LXC Debian containers and unknown GPG signing keys

Posted: 2021-06-04 10:14:27 by Alasdair Keyes

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I needed to create a Debian Buster LXC container on my laptop and when running the following LXC create command I received the following error

# lxc-create -t debian -n testcontainer -- -r buster
debootstrap is /usr/sbin/debootstrap
Checking cache download in /var/cache/lxc/debian/rootfs-buster-amd64 ...
gpg: key 7638D0442B90D010: 4 signatures not checked due to missing keys
gpg: key 7638D0442B90D010: "Debian Archive Automatic Signing Key (8/jessie) <ftpmaster@debian.org>" not changed
gpg: Total number processed: 1
gpg:              unchanged: 1
Downloading debian minimal ...
I: Retrieving InRelease
I: Checking Release signature
E: Release signed by unknown key (key id DCC9EFBF77E11517)
   The specified keyring /var/cache/lxc/debian/archive-key.gpg may be incorrect or out of date.
   You can find the latest Debian release key at https://ftp-master.debian.org/keys.html
Failed to download the rootfs, aborting.
Failed to download 'debian base'
failed to install debian
lxc-create: testcontainer: lxccontainer.c: create_run_template: 1626 Failed to create container from template
lxc-create: testcontainer: tools/lxc_create.c: main: 319 Failed to create container testcontainer

This is telling me that the key used to sign the Debian release is unknown to LXC. It also shows that LXC is using the file /var/cache/lxc/debian/archive-key.gpg as the GPG keyring.

We can check the keys listed in that keyring with the following command. As a break down, this is running the regular gpg utility, but the --no-default-keyring and --keyring arguments are telling gpg to manage just the keyring file that LXC is using.

# gpg --no-default-keyring --keyring /var/cache/lxc/debian/archive-key.gpg --list-key
/var/cache/lxc/debian/archive-key.gpg
-------------------------------------
pub   rsa4096 2014-11-21 [SC] [expires: 2022-11-19]
      126C0D24BD8A2942CC7DF8AC7638D0442B90D010
uid           [ unknown] Debian Archive Automatic Signing Key (8/jessie) <ftpmaster@debian.org>

Which shows it only has the key for Debian 8 - Jessie...

To get the latest version we need to check that the key listed in the error is a valid Debian key, otherwise we could be opening ourselves up to downloading malicious files.

Visiting https://ftp-master.debian.org/keys.html shows that the GPG key with fingerprint DCC9EFBF77E11517 listed in the error is the valid Debian 10 Buster release key.

Now that we're satisfied that nothing shady is going on, we can import the key to the keyring.

Download the key from the Debian site...

# wget "https://ftp-master.debian.org/keys/release-10.asc"
...
2021-06-04 10:51:53 (35.6 MB/s) - ‘release-10.asc’ saved [1200/1200]

Then import into the keyring...

# gpg --no-default-keyring --keyring /var/cache/lxc/debian/archive-key.gpg --import release-10.asc 
gpg: key DCC9EFBF77E11517: public key "Debian Stable Release Key (10/buster) <debian-release@lists.debian.org>" imported
gpg: Total number processed: 1
gpg:               imported: 1

Running the --list-key command we ran before shows the new key in the the LXC keyring

# gpg --no-default-keyring --keyring /var/cache/lxc/debian/archive-key.gpg --list-key
/var/cache/lxc/debian/archive-key.gpg
-------------------------------------
pub   rsa4096 2014-11-21 [SC] [expires: 2022-11-19]
      126C0D24BD8A2942CC7DF8AC7638D0442B90D010
uid           [ unknown] Debian Archive Automatic Signing Key (8/jessie) <ftpmaster@debian.org>

pub   rsa4096 2019-02-05 [SC] [expires: 2027-02-03]
      6D33866EDD8FFA41C0143AEDDCC9EFBF77E11517
uid           [ unknown] Debian Stable Release Key (10/buster) <debian-release@lists.debian.org>

We can now run the create container command...

# lxc-create -t debian -n akeyescouk -- -r buster
debootstrap is /usr/sbin/debootstrap
Checking cache download in /var/cache/lxc/debian/rootfs-buster-amd64 ... 
gpg: key 7638D0442B90D010: 4 signatures not checked due to missing keys
gpg: key 7638D0442B90D010: "Debian Archive Automatic Signing Key (8/jessie) <ftpmaster@debian.org>" not changed
gpg: Total number processed: 1
gpg:              unchanged: 1
Downloading debian minimal ...
I: Retrieving InRelease 
I: Checking Release signature
I: Valid Release signature (key id 6D33866EDD8FFA41C0143AEDDCC9EFBF77E11517)
I: Retrieving Packages 
I: Validating Packages 
...


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