DNS Cache and Sendmail Msg-Id

mikea mikea at MIKEA.ATH.CX
Tue May 20 18:58:19 IST 2003

On Tue, May 20, 2003 at 01:31:25PM -0400, Derek Winkler wrote:
> Couple of kinda off topic questions...
> Is setting up a caching DNS server for the MailScanner machine a good idea?
> Not sure how this would effect RBL lookups.
> How unique is the sendmail message id, is it unique for that day or until
> the process ids rollover or some other method used? Searched the internet
> for this but couldn't find anything.

Done properly, the caching DNS server will retain records for their
TTL, so that you won't have to go all the way to DNSbl server each
time you look up, e.g., flowgo.com. Unless, that is, I'm much

The sendmail message ID is intended to uniquely identify the message
in the universe of all possible messages sent by all sendmail MTAs.
It may even succeed at that. Certainly the date and time, sendmail
process ID, and so on, are encoded in the 8.12 Message-ID -- down to
a very small quantum of time. IIRC, earlier sendmails used a similar
method, but the quantum was larger. Again, that's the way I understand
it, but ICBW.

Here's a randomly-chosen Message-ID from my system:
Message-Id: <200305201731.h4KHVapE058947 at mikea.ath.cx>
             <date-->time moretime<pid-> machinename

Mike Andrews
mikea at mikea.ath.cx
Tired old sysadmin since 1964

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