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| 7.3 |  | Troubleshooting TCP/IP Application Layer Protocols |
| 7.3.3 |  | Terminals and consoles |

Telnet is the standard terminal emulation protocol in the TCP/IP protocol stack. Telnet is defined in RFC 854 and operates over the TCP port 23. Telnet and FTP were the first two services available on ARPANET.

To understand how Telnet works, it is necessary to first make a distinction between console and terminal. A console refers to a keyboard and monitor that are directly connected to the computer system. In mainframe computing, a console was also referred to as a dumb terminal since it only operated by using the resources of the remote server. A microcomputer is now more commonly used as a console.

All consoles require a terminal connection to enable users to log in to remote systems and use resources (for example, CPU, applications, and storage) as if they were connected to a local system. A terminal is a console that artificially emulates the physical hookup of a console. The destination host assumes it has a direct connection to the client since the terminal just provides a communication channel for the user's input and output. A terminal program is commonly used to connect to a central server over the network.

The term "terminal emulator" refers to a terminal application that is implemented in software. Clients can use the Telnet program to establish a terminal connection. Other software such as HyperTerminal or TeraTerm can also be used and they typically offer more advanced features.

The following lists commands that could be used to troubleshoot Telnet problems.

> Note: Debug output is normally sent to the console port. This means that if the connection is established through the Telnet port, the debug output will not be seen. Use the terminal monitor IOS command to redirect the output to any of the VTY ports. Keep the amount of debugging that is enabled to a minimum.

> Note: RDP Protocol (Terminal Server) is a little known Telnet port (port 3389) used by Remote Desktop Protocol (RDP). RDP is the remote Windows terminal protocol used by Microsoft Windows NT 4.0, Terminal Server Edition operating system and Windows 2000 Terminal Services. Keep this in mind when troubleshooting.

Telnet for Troubleshooting
Network administrators often overlook Telnet as a troubleshooting tool. However, Telnetting to a host allows better verification of network status than just using ping. Telnet runs on top of the TCP protocol, so it establishes a more reliable indication of accessibility than ICMP echo requests can. It also tests higher-level functions of the destination host system. A server may be inaccessible for application layer functions, but still answers pings since those are handled by the lower layer protocols.

Telnet also has an additional feature that makes it valuable for troubleshooting application layer protocols. Telnet client applications allow the user to select the destination port number to be used. It can be used to connect to other TCP ports on destination hosts to test out other functions. That means that Telnet can contact network application programs other than a Telnet server. This can be useful as a substitute for a client application program.

For example, Telnetting to port 25 (SMTP) will verify that the e-mail server is answering. Telnet to port 80 (HTTP) to verify the Web server is answering.

Source Telnet Interface
Finally, a useful IOS command to use when testing an access list is the ip telnet source-interface command. This specifies the IP address of an interface as the source address for Telnet connections. To reset the source address to the default for each connection, use the no form of this command.

By default, Telnet will use the IP address of the closest interface to the destination as the source address. However, sometimes another interface may be preferred as the source. Conceptually, this is similar to specifying another source IP address when using an extended ping command.

The following example forces the IP address for FastEthernet interface 0/1 as the source address for Telnet connections:

> Router(config)#ip telnet source-interface FastEthernet 0/1

Figure shows some IOS commands used to isolate Telnet problems.