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Re: Weird DNS issues for domains

  • From: John Dupuy
  • Date: Thu Sep 29 14:13:21 2005

I'll defer to you on this. Clearly a failure to resolve is not the same thing as a NXDOMAIN RCODE.

And yet, personal experience has show that the failure of all a customer's DNS servers for a domain does cause swifter mail bouncing than would occur otherwise. I do not know if it was due to the other providers having broken MTAs or broken DNS servers/resolvers... Or maybe they were all flukes. I now wish I had investigated them more thoroughly for the few times I've seen it.


At 12:29 PM 9/29/2005, 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.

-- Todd Vierling <> <> <>

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