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Re: dns and software, was Re: Reliable Cloud host ?
From: Mark Andrews <marka () isc org>
Date: Wed, 29 Feb 2012 08:06:10 +1100


In message <CAP-guGV09HF7in+vZbKpGk0RR1Q4gpMMo5jQREUZVEj+ewzmkg () mail gmail com>,
 William Herrin writes:
On Tue, Feb 28, 2012 at 12:45 AM, Mark Andrews <marka () isc org> wrote:
       getaddrinfo was designed to be extensible as was struct
       addrinfo.  Part of the problem with TTL is not [all] dat=
a sources
       used by getaddrinfo have TTL information.

Hi Mark,

By the time getaddrinfo replaced gethostbyname, NIS and similar
systems were on their way out. It was reasonably well understood that
many if not most of the calls would return information gained from the
DNS. Depending on how you look at it, choosing not to propagate TTL
knowledge was either a belligerent choice to continue disrespecting
the DNS Time To Live or it was fatalistic acceptance that the DNS TTL
isn't and would not become functional at the application level.

No.  Propogating TTL is still a issue especially when you do not always
have one.  You can't just wave the problem away.  As for DNS TTL addresses
are about the only thing which have multiple sources.  You also don't
have to use getaddrinfo.  It really is designed to be the first step in
connecting to a host.   If you need to reconnect you call it again.
 
Still works fine deeper in the query system, timing out which server
holds the records though.


       Additionally for
       many uses you want to reconnect to the same server rather
       than the same name.

The SRV record was designed to solve that whole class of problems
without damaging the operation of the TTL. No one uses it.

You don't need to know the TTL to use SRV.
 
It's all really very unfortunate. The recipe for SOHO multihoming, the
end of routing table bloat and IP roaming without pivoting off a home
base all boils down to two technologies: (1) a layer 4 protocol that
can dynamically rebind to the layer 3 IP address the same way IP uses
ARP to rebind to a changing ethernet MAC and (2) a DNS TTL that
actually works so that the DNS supports finding a connection's current
IP address.

DNS TTL works.  Applications that don't honour it arn't a indication that
it doesn't work.
 
Regards,
Bill Herrin

-- 
William D. Herrin ................ herrin () dirtside com  bill () herrin us
3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
Falls Church, VA 22042-3004
-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka () isc org


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