regarding www.yahoo.com (akamai) TTL values

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regarding www.yahoo.com (akamai) TTL values
\ Vishwas (25 Apr 2007)
. \ John Wobus (25 Apr 2007)
. . \ Vishwas (25 Apr 2007)
. . . \ John Wobus (25 Apr 2007)

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Subject:	regarding www.yahoo.com (akamai) TTL values
Group:	Bind-users
From:	Vishwas
Date:	25 Apr 2007

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.

Following is the scenario that is confusing me.

STEP-1 [get current TTL from local DNS, and find authority for www.yahoo.com]
$ dig www.yahoo.com
...
;; ANSWER SECTION:
www.yahoo.com. 139 IN CNAME www.yahoo-ht3.akadns.net.
www.yahoo-ht3.akadns.net. 60 IN A 209.131.36.158

;; AUTHORITY SECTION:
akadns.net. 17543 IN NS za.akadns.org.
...

STEP-2 [ask the authority about an entry in its zone and observe the
TTL value returned]
$ dig @za.akadns.org www.yahoo-ht3.akadns.net
...
;; ANSWER SECTION:
www.yahoo-ht3.akadns.net. 60 IN A 209.131.36.158
...

STEP-3 [check the SOA setting of the authority]
$ nslookup
> set type=soa
> akadns.net
Server: 192.122.134.35
Address: 192.122.134.35#53

Non-authoritative answer:
akadns.net
origin = internal.akamai.com
mail addr = hostmaster.akamai.com
serial = 1177489512
refresh = 19200
retry = 19200
expire = 19200
minimum = 180

Authoritative answers can be found from:
akadns.net nameserver = za.akadns.org.
...

From STEP-1, I deduce that www.yahoo.com is a CNAME controlled by
akamai.net. Therefore, I make a query to akamai.net to see what TTL it
returns

I have following questions:
1. Which server decides the max. TTL values (the upper bound) for the
"CNAME" and "A" entries in STEP-1? I found that for CNAME the max. TTL
is 300 and for the A record its 60.

2. Since, www.yahoo.com. = www.yahoo-ht3.akadns.net. (CNAME) and
AUTHORITY is akadns.net. www.yahoo-ht3.akadns.net entry exists in the
zone controlled by akadns.net server. As STEP-3 SOA records show that
for akadns.net the TTL = 180, why the reply from akadns.net for
www.yahoo-ht3.akadns.net gives TTL = 60 (as seen in STEP-2)

Thank you.
PS: The same steps for www.google.com show no mismatch in TTL values
(specified in SOA and direct replies from ns4.google.com)

--
Best Regards,
Vishwas.
ivishwas.googlepages.com

A writer needs three things, experience, observation and imagination,
any two of which, at times any one of which, can supply the lack of
the others. -- William Faulkner