Ubuntu Fundamentals: do-release-upgrade

Mastering do-release-upgrade: A Production-Grade Deep Dive

Introduction

Maintaining a secure and performant infrastructure often necessitates OS upgrades. In a large-scale cloud environment running hundreds of Ubuntu VMs supporting a critical microservices architecture, a rolling upgrade strategy is paramount. Downtime is unacceptable, and a failed upgrade can cascade into a major incident. do-release-upgrade is the core tool for managing these upgrades, but its seemingly simple interface belies a complex underlying process. Mastering its nuances – from pre-flight checks to post-upgrade validation – is critical for minimizing risk and ensuring a smooth transition to newer Ubuntu releases, particularly Long Term Support (LTS) versions. This post dives deep into the tool, focusing on operational excellence and system internals.

What is "do-release-upgrade" in Ubuntu/Linux context?

do-release-upgrade is a command-line utility in Ubuntu (and Debian-based distributions) designed to upgrade the entire operating system to a newer release. It’s a wrapper around apt, dpkg, and other system tools, automating the process of updating package lists, resolving dependencies, and installing new packages. Unlike a simple apt upgrade, do-release-upgrade handles changes to the distribution release itself, including modifications to /etc/apt/sources.list and the addition of new repositories.

Ubuntu’s release cycle dictates the behavior. LTS releases (e.g., 20.04, 22.04) receive five years of standard support and extended security maintenance. Non-LTS releases have nine months of support. do-release-upgrade checks for available releases based on this cycle and the configured update settings.

Key components involved:

• APT (Advanced Package Tool): The core package management system.
• dpkg: The low-level package manager.
• /etc/apt/sources.list: Defines the repositories used for package updates.
• /usr/bin/do-release-upgrade: The main executable.
• /var/lib/apt/preferences: APT pinning and preference settings.
• Prompt: Interactive prompts guide the user through the upgrade process, but can be bypassed with flags.

Use Cases and Scenarios

1. Rolling LTS Upgrade: Upgrading a cluster of web servers from 20.04 to 22.04 in a phased manner to minimize downtime. This requires careful orchestration and monitoring.
2. Cloud Image Updates: Automating the creation of new cloud images (e.g., using Packer) based on the latest Ubuntu LTS release, ensuring consistent base images for deployments.
3. Security Patching & Release Upgrade Combo: Combining a regular security patching cycle with a major release upgrade during a scheduled maintenance window.
4. Container Base Image Updates: Updating the base Ubuntu image used for Docker containers to benefit from the latest security fixes and kernel improvements.
5. Secure Server Hardening: Upgrading to a newer release to leverage updated security features and kernel hardening improvements, followed by a comprehensive security audit.

Command-Line Deep Dive

• Checking for Available Upgrades:
  

  do-release-upgrade -c
  

  This command checks for a new release without actually initiating the upgrade. The -c flag (check) is crucial for pre-flight assessment.

• Initiating an Upgrade (Non-Interactive):
  

  do-release-upgrade -d -f
  

  -d forces the upgrade even if it's not a development release. -f forces the upgrade, bypassing prompts. Use with extreme caution in production!

• Viewing APT Preferences:
  

  cat /etc/apt/preferences
  

  This file is critical for controlling package versions and preventing unintended downgrades during the upgrade process. Incorrect pinning can break the upgrade.

• Monitoring Upgrade Logs:
  

  tail -f /var/log/dist-upgrade/main.log
  tail -f /var/log/apt/history.log
  

  These logs provide detailed information about the upgrade process, including package installations, removals, and errors.

• Checking Systemd Status:
  

  systemctl status apt-daily-upgrade.timer
  systemctl status apt-daily.timer
  

  These timers control automatic updates. Disabling them before a manual upgrade is recommended to avoid conflicts.

System Architecture

graph LR
    A[User/Automation Script] --> B(do-release-upgrade);
    B --> C{APT};
    C --> D[Package Repositories];
    C --> E[dpkg];
    E --> F[Installed Packages];
    B --> G[sources.list Modification];
    G --> C;
    B --> H[systemd];
    H --> I[Services Restart];
    B --> J[Kernel Modules];
    J --> K[Running Kernel];
    B --> L[journald];
    L --> M[System Logs];

do-release-upgrade orchestrates interactions between APT, dpkg, and the configured package repositories. It modifies /etc/apt/sources.list to point to the new release's repositories. Systemd is used to restart services affected by the upgrade. The kernel may be updated, requiring module reloads. All events are logged via journald.

Performance Considerations

do-release-upgrade is I/O intensive. Network bandwidth is consumed downloading packages, and disk I/O is high during package installation and removal.

• Monitoring: Use htop to monitor CPU and memory usage. iotop reveals disk I/O bottlenecks.
• Tuning: Increase vm.swappiness temporarily to allow more memory to be swapped to disk if memory is constrained. sysctl -w vm.swappiness=60
• Disk Space: Ensure sufficient free disk space (at least 20GB) in / and /var.
• Network: A stable, high-bandwidth network connection is essential.
• Benchmarking: Before and after the upgrade, run benchmarks (e.g., sysbench) to assess performance impact.

Security and Hardening

• Disable Unnecessary Services: Before upgrading, disable or remove any unnecessary services to reduce the attack surface.
• Firewall: Ensure ufw or iptables is enabled and configured correctly.
• AppArmor/SELinux: Verify AppArmor or SELinux profiles are active and enforcing security policies.
• Auditd: Enable auditd to log system calls and track changes made during the upgrade. auditctl -w /etc/apt/sources.list -p wa -k do-release-upgrade
• Fail2ban: Configure Fail2ban to protect against brute-force attacks.
• Post-Upgrade Audit: Run a vulnerability scan (e.g., using lynis or OpenVAS) after the upgrade to identify any new security issues.

Automation & Scripting

#!/bin/bash
# Script to automate do-release-upgrade with pre/post checks

set -e

# Pre-upgrade checks

echo "Checking disk space..."
df -h / | grep -q '/\s*[0-9]+G'
echo "Checking network connectivity..."
ping -c 3 google.com

# Disable automatic updates

systemctl stop apt-daily.timer
systemctl stop apt-daily-upgrade.timer

# Perform the upgrade

echo "Starting do-release-upgrade..."
do-release-upgrade -d -f

# Post-upgrade checks

echo "Re-enabling automatic updates..."
systemctl start apt-daily.timer
systemctl start apt-daily-upgrade.timer

echo "Upgrade complete."

This script provides a basic example. Ansible can be used for more complex orchestration, including pre-upgrade backups and post-upgrade validation. Cloud-init can be used to automate upgrades during VM provisioning.

Logs, Debugging, and Monitoring

• journalctl: journalctl -u apt and journalctl -u do-release-upgrade provide detailed logs.
• dmesg: Check dmesg for kernel-related errors.
• netstat/ss: Use netstat -tulnp or ss -tulnp to monitor network connections.
• strace: strace do-release-upgrade can help identify issues with specific system calls.
• lsof: lsof /var/lib/dpkg/lock can identify processes holding locks that prevent the upgrade from proceeding.
• System Health Indicators: Monitor CPU usage, memory usage, disk I/O, and network traffic.

Common Mistakes & Anti-Patterns

1. Running do-release-upgrade without a backup: Incorrect: do-release-upgrade -f. Correct: Create a full system backup before upgrading.
2. Ignoring APT Preferences: Incorrect: Modifying /etc/apt/sources.list directly. Correct: Use /etc/apt/preferences for pinning.
3. Upgrading During Peak Hours: Incorrect: Running the upgrade during business hours. Correct: Schedule the upgrade during a maintenance window.
4. Not Disabling Automatic Updates: Incorrect: Leaving automatic updates enabled. Correct: Disable them before the upgrade to avoid conflicts.
5. Assuming a Smooth Upgrade: Incorrect: Not monitoring logs and system health. Correct: Continuously monitor logs and system health throughout the upgrade process.

Best Practices Summary

1. Always Back Up: Create a full system backup before initiating the upgrade.
2. Test in a Staging Environment: Thoroughly test the upgrade process in a staging environment before applying it to production.
3. Use APT Preferences: Manage package versions using /etc/apt/preferences.
4. Disable Automatic Updates: Disable automatic updates before the upgrade.
5. Monitor Logs: Continuously monitor logs during the upgrade process.
6. Schedule Maintenance Windows: Perform upgrades during scheduled maintenance windows.
7. Validate Post-Upgrade: Run comprehensive tests after the upgrade to ensure functionality.
8. Automate with Ansible/Cloud-init: Automate the upgrade process using Ansible or cloud-init.
9. Implement Rollback Plan: Have a clear rollback plan in case of failure.
10. Document Everything: Document the upgrade process, including any customizations or troubleshooting steps.

Conclusion

do-release-upgrade is a powerful tool, but it requires careful planning and execution. Mastering its intricacies is essential for maintaining a reliable, secure, and up-to-date Ubuntu infrastructure. Regularly audit your systems, build robust automation scripts, monitor upgrade behavior, and document your standards to ensure a smooth and successful upgrade experience. Proactive management of this process is a cornerstone of effective systems engineering.