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Re: Weird DNS issues for domains

  • From: Crist Clark
  • Date: Thu Sep 29 14:04:57 2005

Todd Vierling wrote:
On Thu, 29 Sep 2005, John Dupuy wrote:

If you are talking about strictly http, then you are probably right. If you
are hosting any email, then this isn't the case. A live DNS but dead mail
server will cause your mail to queue up for a later resend on the originating
mail servers. A dead DNS will cause the mail to bounce as undeliverable.

If a mail server is bouncing immediately on a DNS SERVFAIL (which is what
you'll get when a remote DNS server is down), then that mail server is badly
broken and will break quite a bit during tier1 failure situations.

Failure to resolve != resolves to NXDOMAIN/empty.  A failure to resolve
(SERVFAIL) should result in the same queueing behavior that the remote SMTP
server uses for failure to establish a TCP connection.
The problem I've seen is when an SMTP server does not accept emails
which have non-resolvable MAIL FROM domain. When the sender is a dumb
SMTP client, not an MTA, this can cause problems.

(I noticed this happen to a high traffic customer who had both of their
DNS servers in the same /24 located in Slidell, LA. Needless to say, they
were down for more than a few hours when Katrina rolled through.)
Crist J. Clark                     
Globalstar Communications                                (408) 933-4387

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