Here's how to fix a "Cannot start ntop since USER is not defined" error.

I noticed that when my Debian laptop boots up, I would always see the following error message:

Starting network top daemon: ERROR: Cannot start ntop since USER is not defined, check the configuration file /etc/default/ntop will not start network top daemon!

It's been a while since I even installed ntop and I can't remember why I did, but what follows is how I fixed this error.

Obviously a user that NTOP runs under is not defined. The error message references the file /etc/default/ntop. I don't feel like editing the file and I'm using Debian, so the way I fixed it was like this:

I ran the command:

dpkg-reconfigure ntop

And that opened up the pretty blue screen where I was first asked which interfaces I would like ntop to listen on, separated by commas. I entered in the following interfaces: 

eth0,wlan0

Then I was asked to enter the name of the user the ntop daemon would run under. I just chose 'ntop' as the username for the ntop daemon to run under. That seemed like it would make sense. Keep in mind that the script will autumatically create the ntop user if it doesn't exist.

Once you've done those two steps, ntop will be configured and, if you're problem was the same as mine, then you won't see the "Cannot start ntop since USER is not defined" error.