I have a dual-homed Debian server running squid, but not acting as a router. Simplied network diagram is below - there are other local hops between the gateways and the Internet.

Using Gateway1 gives a very fast, but not always reliable route to the Internet. Using Gateway2 gives a slower, but more reliable route to the Internet. The server uses Gateway1 as the default gateway.

I have written a script that pings three hosts on the Internet, and if all three are down, switches the default gateway to Gateway2. This part seems to be easy, but I'd like know if there is a way of routing a ICMP/ping out eth0 to a host, with all other traffic to the host going out eth1, so I can determine if the Internet is reachable via Gateway1 again.

Thanks,
slacky

Why not just set up your routing table using metrics? Then the kernel's internal routing should handle it from there. Set a lower metric value to your preferred route.

That aside, you can use the -I option to ping to specify a local interface to use as the source for your packets.

Thanks for your quick reply. I am unsure if this should work, the manpage for route says metric is "not used by recent kernels, but may be needed by routing daemons." If it does, does it require upstream routers to send ICMP unreachable messages back or anything?


ping -Ieth1 www.google.com gives me the "Destination Host Unreachable"