Previous | Contents | Next

Section 3.12: The Telnet panel

The Telnet panel allows you to configure options that only apply to Telnet sessions.

3.12.1 "Terminal-speed string"

Telnet allows the client to send a text string that describes the terminal speed. PuTTY lets you configure this, in case you find the server is reacting badly to the default value. (I'm not aware of any servers that do have a problem with it.)

3.12.2 Setting environment variables on the server

The Telnet protocol also provides a means for the client to pass environment variables to the server. Many Telnet servers have stopped supporting this feature due to security flaws, but PuTTY still supports it for the benefit of any servers which have found other ways around the security problems than just disabling the whole mechanism.

To add an environment variable to the list transmitted down the connection, you enter the variable name in the "Variable" box, enter its value in the "Value" box, and press the "Add" button. To remove one from the list, select it in the list box and press "Remove".

3.12.3 "Handling of OLD_ENVIRON ambiguity"

The original Telnet mechanism for passing environment variables was badly specified. At the time the standard (RFC 1408) was written, BSD telnet implementations were already supporting the feature, and the intention of the standard was to describe the behaviour the BSD implementations were already using.

Sadly there was a typing error in the standard when it was issued, and two vital function codes were specified the wrong way round. BSD implementations did not change, and the standard was not corrected. Therefore, it's possible you might find either BSD or RFC-compliant implementations out there. This switch allows you to choose which one PuTTY claims to be.

The problem was solved by issuing a second standard, defining a new Telnet mechanism called NEW_ENVIRON, which behaved exactly like the original OLD_ENVIRON but was not encumbered by existing implementations. Most Telnet servers now support this, and it's unambiguous. This feature should only be needed if you have trouble passing environment variables to quite an old server.

3.12.4 Passive and active Telnet negotiation modes

In a Telnet connection, there are two types of data passed between the client and the server: actual text, and negotiations about which Telnet extra features to use.

PuTTY can use two different strategies for negotiation:

The obvious disadvantage of passive mode is that if the server is also operating in a passive mode, then negotiation will never begin at all. For this reason PuTTY defaults to active mode.

However, sometimes passive mode is required in order to successfully get through certain types of firewall and Telnet proxy server. If you have confusing trouble with a firewall, you could try enabling passive mode to see if it helps.

3.12.5 "Keyboard sends telnet Backspace and Interrupt"

If this box is checked, the Backspace key on the keyboard will send the Telnet special backspace code, and Control-C will send the Telnet special interrupt code. You probably shouldn't enable this unless you know what you're doing.

Previous | Contents | Next


Comments to putty@projects.tartarus.org
[$Id: pubkey.but,v 1.6 2001/09/25 19:59:14 simon Exp $]