The sources.list file includes the list of packages that can be installed on the system.
Below is an example debian etch one
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deb http://ftp.uk.debian.org/debian/ etch main contrib non-free
deb-src http://ftp.uk.debian.org/debian/ etch main contrib non-free
deb http://security.debian.org/ etch/updates main contrib non-free
deb-src http://security.debian.org/ etch/updates main contrib non-free
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If you are using Plesk it is a good idea to include the Plesk .deb repository with in the sources.list, below is an example for Plesk
8.6.0
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deb http://autoinstall.plesk.com/debian/PSA_8.6.0/ etch all
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(you can check what version are availble by visiting http://autoinstall.plesk.com/debian/ )
So a complete sources.list file for - Debian Etch - Plesk 8.6.0 would be
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deb http://ftp.uk.debian.org/debian/ etch main contrib non-free
deb-src http://ftp.uk.debian.org/debian/ etch main contrib non-free
deb http://security.debian.org/ etch/updates main contrib non-free
deb-src http://security.debian.org/ etch/updates main contrib non-free
deb http://autoinstall.plesk.com/debian/PSA_8.6.0/ etch all