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Telnet to port 25

By Choppit ·
I'm experiencing difficulty connecting to my mail server by telnet on port 25. This problem only occurs if I connect from the LAN, in which case the connection is dropped instantly. If I connect from the internet to the firewall IP, the connection is successful. Server is Exchange 5.0(SP2) on NT4(SP6a). TCP Port 25 is forwarded from firewall/router. Any ideas?

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by Choppit In reply to Telnet to port 25

I can successfully telnet to port 110 from the LAN side.

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by CG IT In reply to Telnet to port 25

well you got a problem if you can not telnet exchange.

need some information. What shows up in the event viewer for exchange under applications and under system? Next is how is your SMTP virtual server properties configured? Under SMTP connector properties, general tab, what shows for local bridgeheads? Under the Access tab, access control, what are the settings for authentication, connection control? Next what are the settings listed for conntectors, SMTP connector, properties, General tab? What are the settings for Address space tab?

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by CG IT In reply to

I'm gonna add this question. When you run the telnet test what are you using? an IP address or the Domain name? IMHO without any information, the problem might be with DNS rather than exchange itself, if exchange is configured properly.

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by CG IT In reply to

last comment, are you using POP3? or SMTP? port 25 is SMTP you mention being able to telnet port 110 which is POP3. So the question really is what mail service is Exchange?

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by Choppit In reply to

Authentication is not enabled.
Address space is SMTP * (i.e. anything)
SMTP virtual server is mapped WAN IP TCP 25 > EXCHSVR IP TCP 25
All tests have been performed using the host relevant IPs
I can't find any settings relating to SMTP. (Are later versions of exchange different?)
Server is SMTP, I used the reference to port 110 to illustrate that telnet connection to the IMC was possible from the LAN (on ports other than SMTP). Thanks for your input.

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by JIM-H In reply to Telnet to port 25

It does sound like an access level problem. Not knowing your topology it is hard to point you in a direction to troubleshoot.

You mention that port 25 is forwarded from your router/firewall. From the wording I am assuming your doing a static NAT or PAT? If this is the case when you telnet from your Local LAN are you using the publicly known IP or the privately known IP as the destination address? Try using the private address only to bypass your firewall/router if possible.

You might also want to consider running a sniffer on the LAN to see if it the Sever ever gets the packet at all. The problem may not be that the packet from your machine is not getting to the mail server but the reply from the mail server is not coming back correctly.

Good Luck and we can investigate with more information about the setup.

-jim

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by Choppit In reply to

I'd agree that this appears to be an access level problem (the IMC is rejecting all connections from the LAN to the SMTP port). I've added more information to the post.Thanks for your input.

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by jimmy In reply to Telnet to port 25

Are you able to send/receive mail?
If not may well be that another application is listening on port 25 on the LAN.

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by Choppit In reply to

I am able to send and receive mail (i.e the IMC is functioning for SMTP). I have posted more detail to clarify this. Thanks for your input.

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by Choppit In reply to Telnet to port 25

To clarify:
The IMC is working OK. (i.e I can send and receive mail). Exchange is listening on port 25 which is forwarded from the NAT router/firewall.
I CAN telnet to the mail server on port 25 but ONLY from outside the local network using the WAN IP. I CANNOT telnet to port 25 from any host on the LAN using the server IP.

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