TTL of MX records

John Horne john at plymouth.ac.uk
Wed Jan 12 15:37:55 UTC 2000


This is not a problem as such, but I am curious as to what may
happen. Suppose a site (domain) has 3 MX records and they
assign different TTL values to them. E.g:

     abc.example.com.        IN  A  10.0.0.1
                        5    IN  MX 10 abc.example.com.
                        10   IN  MX 20 def.example.com.
                        20   IN  MX 30 xyz.example.com.

Now another site wants to send mail to abc.example.com. The MTA looks
up the MX records and the local name server caches them. After 20
seconds the MX 30 xyz.example.com. record should timeout. Another message
is to be sent, the MTA looks for the MX records but the cache now has 2
records.

Does the nameserver just send those 2 records back to the MTA? And if
so, and both remaining sites are unavailable then the mail will be
held up. Alternatively does the nameserver realise that this is an MX
list and expire all the records? (I don't hink this happens as at all,
but I could be wrong.)

I'd be interested to know what should/would happen. I should add that I
know of no site which has set differing TTL values, and I suspect the
answer would be that if they do then they should expect possible
problems.

Thanks,

John.
----------------------------------------------------------------------
John Horne, University of Plymouth, UK         Tel: +44 (0)1752 233914
E-mail: jhorne at plymouth.ac.uk
Finger for PGP key: john at jhorne.csd.plymouth.ac.uk



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