Telnet LAN Primer

Dialog of a telnet session on the LAN.

The Nodes:
Ubuntu Linux Box
MacBook OS X

The Protocol:
Telnet from the TCP/IP suite

Network Addresses:
Ubuntu on static IP 192.168.1.xx
MacBook on DHCP, currently assigned to 192.168.1.101

Prerequisites:
Available default port of 23
Client must be running Telnet daemon

1. Test port 23 on MacBook from Ubuntu Terminal window using nmap:



jason@thecube:/etc/bind$ nmap -p 23 192.168.1.101
Starting Nmap 5.21 ( http://nmap.org ) at 2011-09-08 16:04 EDT
Nmap scan report for 192.168.1.101
Host is up (0.084s latency).
PORT   STATE  SERVICE
23/tcp closed telnet

Nmap done: 1 IP address (1 host up) scanned in 0.27 seconds


Port is closed because no Telent service is running. Run Telnet service on Mac:



macbook:sudo launchctl load -w /System/Library/LaunchDaemons/telnet.plist

Scan again:



jason@thecube~$ /etc/bind$ nmap -p 23 192.168.1.101

Starting Nmap 5.21 ( http://nmap.org ) at 2011-09-08 16:15 EDT
Nmap scan report for 192.168.1.101
Host is up (0.052s latency).
PORT   STATE SERVICE
23/tcp open  telnet

Nmap done: 1 IP address (1 host up) scanned in 0.24 seconds


Prerequisites met.


2. Establish telnet connection:



jason@thecube:~$ telnet 192.168.1.101
Trying 192.168.1.101...
Connected to 192.168.1.101.
Escape character is '^]'.

Darwin/BSD (macbook.local) (ttys001)

login:mylogin
password:
Last login: Wed Jul 27 21:54:24 on console

3. Success. Logout. End telnet connection.



macbook:sudo launchctl unload -w /System/Library/LaunchDaemons/telnet.plist

A final word: Best not to use telnet as the data sent is unencrypted. Consider instead using SSH on a nonstandard port for comprehensive security.