[maemo-users] Where is telnet?

From: n810 at merctech.com n810 at merctech.com
Date: Thu May 14 18:11:10 EEST 2009

In the message dated: Thu, 14 May 2009 14:35:46 +0200,
The pithy ruminations from Ralph Angenendt on 
<Re: Where is telnet?> were:
=> James Knott wrote:
=> > Any reason why you're using telnet instead of ssh?  Ssh has many
=> > advantages over telnet.

Yes, ssh has many advantages over telnet if the aim is to establish a remote 
login session. However, ssh is not an acceptable substitute for telnet if the 
aim is to establish an unencrypted TCP/IP connection to an arbitry port in 
order to exchange ASCII data. This is a common use of telnet as a network 
troubleshooting tool.

=> ssh -p 25 mail-server.example.com
=> ssh -p 110 mail-server.example.com
=> ssh -p 80 www.example.com

Did you actually try those examples? They will fail when the 
ssh_exchange_identification part of the client-side protocol negotiation times 

=> Need more reasons?

In the examples you gave, after the ssh client establishes a connection up 
through OSI layer 4 (TCP) to the destination port, it then tries to negotiate 
the layer 7 (presentation) protocols. These include ssh_exchange_identification 
of the remote host, ssh key exchange, etc.

In the examples given above, the common daemons listening on those ports (SMTP, 
POP3, HTTP) will not be able to complete the protocol negotiation with the ssh 
client, so the session will fail. I am not aware of any ssh options to disable 
all identification and encryption mechanisms so that ssh can function like 

This means that ssh cannot be used to, for example, connect to an SMTP server 
on port 25 an issue the commands:

	mail from: <user at example.com>
	rcpt to: <foobar at example.com>
	this is a mail message

I'd suggest using "socat" (a "netcat" work-alike) if telnet is not available.


=> Ralph

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