This simple software is designed for a very basic low-level subnet monitoring and is mainly intended to run on a server in continuous mode.
The idea is to collect all MAC addresses in subnets connected to the server and as well as associated hostnames in different protocols such as: DNS, mDNS, UPnP, DHCP, NBNS, LLDP. Note that not all hosts have public names and do not always report their names, even if they are present.
You can use server console or a web browser to view the collected data. Simple SMTP notification service can be configured.

This free software comes with ABSOLUTELY NO WARRANTY

Building instructions:

1. Update and upgrade packages (strongly recommended):
   sudo apt update
   sudo apt upgrade
2. Install SQLite package, development libraries and header files:
   sudo apt install sqlite3 libsqlite3-dev
3. Install OpenSSL package, development libraries and header files:
   sudo apt install openssl libssl-dev
4. From ‘snmonitor’ main folder execute:
   make all

Usage:

1. Fill in ‘snmonitor.ini’ file
2. You can upload MAC vendor database. To get more information run:
   snmonitor -h
3. Run ‘snmonitor’ (Note: using ‘tmux’ may be convenient)
4. Press: h + <Enter> to help
5. Press: q + <Enter> to quit

Note: I'm running this software on a Raspberry Pi 4 Model B, Ubuntu Server 22.04 LTS (64-bit)