Forwarding all UDP Traffic to specific WAN in Dual-WAN Setup

[stevec]

Hi there,

I have already been reading up on different topics in this forum for around a year and now have finally hit a problem I just can't manage to find a solution to. So I decided to join ans hopefully one of you can give me some tipp on how to get this to work.

Here is the situation:
My ISP drops all UDP traffic by default. The network accepts UDP internal but the internet gateway will drop it, ergo applications won't recognize that something isn't working. The only way around this so far has been a VPN. Worked great for around a year but I have never been really satisfied with the solution due to the fact that I never managed to route selectively on an application base but rather only on a source IP base.
A couple of weeks ago I got a new cheap TP-Link Router and set it up to work as a Router to another Network (directional wireless link to a friends wifi who lives a just close by), then connected it to my AC68U and configured a dual WAN. Both Internet connections work just fine, but the wireless link offers only a maximum of 10-20Mbit/s and I don't want to max out my friends connection, only whats barely necessary. Needlessly to say he knows about it.)
I know I could do source routing, but thats not a nice solution.
Now I just can't figure out how to setup the AC68U to send all UDP traffic through the TP-Link Internet connection.
I looked at iptables and tried something like this:
iptables -A PREROUTING -t mangle -i br0 -p udp -j MARK --set-mark 1
ip rule add from all fwmark 1 table wan1

Unfortunately it doesn't seem to work. I searched around a little more and tried to add source ports to the rule, but didn't work either. Now I am looking for a new idea, hopefully one of you guys can help me out with a new idea.

BR,

SteveC

Update, 07-04-2016:
It seems PortForwarding is the way to go for me.
I will try the following now:
iptables -A PRETROUTING -t mangle -j ROUTE --gw 10.0.2.1 -p udp --dport xxxxx

where 10.0.2.1 is the TP-Link gateway
Is the mangle table the "good" way to go?
Furthermore, is there a more efficient way to forward the packets to the right gateway instead of entering all the ports manually?

Update, 07-05-2016:
I tried my above solution but for some reason it won't work.
Any suggestions?

 

[stevec]

Hey all,
after messing up my configuration on both routers, I reset everything to default. Then after re-configuring everything I discovered by coincidence that it seemed to be working. Ta-daahhh.
Some applications still seem to have trouble with it, e.g. FaceTime and Skype, L2TP VPN.
To be honest I don't understand how it is working now and I don't know why some work and some don't.
My only guess is that ISP 1 (the UDP block/drop) is rejecting the Pakets and therefore the AC68U will try Gateway 2 (my friends wireless link). But this would mean before rejecting the node would have to check wether its a local link or if he's supposed to forward to the gateway. Pretty confusing for me.
Maybe someone can explain to me if I'm right or of not what mechanism is making this work?