Priority of MX record

[Barry Margolin]

In article <dos0ha$1gp8$1 at sf1.isc.org>,
 leicheong.bbs at bbs.sayya.org (²z©÷) wrote:

> ¡° ¤Þ­z¡mbarmar at alum.mit.edu (Barry Margolin)¡n¤§»Ê¨¥¡G
> > In article <dohgnf$vqt$1 at sf1.isc.org>,
> >  leicheong.bbs at bbs.sayya.org (²z©÷) wrote:
> > > Hello,
> > >     Say we have a domain name of abc.com. The DNS entries are as
> > > the following.
> > > abc.com MX 10 mail.abc.com
> > > abc.com MX 20 mail2.abc.com
> > >     This morning, we find that our internet connection of mail.abc.com
> > > is not working. But while mail2.abc.com works normally, it seems we're
> > > still not getting any emails. So finally I have to switch the priority
> > > of them to make it work.
> > > Aren't the "priority" of a MX record be used as a fallback when one
> > > mail host is inaccessable? Or do I have some misunderstanding on this
> > > matter?
> > Your understanding is correct.  Are you sure that mail2 is set up
> > properly to deliver the mail?  If it's trying to connect to mail via its
> > Internet connection rather than an alternate path, the mail will still
> > be stuck on mail2.
> I think it should be. Because I just exchange the priority values
> of the two MX records to make it work. That means the settings should
> be correct.
> 
> Really strange.
> 
> P.S.: The network problem is cased by a malfunctioned CISCO swtich in
> in the building... Could it be that the nearest router somehow can't
> deliver "Destination not reachable" RIP signal so the fallback fails?
> (I've not tried the response from outside, so this is juat a guess)

No, that shouldn't be necessary.  The failover should occur when the 
connection attempts time out.  TCP never depends on routers sending back 
ICMP messages.

-- 
Barry Margolin, barmar at alum.mit.edu
Arlington, MA
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