TTL of MX records

[John Horne]

On 13-Jan-00 at 15:03:54 Barry Finkel wrote:
> john at plymouth.ac.uk (John Horne) wrote:
>>     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.
>>
> The values in the MX record are not TTL values; they are preference
> values.  See pages 95ff of The BIND book (third edition).  In the
> above example, a mailer would attempt to send mail to abc (preference 
> 10) before trying def (20) and then xyz (30).
> 
You are right and wrong. They *are* TTL values. If you do this, then use
'dig mx abc.example.com' it will show you the TTL values (5, 10, 20) and the
MX preferences (10, 20, 30). Repeating the command shows the TTL values
going down as well. However, as previously said bind actually sets the TTL
value to that of the lowest preference hence the timeout problem doesn't
occur (they all timeout at once).

*BUT* The Bind book (page  419) shows that the syntax for the MX record is
'owner class ttl MX preference exhange-dname' which is NOT what I had in the
zone file. Changing the records to the syntax above gives an error. So it
looks like the book is wrong in that the syntax should be 'ttl class'.
This also applies to the other record syntaxes in the book it seems.

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