the source address of the packets for it to come back on the correct
> interface?
> 

Yes that's exactly what you need to do, or the other end will just reply with 
the original (wrong) destination address.

Something like:

iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE

you can use -j SNAT if you want to explicitly specify the source address (eg 
because you have several), but MASQUERADE should suffice.

James
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