TTL of MX records

Barry Finkel b19141 at achilles.ctd.anl.gov
Thu Jan 13 15:03:54 UTC 2000


john at plymouth.ac.uk (John Horne) wrote:

>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.

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).
----------------------------------------------------------------------
Barry S. Finkel
Electronics and Computing Technologies Division
Argonne National Laboratory          Phone:    +1 (630) 252-7277
9700 South Cass Avenue               Facsimile:+1 (630) 252-9689
Building 221, Room B236              Internet: BSFinkel at anl.gov
Argonne, IL   60439-4844             IBMMAIL:  I1004994




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