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Telnet is a simplwe remote terminal protocol that is part of the Internet Protocol (IP) suite. Telnet allows a user at one site to establish a TCP connection to a login server at another site, then it passes the keystrokes from one system to the other. Telnet can accept either an Internet address or a domain name as the remote system address.
Use the commands in this chapter to configure Telnet support on your communication server. For configuration information, refer to the Communication Server Configuration Guide. For information about making connections, see the Communication Server and Protocol Translator Connection Guide.
To assign an Internet address to the service provided on a TCP port, use the ip alias interface configuration command. Use the no ip alias command to remove the specified address for the communication server.
ip alias internet-address TCP-port
| internet-address | Specifies the Internet address for the service. |
| TCP-port | Specifies the number of the TCP port. |
None
Interface configuration
A user attempting to establish a connection is connected to the first free line in a rotary group using the Telnet protocol.
The Internet address must be on the same network or subnet as the communication server's main address, and must not be used by another host on that network or subnet. Connecting to the Internet address has the same effect as connecting to the communication server's main address, using
TCP-port as the TCP port.
You can use the ip alias command to assign multiple Internet addresses to the communication server. For example, in addition to the primary alias address, you can specify addresses that correspond to lines or rotary groups. Using the ip alias command in this way makes the process of connecting to a specific rotary group transparent to the user.
When asynchronous mode is implemented, the communication server creates the appropriate IP aliases, which map the asynchronous addresses to the lines they are connected to. This process is automatic and does not require configuration.
The following example configures connections to IP address 131.108.42.42 to act identically to connections made to the server's primary IP address on TCP port 3001. In other words, a user trying to connect is connected to the first free line in rotary group 1 using the Telnet protocol.
ip alias 131.108.42.42 3001
To enable a faster response to user interrupt characters, use the ip tcp chunk-size global configuration command.
ip tcp chunk-size number
| number | The number of characters output before the interrupt executes. The suggested value is 80, which will typically abort output within a line or two of where the user types the interrupt character. Values of less than 50 are not recommended for efficiency reasons. |
None
Global configuration
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.
Changing the chunk size (the number of characters output before the interrupt executes) 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.
The following example allows the communication server to execute an interrupt after an output of 100 characters, when an interrupt character or sequence is entered (Ctrl-C, for example).
!
ip tcp chunk-size 100
!
To define a string of characters that the communication server sends to a host after a successful connection, use the login-string global configuration command. This command applies only to rlogin and Telnet sessions. The no login-string global configuration command removes the login string.
login-string hostname d message [%secp] [%secw] [%b] [%m] dNone
Global configuration
Follow the command with one or more blank spaces and a delimiting character of your choice. Then enter one or more lines of text, terminating the message with the second occurrence of the delimiting character. To use a percent sign in the login string, precede it with another percent sign; that is, type the characters "%%." The options can be used anywhere within the message string.
In the following example, the value %5p causes a five-second pause.
login-string office #ATDT 555-1234
%5p hello
#
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 (Telnet IP) command is received on that connection, use the telnet break-on-ip line configuration command.
telnet break-on-ipThis command has no arguments or keywords.
None
Line configuration
Use this command to control the translation of Telnet IP commands into X.25 BREAK signals. It is also a useful workaround in the following situations:
A hardware Break signal is generated when a Telnet BREAK signal is received.
In the following example, line 5 is configured with the telnet break-on-ip command. The location text notes that this refers to the high-speed modem. The telnet transparent command sets end-of-line handling.
line 5
location high-speed modem
telnet transparent
telnet break-on-ip
telnet transparent
To set a line using Telnet to refuse to negotiate full duplex, remote echo options on incoming connections, use the telnet refuse-negotiations line configuration command.
telnet refuse-negotiationsThis command has no arguments or keywords.
None
Line configuration command
Use this command on reverse Telnet connections to allow the communication server to refuse full-duplex, remote echo option connection requests from the other end. This command suppresses negotiation of the Telnet Remote Echo and Suppress Go Ahead options.
The following example shows how to set line 5 to refuse full duplex, remote echo requests.
line 5
telnet refuse-negotiations
To negotiate speeds on reverse Telnet lines, use the telnet speed line configuration command.
telnet speed default-speed maximum-speed
| default-speed | The line speed the communication server will use if the device on the other end of the connection has not specified a speed. |
| maximum-speed | The maximum speed the communication server will use. |
None
Line configuration
Use the telnet speed command to match line speeds on remote systems in reverse Telnet, on host machines hooked up to a communication server to access the network, or on 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.
The following example allows line 5 to negotiate a bit rate using the Telnet option. If no speed is negotiated, the line will run at 2400 bits per second. If the remote host requests a speed of greater than 9600 baud, then 9600 will be used.
line 5
telnet speed 2400 9600
To cause a reverse Telnet line to send a Telnet Synchronize signal when it receives a Telnet Break signal, use the telnet sync-on-break line configuration command. The TCP Synchronize signal clears the data path, but will interpret incoming commands.
telnet sync-on-breakThis command has no arguments or keywords.
None
Line configuration
The following example shows how to cause reverse Telnet line 8 to send a Telnet Synchronize signal when it receives a Telnet Break signal.
line 8
telnet sync-on-break
To send a Return (CR) as a CR followed by a NULL instead of a CR followed by a Line Feed (LF) on Telnet lines, use the telnet transparent line configuration command.
telnet transparentThis command has no arguments or keywords.
None
Line configuration
This command is useful for coping with different interpretations of end-of-line handling in the Telnet protocol specification.
The following example shows how to change the end-of-line characters on line 7.
line 7
telnet transparent
telnet break-on-ip
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