[gentoo-user] OT: default route dependent on dest port?

[Grant Edwards]

Let's posit two network interfaces net1 (192.168.x.y/16) and net2
(172.16.a.b/16).  There's a NAT/gateway available on each of the
networks. I want to use the 172.16 gateway for TCP connections to port
80 and the 192.168 gateway for everything else.

I'm primarily following this example:

  http://www.tldp.org/HOWTO/Adv-Routing-HOWTO/lartc.netfilter.html

My "main" routing table contains all directly accessible subnets plus
a default route via the 192.168 gateway.
  
I created a second route table named "pmain" which is identical to
"main" except it has a different default route via the 172.16 gateway.

My ip rules are:

  0:      from all lookup local 
  10000:  from all fwmark 0x1 lookup pmain 
  32766:  from all lookup main 
  32767:  from all lookup default 

I then add an iptables rule like this:

  iptables -A OUTPUT -t mangle -p tcp --dport 80 -j MARK --set-mark 1

Now all TCP packets destined for port 80 are sent to the 172.16
gateway, _but_ they're being sent with a 192.168 source address. The
TCP stack is apparently unaware of the advanced routing tricks and
thinks that the packets are going out via the 192.168 gateway.

IOW I've succesfully re-routed TCP _packets_ but not the TCP
_connection_.

How do I tell the TCP stack that it's supposed to use the 172.16
inteface/gateway for connections to port 80?

-- 
Grant Edwards               grant.b.edwards        Yow! I feel partially
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