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Re: Help Needed: My RHEL5 box suddenly stopped accepting e-mails



1.  Add an iptables logging rule that logs and connections to port 25 not
from localhost.  Something like:
iptables -I INPUT -i ! lo -p tcp -m tcp --dport 25 -j LOG


I am going to wait on the change because I don't feel comfortable doing this
just yet.  Note
that we have established that systems on my subnet can successfully telnet
into port 25 of
my system; whereas systems on other subnets cannot.  Would the logging rule
above provide
additional information regarding the failed connection attempts to port 25?

You're not blocking/allowing anything .. just logging, before any ACCEPT rules. If you try to telnet to port 25 from another subnet with this rule in place and you don't see connections getting logged, they're not getting to your server.

6.  Verify other Internet communications work .. perhaps you've got a bad
route of some kind.


I seem to be able to do other internet activity without any problems.

What about connecting to other internal hosts that are on a different subnet. I still think this could be routing related. Have you verified your routing table with IT?

7.  Run some tests with swaks <http://jetmore.org/john/code/swaks/>

I'm not familiar with swaks; but I'll look into it.

I usually manually telnet to port 25 and have an SMTP conversation with the mail server. If you don't speak fluent SMTP, swaks can help.

8.  Use system-switch-mail to verify that your system is using sendmail.

My system is running sendmail.  However, I'm not familiar with
system-switch-mail, nor could
I find that command on my system.

If you ever had postfix or qmail installed from RH it installs in a way that allows you to switch between MTAs. system-switch-mail manages symlinks to make sure everything lines up correctly. You can install the system-switch-mail package if you like. Probably not needed.

Barry

On Sat, 7 May 2011, Mun wrote:

 Hi   Bohdan,


On Sat, May 7, 2011 at 10:21 PM, Bohdan Sydor <bohdan harazd net> wrote:

 On 05/08/2011 06:30 AM, Mun wrote:

 Does everything above look okay?


Yes, they all seem to be alright.

Next, let's try to telnet to the smtp port:

- from the localhost. Simply telnet localhost 25 and try to submit a
sample msg.


You are now beyond my understanding of sendmail.  After telnetting,
what is the command I should enter?


 - from any other machine that is in the same subnet as the mail server


 From any other remote hosts we already know that it fails. But do you

refer to the MTA by address or by name? Check the DNS entries for the
MTA:

host -t mx yourDomainName


This returned a name (not an address).  Let say "xyz1.domain"



 host -t a theResultNameFromPreviousCmd
Is it the same IP as assigned to the server?


Yes, the IP does match that of "xyz1.domain"

But this piqued my interest, and when I look in sendmail.cf I see the
following lines:
# "Smart" relay host (may be null)
DSabc1.domain

Should this entry be "xyz1.domain" (to match the the 'host -t mx'
command's
output)?
Or is it okay that the line in sendmail.cf refers to a different server?

Best regards,

--
Mun





--
regards

Bohdan Sydor
www.sydor.net

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