ubuntu-slim APT configuration not consistent with normal runners

[iskunk]

Description

There are two issues with the default APT (package manager) configuration on the ubuntu-slim runner, compared to the standard ubuntu-24.04:

• It pulls from the global archive.ubuntu.com server instead of the Azure-local azure.archive.ubuntu.com;

• The APT list files are not pre-populated in the image. That is, if I run e.g. sudo apt-get install tcsh, I get the error E: Unable to locate package tcsh. I have to run sudo apt-get update to download the package lists first. From my runs, I see this downloads about 43 MB of data, which is not insignificant when everyone has to do it to install packages.

Platforms affected

• [ ] Azure DevOps
• [x] GitHub Actions - Standard Runners
• [ ] GitHub Actions - Larger Runners

Runner images affected

• [ ] Ubuntu 22.04
• [ ] Ubuntu 24.04
• [x] Ubuntu Slim
• [ ] macOS 13
• [ ] macOS 13 Arm64
• [ ] macOS 14
• [ ] macOS 14 Arm64
• [ ] macOS 15
• [ ] macOS 15 Arm64
• [ ] macOS 26 Arm64
• [ ] Windows Server 2019
• [ ] Windows Server 2022
• [ ] Windows Server 2025

Image version and build link

Current runner version: '2.329.0'
Runner Image Provisioner
  Hosted Compute Agent
  Version: 20251202.455
  Commit: 6c10caca4910e198df60de23adf20ad317c474e3
  Build Date: 2025-12-02T15:56:59Z
  Worker ID: {54b36ee2-a693-44f1-962f-4b426472c8a5}
VM Image
  - OS: Linux (x64)
  - Source: Docker
  - Name: ubuntu:24.04
  - Version: 20251202.31.1

Is it regression?

No

Expected behavior

Same as ubuntu-24.04, which uses the Azure mirror, and has pre-populated APT lists.

Actual behavior

Uses global mirror, no pre-populated APT lists :-(

Repro steps

(see description)

[GitPaulo]

Hi, let me help you with clarifying a few points:

1. ubuntu-slim configures azure.archive.ubuntu.com into /etc/apt/apt-mirrors.txt
2. ubuntu-slim runs install but removes packages as intended (slim)

If any unintended behaviour has been observed a repro is appreciated. Thanks!

[iskunk]

Hello @GitPaulo,

I have recent workflow runs demonstrating both issues with ubuntu-slim:

Run step showing that apt-get update contacts archive.ubuntu.com and not azure.archive.ubuntu.com;

Run where apt-get install fails when not preceded by apt-get update. (Note that the run step linked above is able to install the package at issue.)