Re: OT: DNS, MX Records and TTL

At 11:59 AM 3/8/04, Greg Connor wrote:
> > On Mon, Mar 08, 2004 at 09:48:26AM -0500, Marc Alaia wrote:
> > > Hey, everyone.  I have an 'off topic' DNS question and know you all can
> > > help with the answer.  Specifically, how do MX records work with TTL?
> > > And how does a mail sender know when to look for new records?
> > >
> > > Example:
> > > alaia.net has two MX records, but let's say I make a mistake and have
> > > different TTL's
> > > alaia.net MX 10 mx.spfproxy.com ; TTL=14400
> > > alaia.net MX 20 gw.alaia.net ; TTL=86400
> > >
> > > So, first email to alaia.net goes to mx.spfproxy.com, of course, but what
> > > happens 5 hours later after MX 10 has expired?  Similarly, what if I add
> > > another MX record with priority 5 immediately after an email is sent to
> > > me? I presume that that sender would not check for new records until all
> > > prior records have expired?
>
> --"Arnold K." <aksup(_at_)voicenet(_dot_)com> wrote:
> > Marc,
> >
> > A record can only have one TTL.  The TTL  either will be the lower
> > of the two or the first seen when the record is read-in have.  I am
> > inclined to say that it will be the first one seen, but have not tested
> > it.
>
> I think it's correct that one record can have only one TTL, but in Marc's 
> example there are two MX records.  I don't actually know what happens at 
> the cacheing server if one record expires and the other does not.
> I could see having two different TTLs, maybe if the secondary doesn't 
> change but the primary might change often.  But, I would probably want to 
> test this out, or else use the lower ttl for both to be on the safe side.
> Also, you can have a long TTL time for the MX and a short TTL for the 
> corresponding A records, which would allow you to change the IP address 
> quickly if you need to.
> --
> Greg Connor <gconnor(_at_)nekodojo(_dot_)org>
>
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Greg,

The example deals with an MX record.

i.e.
domain TTL IN MX is the record.
The preference and server are instances within the record.

You're right that you can have one TTL for the MX record and a different 
TTL for the A record.
Arnold