I've configured a PPTP VPN in a small company which so far is running well. In the VPN logs, they've found several logs of attempted connections from unkwoun users, one attempted connection every second during a few minutes. Standard names were used like helpdesk, anyuser, interface and so on. The VPN server and router firewalls are enabled. There are only a few users with strong passwords with provileges to connect to the VPN. IP range is also defined. Is there any other security measures that could be taken? Thanks!
As @ChristopherPerrin said, it's the background noise of the internet. Scripts and bots are constantly scanning the internet, doing the equivalent of jiggling door handles looking for ones that are unlocked or poorly locked. I do however disagree that there is nothing you can do about it.
Use those logs to block them at the firewall level. On linux I use fail2ban do to this. There may be similar solutions for windows, or you could write your own script to do it.
There are a few reasons for doing this:
The way fail2ban works is to watch the log files, and when it sees 5 failed login attempts from the same IP address within 5 minutes, it runs an iptables command to add a firewall rule blocking them for 30 minutes. After the ban expires it removes that firewall rule. The numbers are configurable, as are the actions.
You may also want to set a few addresses that it will NEVER block, no matter how many failed tries there are - like your local LAN, or some public addresses of servers you control, so there's no risk of you being locked out.
It is normal that unknown users try to connect with standard usernames. You can view it as the noise of the internet.
If you have strong passwords there is no need to worry. Take it as it is and keep your software updated. There is about nothing you can do.