Binary package hint: gufw
When you add the cups exception rule the using the predefined service in gufw the rule "631/TCP allow EVERYWHERE" is added. I believe that the correct thing would be "631 allow everywhere" which means it can allow both TCP and UDP protocols. This is needed because in many cases printers broadcast UDP messages to inform the computers on the network that they exist.
I am not sure about this, but I have come up to this conclusion watching the listening ports of my computer and capturing packages with wireshark.
ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: ufw 0.30.1-1ubuntu1
ProcVersionSignature: Ubuntu 2.6.38-8.42-generic 2.6.38.2
Uname: Linux 2.6.38-8-generic x86_64
NonfreeKernelModules: nvidia
Architecture: amd64
Date: Tue May 3 17:54:25 2011
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release amd64 (20110426)
PackageArchitecture: all
ProcEnviron:
LANGUAGE=en_US:en
LANG=en_US.UTF-8
SHELL=/bin/bash
SourcePackage: ufw
UpgradeStatus: No upgrade log present (probably fresh install)
True! This will fixed in Gufw 11.10 ;) Thanks Orestis!