Cannot connect to telnet server

So, I can't use telnet to connect to any server but it works fine from a different computer. It just says it can't connect. I tried the following things:

Disable firewall and AV protection. (Basically, there was no security feature left online)
Telnet is set to "Trusted" in my AV protection. (Kaspersky Internet Security 2011)
Using Putty to telnet, but apparently Putty's connection is also inhibited. (Says it can't connect to host)
Disabling the telnet client in Control Panel and then re-enabling it. (Windows 7 Ultimate)
hosts file is clean.
Checked for nasties using MBAM and KIS 2011 as well as going though my HijackThis logs, nothing found.

I can connect to the same machines/servers through the web browser, ping, tracert, etc. Only telnet seems to be blocked.

Any other thoughts?

BloodPhilia

Posted 2011-01-10T00:36:32.997

Reputation: 27 374

can you reach it via other means from this machine (ping/ftp/tracert)? – Rhys Gibson – 2011-01-10T00:40:28.947

@Rhys Yes I can – BloodPhilia – 2011-01-10T00:41:27.390

Is your specific client blocked by the telnet server or is telnet running on a different port? – Rhys Gibson – 2011-01-10T00:55:06.750

@Rhys It works fine from machines operating within the same subnet and sharing the same external IP. Furthermore, it works neither with port 25 nor with custom ports. – BloodPhilia – 2011-01-10T00:59:01.317

Weird. So what's different about that machine/user account? Any default telnet settings that are different (does display return any differences)? If you start a telnet server can it telnet to itself? – Rhys Gibson – 2011-01-10T01:29:04.380

@Rhys No, the machine WILL NOT telnet to itself... – BloodPhilia – 2011-01-10T01:33:33.520

Running out of ideas now. You're sure nothing on the local machine is blocking SMTP (assuming you are telneting to a mail server as you said port 25 not port 23) – Rhys Gibson – 2011-01-10T02:03:23.273

@Rhys Sorry, I meant 23, I used custom telnet ports 25, 550 and 2021! Thanks for trying anyway! ;) – BloodPhilia – 2011-01-10T02:05:28.720

No probs. Don't suppose you have just installed Visual Studio have you? I've heard rumours that it can interfere with telnet. – Rhys Gibson – 2011-01-10T02:10:38.587

@Rhys Nope, it's a clean, factory installed machine – BloodPhilia – 2011-01-10T02:41:42.593

File this under "other thoughts": Don't telnet if you can find a way to ssh instead. – Iszi – 2011-03-17T19:10:47.337

@Iszi I can't use telnet to raw interface with an smtp server. – BloodPhilia – 2011-03-17T20:29:31.360

@BloodPhilia - this topic just came to top! When you say other computers can access, are they on the same network? Also, have you made sure that the ports you are trying to access are not being blocked by your ISP? – William Hilsum – 2011-07-01T22:07:10.247

@will yes, other computers on the same network can connect. My ISP has no ports blocked whatsoever. – BloodPhilia – 2011-07-01T22:50:22.030

Ok... I will "attempt" to write an answer... but you won't like it :/ – William Hilsum – 2011-07-01T23:23:10.587

Answers

Have you tried disabling the auto-tuning diagnostic in Windows 7?

netsh int tcp set global autotuninglevel=disabled

Microsoft Support KB article:

When the receive window autotuning feature is enabled, older routers, older firewalls, and older operating systems that are incompatible with the receive window autotuning feature may sometimes cause slow data transfer or a loss of connectivity.

The default auto-tuning level is "normal", and the possible settings for the above command are:

disabled: uses a fixed value for the tcp receive window. Limits it to 64KB (limited at 65535).
highlyrestricted: allows the receive window to grow beyond its default value, very conservatively
restricted: somewhat restricted growth of the tcp receive window beyond its default value
normal: default value, allows the receive window to grow to accommodate most conditions
experimental: allows the receive window to grow to accommodate extreme scenarios (not recommended, it can degrade performance in common scenarios, only intended for research purposes. It enables RWIN values of over 16 MB)

PenguinCoder

Posted 2011-01-10T00:36:32.997

Reputation: 628

Run Wireshark and see if the telnet command produces any traffic at all.

SlimShaggy

Posted 2011-01-10T00:36:32.997

Reputation: 218

Sorry, I know you won't like this, but, I think it is your security software somehow, somewhere.

This really is not normal behaviour, and the fact that you said other machines on the same network can access means we have isolated it to your machine.

Something must be blocking it. Depending on how low level your firewall is, possibly try to enter safe mode with networking and see if you can connect to a target.

However, whether or not this works, I still think it is related to security software.

William Hilsum

Posted 2011-01-10T00:36:32.997

Reputation: 111 572

Nah, still the same when I uninstalled all my AV protection! :P I've basically given up on this one and waiting for my next reinstall of Windows... – BloodPhilia – 2011-07-03T12:00:10.713