Configuring different TTLs in multiple RRs for the same domain name, TYPE, and CLASS

[John Wobus]

On Mar 24, 2016, at 12:18 PM, Ben Bridges <bbridges at springnet.net> wrote:
> 
> TXT records are multiple-purpose.  They can be used for SPF records, Office 365 “MS” records, DMARC records, or whatever arbitrary uses someone dreams up, all for the same domain name.  Microsoft wants a short TTL for their Office 365 records, but I would prefer to generally use a longer TTL for most records (including other TXT records) in order to reduce the query load on our servers.  It would be nice to be able to set a short TTL for the Office 365 record but a longer TTL for other TXT records for the same domain name.
>  
> Thanks,
> Ben

From the caching server's point of view: if among two records, it expires just one of them, keeping the other one, then when another query comes:

Strategy 1: The caching server just returns the record it has?
Strategy 2: The caching server re-queries the auth server?

Strategy 1 implies this next query doesn’t get all the data, perhaps not the data the client needs.
Strategy 2 acts exactly the same as if all the records expired at once.  Except you also have this
new weird status to remember, with no benefit.

Knowing when a query to the auth server is necessary for the client’s usage would require magic knowledge, i.e. which of the two records record the client is looking for.  The RFCs take this into account and require the TTLs to be the same.

John Wobus
Cornell IT