Multi-homed network?

Question

I have a network with a layout like this:

*******           ******            *******            ******
* SW1 * --------- * R1 * ---------- * SW2 * ---------- * R2 *
*******           ******            *******            ******
                     |                                    |
                     |                                    |
                 ********                             ********
                 * ISP1 *                             * ISP2 *
                 ********                             ********

SW1 = Switch with multiple clients connected.
R1 = Primary router on the network. This router handles DHCP.
ISP1 = Primary Internet uplink. All clients should use this by default.
SW2 = Switch with multiple clients connected.
R2 = Secondary router on the network.
ISP2 = Secondary Internet uplink. Clients should use this for specific traffic and if ISP1 fails.

By default all clients have R1 as default gateway, and connects to Internet through ISP1.

However, all traffic with destination port 995 or 465 should go to ISP2, and if ISP1 goes down, all Internet traffic on the network should go to ISP2.

I'm not sure how to accomplish this. What would be the correct strategy to do this?

R1 and R2 are regular computers with multiple network cards and Debian installed.

Answer 1

In theory, the correct strategy would be:

• Use iptables --mark to add tags to the special packages you have (e.g. iptable -A PREROUTING -t mangle -p tcp --dport 995 -j MARK --set-mark 1)
• Use this mark to set a special route via R2 (see http://nerdboys.com/2006/05/05/conning-the-mark-multiwan-connections-using-iptables-mark-connmark-and-iproute2/ or http://blog.khax.net/2009/11/28/multi-gateway-routing-with-iptables-and-iproute2/)
• Route the other packets via R1, but give them also R2 with a different weight/prio.

The proper term for searching is "multihomed routing". I don't have a good howto at hand right now.