[ale] Bellsouth Sendmail Fix

[Joe Steele]

On Wednesday, July 02, 2003 1:35 AM, Ryan Neily wrote:
>
> I tried this below, and it seems to work for me as well.  Anyone know why?
>

Every time sendmail parses a recipient address, the end result is a 
{MAILER, HOST, USER} 3-tuple.  The processing of the message is then 
passed to the selected MAILER, along with the HOST and USER info.  If 
the resulting MAILER is an '[IPC]' mailer (e.g., when the mailer is  
'smtp' or 'relay'), then the HOST name has MX expansion performed via 
DNS.  However, this MX expansion can be bypassed by putting square 
brackets around the host name.

All of the above is irrespective of whether a SMART_HOST is used, 
because the sendmail program doesn't actually know anything about 
SMART_HOSTs.  SMART_HOSTs are a feature of the m4 processing which 
adds rulesets that end up putting the SMART_HOST name into the HOST 
name which is passed to the MAILER.  Consequently, SMART_HOSTs are 
subject to MX expansion just like any other host name.

The Bellsouth server which you are using as a SMART_HOST 
(mail.bellsouth.net) has the following MX records:

mail.bellsouth.net      preference = 0, mail exchanger = mx01.mail.bellsouth.net
mail.bellsouth.net      preference = 0, mail exchanger = mx00.mail.bellsouth.net

So as a consequence of MX expansion, sendmail is attempting to relay 
your mail to mx0[01].mail.bellsouth.net, rather than to 
mail.bellsouth.net.  The problem is that mail.bellsouth.net is 
configured to relay mail received from Bellsouth IP addresses, 
whereas mx0[01].mail.bellsouth.net are configured to accept mail 
addressed to Bellsouth customers but not to relay mail.

As Jeff Brodnax has shown, using [mail.bellsouth.net] as the 
SMART_HOST solves the problem by avoiding MX expansion so that 
sendmail relays the mail directly to mail.bellsouth.net.

Ryan Neily solved it in a more circuitous fashion by using port 
forwarding and by using localhost as the SMART_HOST.  Sendmail thinks 
it's relaying the mail to localhost, but in reality the connection is 
forwarded to mail.bellsouth.net.

--Joe



> On Wed, 2 Jul 2003, Ryan Neily wrote:
>
> >
> > So you're SMART_HOST line in your sendmail.mc file reads:
> >
> > define(`SMART_HOST', [mail.bellsouth.net])
> >
> > ???
> >
> > On Wed, 2 Jul 2003, Jeff Brodnax wrote:
> >
> > > On Tue, Jul 01, 2003 at 11:26:49PM -0400, Ryan Neily wrote:
> > > >
> > > > If this gets posted, I've fixed the Bellsouth no-relay problem that people
> > > > has been complaining about.  I used some of the tips of James CE Johnson
> > > > which is using Postfix.  My solution is using Sendmail, but works much the
> > > > same...
> > > >
> > > > I added a redirect rule to my IPtables firewall script that redirects any
> > > > traffic coming from port 26 on the localhost interface to the bellsouth
> > > > SMTP server on port 25.  I used this iptables command:
> > > >
> > > > $IPTABLES -t nat -A OUTPUT -p tcp -d 127.0.0.1 --dport 26 -j DNAT
> > > > --to-destination 205.152.59.16:25
> > > >
> > > > and then added the following directives to my sendmail.mc, m4'd it to
> > > > sendmail.cf and it seems to be working:
> > >
> > > Hi I am a lurker, just wanted to share the work around I found for the
> > > no relaying by bellsouth.net.  I added brackets.. ie.
> > > [mail.bellsouth.net] to my sendmail.mc as the smart host.  I have no
> > > idea what this does, other than it doesn't use the mx anymore just right
> > > through the main smtp server.   Relaying works again.
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