net-bouncer

This is a honeypot program that logs connection attempts to a specified port (any address) and then disconnects. No data is received or sent.

The idea is to use it in conjunction with fail2ban to block IP addresses attempting to exploit known services, such as SSH, while the authentic service is offered on a non-standard port. This way, you can prevent malicious or unknown actors from having any type of communication with your services.

Build

To build the software, all you need is a C99-compatible compiler and make.

First, clone the repository using git or download the source code in the project's GitHub page:

$ git clone https://github.com/brunexgeek/net-bouncer.git

Go to the source directory and run make:

$ cd net-bouncer
$ make

The executable net-bouncer will ge created. Use make install to install the program in the system or any other location.

Running

To start the honeypot, run net-bouncer specifying the port to listen on using the -p parameter. At least one port must be provided. The destination file for the log can be specified with -l; if no log file is provided, the output will go to stderr.

$ net-bouncer -p 22 -p 23 -l /var/log/net-bouncer.log

The command above will listen on ports 22 (SSH) and 23 (Telnet) and store logs in /var/log/net-bouncer.log. The log looks like the following:

net-bouncer 0.1.0
2024-07-08 21:26:44.730 [INFO] Listening to any address on the port 22
2024-07-08 21:26:44.730 [INFO] Listening to any address on the port 23
2024-07-08 21:26:48.474 [INFO] Connection from 127.0.0.1 on port 22
2024-07-08 21:26:50.114 [INFO] Connection from 3.3.1.20 on port 23
2024-07-08 21:26:50.738 [INFO] Connection from 64.25.33.120 on port 22

The program will generate a log entry for each connection, displaying the remote address and the local port that the remote actor attempted to access.

Running as service with systemd

The best way to run net-bouncer is using systemd. You can use a service description like the following:

[Unit]
Wants=network-online.target
After=network-online.target

[Service]
User=net-bouncer
Group=net-bouncer
ExecStart=net-bouncer -p 22 -p 23 -l /var/log/net-bouncer.log

[Install]
WantedBy=default.target

The service above assumes you have a user and group named net-bouncer, which is the recomended thing to do. If you don't want to create a specific user to run net-bouncer, you can omit the fields User and Group.

Monitoring the log with fail2ban

You can use the information from the net-bouncer's log to instruct fail2ban to block the IP addresses of machines that triggered the honeypot. I'll assume you already have fail2ban installed and operational in your environment. For detailed configuration instructions, refer to the official fail2ban documentation.

First, set up a filter that correctly identifies the relevant lines in your log. To do this, create a file named filter.d/net-bouncer.conf in the fail2ban configuration directory (usually located at /etc/fail2ban). Here’s the content for that file:

[Definition]
failregex = ^.*Connection from <HOST>.*$
ignoreregex =
datepattern = ^%%Y-%%m-%%d %%H:%%M:%%S

Next, we’ll configure the jail settings to associate the filter with the actual log file. Create a file named jail.d/net-bouncer.conf in the same configuration directory, adjusting the content to match your specific scenario:

[net-bouncer]
enabled = true
logpath = /var/log/net-bouncer.log
bantime = 1w
maxretry = 1

If you’re monitoring multiple logs (for example, if you have more than one instance of net-bouncer), you can use wildcards (*) in the log path.

Finally, restart the fail2ban service:

$ systemd restart fail2ban.service

License

This program is distributed under Apache License 2.0.