[John King]

I have an interface created via openvpn. This is called tun_vpn.

I have tried to enable source based routing in two ways. Both fail in the same way. I can ping and I cant send udp packets to open an outgoing connection but as soon as the second udp packet comes in the flow stops. With TCP I can see syn packets sent, and only the ack is received by the other side. All other packets are blocked by DDWRT until the close comes from the my computer.

The second way was to use the same IP rule and mark the packets with the fwmark. This only happens with the broadcom hardware

[egc]

I agree with @jxm (I ususally do).
But to take a long shot: disable Shortcut Forwarding Engine on setup page

[John King]

Thanks that worked for tcp.

I didn't want to post iptables nor ip rules but the gist is;
ip rule add from ip table xxx
for source routing, which is the same as
iptables -t mangle -A PREROUTING -s ip -j MARK --set-mark xxx
and for both creating the routing table xxx.

Anyhow thanks for that.