SV: TTL ?

[Johnny Damtoft]

Hi Jim.

The $TTL value has been 300 for the last 2 months.

/ Johnny

-----Oprindelig meddelelse-----
Fra: Jim Reid [mailto:jim at rfc1035.com]
Sendt: 30. november 2000 14:39
Til: Johnny Damtoft
Cc: bind-users at isc.org
Emne: Re: TTL ? 


>>>>> "Johnny" == Johnny Damtoft <JOD at sonofon.dk> writes:

    Johnny> When i set $TTL in top of the zone-file, it should tell
    Johnny> how the TTL is right???

Sorry, I cannot parse this or even guess at what you mean.

    Johnny> Well...!  I have changed the IP for mail.itwebnet.com, and
    Johnny> that was = almost 1=BD days ago...

    Johnny> root at dns1/etc/dns> host -a mail.itwebnet.com ns.tele.dk
    Johnny> Using domain server: 
    Johnny> Name: ns.tele.dk
    Johnny> Address: 193.162.159.194

    Johnny> Trying null domain 
    Johnny> rcode =3D 0 (Success), ancount=3D1
    Johnny> The following answer is not authoritative: 
    Johnny> The following answer is not verified as authentic by the server:
    Johnny> mail.itwebnet.com 35478 IN A 194.239.12.3
    Johnny> For authoritative answers, see: 
    Johnny> itwebnet.com 172704 IN NS DNS1.ITWS.DK
    Johnny> itwebnet.com 172704 IN NS DNS2.ITWS.DK

The answer is staring you in the face. Your query went to the name
server at ns.tele.dk. It gave a non-authoritative answer. So that name
server must have already cached the DNS data for mail.itwebnet.com.
If it hadn't, it would have looked up mail.itwebnet.com and presumably
found your new data with the 5 minute TTL. However the cached data
that ns.tele.dk answered with had a TTL of 35478 seconds: about half a
day. So it'll take that server about 12 hours to discard the stale
data it has cached from an earlier lookup of that name.

This means that the administrator of the itwebnet.com domain did not
properly co-ordinate the renumbering of mail.itwebnet.com. The TTL on
the old DNS data should have been reduced long before the host was
renumbered. That would have meant that the old, stale information
wouldn't hang around in the caches of the rest of the world's name
servers for too long. [It's going to take half a day before people
querying ns.tele.dk will be directed to the new, correct address of
mail.itwebnet.com instead of the old one. Who knows how long that
stale and incorrect data will be cached by other name servers?] So if
jod at itws.dk had dropped the TTL for mail.itwebnet.com to 5 minutes a
day or two before the host was renumbered, the old details would only
have been cached by other servers for five minutes. This would have
meant more lookups for the name, but a smaller window of disruption
for the rest of the world when mail.itwebnet.com was renumbered.