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

[Kerin Millar <kerframil@fastmail.co.uk>]

On 04/10/2013 21:55, Grant Edwards wrote:
> 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

It would help if you were to also supply the details of:

   * ip -f inet -o a s
   * ip route show table main
   * ip route show table pmain

>
> 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?

--Kerin