sidedoor

sidedoor maintains a reverse tunnel to provide a backdoor. sidedoor can be used to remotely control a device behind a NAT.

sidedoor is packaged for Debian-based systems with systemd or upstart. It has been used on Debian 8 (jessie) and Ubuntu 14.04 LTS (trusty).

The sidedoor user has full root access configured in /etc/sudoers.d.

Installation

If sidedoor is in your package repositories, simply install it, e.g., sudo apt-get install sidedoor.

Otherwise, you will need to build a Debian package and install it. First, install build dependencies.

sudo apt-get install debhelper dh-systemd

Then, from the directory containing this README file, build and install a package.

rm -f ../sidedoor*.deb # remove old package builds
dpkg-buildpackage -us -uc -b
sudo dpkg -i ../sidedoor*.deb

Configuration

The remote server and tunnel port are configured in /etc/default/sidedoor. SSH configuration files are located in the /etc/sidedoor directory. ~sidedoor/.ssh is a symlink to /etc/sidedoor.

• Configure REMOTE_SERVER and TUNNEL_PORT in /etc/default/sidedoor.
• Create SSH configuration files under /etc/sidedoor.
  • authorized_keys: SSH public key(s) to control access to the local sidedoor user.
  • id_rsa: SSH private key to access the remote server. Can be generated with sudo ssh-keygen -t rsa -f /etc/sidedoor/id_rsa (press enter when prompted for passphrase to leave empty). Needs read permission by the sidedoor user or group, e.g., sudo chown root:sidedoor /etc/sidedoor/id_rsa and sudo chmod 640 /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.
  • config (optional): Additional SSH config, see man ssh_config.

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 backdoor with ssh, scp, rsync, etc.

License

Copyright 2015 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 http://www.gnu.org/licenses/.