regarding www.yahoo.com (akamai) TTL values

[John Wobus]

On Apr 25, 2007, at 12:31 PM, Vishwas wrote:
> On 4/26/07, John Wobus <jw354 at cornell.edu> wrote:
>> On Apr 25, 2007, at 6:35 AM, Vishwas wrote:
>> > Hi All,
>> >
>> > My understanding is that TTL values for all entries specified in a
>> > zone are equal to the TTL value specified in the SOA section of that
>> > zone.
>> I think the problem is with your understanding.
>> Each and every record in the DNS has its own individual TTL.
> !
> Does this mean WWW.yahoo.com and MAIL.yahoo.com entries from zone file
> zone-yahoo-com can have different TTL values even if we are querying
> the authoritative DNS directly?

Yes, and this is standard practice, for example, when you (i.e., the
admin of nameservers authoritative for a zone) have a name
that you don't want cached for very long because you plan to change its
address.  If, for zone 'example.com', I have records:

www1   86400  A 10.10.10.1
www2   86400  A 10.10.10.2

and I planned to change www2.example.com to use some new IP address, I 
might
initially revise the zone file so it has these records:

www1   86400  A 10.10.10.1
www2   300  A 10.10.10.2

Then, after 86400 seconds, all the world's caching servers would be 
caching the
name www2.example.com for no more than 300 seconds at a time.  Then, 
when I
load another zone file modification with my desired address change, 
within 5 minutes, the
old address would be gone from all the world's caching name servers.  
Caching
times for www1.example.com remain unaffected throughout the process.

You may find it valuable to read about the DNS and zone files.

John Wobus