The procedure below uses flashplugin-nonfree as an example package, but this process could be used to edit any .deb with a little care – just change the package name.

Having said that, it’s a quick-and-dirty fix-up and as such only really suitable if you only need to make simple changes to the control scripts such as postinst, prerm, etc, in an existing package. Major changes to a package’s structure or contents will need more care and you should take a look at the Debian Maintainers Guide or any of the other HOWTOs and FAQs available on the web for detailed information on how to do this.

Needless to say you’ll need to do much of this as root. Have a care.

1. Download the updated debian package using apt-get -d install flashplugin-nonfree. This will place the latest version of the package in /var/cache/apt/archives without actually installing it. Note that if you have multiple updates to do, you can use apt-get -d upgrade instead; this will also download any other packages that are currently due an upgrade at the same time – this is fine, they will be installed normally at the end of the process along with the modified package.
2. Change your working directory to /var/cache/apt/archives then make a backup: cp flashplugin-nonfree_$version_i386.deb /root/
3. Create a tempory directory structure where you can unpack the archive: mkdir -p /tmp/flashplugin-nonfree_$version_i386/debian/DEBIAN
4. Extract the contents of the .deb: ar -x flashplugin-nonfree_$version_i386.deb. This will result in three files, debian-binary, data.tar.gz and control.tar.gz. You can delete the debian-binary file.
5. Move data.tar.gz into /tmp/flashplugin-nonfree_$version_i386/debian/ and the control.tar.gz file into /tmp/flashplugin-nonfree_$version_i386/debian/DEBIAN. Unpack the archives in these locations and delete the tarballs.
6. You can now edit the postinst script in /tmp/flashplugin-nonfree_$version_i386/debian/DEBIAN to include the proxy settings as outlined in Installing the Flash plug-in on Ubuntu from behind a proxy.
7. Now you are ready to rebuild the package. Change directory to /tmp/flashplugin-nonfree_$version_i386/ and run dpkg-deb --build debian. This should create a file in the debian/ subdirectory called debian.deb. You may see some warnings about the control file containing user-defined fields – these can be safely ignored.
8. Now move the debian.deb file into /var/cache/atp/archives using the same filename as the original package: mv debian.deb /var/cache/atp/archives/flashplugin-nonfree_$version_i386.deb.
9. You should now be able to run apt-get install flashplugin-nonfree or apt-get upgradeand the package will be installed using the new .deb file complete with proxy information in the postinst script to enable downloading the binary.

Now I’m quite new to Debian-based distros (I learned on and still use Slackware for the most part) but this is obviously some kind of server issue (I’ve updated before without problems). Thankfully there’s a list of mirrors available at the Debian website and ftp.uk.debian.org/debian-non-us works fine. Maybe I’ll do a bit of digging when I’ve got the time.

Update

Solved: sounds this is due to non-us being obsoleted with the release two days ago of Debian 3.1. I assume the mirrors have yet to catch up.