Firstly, you have to setup VSFTD to display the number of connections, to do this, you have to add this variable setproctitle_enable=YES in the vsftpd.conf file and restart the service.
This setting allows you to monitor the clients, now you can see the connection in the output of ps command, in this case the output looks like this:
vsftpd_daemon_user 11203 1.0 0.3 56320 1548 ? Ss 15:12 0:00 vsftpd: 90.IP.IP.IP: connected
Counting the number of connections is very easy, one script like this is enough:
It will return the number of the connected users.
#!/bin/bash ps aux | grep vsftp | grep connected | wc -l
We are going to save it as /scripts/ftp-who.sh to use it in the next examples.
Now is time to integrate it on SNMPD config.