When I need to check whether a firewall rule has been opened or whether a service is up and running, I usually run a telnet test, basically "telnet 80" for e.g. If a firewall is dropping packets in between, I would get a timeout and if the service port is not up, I would get a connection refused (If I was using a Linux or Unix telnet client).

The problem is that the telnet client on Windows reports timeouts and connection refused errors using the same error message (i.e. a connection cannot be established). The only differentiating factor is that the timeout error only appears after 20 seconds of waiting. That information cannot be captured in a screenshot.

Is there a third party telnet-like tool that I can use for this purpose with better error reporting capabilities?

By the way, I'm not looking for a telnet terminal client per se, I'm only using the telnet protocol to do connectivity testing. Putty, for example, is not very useful for me in this instance, because when it encounters a connection refused, it will just close the whole window, leaving me with no troubleshooting evidence.

Thanks, Wong

  • 2
    Telnet isn't the right tool for what you are doing. netcat is the correct tool, on both Windows and Linux.
    – Zoredache
    Nov 7, 2013 at 6:20
  • I can't comment on whether it's appropriate on windows, but why on Linux? telnet is lightweight and installed nearly everywhere, so I'm curious what flaws it has for simple TCP connectivity testing.
    – MadHatter
    Nov 7, 2013 at 7:20
  • @MadHatter a) udp support b) netcat is clean in that it sends exactly what you send, unlike telnet, which modifies what you send in some cases c) netcat doesn't try to do telnet negotiation, which makes it better for non-telnet protocols.
    – Zoredache
    Nov 7, 2013 at 8:11
  • a) is true, but irrelevant for the usage case I gave (though I freely concede that there's merit in using a single tool for both protocols). b) is interesting; I've never seen telnet send anything except the TCP three-way handshake. Do you have counterexamples, from a Linux implementation?
    – MadHatter
    Nov 7, 2013 at 9:35

1 Answer 1


One of the best tools for creating firewall rules and determining which services (TCP and UDP conversations) that are running based on captured packets is Wireshark (formerly known as Ethereal).

Wireshark alone is better for seeing whats happening between your firewall and your client. If you want to see what TCP and UDP conversations that are taking place there are tremendous tools within Wireshark.

Also, it sounds like you may want to know how effective your rules are. Once downloaded and installed; look under tools> Wireshark’s Firewall ACL Rules.

The tool supports ACL development and filtering for Cisco IOS, windows, Linux and others.

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