I am trying to setup network fail-over on my home server but am at a
loss as to the best way to do it. Google is no help as either my
situation is unique or (more likely) my search terms are horrid.
Here's my situation: I have a 100Mbps Ethernet card on eth0 and a
802.11b wireless card on eth1. The server is on a UPS that will keep
it running for some time during a power outage. My cable modem and
wireless router/ap are also on a (separate) UPS that will keep them
up for over an hour after power loss. So if the power goes off, my
server will stay up and my Internet connection will stay up.
Unfortunately, my server is on the opposite side of the house
compared to my Internet router and goes through two other switches
before getting there. Neither of these switches are UPS protected.
So if the power goes out, the server stays up and the 'net connection
stays up... but the server can't access the 'net because the switches
in between are now dead.
What I want to do in this situation is automatically switch to the
wireless eth1. It's much much slower, but it will still work in this
case. The question that I'm being stumped on is how to do that.
I read up a bit on NIC bonding or teaming and I like the fact that it
presents a unified front-end to the sockets so if one NIC goes down,
no socket connections need to be broken. However, bonding treats
both NICs as equals and sends packets round-robin. I don't want the
wireless eth1 to be used at all unless it absolutely has to. So
bonding/teaming is out.
I then found a number of references to setting up a fail-over router
by setting two routes and having the kernel automatically switch from
the primary to the secondary route. The exclusive nature of that
appeals to me, but that won't work either since both eth0 and eth1
will have the same default route in my setup. I had the brief
thought of maybe setting up an embedded router using UML or Xen or
VMWare and having eth1 point to the router as the default route...
but that's such a hack. And besides, I would like whichever is the
dominant NIC to have the main server IP.
I could probably approximate this with a script that just checks the
link status on eth0 and if it drops for more than a few seconds, it
would 'if eth0 down' and 'if eth1 up'. That would likely work but
would have the unfortunate problem of terminating any existing socket
connections and would be, IMO, ugly.
So is there some elegant solution to this that I am missing?
Kurt
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