Which TTL will bind use ?

Kevin Darcy kcd at daimlerchrysler.com
Tue Apr 16 19:49:25 UTC 2002


Torsten Mueller wrote:

> I have a question regarding the TTLs.
>
> Assume that you have an A record for
> test.mydomain.org with the TTL 86400 and the IP
> 192.168.1.1
>
> Now you try to add via dynamic dns update a second A record
> with a different TTL i.e.
> test.mydomain.org with TTL 20000 and the IP
> 192.168.33.10
>
> I know it's "illegal" and bind only accepts only 1 TTL for
> the same host with the same record type.
>
> My question is, which of the TTLs bind will use for the 2 records ?
>
> The TTL with the highest value ?
> The TTL which was "the first" TTL ?
> The TTL of the last submitted record ?
> one of the TTLs (random decision of bind) ?

According to RFC 2181, Section 5.2, nameservers cannot *send* an RRset
with differing TTLs. If they *receive* such an RRset, then they
"minimize" the TTLs to the lowest TTL value. This also appears to be
what BIND does when receiving a Dynamic Update which causes differing
TTLs in an RRset (I just tested it).


- Kevin




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