backconn - a "back connection" to your devices behind firewalls and NAT Copyright (C) 2017-2018 Mateusz Viste backconn is a Linux networking tool that is designed to solve the problem of connecting to devices that are behind a firewall or a NAT box. It is a set of two programs: backconn-cli and backconn-adm. backconn-cli needs to be installed on the device you wish to have access to, while backconn-adm is the server part, and needs to be installed on a publicly available host. backconn-cli connects then to backconn-adm. backconn-adm opens a new local socket on the server machine. Anything that is sent to the backconn-adm socket is pushed to backconn-cli, and forwarder by backconn-cli to a predefined localhost port. Note: a very similar thing can be achieved using reverse SSH tunneling. backconn is a much simpler alternative to that, and supports multiple parallel connections. First, backconn-cli establishes a connection to backconn-adm: backconn-cli --> internet --> backconn-adm Then, backconn-adm opens a local socket and proxifies it with the backconn-cli connection. you a TCP port on localhost | ^ V | backconn-adm --> internet --> backconn-cli ====== How to use it ====== 1. Launch the backconn-adm process on a publicly reachable server, providing it with a configuration file: backconn-adm bc.conf 2. Execute the backconn-cli process on the device you'd like to have access to. Assuming your public server's address is and that it listens on port 999, the command would be this: backconn-cli 999 3. Connect to the socket you attributed to backconn-adm ====== Limitations ====== backconn is a plain-text protocol, hence all your data is exposed on the wire. You can optionnaly let backconn 'encrypt' its connection with an 8-bit xor key, but this obviously shouldn't be considered a security feature, rather only a primitive obfuscation method designed to fool deep packet inspection systems. ====== Configuration file ====== backconn-adm reads its configuration from a configuration file. This file is meant to provide the process with the list of IP addresses it should allow connections from, the local ports it should assign to each IP, the remote port we wish tunneling to, and the xor key to use (if any). ====== Protocol ====== The protocol used between backconn-cli and backconn-adm 1. backconn-cli connects to backconn-adm through a standard TCP connection 2. backconn-cli immediately sends its 'name' in the following form: Lname where L is the length of the name (can be zero), and the actual name follows. 3. backconn-adm opens then a tunneling socket on its side, and awaits for a connection on it. In the meantime, it sends every few seconds a keepalive request to backconn-cli, in the form of two zero bytes (0x00 0x00). backconn-cli is expected to answer with a single 0xAA byte. 4. when backconn-adm gets a tunneling connection, it then sends a tunneling request to backconn-cli. A tunneling request is nothing more than the TCP port number backconn-cli is supposed to connect to on its side. This TCP port is sent as a 16bit number in networking byte order. 5. from now on, the tunnel is established Note: all communication between backconn-adm and backconn-cli can be optionally obfuscated through an 8 bit xor key. This xor key, if set, is applied to all communication: keepalives, tunneled data, etc, but NOT to the step 2 (ie. advertising client's name). ====== Download ====== backconn's source code is available through the project's SVN repository: svn checkout backconn ...or download the latest version right here: backconn-20180206.tar.xz ====== License ====== The backconn suite is released under the MIT license, as listed below. Copyright (C) 2017, 2018 Mateusz Viste Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.