pax_global_header00006660000000000000000000000064130372446460014523gustar00rootroot0000000000000052 comment=05fff43851391d663459c617094a183a9a0258d8 sidedoor-0.2.0/000077500000000000000000000000001303724464600133325ustar00rootroot00000000000000sidedoor-0.2.0/COPYING000066400000000000000000001045131303724464600143710ustar00rootroot00000000000000 GNU GENERAL PUBLIC LICENSE Version 3, 29 June 2007 Copyright (C) 2007 Free Software Foundation, Inc. Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. Preamble The GNU General Public License is a free, copyleft license for software and other kinds of works. The licenses for most software and other practical works are designed to take away your freedom to share and change the works. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change all versions of a program--to make sure it remains free software for all its users. 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Copyright (C) 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 . Also add information on how to contact you by electronic and paper mail. If the program does terminal interaction, make it output a short notice like this when it starts in an interactive mode: Copyright (C) This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'. This is free software, and you are welcome to redistribute it under certain conditions; type `show c' for details. The hypothetical commands `show w' and `show c' should show the appropriate parts of the General Public License. Of course, your program's commands might be different; for a GUI interface, you would use an "about box". You should also get your employer (if you work as a programmer) or school, if any, to sign a "copyright disclaimer" for the program, if necessary. For more information on this, and how to apply and follow the GNU GPL, see . The GNU General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Lesser General Public License instead of this License. But first, please read . sidedoor-0.2.0/README.md000066400000000000000000000144731303724464600146220ustar00rootroot00000000000000# 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](https://en.wikipedia.org/wiki/Bastion_host) by local forwarding them. * [Melt Evil Corp's tape backups][mrrobot] 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-based systems like Raspbian and Ubuntu, 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](https://github.com/daradib/sidedoor/releases). 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][portforwarding] and the [`ssh` man page](https://linux.die.net/man/1/ssh). 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. ## Comparison with autossh [autossh](http://www.harding.motd.ca/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](https://bugs.debian.org/698390)). * sidedoor disables remote commands and pseudo-tty allocation. For interactive use, consider autossh with SSH keepalives or [Mosh](https://mosh.org/). * 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 sidedoor is intended as a lightweight solution to tunneling ports with minimal dependencies. Some more featured alternatives include: * [OpenVPN](https://en.wikipedia.org/wiki/OpenVPN) * [PageKite](https://github.com/pagekite/PyPagekite/) * [ssh_tunnel](http://sshtunnel.sourceforge.net/) ## License Copyright 2015-2016 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 . [mrrobot]: https://www.forbes.com/sites/abigailtracy/2015/07/15/hacking-the-hacks-mr-robot-episode-four-sam-esmail/ [portforwarding]: https://blog.trackets.com/2014/05/17/ssh-tunnel-local-and-remote-port-forwarding-explained-with-examples.html sidedoor-0.2.0/config000066400000000000000000000010441303724464600145210ustar00rootroot00000000000000Host * # SSH private key to access the remote server. IdentityFile /etc/sidedoor/id_rsa # SSH host key of the remote server. UserKnownHostsFile /etc/sidedoor/known_hosts # If tunneled traffic is encrypted (e.g., SSH), # it probably does not compress well. Compression no # Disable password authentication. BatchMode yes # Terminate if unable to set up port forwarding. ExitOnForwardFailure yes # Enable SSH keepalives. ServerAliveInterval 30 # Disconnect after unresponsive SSH keepalives. ServerAliveCountMax 2 sidedoor-0.2.0/sidedoor000077500000000000000000000042031303724464600150670ustar00rootroot00000000000000#!/bin/sh set -eu if [ "$#" -eq 0 ]; then echo "Usage: $(basename "$0") [SSH_ARGS...] [user@]hostname" echo echo "Maintain an SSH connection or tunnel." echo "No arguments provided. Exiting." exit 0 fi # POSIX arithmetic expressions do not support exponentiation, # so define a function for integer exponentiation. pow() { set "$1" "$2" 1 while [ "$2" -gt 0 ]; do set "$1" $(($2-1)) $(($1*$3)) done echo "$3" } # Wait for a child process to complete and resume waiting after trap. # Repeat wait builtin if it returns an exit status of 128 + SIGUSR1, # which corresponds to wait returning early to handle that signal. wait_continue_after_trap() { while true; do if wait "$1"; then return else retval="$?" fi if [ "$retval" -eq "$((128 + $(kill -l | sed -n /USR1$/=) - 1))" ]; then continue else return fi done } # SIGINT, SIGTERM: terminate child processes, then exit. cleanup() { if [ -n "${ssh_pid-}" ]; then kill "${ssh_pid-}" || true ssh_pid= fi if [ -n "${sleep_pid-}" ]; then kill "${sleep_pid-}" || true sleep_pid= fi wait exit } trap cleanup INT TERM EXIT # SIGUSR1: wakeup from sleep to retry ssh immediately. wakeup() { if [ -n "${sleep_pid-}" ]; then echo "Received SIGUSR1. Retrying ssh immediately." >&2 tries=0 kill "${sleep_pid-}" || true; sleep_pid= else echo "Received SIGUSR1, but ssh is running. Ignoring." >&2 fi } trap wakeup USR1 tries=0 while true; do # CLOCK_MONOTONIC would be resilient to administrator changes, # but timestamp is good enough. timestamp="$(date +%s)" ssh -NT -F /etc/sidedoor/config "$@" & ssh_pid="$!" if wait_continue_after_trap "$ssh_pid"; then exit fi ssh_pid= # If ssh exits quickly, retry with exponential backoff. if [ "$(date +%s)" -lt "$((timestamp + 30))" ]; then tries="$((tries + 1))" timeout="$(pow 2 $tries)" [ "$timeout" -gt 600 ] && timeout=600 echo "ssh exited too quickly. Retrying in $timeout seconds." >&2 sleep "$timeout" & sleep_pid="$!" wait_continue_after_trap "$sleep_pid" || true sleep_pid= else tries=0 fi done sidedoor-0.2.0/ssh_client_config_example000066400000000000000000000016121303724464600204500ustar00rootroot00000000000000# Example ~/.ssh/config file to connect to an SSH server tunneled by sidedoor. # # Replace these placeholders below: # - MY_HOSTNAME: hostname argument to match on the command-line, e.g., # if you want to access a server by running `ssh pi`, # the hostname would be pi # - TUNNEL_PORT: tunneled port on the "bastion" to connect to, e.g., # if you passed `-R 8022:localhost:22` to sidedoor, # the tunnel port would be 8022 # - REMOTE_SERVER: SSH reverse proxy that sidedoor maintains a tunnel to Host MY_HOSTNAME # Access the SSH server through the SSH reverse proxy, # with end-to-end SSH encryption. ProxyCommand ssh -W localhost:TUNNEL_PORT REMOTE_SERVER # If authorized_keys is populated, log in as the sidedoor user. User sidedoor # Optionally, specify a different private key. #IdentityFile sidedoor-0.2.0/sudoers000066400000000000000000000000411303724464600147340ustar00rootroot00000000000000sidedoor ALL=(ALL) NOPASSWD: ALL