Sendmail and DNS lookups

[Paul Howarth]

On Mon, 2005-05-09 at 19:22 -0500, Gregory P. Ennis wrote:
> Everyone,
> 
> Thought I could solve this problem, but Looks like I need some help.
> 
> I am trying to replace our R.H 8.0 internal e-mail server with a faster
> machine using FC3.  I set a FC3 unit up as a desktop PC in order to do
> the setup and testing before I put it into production.
> 
> I have set up other machines using sendmail with only the usual
> problems, but this one has me stumped.  The internal mail server
> 'mailin.domain.com' needs to pass its e-mail to our external server
> 'mailout.domain.com' while at the same time processing mailin needs to
> process local mail.  
> 
> define(`SMART_HOST',`mailp.pomec.net')dnl
> 
> The HUB feature was commented because I want it to process some local
> mail
> dnl define(`MAIL_HUB', `mailp.pomec.com')dnl 
> 
> Both of the below are commented out
> 
> dnl FEATURE(`accept_unresolvable_domains')dnl
> dnl #
> dnl FEATURE(`relay_based_on_MX')dnl
> 
> 
> I was using the mail coomand in the form of
> 
> mail -s 'test trasnmissions to mailout' Name at mailout.com < ./textfile
> 
> The maillog on mailin demonstrate that mailout refused the connection.
> 
> I checked my host, host.allow files, turned off iptables on both units,
> and still no solution.
> 
> I decided to watch what was happening using ethereal, and found that
> sendmail was doing a dns lookup for domain.com and found the internet IP
> address for mailout.domain.com.  Of course when sendmail uses this
> address it fails a connection which is related to our firewall.  
> 
> I tried using the internal IP address with 'telnet mailout.domain.com
> 25' and it connects easily.  'telnet "external IP address" 25' does not
> connect and this is what sendmail is doing. 
> 
> I tried changing the name of of mailout.domain.com in the host file of
> mailin to something like mailoutp.domain.com but sendmail continues to
> do DNS searches on 'domain.com' giving it the external IP address that
> fails.
> 
> I need sendmail on the new mailin server to stop doing dns searches for
> the IP address and just use the host file for the internal IP address.
> I think that would solve this problem.  
> 
> I am sure there is a simple solution, but so far I can not find it.
> Your help would sure be appreciated.

You might be better off taking a different approach. Try specifying the
name of the smart host inside square brackets, which turns off MX
lookups for the name:

If mailp.pomec.net resolves only to the internal IP address:
define(`SMART_HOST',`[mailp.pomec.net]')dnl

Otherwise:
define(`SMART_HOST',`[mailout.internal.ip.addr]')dnl

Paul.
-- 
Paul Howarth <paul at city-fan.org>