[pingec]

I am super scared of accidentally exposing a port to the internet. Is there a service / tool that I could provide with a list of all my public IP machines and it would keep port scanning them periodically, sending me a report of all open ports by email each month and sending me an email each time a new port becomes open to the public internet?

[bauruine]

Shodan has a monitor feature. [0] I haven't used it myself but the description looks like it's about what you want.

[0] https://monitor.shodan.io/

[schipplock]

I use "nmap" for this.

  $ nmap example.com
  PORT     STATE  SERVICE
  80/tcp   open   http
  443/tcp  open   https
  1119/tcp closed bnetgame
  1935/tcp closed rtmp

[exciteabletom]

A more complete answer using nmap and cron:

    MAILTO="[e-mail]"
    */30 * * * * nmap yourdomain.com | grep open > nmap.log.tmp; diff nmap.log nmap.log.tmp; mv nmap.log.tmp nmap.log

[jam48]

I think that shodan.io can do this, if you give it an IP it will monitor it and email you about services it finds.