How to find connected hosts at network (vpn or lan)

Question

Hello, I'm looking for possible solutions to the following need:

I have a VPN configured (using openVPN over Linux, BTW), and I want to know at any moment which hosts are connected to it. I recognize that it probably is the same thing as trying to know which hosts are connected to a lan, so any of the solutions might do the job...

The fact is that I once used a hamachi vpn on linux and with it I had the chance to know which hosts were connected to a particular network where I belonged, so I was wondering if something similar might be possible in openVPN (or even any VPN and/or any LAN).

Preferably, I'm looking for opensource/free sw solutions, or maybe the hints to program it myself (in the most simple way if possible, not that I don't know how to program, but I'm trying to achieve this in a simple manner). But anyway, if there are no os/fsw solutions, any other one might do...

Thanks a lot!

Javier, Mexico city

Answer 1

An easy way to do this with OpenVPN in linux is to use the client-connect and client-disconnect scripts on the server end to maintain a list for you. The client-connect script can log the $common_name environment variable (and also its $trusted_ip, if you like) each time a client connects, and the client-disconnect script can remove that client from the list.

If you also write both connections and disconnections to a different time-stamped log, you'll have a permanent record of the time and duration of each connection.