Route return traffic to correct gateway depending on service

Posted by Marnix van Valen on Server Fault See other posts from Server Fault or by Marnix van Valen
Published on 2011-01-21T15:31:34Z Indexed on 2012/06/04 4:41 UTC
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linux

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iptables

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routing

On my office network I have two internet connections and one CentOS server running a website (HTTPS on port 443). The website should be publicly accessible through the public IP of the first internet connection (ISP-1). The other internet connection, ISP-2, id the default gateway on the network. Both internet connections have routers (the household-kind) with NAT, SPI firewalls etc. The router on ISP-2 is a Netgear WNDR3700 (aka N600) with original firmware.

The problem is that the website is unreachable. Looks like incoming traffic on ISP-1 will reach the server but the returning traffic is routed through ISP-2, effectively making the site unreachable. As far as I can tell I can't do port based routing on the WNDR3700.

What are my options to make this work? I've been looking at implementing an iptables / routing based solution on the server itself but haven't been able to make that work.

Update: Note that the server has one network interface connecting it to both routers.

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