[vox-tech] Redundant DSL Question

[Jeff Newmiller]

On Sun, 22 Dec 2002, Rod Roark wrote:

> I'm inspired to ask this after waking up to a dead DSL line
> this morning.  Fortunately it revived shortly thereafter,
> but concerns linger as I provide mail, WWW and some other
> services that I REALLY want to stay up.
> 
> Suppose I have two DSL lines attached to my multi-homed 
> server.  I can think of two different ways to handle the
> DNS:
> 
> (1) Round-robin DNS.  Advertise both external IP addresses
> for the same domains.  DNS lookups have an equal chance of
> returning either IP.  This approach is suggested by:
> http://www.samag.com/documents/s=1824/sam0201h/0201h.htm.
>
> (2) Run a separate instance of BIND on each of the two
> interfaces, one as the primary and one as the secondary
> nameserver.  Each instance resolves names to its own IP
> address.  This way if the primary DSL line dies, so does its
> nameserver and the outside world will presumably try the 
> secondary nameserver, which in turn will give out only the
> remaining IP.
> 
> #1 seems like a nice solution for load balancing, but I'm
> not sure how well it works for failover which is my primary
> concern.  In particular, will half of all HTTP requests fail
> if either line goes down?  I suspect so.

As you point out, this is a load balancing technique, not a failover
technique.
 
> #2 I just made up.  Comments?

You aren't the first.

http://boudicca.tux.org/mhonarc/ma-linux/2000/2000-Jun/msg00713.html

> Any other ideas?

Not I.  For most tinkerers, option 2 would seem to make the most sense.  

Another alternative is an EGP that sends connections from your downed ISP
to your other ISP when necessary. However, ISPs probably aren't too
enthusiastic about offering icmp redirects when their connection to you
goes down due to the waste of bandwidth for their upstream connections,
and problems with flapping routes if both connections go down.  However,
having not attempted such arrangements, I don't know just how much bribe
it would take to get them to cooperate... you never know until you try. :)

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