sidedoor

sidedoor maintains an SSH connection or tunnel with a shell script daemon.

The primary use case is maintaining a remote port forward to the local SSH server (or another port). Thus, the local device can be accessed without using incoming connections that may be blocked by a NAT or firewall or otherwise impractical with mobile devices.

SSH clients can connect to the device via the reverse SSH proxy that sidedoor tunnels to. This proxy server can be untrusted and run by a third party or cloud service.

sidedoor enables SSH keepalives and retries SSH with exponential backoff. In order to reconnect as soon as possible, it resets the backoff when a network interface is brought up (or changed).

Other use cases:

• Access a web application behind a NAT by remote forwarding the local web server (e.g., port 80). A remote server can host a reverse proxy to the web application and handle SSL/TLS termination.
• Stay connected to office network services behind an SSH bastion host by local forwarding them.
• Melt Evil Corp's tape backups by remotely controlling a Raspberry Pi (not recommended!).

Are you using sidedoor? Bugs reports, feature requests - please open an issue! Pull requests are welcome.

Installation

sidedoor is packaged for Debian and Debian-based systems like Raspbian, Ubuntu, and VyOS/EdgeOS, but should work in any POSIX environment with an (OpenSSH) SSH client.

If sidedoor is in your distribution repositories (Debian 9+, Ubuntu 17.04+), simply install it with your package manager.

sudo apt install sidedoor

Otherwise, you can manually download debs from the Releases page.

To grant the sidedoor user full root access, install the sidedoor-sudo package.

Configuration

The remote server and port forwards are configured in /etc/default/sidedoor. SSH configuration files are located in the /etc/sidedoor directory.

1. Configure REMOTE_SERVER and OPTIONS in /etc/default/sidedoor. For some arguments to pass in OPTIONS, see the blog post Local and Remote Port Forwarding Explained With Examples and the ssh man page.

2. Edit SSH configuration files under /etc/sidedoor.

   • id_rsa: SSH private key to access the remote server. Can use ssh-keygen to create this key (press y when prompted to overwrite the existing file):

     sudo ssh-keygen -t rsa -N '' -f /etc/sidedoor/id_rsa
     

     The corresponding public key id_rsa.pub will need to be included in the remote user's ~/.ssh/authorized_keys file.

   • known_hosts: SSH host key of the remote server.

3. Optionally, grant remote access to the local sidedoor user by adding SSH public key(s) to the file /etc/sidedoor/authorized_keys. /etc/sidedoor/authorized_keys is a symlink to ~sidedoor/.ssh/authorized_keys. The sidedoor-sudo package, if installed, provides full root access to this user.

4. Restart the sidedoor service to apply changes.

   sudo service sidedoor restart
   

Recommendations

• Lock down the local SSH server by editing /etc/ssh/sshd_config.
  • Disable password authentication (ChallengeResponseAuthentication no and PasswordAuthentication no).
  • Limit daemon to only listen on localhost (ListenAddress ::1 and ListenAddress 127.0.0.1).
  • To apply changes, restart or reload sshd, e.g., sudo service ssh reload.
• Modify the ssh_client_config_example file and include it in a client's ~/.ssh/config file to easily access the tunneled SSH server with ssh, scp, rsync, etc.

Alternatives

sidedoor is intended as a lightweight solution to tunneling ports with minimal dependencies, but there are some alternatives with more features.

Tor hidden service

Tor provides anonymity to servers run as hidden services, but also handles NAT traversal.

Advantages:

• Metadata, including the IP address of the local device and its connection state (on/off), is less exposed to an intermediary like the reverse SSH proxy.

Disadvantages:

• Tor must be installed and running on both the local device and clients.
• Tor has higher latency so terminal feedback (input echo) is slow.

On both the device and clients, install Tor.

sudo apt install tor

On the device that is being exposed, edit /etc/tor/torrc to create a hidden service on port 22.

HiddenServiceDir /var/lib/tor/sshd/
HiddenServicePort 22 127.0.0.1:22
HiddenServiceAuthorizeClient stealth client

Replace "client" with a comma-separated list of client names to generate multiple authorization secrets.

Then reload Tor and get the onion hostname and authorization data.

sudo service tor reload
sudo cat /var/lib/tor/sshd/hostname

On clients, edit /etc/tor/torrc to add the onion hostname and authorization data seen in the hostname file.

HidServAuth <hostname>.onion <secret>

Then reload Tor and run torsocks ssh <hostname>.onion or set ProxyCommand in the ~/.ssh/config file.

ProxyCommand torsocks nc <hostname>.onion 22

autossh

autossh, like sidedoor, starts ssh and restarts it as needed.

Some differences include:

• sidedoor is a minimalistic shell script daemon. autossh is a more extensive and configurable C program.

• sidedoor enables SSH keepalives (ServerAliveInterval and ServerAliveCountMax), which are available in modern versions of OpenSSH. autossh monitors ssh by sending data through a loop of port forwards (this feature predates SSH keepalives), though this can be disabled with the -M 0 option.

• sidedoor is intended to run automatically as a service, so the package includes init/systemd scripts and config files. autossh does not include an init/systemd script (Debian bug #698390).

• sidedoor disables remote commands and pseudo-tty allocation. For interactive use, consider autossh with SSH keepalives or Mosh.

• sidedoor always retries if ssh exits with a non-zero exit status. autossh does not retry if ssh exits too quickly on the first attempt, which can happen when network connectivity or DNS resolution is broken, particularly on mobile devices. Both sidedoor and autossh have retry backoff logic.

• sidedoor resets retry backoff when a network interface is brought up, to attempt to reconnect as soon as possible, by receiving SIGUSR1 from an if-up.d script. autossh does not have network state hooks.

Other alternatives

• OpenVPN
• PageKite
• ssh_tunnel

License

Copyright 2015-2017 Dara Adib.

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 https://www.gnu.org/licenses/.