Re: [LARTC] Redundant internet connections.

[Peter Rabbitson <rabbit@rabbit.us>]

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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