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273 posts

Ultimate Geek
+1 received by user: 7


Topic # 13257 30-Apr-2007 22:51
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Hi there!

I'm trying using two ISPs together (Xnet and Woosh) to give some redundancy to my net connection, both for outgoing and incoming traffic.

Outgoing is sorted - the usual default route goes to my ADSL router with internal IP The ADSL router itself has been set up with a default route pointing to (ie back into the LAN) which is another router which talks PPPoE to the Woosh modem plugged into it.

So if the ADSL line is down, then the ADSL router sends outgoing traffic via the Woosh link. Eventually I'll set it up gc_timeout on my Linux boxen to improve on this a bit.

However it isn't working how I'd like it to with incoming traffic.

I use SSH to get into my machine remotely, and there's a webserver which can be seen by a couple of clients of mine. I want to be able to get in using the public IP of either connection. Trouble is, when the incoming connection comes via Woosh the server tries to route the reply out via the ADSL as it's the default route.

I can find online some stuff regarding using iptables and iproute2 to tag and route traffic but it seems to depend on having multiple interfaces on the server (eth0, eth1) whereas with my setup there's just eth0 and then two routers on that LAN.

Anyone got any tricks for routing traffic out on the same connection it came in on? Google hasn't been as much my friend as I'd hoped!



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21300 posts

Uber Geek
+1 received by user: 4299


  Reply # 68998 1-May-2007 00:36
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Second IP on the nic in a different subnet and put one of your external routers in that network. Seems to work mostly when I tried it, but then traffic went out on both of them. I didnt have 2 connections for long enough to bother trying to sort it any more then that.


836 posts

Ultimate Geek


  Reply # 69002 1-May-2007 01:02
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Thats an odd setup, are you sure it actually routes out the ADSL when this is up? Because it should not be from your description, if the default route points to the Woosh router then this should take precidence over any implicit outbound route if a PPPoA connection is up.

Anyhow... you could try double NAT if your router supports it. Then the incoming packet source address is the source routers internal IP, your web box will route back to the correct router and then this will double NAT back again. Simple, but you normally need a higher end router to support it.

I presume you could probably get a linux box to sit inbetween the existing routers and the Web box and do this, however it wouldnt be simple and is asking for trouble.


273 posts

Ultimate Geek
+1 received by user: 7


  Reply # 69442 3-May-2007 20:32
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Hi Fraktul

Yes it does indeed work - the ADSL router assigns a higher metric to the Ethernet router than to its own PPP connection, so the PPP route takes precedence when it exists.

I might try richms' idea of a second subnet on the network and see where that gets me. The Ethernet router is not up to double NATting I'm afraid

Thanks for the replies :) I'll try it this weekend and post how it goes.


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