9.2.7 The Telnet utility is a virtual terminal protocol that is part of the TCP/IP protocol suite. It allows verification of the application layer software between source and destination stations. This is the most complete test mechanism available. The Telnet utility is normally used to connect remote devices, to gather information, and to run programs.
The Telnet application provides a virtual terminal connection to routers that use TCP/IP. For troubleshooting purposes, it is useful to verify that a connection can be made using Telnet. This proves that at least one TCP/IP application is able to connect end-to-end. A successful Telnet connection indicates that the upper-layer application and the services of lower layers are functioning properly.
If an administrator can Telnet to one router but not to another router, verify lower layer connectivity. If connectivity has been verified, it is likely that the Telnet failure is caused by specific addressing, naming, or access permission problems. These problems can exist on the administrator's router or on the router that failed as a Telnet target.
If the Telnet to a particular server fails from one host, Telnet from a router and other devices. If a login prompt is not achieved during Telnet, check the following:
- A reverse DNS lookup may not be found on the client address. Many Telnet servers will not allow connections from IP addresses that have no DNS entry. This is a common problem for DHCP-assigned addresses if the administrator has not added DNS entries for the DHCP pools.
- It is possible that a Telnet application cannot negotiate the appropriate options and will not connect. On a Cisco router, this negotiation process can be viewed with the debug telnet command.
- It is possible that Telnet is disabled or has been moved to a port other than 23 on the destination server.
This page concludes this lesson. The next lesson will teach students how to troubleshoot router issues. The first page will discuss show commands.
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