Ubuntu Server: Missing Signed-By in the sources.list(5) entry for https://esm.ubuntu.com

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Problem

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~$ lsb_release -a
...
Description:    Ubuntu 24.04.1 LTS
...
~$ pro status
...
Subscription: Ubuntu Pro - free personal subscription
...
~$ sudo apt update
...
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
All packages are up to date.
N: Missing Signed-By in the sources.list(5) entry for 'https://esm.ubuntu.com/apps/ubuntu'
N: Missing Signed-By in the sources.list(5) entry for 'https://esm.ubuntu.com/apps/ubuntu'
N: Missing Signed-By in the sources.list(5) entry for 'https://esm.ubuntu.com/infra/ubuntu'
N: Missing Signed-By in the sources.list(5) entry for 'https://esm.ubuntu.com/infra/ubuntu'

The problem is caused by the missing “Signed-By” option in the ESM (Expanded Security Maintenance) sources.list file.

  • Signed-By (signed-by) is an option to require a repository to pass apt-secure(8) verification with a certain set of keys rather than all trusted keys apt has configured.

Solution

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~$ sudo sh -c "echo \"Signed-By: /usr/share/keyrings/ubuntu-pro-esm-apps.gpg\" >> /etc/apt/sources.list.d/ubuntu-esm-apps.sources\"\""
~$ sudo sh -c "echo \"Signed-By: /usr/share/keyrings/ubuntu-pro-esm-infra.gpg\" >> /etc/apt/sources.list.d/ubuntu-esm-infra.sources\"\""

Reference