padde/SSH-Remote-Port-Forwarding.markdown

!!! WIP !!!

SSH Remote Port Forwarding

Provision Server

DigitalOcean
512 MB RAM
Ubuntu 14.04

SSHD Config

SSH allows you to share ports forwarded to the server with other remote machines, not just the server itself. By default, SSH will open ports on the loopback address 127.0.0.1 on the server, but can be configured to use th wildcard address 0.0.0.0 instead. This means we can ultimately reach the forwarded port from the internet. On the server, add the following line to /etc/ssh/sshd_config:

GatewayPorts yes

Then restart the ssh daemon

sudo service ssh restart

Basic Firewall

We only allow the following connections:

Connections present at the time running the setup
Port 20 SSH
Port 80 HTTP

On the server run:

sudo iptables -I INPUT -i lo -j ACCEPT
sudo iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
sudo iptables -A INPUT -p tcp --dport ssh -j ACCEPT
sudo iptables -A INPUT -p tcp --dport http -j ACCEPT
sudo iptables -A INPUT -j DROP
sudo apt-get install iptables-persistent
sudo service iptables-persistent start

Check if forwarding works

On your local machine, start a web server to test out forwarding. For example, cd to a directory and start a simple webserver with Python that serves the files:

cd ~/lolcats/
python -m SimpleHTTPServer
# Serving HTTP on 0.0.0.0 port 8000 ...

Then set up remote port forwarding via SSH:

ssh root@<your-server-ip> -R 80:localhost:8000

If everything is ok, ssh should now listen on tcp 0.0.0.0:80 on the server and forward requests to port 8000 on your local machine, where the Python webserver is running. You can check whether SSHD uses the correct address on the server:

netstat -tunelp

It should look like this:

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       User       Inode       PID/Program name
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      0          10643       1223/sshd
tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN      0          11653       1412/0
tcp6       0      0 :::22                   :::*                    LISTEN      0          10645       1223/sshd
tcp6       0      0 :::80                   :::*                    LISTEN      0          11654       1412/0

Now open a web browser, navigate to http://<your-server-ip> and you should see a directory listing produced by the Python webserver.