Solving SSH “channel 3: open failed: administratively prohibited” error when tunnelling

A couple of days ago, I’ve encountered this error when I was trying to create a SSH tunnel from my office LAN to a remote server.

After looking for a solution, I found out that the remote SSH server must add “PermitTunnel yes” line in “/etc/ssh/sshd_config” file.

To do that, you need to:

sudo echo "PermitTunnel yes" >> /etc/ssh/sshd_config
service sshd restart

The second line is used to restart the ssh service in order to enable the changes.

How to send Email with SMTP using Telnet in GNU/Linux

SMTP is a protocol to send email over the internet and it is documented in RFC821. Sending email using Telnet is straight-forward if you know how to do it.

telnet 25
MAIL FROM: <example_from @>
RCPT TO: <example_to @>
From: [John Doe] <example_from @>
To: [Jane Doe] <example_to @>
Subject: This is a test message....

This is an example email content to demonstrate email sending using Telnet.



Note that you need to type "." and QUIT after you've finished writing the email content. You should change the SMTP server domain and the email used in the example accordingly.