Q:

Why did port 3000 suddenly start getting used by PPP? How to disable?

I run a rails development environment, which runs a server I can access at localhost:3000.  I was doing this again today, and went to restart the webserver and it started timing out.
After some time I ran nmap localhost, and realised that 'ppp' is running on port 3000:
3000/tcp open  ppp

I've never used PPP, and it's stopping me getting my work done.  I tried service pppd-dns stop, which appears to have no effect.  I even tried sudo apt-get remove ppp, but the port is still open, and I still can't start my rails server.

A:

Well, @whiskers75 answer seemed to work at first, but I've now witnessed the behaviour again and ppp was still not installed - it must have been the reboot that did the trick before.
I used netstat -tulpn to check which processes where listening to which ports, and it turns out that nmap was giving misleading information - it wasn't ppp, but ruby.  A rogue ruby process had been left over by a dead rails instance and was still listening to port 3000.
killall ruby did the trick, the port was freed and I could restart my rails server.

A:

Try sudo apt-get purge ppp then reboot.

