Having a very strange issue here...

The built in telnet client in OS X is giving me some strange results. I normally use it for testing whether a TCP connection can be established: telnet google.com 80

If it's successful, you should expect:

Connected to google.com.
Escape character is '^]'.

And from there you can send a HTTP message, or whatever you want to do.

What I'm seeing, which is confusing me to no degree, is connections being connected, regardless of whether it actually was able to connect to the server.

For example.

mac:~ user$ telnet 12345
Connected to
Escape character is '^]'. is obviously not a real server, and there's definitely no service listening on port 12345.

A Wireshark capture confirms that there's not something weird going on. I see TCP syn's go to on 12345, but no response returned.

Note: As I was writing this question, I found the answer, so I'll be posting it shortly.


Sophos Anti-Virus was causing this issue.

The Web Protection feature of Sophos seemed to be causing it to proxy all TCP connections through itself, hence the instant connection, whether or not the destination server was available.

I turned off both of the Web Protection settings, and the strange behavior stopped.

If anybody else is having this issue on Mac/Windows... check to see whether you have an AV installed with web filtering/protection enabled.

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