Telnet to port

UNIX AIX
1

can some one help me ?

what happens when we telnet to a port ?
example
telnet 192.168.0.xx 1234

where 1234 is a port number

Thank you

2

You'll have to be more specific on what you want to know. Basically, the telnet command will try to establish a TCP connection to the gives address (IP+Port) using the underlying network stack. What happens afterward depends on the hardware in between, any firewalls and ACLs in effect, and the service listening on the other side (if any).

3

Telnetting to a particular port rather than the standard port 23 for telnet can be a useful test to see if something is listening on a port, e.g. to see if a machine is running a SSH daemon on port 22 the you do:

$ telnet localhost 22
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
SSH-2.0-OpenSSH_5.1p1 Debian-6ubuntu2
^]
telnet> quit
Connection closed.
$

Whereas testing to see if my machine (or a remote machine) is serving DNS:

$ telnet localhost 53
Trying 127.0.0.1...
telnet: Unable to connect to remote host: Connection refused
$

in this case the answer is not...

4

try to run the following command on the DNS Server

 netstat -an

and check if bind is started and listening on the 53 port number

5

Running

$ telnet <remotemachine> netstat

can be useful on machines that have not been hardened.

6

Thank you