[dns-operations] Prime TTL values for TLD and root server delegations.

[Colm MacCárthaigh]

2009/12/21 Colm MacCárthaigh <colm at stdlib.net>:
> No doubt, this question has been asked before - but I can't seem to
> find a good, documented, answer for it;
>
> Why aren't the TTLs on NS delegations served by root servers and TLD
> operators prime numbers?
>
> colmmacc at caer (~) $ dig www.example.com @k.root-servers.net
> ...
> com.                    172800  IN      NS      a.gtld-servers.net.
>
> colmmacc at colmmacc (~) $ dig www.example.com @a.gtld-servers.net.
> ...
> example.com.            172800  IN      NS      a.iana-servers.net.

I forgot to add some more detail;

The TTL on the A records for the gtld servers is also 172800;

a.gtld-servers.net.     172800  IN      A       192.5.6.30

The TTL on the A records for the zone servers is a whole divisor -  21600;

a.iana-servers.net.     21600   IN      A       192.0.34.43

and finally, the TTL for www.example.com is itself 172800;

www.example.com.        172800  IN      A       192.0.32.10

So, when the priming query is in-phase, the following all expire at
the same time;

 NS records for .
 NS records for com.
 NS records for example.com.
 A record for a.gtld-servers.net.
 A record for a.iana-servers.net.
 A record for www.example.com

making www.example.com a great example of the disastrous caching
mis-alignment anti-pattern. It only affects 1 query in very many, but
the behaviour becomes very bi-modal. If these were each prime numbers,
the occurrence would only happen a few times per universe-lifetime.
Just wondering :-)

-- 
Colm