Today I ran into a strange error when I attempted to install a package on a debian server. When I tried to run apt-get install I encountered this error:

Media change: please insert the disc labeled
 'Debian GNU/Linux 9.3.0 _Stretch_ - Official amd64 xfce-CD Binary-1 20171209-12:11'
in the drive '/media/cdrom/' and press [Enter]

Essentially what this is saying is that it expects to install software from a cd that is not in the cd drive. That is problematic. We want to install new programs from the debian repository, as well as updates.

The fix was to edit the source list located at /etc/apt/sources.list. For me there was a line that said

deb cdrom:[Debian GNU/Linux 9.3.0 Stretch - Official amd64 xfce-CD Binary-1 20171209-12:11]/ stretch main

that must have been enabled when setting up the server from CD. All I had to do was to comment it out by putting a # sign at the beginning of the line.

Then when I again ran apt-get install the system knew to grab the package from the appropriate repository. Crisis averted.