Re: /etc/resolv.conf and sendmail

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On Wed, Jul 14, 2004 at 11:06:16PM -0400, Gene Heskett wrote:
> On Wednesday 14 July 2004 14:05, Alexander Dalloz wrote:
> [...]
> >
> >Of course, if Philippe did give his Sendmail a hostname which is not
> >resolvable, neither by the hosts file nor a DNS, his MTA will not
> > work proper. 
> All of which makes sense (I think, I'm having trouble with he 
> "internal" status of AAA yet)

I recall reading a comment that local host names for sendmail need "enough"
dots. i.e.   

      box.bogustopdomain		# will not work for some.
      box.subdomain.bogustopdomain	# can work.

Since there is a, and resolver code could be 
looking any of these up because you do not have enough dots.

I recommend the top two levels of a private host name be researched
for conflicts.  I once found a nifty but flawed how-to for sendmail
and dns that said use "".  It turns out that there is a
registered domain by that name.  Following the directions in this case
did unpredictable and possibly bad things.

In part the root of this is the ndots of resolver code (not really a
sendmail problem).  In some cases /etc/hosts can use dots to force
termination and closure.  i.e.  DNS resolver code should understand
that a trailing dot terminates the fully qualified domain name.	 boxa

Also "Mail -v fred@xxxxxxxxx"  may respond differently than
without the trailing dot "Mail -v fred@xxxxxxxx".  If things are correct
they should be the same.  Compare and contrast...

     Mail -v fred@xxxxxxxxx
     Mail -v fred@xxxxxxxx

next do a reverse lookup on the host results.  Forward and reverse lookup
should be consistent.

      $ host has address
      $ host domain name pointer
Since aaa is not a subdomain of a .com .net or other high level domain
this dot count thing could be important.

In addition sendmail has a need to locally resolve private network

The point here is that if the reverse lookup fails for a private
network as it will without local interaction mail will queue.  This
"should be OK" with hosts first in resolv.conf but I have not found it
to be satisfactory.
Also there may be issues with path MTU discovery for some ISP services.

It can be useful and diagnostic to force your outgoing MTU to be
modestly sized.

Lastly if you are on a DHCP assigned address you will find that
setting up sendmail is painful.  The short TTL for the domain name is
a signature of dynamic DNS.  Use the ISP's mail host as a smart host.
This may require authentication.  Set 'reply to:' headers on mail to
be the ISP's assigned mail address.  Collect mail from the ISP with
fetchmail/ pop/ imap for local access.

I once thought that MX records would be helpful but there is a window
when your old IP address and DNS records could point to a different
box.  If that box answers then rejects mail, mail will bounce back to
the sender.

in etc/hosts	a.aa.priv.notpublicnet	a
    not	a.aa			a

	T o m  M i t c h e l l 
	/dev/dull where insight begins.

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