[80085] in North American Network Operators' Group
DNS Round Robin
daemon@ATHENA.MIT.EDU (Roy)
Sat Apr 23 17:55:51 2005
Date: Sat, 23 Apr 2005 14:55:22 -0700
From: Roy <garlic@garlic.com>
To: nanog@merit.edu
Errors-To: owner-nanog@merit.edu
Something I seem to have found and wonder if anyone else sees this.
One of my users has been using round robin DNS to attempt to load
balancing using two IP addresses. A query for www.whatever gives both
addresses with a TTL of zero. One address is obviously less than the
other numerically. Subsequent queries show alternating results where
the first address given switches back and forth. This is the desired
result.
Here's where is goes weird. If I do the queries through a caching NS
running bind 9.3.0, the order that the addresses is always the same with
the lower one first which clearly defeats the purpose of the load balancing.
If I specify "rrset-order {order random;};" as an option in the caching
NS then queries come back with random results.
My theory is as follows. The query causes the caching NS to get the two
answers but the software stores them in numerical order. The default
for bind is to "round-robin" so it choses the first (and thus the lower
IP address) as the first value. Since the TTL is zero, the software
then discards the data so it never gets to select the second value in
its robin robin scheme.
Does this sound plausible? Has anyone else observed this? Is it a bug
or a feature?
Roy Engehausen
