DNS TTL Question
 

Hi,

The person who handles our DNS is out for a short while so I'm trying to find the answer to a question.

I was asked -- if I understand the question correctly --if our nameserver has a minimum TTL which overrides the TTL from authoritative nameservers for cached records. Apparently, an ISP which hosts a domain has a very short TTL in order to work correctly for failover purposes. A lot of people at our institution check this website so he wanted to verify that we don't ignore the TTL from the ISP's DNS server because it's too low and use a higher TTL instead. I'm not too sure how to find out this info. Does anyone know how?

Thanks,
Jason

well essentially you can see the TTL's and other associated information about resolved dns names with the "dig" tool. do a dig against your local server, and the remote server and compare the TTL's. note that TTL's (obviosuly) change over time, and are nopt synchronous across servers, so you need to find the higheste TTL returned.

Try using dig to view the TTL of www.google.com. or www.yahoo.com. Both of them have CNAME records for www that have a certain TTL, and the CNAME resolves to several A records with even shorter TTLs. If you keep doing dig over and over again you will notice the TTL getting lower (as it expires from your nameserver's cache).

You should get a TTL of 300 for the A records from the CNAME of www.google.com. and TTL of 60 for the A records resolved from the CNAME of www.yahoo.com. (assuming it's not already cached).