/etc/resolv.conf and sendmail

[Nifty Hat Mitch]

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 AAA.com.,  aaa.net. and aaa.org 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 "invalid.com".  It turns out that there is a
registered domain by that name.  Following the directions in this case
did unpredictable and possibly bad things.

  http://www.interex.org/tech/9000/Tech/sun_hpux_interop/chap10_dns.html

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.

      192.188.1.5    boxa.aaa. boxa.aaa	 boxa

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

     host boxa.aaa.
     host boxa.aaa
     Mail -v fred at boxa.aaa.
     Mail -v fred at boxa.aaa

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

      $ host boxa.aaa.
      boxa.aaa. has address 192.168.0.51
      $ host 192.168.0.51
      51.0.168.192.in-addr.arpa domain name pointer boxa.aaa.
     
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
addresses.

   http://sendmail.org/tips/private-dns/

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.

    http://sendmail.org/tips/pathmtu.html

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.

Summary:
in etc/hosts

	192.168.0.10	a.aa.priv.notpublicnet	a
    not
	192.168.0.10	a.aa			a


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