libc6 dependency mismatch in Debian 9.12 stretch

Question

I have the following libc6 dependency problem in Debian 9.12 stretch.

sudo apt install libc6-dev

Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 libc6-dev : Depends: libc6 (= 2.24-11+deb9u4) but 2.27-6 is to be installed
             Depends: libc-dev-bin (= 2.24-11+deb9u4) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

How can I fix it?

EDIT1: apt policy libc6

libc6:
  Installed: 2.27-6
  Candidate: 2.27-6
  Version table:
 *** 2.27-6 100
        100 /var/lib/dpkg/status
     2.24-11+deb9u4 500
        500 http://deb.debian.org/debian stretch/main amd64 Packages
     2.24-11+deb9u1 500
        500 http://security.debian.org/debian-security stretch/updates/main amd64 Packages

EDIT2:

apt install libc6/stretch libc6-dev/stretch libc-dev-bin/stretch
Reading package lists... Done
Building dependency tree
Reading state information... Done
Selected version '2.24-11+deb9u4' (Debian:9.12/oldstable [amd64]) for 'libc6'
Selected version '2.24-11+deb9u4' (Debian:9.12/oldstable [amd64]) for 'libc6-dev'
Selected version '2.24-11+deb9u4' (Debian:9.12/oldstable [amd64]) for 'libc-dev-bin'
The following additional packages will be installed:
linux-libc-dev
Suggested packages:
glibc-doc locales
The following packages will be REMOVED:
libc-bin locales sudo
The following NEW packages will be installed:
libc-dev-bin libc6-dev linux-libc-dev
The following packages will be DOWNGRADED:
libc6
WARNING: The following essential packages will be removed.
This should NOT be done unless you know exactly what you are doing!
libc-bin
0 upgraded, 3 newly installed, 1 downgraded, 3 to remove and 0 not upgraded.
Need to get 6,797 kB of archives.
After this operation, 1,437 kB disk space will be freed.
You are about to do something potentially harmful.
To continue type in the phrase 'Yes, do as I say!'
?]

Solution

You configured your system to use packages from several distributions at once (a so-called Frankenstein-Debian), so dependency errors are normal. Run cat /etc/apt/sources.list{,.d/*.list} in bash to list all sources.

Your libc6 (version 2.27-6) is stuck somewhere between stretch (version 2.24-11+deb9u4) and buster (version 2.28-10). My guess is you pulled some packages from buster, while it was still the testing distribution. The easiest way to solve this problem is:

1. Find in your APT sources a reference to testing or buster. If you find something like:

   deb http://deb.debian.org/debian testing main non-free contrib
   

   replace it with:

   deb http://deb.debian.org/debian buster main non-free contrib
   

   since buster isn't the testing distribution any more (bullseye is).

2. Update the list of packages:

   apt update
   

3. Upgrade your libc6 to the version from buster:

   apt install libc6/buster libc6-dev/buster libc-dev-bin/buster
   

4. Consider upgrading to Debian 10 in the near future to have all packages from the same distribution.

Edit: It seems you have few packages with a version higher than Stretch, so you can just downgrade them:

apt install libc6/stretch libc6-dev/stretch libc-dev-bin/stretch libc-bin/stretch locales/stretch sudo/stretch

and remove buster and company from APT sources.

Comments

[Nasir Riley]

You already have this question on Unix Stackexchange: unix.stackexchange.com/questions/570133/…

[A.B]

and also on SU: superuser.com/questions/1528878/…

[klor]

Thanks for your answer! Downgrading libc6 isn't an option? Could be possible?

[Piotr P. Karwasz]

Sure, you can downgrade it (just replace /buster with /stretch), but I believe you have packages on your system that forced the upgrade in the first place. You'll need to downgrade them too.

[klor]

Added EDIT2 to OP. The downgrade wants to remove sudo, too. Would not cause any harm? May I continue the downgrade with yes?

[Piotr P. Karwasz]

apt install libc6/stretch libc6-dev/stretch libc-dev-bin/stretch libc-bin/stretch locales/stretch sudo/stretch

[klor]

Thank you very much! The downgrade went successful with the last command! I'm so happy now!

[klor]

If you include the last command into the answer, I will accept your answer as solution.

[klor]

But please keep the upgrade part of your answer, too.

[Fel]

If you want a bounty, post this answer in my question.