Grant Taylor wrote:
I need a way for the Linux kernel to try to use a default gateway and
switch to another one if it does not see any traffic.
I don't know about any working in-kernel solutions, but you can do it
trivially with netfilter and a cronjob:
* In netfilter do this:
-t mangle -N ispA
-t mangle -A ispA -j RETURN
-t mangle -N ispB
-t mangle -A ispB -j RETURN
-t mangle -A PREROUTING -i $ifA -s ! a.a.a.a/aa -j ispA
-t mangle -A PREROUTING -i $ifB -s ! b.b.b.b/bb -j ispB
where a.a.a.a and b.b.b.b are subnets describing your first 1 - 2 hops,
so traffic from your upstream router will not count.
* Then make a cron job that run this every minute:
iptables -t mangle -vnxZL isp[AB]
and will look for the first number on the third line. If it is not 0 -
the link is alive, otherwise change the routing tables accordingly.
Of course you can have up to 1 minute of downtime, but it does not look
so bad IMO.
HTH
Peter
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