Re: [gentoo-server] advanced routing question

[Eduardo Tongson]

>         I believe the author is counting on a total failure of eth0 to drop the
> primary route. Once eth0 goes down that route is withdrawn from the
> routing table leaving only the eth1 route. Linux without turning on IP:
> advanced router in the kernel will use only one gateway. I assume it
> uses the first one you config and once that disappears the second will
> be used.
>         The problem is that eth0 will hardly every fail completely. The only
> way it would is if it is directly connected to say a DSL device and the
> device became unplugged. That would completely drop eth0 and then the
> failover would happen. If only the connection upstream from the DSL
> device went down then the Linux box would happily send data to the up
> ether interface of the DSL, router, switch, etc.
>         It would also work if you're terminating connections directly into the
> Linux box... using a ds-1 card or something like that. Again this
> assumes that the connections fail totally. A ds-1 usually will, but in
> offices you're sometimes handed ether which terminates on a switch. That
> switch will be up, but the router or upstream connection has failed.
>         I can't find much on the /proc/sys/net/ipv4/route/gc_timeout setting,
> but what I did find suggests that it only provides timing as to when the
> route is dropped. It doesn't add anyway to monitor the connection.
>
> It's an interesting little hack assuming I'm right about what he's doing
> with it, but I wouldn't go around implementing it on most networks.
>

I have yet to try this (no test machines), but if you are correct this
is pretty much useless to  ~90% of those trying to achieve real
redundancy on dual links. I didn't find any concrete details regarding
gc_timeout's real purpose. If anyone here can easily read linux code
they may peruse net/ipv4/route.c which contains the code relevant to
gc_timeout.

-- 
                                                    Eduardo Tongson     
                                                    <pornadmin.net/~tongson>

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