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.

Limiting the number of connections to SSH Server using Iptables

This is the quickest way to limit the number of connection to your SSH server with iptables.

[bash]
sudo /sbin/iptables -A INPUT -p tcp –syn –dport 22 -m connlimit –connlimit-above 5 -j REJECT
[/bash]

This will only allow up to 5 concurrent connections to the SSH server, subsequent connections will be rejected by iptables, thus this can thwarts Brute-force attempts to your server.

More Articles About Securing SSH Server

How to Secure SSH server from Brute-Force and DDOS with Fail2ban ( Ubuntu )

Fail2ban is a security tool used for preventing brute-force attack and Distributed Denial of Service (DDoS) attack to your GNU/Linux box.

Fail2ban monitors failed login attempts and subsequently blocks the ip address from further logins. Although Fail2ban can also be used to secure other services in Ubuntu server, in this post, I will only focus on securing SSH server.

Step 1: Install Fail2ban and (optionally) sendmail

sudo apt-get install fail2ban
sudo apt-get install sendmail-bin

Step 2: Setting up Fail2ban

Next, you need to configure fail2ban by creating a copy of ‘jail.conf’ to ‘jail.local’

cd /etc/fail2ban
sudo cp jail.conf jail.local

Step 3: General fail2ban configuration

Edit fail2ban configuration file using your favorite text-edito (I personally use ‘nano’)

sudo nano /etc/jail.local

You can set IP address for fail2ban to ignore, IP addresses can be separated by space.

Bantime is the duration of time that you want fail2ban to block suspicious attempt, the value is in seconds
Maxretry is the number of failed attempts before fail2ban block the IP-address, in this case 3600 means 1-hour ban

# "ignoreip" can be an IP address, a CIDR mask or a DNS host
ignoreip = 127.0.0.1 192.168.1.1
bantime  = 3600
maxretry = 3 

Step 4: Enabling ssh and ssh-ddos protection
Find ssh configuration under [ssh] heading, and enable it.

[ssh]
enabled = true
port    = ssh
filter  = sshd
logpath  = /var/log/auth.log
maxretry = 3 

Similarly, you can also enable [ssh-dos] protection by changing the enabled value to “enabled = true

[ssh-ddos]
enabled = true
port    = ssh
filter  = sshd-ddos
logpath  = /var/log/auth.log
maxretry = 2

Step 5: Enable Sending Notification Email (optional)
Optionally you can have fail2ban sends you notification email in case of suspicious login detected. To do that, you need to locate destemail settings and changed it to your email

destemail = security@mypapit.net

Fail2ban can use ‘sendmail’ and ‘mail’ application to send notification email

Step 6: (Re-)start Fail2ban
After all is done, you may save the file, and (re)start the fail2ban service

sudo /etc/init.d/fail2ban restart

You can test the configuration by trying to login into your box. You may also check fail2ban log in /var/logs/auth.log (or in other directory specified in jail.local)

For more information about fail2ban, you can read : the official fail2ban manual

Recommended Reading

Iptables rule to safeguard SSH server from crackers

Secured Shell or SSH is a service to enable users to access remote system securely. However, SSH servers depending on password-based authentication might be vulnerable to dictionary-based (or brute-force) attacks by crackers.

Luckily iptables can be used with ‘–limit-burst‘ and ‘–limit’ option to reduce the number of attempts and connection that a cracking tool can make in a period of time.

For example, in order to limit an IP address to making only 5 connections per minute in burst of 2 connections, you can use this iptables rules:

iptables -A INPUT -p tcp --dport ssh -m limit --limit 5/minute --limit-burst 2 -j ACCEPT

This will result in the iptables will only allow up to 5 connections per minute with 2 maximum initial number of connections, which will make any brute-force or dictionary-based attack uneconomical/unfeasible for the server.

Read more about iptables –limit and –limit-burst in Linux Iptables Limit the number of incoming tcp connection / syn-flood attacks