Telnet - TCP , UDP ports

[grapevine]

Unlike TCP connections, why can Telnet NOT be used to verify UDP connections?

Why does the below command fail to show as connected even if there is a connection?

C:\Users\ABC>telnet 10.0.0.1 169

[Steve Fuller · Accepted solution]

Hi,

Telnet can't verify UDP connections simply because Telnet was only designed to use TCP as its underlying transport layer.

Take a read of the RFC 854, Telnet Protocol Specification that states:

A TELNET connection is a Transmission Control Protocol (TCP) connection used to transmit data with
interspersed TELNET control information.

Regards

[Richard Burts · Accepted solution]

Perhaps there is something in your question that I am not understanding. But it seems straightforward to me that telnet was designed as a communications protocol using TCP as its transport layer. There is no provision in the protocol design for using UDP as the transport for telnet.

HTH

Rick

[Karsten Iwen]

You can use the tool Netcat (there are also Windows-versions available) to connect to UDP-servers. But a problem remains, the server needs some input that is understood. And that has to be simulated by you.

This input can also be generated by tools like Scapy, but that needs quite some learning to use it.

[Joseph W. Doherty]

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Posting

Also understand TCP and UDP are two different protocols with different rules of usage.  The reason telnet often works with other non-telnet ports, is because TCP shares a common connection setup (the 3 way handshake).  Beyond that, some other TCP protocols will take text commands, so if you know what they are, you can also interact with such non-telnet protocols using telnet.

UDP, though, does not have a common setup, if fact, the protocol doesn't support the concept of having a session.  The latter, if needed, is provided by the application using UDP.

[grapevine]

Thanks for the insight.