Sendmail takes ages to start at bootup

[John Summerfield]

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.

Note that "example.com" is reserved (IANA I think) for documentation. 
When one sees it, one should substitute something appropriate to one's 
own circumstances.


> 
> 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.
> 
Some are reserved for private use, so your use of them does not conflict 
with mine:
192.168.0.0/16 (/24 networks)
172.16.0.0/12 (/16 networks)
10.0.0.0/8 (/8 network).
These may be subdivided as you wish. I mostly take a 192.168 network.
Some are commonly used in consumer routers: 192.168.0.0 and 182.168.1.0 
are common, and probably low numbers in the other ranges too).

/n above refers to the number of bits used in the network address, /24 
corresponds to the netmask 255.155.255.0.

-- 

Cheers
John

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