Re: [Sheflug] sendmail & cannons

[Alex Hudson <xxxxx [at] xxxxxxxxxxxxxx>]

On Tue, 2002-01-22 at 20:28, Andrew Basterfield wrote:
> Can someone explain the action of the sendmail FEATURE('nocanonify')?

It does reverse look-ups on IP addresses (canonify, that is ;) - it
returns the 'canonical name', hence canonifying. nocanonify turns this
off.

Reverse look-ups require a DNS request every time you do it - as you
realised - but can also cause problems, because it requires that the
'reverse zone' for the IP address is setup. For example, the zone
178.168.192.in-addr.arpa is authoratitive for all my local IP addresses
(192.168.178.0/24). To get the name for the machine with IP address
192.168.178.1, it attempts to get the PTR record for
1.178.168.192.in-addr.arpa, which will work here. However, if this zone
isn't delegated (er... 'configured' ;) properly, it could be that the
answer won't return and you're waiting the 15-odd seconds for a DNS
timeout. This would cause things to slow down quite badly. 

The time to do a DNS request might be long anyway, because if you're
local nameserver is not authoratitive for the local subnet, or is only a
recursive dns cache or something, then it's going to refer the request
(probably recursively) to the next upstream DNS server. This will keep
happening until it hits a 'not exist' or it times out. This will
probably be over your dial-up link or something, which will have a
highish latency anyway.

Did you want to know all that lot ? :)

Cheers,

Alex.


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