Subject: Re: [dnsop] Re: getaddrinfo/TTL and resolver application-interface

From: Edward Lewis <[email-masked]>
Date: Wed, 4 Aug 2004 08:40:39 -0700

At 17:22 -0700 8/3/04, Erik Nordmark wrote:

>Do you have example of where an application would actually believe that the
>data would change after the TTL expires?

No, I don't.  I was just imagining the worst-case scenario of an 
application developers "wild imagination." ;)  The last time I did 
any work myself in this area, it was the 80's and I didn't have a TTL 
but did have changing data.

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Edward Lewis                                            +1-703-227-9854
ARIN Research Engineer

"I can't go to Miami.  I'm expecting calls from telemarketers." -
Grandpa Simpson.

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From: Paul Vixie <[email-masked]>
Date: Wed, 04 Aug 2004 16:54:43 +0000

(i'm not sure how this thread got started on both namedroppers and dnsop,
or whether it's wise for any thread to ever be in both places, but i'm not
going to be the one to remove either list from the distribution here.)

> > The TTL isn't a necessarily a measure of the volatility of the 
> > data - e.g., it's not an indication of the "mean time to next 
> > change," say before the next dynamic update.  Another example, the 
> > TTL isn't a measure of the useful life of the data - just the amount 
> > of time that a cache holding of it is "wise."
> 
> Agree.

me too.

> data would change after the TTL expires?

when rsalz first wrote INN he complained that a server administrator
would have to say "innctl reload hosts" whenever his cached dns data
became unavailable.  he asked me for an extension to gethostbyname()'s
"struct hostent" to show ttl, but i considered this unwise since there
was no reasonable default for the case where the hostent had come from
YP/NIS or /etc/hosts.

i can only assume that web browser developers would also like to know
how long it was safe to reuse a hostname->address translation before
going "back out to the wire" to see if it had changed.  right now i
suspect that they have a hardwired time threshold measured in minutes,
which would be nonrespectful toward the loadbalancing people who use
violently short TTL's.

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From: Roy Badami <[email-masked]>
Date: Wed, 4 Aug 2004 18:14:25 +0100

>>>>> "Paul" == Paul Vixie <[email-masked]> writes:

    Paul> i can only assume that web browser developers would also
    Paul> like to know how long it was safe to reuse a
    Paul> hostname->address translation before going "back out to the
    Paul> wire" to see if it had changed.

FWIW, I believe that the squid web proxy uses the TTL in this way.

      -roy