Re: Sendmail takes ages to start at bootup

On Fri, 2007-12-21 at 01:25 +1030, Tim wrote:
On Thu, 2007-12-20 at 23:06 +1100, Simon Slater wrote:
Soooo..., going back to kindergarten..., the names I put in /etc/hosts
were made up to describe the box on a purely internal network. Should
the names/aliases be something different? What would/would not
conflict with public FQDN?
Given that intro, I can't resist some fun with domain names...

If you have SMTP server running on a machine called bluecrayons (the
machine's hostname), then that machine needs to be able to resolve
addresses in a way that makes sense to itself at start up. It'll try to
resolve the name as the server starts up. It needs to be able to
resolve those addresses, to itself, at least.

A sample hosts file entry:
192.168.0.1 bluecrayons.example.com bluecrayons mail smtp

That's an IP address, a FQDN, and a list of three aliases that you might
have used (the hostname, a bog standard "mail" hostname, and another
common "smtp" hostname). The latter two aliase being things that some
people pick out of habit, but there's nothing saying you must do so,
though it does make simple sense to users to configure mail.example.com
as their mailserver).

Your mail server can start up disconnected from a network, and that's
all it'll care about (its own addressing - that out of the interfaces
it's using, the addresses resolve). It can start up connected to a
network, and the same applies. Thus far, that's all for internal
purposes.

However, something from outside connecting to your mail server is going
to expect a public name to match a public IP address. You could have
the same hostname/domain names, and external DNS servers use the
external IP address to the machine, and internal DNS servers giving a
different IP address to other local machines.

e.g. An outside DNS server might associate 208.77.188.166 with
bluecrayons.example.com. Outside services would connect to you using
that FQDN or IP, and since they both resolve against each other,
externally, those outside services are happy about it.

As for what won't conflict, don't make use of real domain names
belonging to someone else, or make up ones that might be registered by
someone at some time.

An IP address is how you connect between A and B. A and B might have
more than one address. Which one is used depends on the networking.

The lights have just come on, physically and metaphorically! A
thunderstorm has sat over the area for most of the day with power on and
off all day, so I left the mail til now. Ever read all you can lay your
hands on and still seemed in the dark? The light has turned on now.

I realise now what I have done and how to fix it. Too late to start
now. First thing tomorrow. Many thanks.

--
Regards
Simon

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