My words on free/open source software and Mac

Saturday, November 08, 2008

Don't use APT::Default-Release in Ubuntu unless you know what you are doing

Setting APT::Default-Release in Ubuntu blocks all future security fixes and updates.

This is related to all versions before Hardy (include). I haven't tested this on Intrepid so I'm not sure about those versions after Hardy.

According to apt_preferences manpage, the target release can be set on the apt-get command line or in the APT configuration file /etc/apt/apt.conf, and "APT::Default-Release "stable";" is given out as an example. This is a very common and popular practice used in Debian community to set the default release and using apt-pin, but doing this in Ubuntu leads to serious security impact with no obvious warning.

After setting APT::Default-Release to "hardy", which is the "Suite" name for main hardy source, no security fixes nor updates would be installed unless their priorities are also set explicitly in apt_preferences. This is because that in Ubuntu's world, security fixes are from "hardy-security" source and other updates are from "hardy-updates" source, which bear different "Suite" from the main source. Setting APT::Default-Release rises the priority of packages from main source to 990, but doesn't cover packages from hardy-security and hardy-updates, so the latter are ignored since their packages now has lower priority (priority 500 only) than those old ones in main source (990).

I set APT::Default-Release to "hardy" on Sep this year until I found this problem today. Removed that setting and I'm surprised to found that I can install 46 security fixes and updates accumulated. Which is pretty sad to me that got known I haven't got security fixes for more than 2 months.

This is a radical deviation from the Debian practice. In Debian all security fixes and updates bear the same "Suite" (etch or lenny) so setting APT::Default-Release to "etch" covers all security fixes and updates.

I think it's unlikely that Ubuntu changes the organization of it's source, so at least a fix to this problem is patching the apt_preferences manpage, alerting people not to use APT::Default-Release like they have used this in Debian and the reason and the following impacts.

I've opened a bug about this: https://bugs.launchpad.net/ubuntu/+source/apt/+bug/295448

2 comments:

Bob/Paul said...

Since this showed up as the first result on my search, I thought I'd add this.

There's a community page on the ubuntu help site about pinning:
https://help.ubuntu.com/community/PinningHowto

It recommends you don't use /etc/apt/apt.conf, but rather the /etc/apt/preferences file. This file has a different syntax.

I copied my /etc/apt/sources.list to /etc/apt/sources.list.d/testing.list and replaced "jaunty" with "karmic". In this way I can still pull from karmic-security should I need to.

I install with either "apt-get -t karmic install package" or if that fails due to library conflicts, "apt-get -t karmic build-dep package && apt-get -t karmic -b source package".

Here is my preferences file for example:

Package: *
Pin: release a=jaunty
Pin-Priority: 900

Package: *
Pin: release a=karmic
Pin-Priority: 500

Package: *
Pin: release a=jaunty-updates
Pin-Priority: 900

Package: *
Pin: release a=karmic-updates
Pin-Priority: 500

Package: *
Pin: release a=jaunty-backports
Pin-Priority: 900

Package: *
Pin: release a=karmic-packports
Pin-Priority: 500

Package: *
Pin: release a=jaunty-security
Pin-Priority: 900

Package: *
Pin: release a=karmic-security
Pin-Priority: 500

Package: *
Pin: release a=jaunty-proposed
Pin-Priority: 900

Package: *
Pin: release a=karmic-proposed
Pin-Priority: 500

Yan Li said...

To Bob/Paul:

Thanks for the comment. Your solution is correct for solving this problem. The reason I filed a bug for this is that Ubuntu has changed the Debian way (which is popular among Debian users) for no good reason and caused misunderstanding.

About Me

My Photo
Santa Cruz, California, United States