Linux OpenSSH Tunelling

From Ever changing code

Different types of tunnelling:

  1. Local port forwarding: connections from the SSH client are forwarded via the SSH server, then to a destination server
  2. Remote port forwarding (reverse SSH tunnel): connections from the SSH server are forwarded via the SSH client, then to a destination server
  3. Dynamic port forwarding: connections from various programs are forwarded via the SSH client, then via the SSH server, and finally to several destination servers

Local port forwarding - straight tunnel

Used when only gateway/bastion can talk directly to databases (or other servers). The connection is made through gateway so the database see connection coming from the gateway and allows it.

-L <local-port-to-listen>:<remote-host>:<remote-port> <gateway>


in-front-of-firewall@localhost:$ ssh -f -N -T

Example of connecting to Azure MySQL

# Create a tunnel, localhost will be listening on port 3306
ssh -fNT db@bastion
# Connect, to localhost, by default that port 3306, will forward connection over 
# the tunnel to database using username 'admin@database-eu'
mysql -h -u admin@database-eu -p

Remote port forwarding (reverse SSH tunnel)

-R <sourcePort>:<forwardToHost>:<onPort> <gate>

in-front-of-firewall@server1           //NAT//           behind-firewall@server2       
2. the tunnel listens on             //FW //             1. this server initiates a -R reverse tunnel 
   localhost:2222 port and forwards                         (because is allowed out through FW/NAT)
   any data down via tunnel to port 22 of the other end
       data >>> :2222      >>>>>   tunnel       >>>>>       :22

From the firewalled host:

behind-firewall@server2:$ ssh -f -N -T -R2222:localhost:22

This tells your client to establish a tunnel with a -Remote entry point. Anything that attaches to port 2222 on the far end of the tunnel will actually reach "localhost port 22" (computer that you execute the command).

Then at the in-front-of-firewall@server1 you can connect over ssh to localhost:2222 will send all traffic through the tunnel to behind-firewall@server2. This resolves issue for a remote users that needs a temporary access from theirs machines that are in-front-of-firewall.

ssh -p 2222 behind-firewall@localhost

The other options are:

  • -f tells ssh to background itself after it authenticates, so you don't have to sit around running something on the remote server for the tunnel to remain alive.
  • -N says that you want an SSH connection, but you don't actually want to run any remote commands. If all you're creating is a tunnel, then including this option saves resources.
  • -T disables pseudo-tty allocation, which is appropriate because you're not trying to create an interactive shell.

Local and Remote port forwarding graphs

Local port forwarding


Remote port forwarding


Show current tunnels

Shows -L local forwarding tunnels

netstat -tpln | grep ssh    #t: TCP, p: show process, l: listening, n: numeric values
(header added, tested on Debian wheezy)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0*               LISTEN      4036/ssh        

Which can be read as: SSH (not SSHd) is listening to local TCP port 1443

if you only want to list tunnels created by ssh

$ sudo lsof -i -n | egrep '\<ssh\>'
ssh  19749  user  3u  IPv4 148088244   TCP x.x.x.x:39689->y.y.y.y:22 (ESTABLISHED)
ssh  19749  user  4u  IPv6 148088282   TCP [::1]:9090 (LISTEN)
ssh  19749  user  5u  IPv4 148088283   TCP (LISTEN)

(that would be a -L 9090:localhost:80 tunnel)

Shows -R reverse tunnels

if you want to see the tunnels / connections made to a sshd:

$ sudo lsof -i -n | egrep '\<sshd\>'
sshd  15767  root  3u  IPv4 147401205   TCP x.x.x.x:22->y.y.y.y:27479 (ESTABLISHED)
sshd  15842  user  3u  IPv4 147401205   TCP x.x.x.x:22->y.y.y.y:27479 (ESTABLISHED)
sshd  15842  user  9u  IPv4 148002889   TCP> (ESTABLISHED)
sshd  1396   user  9u  IPv4 148056581   TCP (LISTEN)
sshd  25936  root  3u  IPv4 143971728   TCP *:22 (LISTEN)

the ssh-daemon listens on port 22 (last line), 2 subprocesses are spawned (first 2 lines, login of 'user'), a -R tunnel created on port 5000, and a -L tunnel which forwards a port from my (local) machine to localhost:80 (www).

sudo lsof -i -n | egrep '\<sshd\>' | grep -v ":ssh" | grep LISTEN | sed 1~2d | awk '{ print $2}' | while read line; do sudo lsof -i -n | egrep $line | sed 3~3d | sed 's/.*->//' | sed 's/:......*(ESTABLISHED)//' | sed 's/.*://' | sed 's/(.*//' | sed 'N;s/\n/:/' 2>&1 ;done

Windows Putty ssh tunnelling

The all above can also be achieved using Putty.

Putty Local port forwarding tunnel


Once connected the tunnel is established and from the local PC you can eg. go that in effect pull data from server2:4444. If you don't want to have open CLI session you can disable Pseudo TTY terminal in Putty>Connection>SSH>TTY, tick Don't allocate a pseudo-terminal