Using TLDs in non-TLD places

[Jeff Schreiber]

From: SMTP%"David...@jhuapl.edu" 11-JUN-1996 23:52:33.72
To: bind-...@vix.com
>I had understood that using a top level domain as a part of a domain name
>(eg "www.us.foo.com") was generally considered a Bad Thing, as it could
><cause problems with the resolver search list.

It was originally a large problem, but RFC 1535 by Ehud Gavron proposed
a solution that is widely implemented and you should verify that your
resolvers are 1535 complient.

In a quick nutshell, 1535 changes the algorithm of stripping down the
domain name, and the differences between the two algorithms should be
understood. For example:
mail to xxx.yyy.zzz in domain aaa.bbb.ccc results in the following query
order:
1) MX for xxx.yyy.zzz (with default ndots settings)
2) MX for xxx.yyy.zzz.aaa.bbb.ccc
3) A for xxx.yyy.zzz
4) A for xxx.yyy.zzz.aaa.bbb.ccc

In the old algorithm, the following pattern would occur.
1) MX for xxx.yyy.zzz
2) MX for xxx.yyy.zzz.aaa.bbb.ccc
3) MX for xxx.yyy.zzz.bbb.ccc
4) MX for xxx.yyy.zzz.ccc
5-8) A lookups for each.

In other words, given the following situation. Mail going from process.au
domain to sirius.edu can have the following result in the old method.
1) MX lookup for sirius.edu (fail)
2) MX lookup for sirius.edu.process.au (fail)
3) MX lookup for sirius.edu.au ([wrong] success)
4) A lookup for sirius.edu ([Correct] Success - Not reached)
2) A lookup for sirius.edu.process.au (fail - Not reached)
3) A lookup for sirius.edu.au ([wrong] success - Not reached)

With the assumption that sirius.edu.au has an MX record, but you wanted
sirius.edu that didn't have an MX record, and were waiting for that A
record, you see the problem. With 1535 only the following 4 are used:

1) MX lookup for sirius.edu (fail)
2) MX lookup for sirius.edu.process.au (fail)
1) A lookup for sirius.edu (success)
2) A lookup for sirius.edu.process.au (fail - Not reached)

-Jeff Schreiber
Process Software Corp.