Re: SMTP servers behind firewall....
From: Lars M. Hansen (badnews_at_hansenonline.net)
Date: 05/28/03
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Date: Tue, 27 May 2003 22:27:05 GMT
On Tue, 27 May 2003 21:20:05 GMT, Don Kelloway spoketh
>"Winter" <Respond@InThis.Group> wrote in message
>news:QZKAa.114$vi4.16924@news0.telusplanet.net...
>> Would this eliminate problems with the MX records as well?
>>
>
>If the A record that the MX record is specifying is entered into the
>hosts file, it should alleviate the need to query DNS. This is because
>the hosts file is loaded into memory/cache when the system is restarted.
No, it isn't. If that was true, then changes to the hosts file would not
be immediate, but require a reboot. When a request for a lookup is done,
normally, the content of the hosts file is scanned, then if it's not
found there, it'll make the request to the DNS server(s).
With OS's like W2K and XP, there's a local DNS caching service, which
will cache DNS entries until it's expiration time (or next reboot,
whichever comes first). It is possible (I've never tested it) that the
DNS caching service on W2K and XP will read and cache the content of the
hosts file on startup, but the file is still checked with every lookup.
The order of lookup on these systems are: local cache, hosts, DNS
server.
Un*x based systems works a little different, as you can specify the
order of lookup (default is hosts, bind)
Lars M. Hansen
http://www.hansenonline.net
(replace 'badnews' with 'news' in e-mail address)
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