This week my blog post is inspired by one of the exercises we did in Networking class (teacher: Morten Bo Nielsen) and it relates to Telnet server and services and how the connection between host and client is initiated.
What is Telnet?
Telnet is a client-server protocol, based on a reliable connection-oriented transport. Typically this protocol is used to establish a connection to TCP (Transmission Control Protocol, which provides the service of exchanging data directly between two hosts on the same network) port number 23, where a Telnet server application is functioning. Actually Telnet predates TCP/IP and was originally run over Network Control Program (NCP) protocols. It provides access to a command-line interface (usually, of an operating system) on a remote host.
Telnet – a reliable service, which assures good quality connection. So far, so good, eh? Yes, but the major downside of Telnet, is that, unlike SSH (Secure Shell, another network protocol that allows data exchange), it does not encrypt any data sent over the connection (including passwords – fig.1). You can imagine how annoying can that be and not to mention the threats it can raise for your machine.
How to set up a Telnet server (on a Windows OS)?
A few basic steps to help you create such a server on your personal computer:
First the Telnet Client and Server applications found in your Windows Features menu (Control panel -> Programs and Features) need to be properly installed.
After that everything is based on commands in the Command Prompt. Use the following to create users, give them rights and create the Telnet Clients local group:
net user (username) (password) /add
net localgroup Administrators /add (username)
net localgroup /add TelnetClients
net localgroup TelnetClients /add (username)
A link to the full guide can be found in the References.
Another interesting thing I noticed is that during a package trace with Wireshark the guy listening to the connection between the host and the client of the Telnet server can see every single command in plain text (fig.2).
How to set up a Telnet server?