This is a text-only version of the following page on https://raymii.org: --- Title : Persistent reverse (NAT bypassing) SSH tunnel access with autossh Author : Remy van Elst Date : 05-10-2012 URL : https://raymii.org/s/tutorials/Autossh_persistent_tunnels.html Format : Markdown/HTML --- Situation: you are in a restricted network (company, hotel, hospital) where you have a "server" which you want to access from outside that network. You cannot forward ports to that machine, but you can ssh outside (to your own server). This tutorial solves this problem. You need another server to which you setup a persistent ssh connection with a reverse tunnel. Then if you need to access the machine you ssh into the other server, and from there you ssh through the tunnel to the restriced machine. Make sure you have permission to do this from the administrators. They generally don't like holes in the firewall/security. They don't block it for no reason.

Recently I removed all Google Ads from this site due to their invasive tracking, as well as Google Analytics. Please, if you found this content useful, consider a small donation using any of the options below:

I'm developing an open source monitoring app called Leaf Node Monitoring, for windows, linux & android. Go check it out!

Consider sponsoring me on Github. It means the world to me if you show your appreciation and you'll help pay the server costs.

You can also sponsor me by getting a Digital Ocean VPS. With this referral link you'll get $200 credit for 60 days. Spend $25 after your credit expires and I'll get $25!

#### Naming convention: restricted machine: machine inside the restricted network middleman: machine to which the restricted machine sets up the tunnel, and from which you access the restricted server #### Install the tools We are going to use autossh. This is in the debian/ubuntu repositories. Make sure you also install openssh server. Execute on: restricted machine. sudo apt-get install autossh ssh #### Create your ssh-key. Execute on: restricted machine. ssh-keygen Generating public/private rsa key pair. Enter file in which to save the key (/root/.ssh/id_rsa): */root/.ssh/nopwd* Enter passphrase (empty for no passphrase): *leave empty* Enter same passphrase again: *leave empty* #### Copy your key to the middleman machine Execute on: restricted machine. ssh-copy-id -i .ssh/nopwd.pub "-p 2222 remy@middleman" (replace remy@middleman with your username and middleman ssh server. Also note how you can give a custom port in the ssh-copy-id.) #### Test the connection with autossh Execute on: restricted machine autossh -M 10984 -o "PubkeyAuthentication=yes" -o "PasswordAuthentication=no" -i /root/.ssh/nopwd -R 6666:localhost:22 remy@middleman -p 2222 Explanation"of options: * -M 10984: autossh monitoring port. * -o "PubkeyAuthentication=yes": authenticate with ssh-keys instead of password. * -o "PasswordAuthentication=no": explicitly disable password authentication. * -i /root/.ssh/nopwd: the location of the ssh key to use. * -R 6666:localhost:22: reverse tunnel. forward all traffic on port 6666 on host middleman to port 22 on host restricted machine. * remy@middleman -p 2222: ssh user remy, ssh host middleman, ssh port 2222 If this all goes well you should be logged in to the middleman host without being asked for a password. You might get the question if you want to add the ssh key. Say yes to this. If it does not go well, check the permissions on the ssh key (should be 600), and make sure you have the correct values in the autossh command. #### SSH back in the restricted host From another machine (outside the restricted network preferably) ssh into the middleman host. Execute on: other machine ssh -p 2222 remy@middleman From the middleman, ssh into the restricted host via the reverse tunnel we created: Execute on: middleman ssh -p 6666 remy@127.0.0.1 If all goes well, you should see a prompt to login to the restricted machine. Enter your password and go. If this goes well, you can continue. If this does not work, check the values in the command and the ssh configs. Also make sure you have executed the steps above correctly. #### Enable the tunnel on boot We are going to edit the /etc/rc.local file. This script normally does nothing, but gets executed at boot. If you make any errors in this script, your machine might not boot so make sure to do this correctly. Execute on: restricted machine sudo nano /etc/rc.local Add (and change) the following line autossh -M 10984 -N -f -o "PubkeyAuthentication=yes" -o "PasswordAuthentication=no" -i /root/.ssh/nopwd -R 6666:localhost:22 remy@middleman -p 2222 & We have three new things in this command: * -N: Do not execute a command on the middleman machine * -f: drop in the background * &: Execute this command but do not wait for output or an exit code. If this is not added, your machine might hang at boot. Save the file, and as make it executable: Execute on: restricted machine sudo chmod +x /etc/rc.local And test it: Execute on: restricted machine sudo /etc/rc.local If you get your regular promt back without any output you've done it correct.z #### Forward a website, not ssh You might want to forward a website on the restricted host. Follow the above tutorial, but change the autossh command: autossh -M 10984 -o "PubkeyAuthentication=yes" -o "PasswordAuthentication=no" -i /root/.ssh/nopwd -R 8888:localhost:80 remy@middleman -p 2222 * -R 8888:localhost:80: this forwards all traffic on host middleman to port 80 on host restrictedhost. (port 80 = website). #### Other host inside restricted network You can also forward ports from other restricted hosts in the network: autossh -M 10984 -o "PubkeyAuthentication=yes" -o "PasswordAuthentication=no" -i /root/.ssh/nopwd -R 7777:host2.restrictednetwork:22 remy@middleman -p 2222 This will forward all traffic to port 7777 on host middleman, via host restrictedhost, to host host2.restrictednetwork port 22. [1]: https://www.digitalocean.com/?refcode=7435ae6b8212 --- License: All the text on this website is free as in freedom unless stated otherwise. This means you can use it in any way you want, you can copy it, change it the way you like and republish it, as long as you release the (modified) content under the same license to give others the same freedoms you've got and place my name and a link to this site with the article as source. This site uses Google Analytics for statistics and Google Adwords for advertisements. You are tracked and Google knows everything about you. Use an adblocker like ublock-origin if you don't want it. All the code on this website is licensed under the GNU GPL v3 license unless already licensed under a license which does not allows this form of licensing or if another license is stated on that page / in that software: This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see . Just to be clear, the information on this website is for meant for educational purposes and you use it at your own risk. I do not take responsibility if you screw something up. Use common sense, do not 'rm -rf /' as root for example. If you have any questions then do not hesitate to contact me. See https://raymii.org/s/static/About.html for details.