I've installed Debian bullseye on an older machine to play around with. I just updated all repos to testing with the guide from DebianTesting.

  1. Edit your apt sources, changing 'stable' (or bullseye, the current codename for stable) to 'testing' (or bookworm, the current code name for the next stable release).
  2. Remove, disable or comment out your stable security updates apt sources (anything with security.debian.org in it).
  3. Remove, disable or comment out any other stable-specific apt sources, like *-backports or *-updates.
  4. Verify that your installation is not fixed to a specific release in /etc/apt/apt.conf.d/00default-release

It states I should remove all stable-specific sources, but as I generally don't follow orders, I replaced bullseye-updates and bullseye-backports with the appropriate testing tag. And see: apt update doesn't even complain. I double checked and there is indeed a testing-backports folder in remote repository.

But after running apt upgrade -s -t testing-backports to see what packages have newer version in this repo, it failes with the above mentioned error message:

E: The value 'testing-backports' is invalid for APT::Default-Release as such a release is not available in the sources

I verified that the command works on another Debian system with stable-backports.

# /etc/apt/sources.list.d/backports.list
deb https://deb.debian.org/debian/ testing-backports main

So is it there indeed no repo for testing-backports, because the release file exists.

1 Answer 1


testing-backports is empty (see all the 0-length Packages files in Release), which is why it can’t be used here.

The backports repositories are used to provide packages from testing rebuilt for stable suites, so testing-backports has no purpose.

Note that using a backports repository as the default release (even temporarily using -t) with operations which touch the whole distribution (apt upgrade etc.) is a bad idea since the backports repository doesn’t host a consistent distribution.

  • That's why I used the -s flag
    – Zacki
    Jul 21, 2022 at 16:13
  • Yes, and that’s fine — I’ve seen more than a few users run apt -t foo-backports upgrade and just wanted to make sure that readers in general were aware of the dangers. Jul 21, 2022 at 16:15

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