I've managed to configure openvpn on my DD-WRT router such that the tunnel is up and working, but that it doesn't mess with routes at all (the default configuration wants all traffic to go through it).I would like to port forward port 51413 on tun1 to a specific host on my network (192.168.77.145). I also need to masquerade ports 6881-6890 so that they are routed through tun1. I know this involves packet mangling/marking, and setting up routes manually as well, but it's been 12 years since I've even thought about iptables (I think I still had dialup at the time).Is there some recipe out on the internet somewhere that I'm not stumbling over?[edit]I've been experimenting with tcpdump, and it appear that the client I use (Transmission on OSX) has a curious feature: it uses 51413 (user configurable) as the source port for all outgoing packets, and as the destination for all incoming packets. Even when I look for port 6881-6889 in tcpdump, those are always the source port for the remote end, and it's always being sent to the 51413.So this is a rather simpler than I imagined it would be. I just need to route/masquerade anything with that src port number through the tun1 interface.I think that the iptables rules might be something like the following:# mark bittorrent packetsiptables -t mangle -A OUTPUT -p udp --sport 51413  -j MARK --set-mark 7iptables -t mangle -A OUTPUT -p tcp --sport 51413  -j MARK --set-mark 7# allow responsesiptables -A INPUT -i tun1 -m conntrack --ctstate ESTABLISHED -j ACCEPT# allow bittorrentiptables -A INPUT -i tun1 -p udp --dport 51413 -j ACCEPTiptables -A INPUT -i tun1 -p tcp --dport 51413 -j ACCEPT# block everything incoming on vpn#iptables -A INPUT -i tun1 -j REJECT# masqueradingiptables -t nat -A POSTROUTING -o tun1 -j MASQUERADEDo these look sane? Still no clue on the routing rules.[edit 2]I think -A is wrong, as this appends it to the end of the iptables rules. My router is adding god-knows-what first, and if those rules match, these are ignored. I'm pretty sure they should all be -I.Even with that change though, I'm unable to get anything to work.The packets show up on my private network on br0, the bridge interface. If I change nothing, DD-WRTs configuration will do NAT out over vlan2. And of course, OpenVPN's interface is tun1 (the interface I want to send bt traffic through).[edit 3]The initial two iptables statements are wrong. I was working off of examples where the author was using his desktop machine as a router (I think), and so this is different than a desktop machine that sends packets over a local network to a router with two external interfaces. The correct set of statements is something like this:# routing table setupip route replace default via $VPNGW dev tun1 table 100ip rule add fwmark 7 table 100# mark bittorrent packetsiptables -t mangle -I PREROUTING -p udp --sport 51413 -j MARK --set-mark 7iptables -t mangle -I PREROUTING -p tcp --sport 51413 -j MARK --set-mark 7# allow responsesiptables -A INPUT -i tun1 -m conntrack --ctstate ESTABLISHED -j ACCEPT# allow bittorrentiptables -A INPUT -i tun1 -p udp --dport 51413 -j ACCEPTiptables -A INPUT -i tun1 -p tcp --dport 51413 -j ACCEPT# block everything incoming on vpniptables -A INPUT -i tun1 -j REJECT# masqueradingiptables -t nat -I POSTROUTING -o tun1 -j MASQUERADEThis still isn't perfect. The bulk of bt traffic now goes over tun1, but I continue to see incoming packets for 51413 on vlan2. It's unclear to me why this is happening. 
How do I route bittorrent through a second interface?
iptables;routing;bittorrent;dd wrt