Multihoming using DNS

jfk63 at my-deja.com jfk63 at my-deja.com
Thu Jan 27 11:19:26 UTC 2000


Multihoming Using DNS

We host a web site and we want to ensure greater resilience by using
more than one ISP.

Clearly we can go the whole hog and implement BGP, but this is a major
upheaval and will have very severe consequences if we don't configure
it correctly (so that is a longer term prospect).

We plan - at least in the short term - to employ the DNS to make use of
this second link...as described below:

The DNSs resolve www.oursite.com as 1.2.3.4 which is routed through ISP-
A (what we've got at the moment)
The TTL on this record is set to be very low as is the refresh
In the event that the link goes down or other anomalies occur, a script
(automated or manual) changes the record in the DNS to resolve
www.oursite.com as 5.6.7.8 (which is routed via ISP-B)

Of, course, it's more complicated than that.

We will need to have a nameserver on the 1.2.3 network and on the 5.6.7
network (and probably elsewhere). So if the outside world is used to
getting its info from the NS-A on 1.2.3.x it will have to switch to NS-
B on 5.6.7.x

We will probably use NAT (Network Address Translation) at the edges to
ensure that the Web Farm can remain consistent

As far as I can tell this should allow for a fairly quick recovery in
the event that we lose the link to ISP-A.

Clients should only cache the 1.2.3.4 address for a short time - 5 mins
or so - after which they try and resolve www.oursite.com again. They
cannot find NS-A (because the link is down) so they fall-thru to NS-B
this resolves the address as 5.6.7.8

Of course we will have to put up with a lot more NS traffic but that's
not the end of the world.

So....why am I telling you all this...well I'm hoping that someone may
have already done this, in which case you can tell me whether it is
likely to work.

I'm sure that most of the people in this group have far more DNS
knowledge than I have. If you reckon this is a non-starter please let
me know.



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