How to test a connection without telnet?

Question

I'm trying to test whether machine A can connect to machine B on certain ports. The sysadmins of machine A have seen fit to remove the telnet command. What would be a convenient replacement? Machine A is CentOS.

Answer 1

iirc, telnet isn't installed by default on centos 6 machines anymore. if you don't have tools like telnet or nc available you can always talk to a network socket using python. (not sure if this fits your "convenient" requirement though... )

to simply test if the connection can be established:

[gryphius@ut ~]$ python
Python 2.6.6 (r266:84292, Feb 22 2013, 00:00:18) 
[GCC 4.4.7 20120313 (Red Hat 4.4.7-3)] on linux2                                                                                                                                                                                                                               
Type "help", "copyright", "credits" or "license" for more information.                                                                                                                                                                                                         
>>> import socket                                                                                                                                                                                                                                                              
>>> conn=socket.create_connection(('gmail-smtp-in.l.google.com',25))                                                                                                                                                                                                           

If that didn't throw an error so far the connection is ok. If you also want to test receiving/sending data, you could continue like this:

>>> input=conn.makefile()                                                                                                                                                                                                                                                      
>>> print input.readline()                                                                                                                                                                                                                                                     
220 mx.google.com ESMTP p2si6911139eeg.7 - gsmtp                                                                                                                                                                                                                               
>>> conn.sendall('HELO example.com\r\n')
>>> print input.readline()
250 mx.google.com at your service

>>> conn.sendall('QUIT')
>>> conn.close()
>>> 

Answer 2

Any of: nc (netcat), nmap, hping.

http://linux.byexamples.com/archives/258/when-netcat-act-as-telnet-client-it-becomes-better/

http://nmap.org/

http://www.hping.org/

Answer 3

A method I came up with:

$ ssh -f -N machineA -L 10123:machineB:123

$ telnet localhost 10123

It failed, but I wasn't sure whether that was actually diagnostic or not. Having tested further, it is indeed diagnostic.