[dns-operations] HTTP SRV, was Adding CNAME for the root domain issue

John R Levine johnl at taugh.com
Sat Apr 30 17:43:48 UTC 2016

>> _imap._tcp.example.com. SRV target.example.net.
>> ...

> mostly is, because the SRV target is often in the same zone, so barring 
> truncation, the [AAAA | A] RRsets will already be in cache since they will 
> have been sent with the cache miss response.
> pragmatically speaking, if an SRV client sees only one RRset (AAAA alone, or 
> A alone) it can query for the one it didn't receive. negative caching does 
> the rest. CDN's (low AAAA/A TTL, or ECS) fold in nicely in this model, since 
> the missing RRsets are either popular, or they aren't.

I'd think that the answer from the SRV would most likely either be in the 
same zone, so it can return the A/AAAA at the same time, or it'll be 
pointing at a CDN in the same way that they use a CNAME now.  Either way 
the number of round trips seems the same.

I suppose you could tweak the cache to prefetch A/AAAA records for SRV 
results, since whoever asked for the SRV is very likely to ask for them 


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