Sometimes you require a certain file from a Debian .deb package. For example, because you accidentally deleted a file. Or because the package's installation is faulty and it can't be restored without first adding the missing file. This FAQ explains how to extract the contents of a.deb file downloaded from the Debian or Ubuntu repository.
Save the Debian package file to your computer. I'll download the dovecot package in this example:
cd /tmp apt-get download dovecot
Now extract the .deb package into the directory /tmp/extract/:
dpkg-deb -x *.deb /tmp/extract/