[sf-lug] DNS: last SOA sub-field TTL --> Negative Cache TTL, comments, etc.

Michael Paoli Michael.Paoli at cal.berkeley.edu
Mon Nov 23 21:25:27 PST 2009

> Date: Tue, 17 Nov 2009 12:17:00 -0800
> From: Rick Moen <rick at linuxmafia.com>
> Subject: Re: [sf-lug] SF-LUG DNS
> To: sf-lug at linuxmafia.com
> Message-ID: <20091117201700.GO6625 at linuxmafia.com>
> Content-Type: text/plain; charset=utf-8
> Quoting Michael Paoli (Michael.Paoli at cal.berkeley.edu):
>> # cat var/named/chroot/var/named/sf-lug.com
>> $TTL 86400
>> $ORIGIN sf-lug.COM.
>> @       IN      SOA     ns1.sf-lug.com. jim.well.com. (
>>                         2007102904      ;Serial
>>                         3600            ;refresh period
>>                         3600            ;retry period
>>                         1209600         ;expire period
>>                         10800)          ;minimum TTL period
>> ;
> Minor correction:  The last SOA sub-field hasn't signified "minimum TTL
> period" since BIND4 days.  The above annotation is a dusty holdover,
> probably copied from an old example file, and should be replaced.  The
> new-er purpose of that subfield is "negative TTL" aka "negative
> caching", which is how many seconds a nameserver should cache a NAME
> ERROR (NXDOMAIN) record.

Good catch.  Knowing ... and also taking a quick peek around ... I find
in BALUG.ORG.'s master DNS files, the comment portion indicates
"minimum", or "MINIMUM", with no reference to TTL (but no mention of
cache or negative).  Peeking for a better example, in current Debian
stable, I find "Negative Cache TTL".  I think that works well -
accurate and concise.  Anyway, I've updated the comment data in the
applicable SF-LUG.COM. zone file(s) for that SOA subfield (and will
probably do likewise next time I'm updating BALUG.ORG master
(sub)domain/zone files.

Good comments in code/configurations can be rather to quite helpful (and
generally should be).  But programs, etc., generally ignore comments -
so unfortunately comments may be inconsistent with code/configuration.
Comments at least ought not to lie or mislead - but sometimes they do -
useful to keep in mind when troubleshooting, reviewing
code/configurations, etc.

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