Configuring Telnet

This chapter describes how to configure communication server lines to support Telnet connections. For a complete description of commands in this chapter, see the Access and Communication Servers Command Reference publication. Making Telnet connections is described in the Cisco Access Connection Guide.

Cisco's Implementation of Telnet

The Internet Protocol (IP) suite includes the simple remote terminal protocol called Telnet. Telnet allows a user at one site to establish a TCP connection to a login server at another site, then passes the keystrokes from one system to the other. Telnet can accept either an IP address or a domain name as the remote system address. In short, Telnet offers three main services:

• Network virtual terminal connection

• Option negotiation

• Symmetric connection

The Cisco Systems implementation of Telnet supports the following Telnet options:

• Remote Echo

• Binary Transmission

• Suppress Go Ahead

• Timing Mark

• Terminal Type

• Send Location

• Terminal Speed

• Remote Flow Control

• X Display Location

Telnet Configuration Task Overview

The following sections describe the tasks for configuring Telnet on the communication server:

• Set the Line Speed Range for Remote Modification

• Set the Line to Refuse Full Duplex, Remote Echo Connections

• Set the Line to Handle Different End-of-Line Interpretations

• Synchronize the Break Signal

• Generate a Hardware Break Signal

• Optimize Response to User Interrupt Characters

• Assign an IP Address to a Service

• Define a TCP Login String

Set the Line Speed Range for Remote Modification

You can set the line speed to match line speeds on remote systems in reverse Telnet, host machines hooked to a communication server to access the network, or a group of console lines hooked up to the communication server when disparate line speeds are in use at the local and remote ends of the connection. Line speed negotiation adheres to the Remote Flow Control option, defined in RFC 1080. Perform the following task in line configuration mode:

Set the Line to Refuse Full Duplex, Remote Echo Connections

You can set the line to allow the communication server to refuse full duplex, remote echo connection requests from the other end. This task suppresses negotiation of the Telnet Remote Echo and Suppress Go Ahead options. Perform the following task in line configuration mode:

Set the Line to Handle Different End-of-Line Interpretations

You can configure the line to cause the communication server to send a RETURN (CR) as a CR followed by a NULL instead of a CR followed by a LINE FEED (LF). This is useful for coping with different interpretations of end-of-line handling in the Telnet protocol specification. Perform the following task in line configuration mode:

Synchronize the Break Signal

You can set the line to cause a reverse Telnet line to send a Telnet Synchronize signal when it receives a Telnet Break signal. The Telnet Synchronize signal clears the data path, but will still interpret incoming commands. Perform the following task in line configuration mode:

Generate a Hardware Break Signal

You can set the line to cause the system to generate a hardware Break signal on the RS-232 line that is associated with a reverse Telnet connection, when a Telnet Interrupt-Process command is received on that connection. This can be used to control the translation of Telnet Interrupt-Process commands into X.25 Break indications, and to work around the following situations:

• Several user Telnet programs send a Telnet Interrupt-Process command, but cannot send a Telnet Break signal.

• Some Telnet programs implement a Break signal that sends a Telnet Interrupt-Process command.

• Some RS-232 hardware devices use a hardware Break signal for various purposes.

To set a line to generate a hardware Break signal, perform the following task in line configuration mode:

Optimize Response to User Interrupt Characters

When used with a correctly operating host, communication servers implement the Telnet Synchronize and Abort Output signals, which can stop output within one packet's worth of data from the time the user types the interrupt character. You can configure a faster response to user interrupt characters. Perform the following global configuration task:

Changing the number of characters output, or chunk size, affects neither the size of the packet used nor the TCP window size, either of which would cause serious efficiency problems for the remote host as well as for the communication server. Instead, the Telnet status is checked after the number of characters specified, causing only a relatively minor performance loss.

Assign an IP Address to a Service

You can configure connections to an IP address to act identically to connections made to the server's primary IP address on the TCP port. A user trying to connect is connected to the first free line in a rotary group using the Telnet protocol. Perform the following task in interface configuration mode:

Define a TCP Login String

You can send a specific string to a host computer once a successful Telnet or rlogin connection has been made. The string can be another login command or any string that would be appropriate after logging in to a host. Perform the following task in global configuration mode:

This task is only useful with two-step protocol translation sessions. For more information about protocol translation, refer to the "Configuring Protocol Translation" chapter.