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Windows Server 2008 Server Core : Creating System Connections - Communicating with Telnet

8/12/2011 5:43:02 PM
Telnet provides low-level access to other machines using a standardized protocol and connection. This section describes the command line interface for the Windows version of Telnet, which varies a little from the interface other platforms use. This section won't tell you how to perform management tasks.

1. Administrating Telnet with the TLNTAdmn Utility

The Telnet Administrator utility helps you control Telnet sessions on your machine. You access it using the TLNTAdmn utility. If you use TLNTAdmn alone, you'll see a display of the current server status, as shown in Figure 1. Adding start, stop, pause, or continue to the command line controls the Telnet service state. Note that these commands only work if you set Telnet to manual or automatic mode—the command fails if you disable the Telnet service.

Figure 1. Use the TLNTAdmn utility to display the status of your Telnet server.

The TLNTAdmn utility includes three user-specific commands. Use the -s switch with an optional session identifier to display the user status information. Each user entry includes the user ID, name, remote connection point, and logon time. The idle time column is a good indicator of who has gone to lunch with their Telnet connection intact. Use the -m switch with a session identifier to send the user a message. The third user option is the -k SessionIdentifier switch. Use it to end a user session. This utility uses the following syntax:

TLNTAdmn [ComputerName] [-u user [-p password]] start | stop | pause|
continue | -s sessionid | -k sessionid | -m sessionid message | config

The following list describes each of the command line arguments.


Specifies the name of a remote computer. The default setting uses the local computer.


Specifies the name of the user account used to execute command. The default setting relies on the user account for the currently logged in user.


Specifies the user password.


Starts the Telnet service. This option only works when the service is set to automatic or manual. You can't start a disabled service.


Stops the Telnet service.


Pauses the Telnet service. A pause is temporary when compared to a stop. Don't use pause when you actually mean to stop the service.


Continues the service from a pause.


Lists information about the specified session.


Terminates the specified session. Exercise care in using this option since a terminated session could cause data loss. Generally, you'll use this as a last resort. Try sending a message to the user first to shut down the session from the client side or physically end the session from the client terminal yourself.

sessionid message

Sends a message to the specified session. Make sure you place any message within quotes. Otherwise, the utility sends just the first word of the message to the remote terminal and then displays an error message.


Changes the server configuration. Use one or more configuration options to change the way in which the server works.

Telnet provides a number of configuration options. Each of these configuration options controls an aspect of the way in which Telnet works. Change the server configuration carefully because some options can cause connection errors or make the server unavailable to users who need it. The following list describes each of the configuration options.

dom =

Sets the default domain for checking usernames. If you're using a peer-to-peer configuration, then the only domain is your machine. The only time you can set this to another domain is if you have a Windows server set up as a domain controller.

ctrlakeymap = <Yes|No>

Sets the mapping of the ALT key to Ctrl+A when on. This is the default setting. This setting doesn't affect the VTNT terminal, but does affect other terminal types. See RFC 884 (http://www.faqs.org/rfcs/rfc884.html) for a list of standard Telnet terminal types.

timeout =

Determines how long the Telnet server waits before it logs out a user automatically. You must include the colons between the hours, minutes, and seconds. In addition, if you want to set a value to 0, then include a 0 on the command line. For example, if you want to set the timeout value to 30 minutes, type TLNTADMN config timeout = 0:30:00 at the command line.

timeoutactive = {Yes | No}

Enables idle session timeout counter. Whenever a session reaches the timeout value, the Telnet server disables it automatically.

maxfail =

Sets the maximum number of login failure attempts before disabling the user account. Telnet won't allow disabled user accounts to connect.

maxconn =

Determines the maximum number of connections the Telnet server accepts. Note that the Microsoft documentation states that you can accept a maximum of two sessions. This is incorrect. Using this configuration option allows you to accept the maximum number of connections your machine can handle.

port =

Changes the connection port number. It's always a good idea to change this number to something other than the default to help thwart crackers. Of course, if you leave the port open and use poor security, someone will still get in.

sec = [+/-]NTLM [+/-]PASSWD

Determines the acceptable security (authentication) mechanisms. Allowing NTLM enables the user to log in using their default Windows username and password.

mode = {Console | Stream}

Controls how the server reacts to control character input. Always use console mode to ensure users can use applications such as EDIT.

2. Executing Commands Remotely with the Telnet Utility

The Telnet utility manages Telnet sessions at the command prompt. You'll start the Telnet client at a command prompt by typing Telnet and pressing Enter. Telnet displays a Welcome message, the escape character, and a Microsoft Telnet prompt. This utility uses the following syntax:

TELNET [-a][-e escape char][-f log file][-l user][-t term][host [port]]

The following list describes each of the command line arguments.


Performs an automatic logon using the currently logged on username and password. This option works about the same as the -l option except you don't have to specify the username. Windows XP and above ignore this option if you have NTLM security enabled. It automatically logs on using the currently logged on username and password.


The Windows XP and above versions of Telnet sets Windows NT LAN Manager (NTLM) authentication on by default. This means that it will always attempt to log on using the currently logged on username and password. Using this option makes access somewhat automatic. All you need to do is type Telnet HostName at the command prompt and Telnet connects you, if you have proper rights. However, this option has two unfortunate side effects. The first is that you can't specify another username and password to log onto the system. The second is that the NTLM option appears to interfere with operation of some Telnet clients.

-e Modifies the escape character used to enter the Telnet client prompt from a remote session. Telnet defaults to Ctrl+], which is a good choice because it isn't used by anything else.


Sets the filename for client-side logging. Using this option also turns client-side logging on. Client-side logging doesn't track the commands you type at the Telnet prompt; they only record what you've done at the remote terminal connection. For example, if you type a Dir command at the remote prompt, you'll see the Dir command and results in the log. However, you won't see the command used to open the connection because that occurs at the Telnet prompt.


Specifies the username to log in with on the remote system. You can't specify a password at the command line, so you still have to provide a password before the session will start. Windows XP and above ignores this option if you have NTLM security enabled. It automatically logs on using the currently logged on username and password.


Specifies the terminal type used for command processing and text display. Telnet supports the VT100, VT52, ANSI, and VTNT terminal types. The terminal type determines the characteristics of the session. It dates back to a time when people accessed mainframes using utilities such as Telnet. Using the default ANSI terminal usually works fine. Telnet remembers your preferred terminal type from session to session.

The default terminal type of ANSI does work fine for most connections, especially those with a mainframe. However, the ANSI terminal type causes problems when you run certain Windows XP character mode utilities. Any utility that has a display and a functional menu system will likely require you to use the VTNT terminal. For example, if you normally use the EDIT command to work with text files, you'll want to use the VTNT terminal.

HostName [PortNumber] Specifies the hostname or IP address of the remote computer. You may optionally specify a service name or port number. The only time you need to specify a port number is to access a service other than Telnet or if the Telnet administrator changes the port number.

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