Re: Troubleshooting SMTP/TELNET
From: prg (rdgentry1_at_cablelynx.com)
Date: 01/31/05
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Date: 31 Jan 2005 07:16:35 -0800
Carl Hilton wrote:
> Ok, My SMTP is unable to accept connections from non-local subnets.
Is
> this an SMTP thing or something else... my inetd.conf file has TELNET
> remarked out. So, I can actually NOT TELNET in to this box from
> anywhere,but I CAN telnet in on port 25 from the local subnet. What
can
> block port 25 telnet sessions?
So, you are not running the telnetd, but you are running a SMTP
(sendmail?) daemon that is set to listen for _all_ incoming
connections? Double check what it is prepared to accept. I have to
assume you're running sendmail till you say otherwise ;-)
What do you mean by "non-local" subnets? From the internet or LAN
based subnets different from the subnet of your SMTP server/daemon?
I presume you mean the internet -- this is good that connections are
blocked. Are the SYN packets (to port 25?) blocked by a firewall rule?
Or is this (additionally?) a sendmail configuration? Hopefully,
_both_ are in effect but your config is allowing other _LAN_ hosts to
use your sendmail as a GW MTA. Check that your firewall rules are not
allowing outside traffic through port 25. Most host based firewalls
and GWs allow outgoing SYN packets (including mail on port 25) and
their replies to pass through, which is OK for now.
RH/FC come with sendmail configured to _only_ accept connections from
localhost -- no other hosts can connect, neither local LAN nor
internet. I'm not sure of other distros. The other day someone posted
that their experience was that other distros _allow_ LAN host
connections to the listening sendmail daemon.
Do you have/use a gui tool to configure sendmail or do you use a text
editor? If the former, check for an option that allows sendmail to
accept (or reject) local LAN connections. This (accept local LAN
connections) is usually only done if you are running a sendmail gateway
for the LAN. Check your sendmail.mc file for DAEMON_OPTIONS. Mine (on
a RH box) reads:
DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl
Do NOT make any changes to sendmail.cf or sendmail.mc before making
backups! Repeat: do NOT make any changes without backups. Do not
trust your gui tool to make backups for you (if you are using one).
Read the /usr/share/sendmail-cf/README for some background and check
your distro docs for any info they provide about the MTA setup.
I'm reluctant to go further for fear I'll break your mail. Probably
not what you want. Check your MTA config and any docs with your
distro, then post back if you have questions on how to set things up
the way you want. Be sure to mention your distro and what mailer you
are using for your MTA. Perhaps a copy of your /etc/mail/sendmail.mc
with _all_ the comments stripped out.
If you attempt to make changes to the sendmail configuration via a text
editor, be sure you understand the use of sendmail.mc and the m4
processor to generate the sendmail.cf file. Directly editing
sendmail.cf is not for mere mortals.
If you're using another mailer the setup details will vary
significantly but the options (in this case) will be "conceptually"
similar. I'm just assuming you're using sendmail as that is the most
commonly used MTA.
.
hth,
prg
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