Ubuntu Knowledge Patch

Ubuntu Knowledge Patch

Use this skill when installing, upgrading, configuring, securing, or operating Ubuntu systems and images. Start with the breaking-change checks, then read the topic reference that matches the machine, service, or deployment being changed.

Reference index

Breaking changes and upgrade gates

Check the direct upgrade path

Do not assume every installed release can upgrade directly. For the latest LTS, only the immediately preceding interim release and the prior LTS are direct sources. LTS users receive the automatic offer at the first point release rather than on the initial release date. Read the lifecycle section before automating a fleet upgrade.

Revalidate privileged commands and core utilities

Ubuntu 25.10 changes two foundational command providers:

• sudo-rs provides sudo by default. Classic sudo binaries use a .ws suffix, and sudo-ldap is removed; integrate LDAP authentication through PAM.
• rust-coreutils provides the core operating-system utilities. GNU coreutils remain selectable through managed diversions, but scripts that rely on edge behavior need compatibility tests.

Also stop assuming a server image directly contains screen, wget, byobu, cloud-guest-utils, or dirmngr. Use the seeded wcurl for simple downloads or install the package a workload actually requires.

Migrate APT keys and solver assumptions

apt-key is removed. Store repository keys as files and configure verification according to apt-secure(8) instead of scripting the deleted command.

APT 3.0 tries the new solver only after the classic solver fails; APT 3.1 makes the new solver the default. Use these commands to explain its choices:

apt why PACKAGE
apt why-not PACKAGE

Deb822 repository definitions can use Include and Exclude to constrain which packages a repository supplies. Account for the automatic pager used by apt show and apt list in scripts and captured output.

Convert removed service configuration

Treat these as migrations, not restart-only upgrades:

• Dovecot 2.4 substantially changes and renames 2.3 configuration syntax.
• QEMU 9.2 removes old block-migration, compression, and 9pfs proxy interfaces; QEMU 10.1 also removes old x86 and s390x machine types.
• Samba 4.22 removes nmbd proxy logon, cldap port, and fruit:posix_rename.
• OpenSSH 10 removes DSA support, and strongSwan 6.0 removes NTRU.
• HAProxy 3.0 rejects several previously tolerated forms and renames tune.ssl.ocsp-update to tune.ocsp-update.
• Containerd 2.0 removes 1.x-deprecated functionality, while runc 1.2 changes mount validation and flag handling.

Complete any guarded Redis-to-Valkey migration before installing a release that removes valkey-redis-compat. Never delete /etc/valkey/REDIS_MIGRATION until the migrated configuration and data have been reviewed.

Declare cloud-init datasources on non-x86 images

Cloud-init 25.3 disables itself on non-x86 systems unless a known datasource is declared during early boot through DMI, kernel parameters, filesystem configuration, or environment files. Image builders must make the declaration; otherwise a newly launched machine may be unreachable over SSH.

Invoke cloud-init boot stages through their systemd units. The four stages share one Python process through cloud-init-main.service, and direct post-production stage invocation produces warnings and bypasses the intended lifecycle.

Account for boot-stack changes

Systemd 256.5 refuses cgroup v1 unless the kernel command line contains:

SYSTEMD_CGROUP_ENABLE_LEGACY_FORCE=1

It also ships tmp.mount, making /tmp a tmpfs, and places cryptsetup-related helpers in the separately installed systemd-cryptsetup package.

Ubuntu Desktop 25.10 uses Dracut and systemd for initramfs generation. Server and Raspberry Pi desktop installations retain initramfs-tools; choose hooks and recovery procedures for the stack actually installed.

Installation quick reference

Make offline behavior explicit

For an autoinstall that may have no reachable package mirror, configure:

apt:
  fallback: offline-install

Selecting NVIDIA drivers while offline installs Nouveau instead. Complete that installation with network access or change the graphics driver afterward.

Avoid known storage and firmware traps

• TPM-backed full-disk encryption cannot boot while Absolute/Computrace is enabled and can omit hardware-specific modules such as vmd, hiding NVMe RAID.
• ZFS encryption can fail to activate cryptoswap. OEM installation is not supported in the affected desktop installer generation.
• NVMe-over-TCP firmware installation can leave the target unbootable; apply the dedicated autoinstall workaround or repair it before the first reboot.
• U-Boot targets must have the matching newer U-Boot before installation because Subiquity does not run flash-kernel or update-grub there.
• Physical GRUB-to-iPXE boot needs grub-ipxe and the correct ipxe.efi or ipxe.lkrn; QEMU's driver-only UEFI ROM still has to chainload full iPXE.

Prevent installer network hangs

The installer treats otherwise-unconfigured interfaces as DHCPv4. Remove disconnected interfaces from /etc/netplan/50-cloud-init.conf or mark them optional: true. A Raspberry Pi server that needs network-provided cloud-init data must instead leave at least one interface optional: false.

Networking and security quick reference

Make DNS and search-domain policy explicit

General systemd-networkd installations no longer add DHCP search domains unless UseDomains=true is configured. Ubuntu cloud images deliberately restore that behavior in /etc/systemd/networkd.conf.d/50-cloudimg-settings.conf.

Netplan wait-online semantics also evolved: link-local and a routable interface matter, and configured DNS servers must be reachable before an interface is online. Mark interfaces and DNS dependencies deliberately.

Update time synchronization rules

Ubuntu's Chrony pool definitions live in /etc/chrony/sources.d/ubuntu-ntp-pools.sources. Avoid keeping duplicate pool entries in chrony.conf after an upgrade.

Ubuntu NTS uses TCP 4460 for key exchange and UDP 123 for time. Networks that cannot reach both need ordinary NTP sources. Chrony later becomes the seeded default; migrate an existing system with:

apt-mark auto systemd-timesyncd
apt install chrony

Rehome SSH environment configuration

SSH sessions no longer read ~/.pam_environment. Move required values to a supported mechanism such as server AcceptEnv plus client SendEnv. Review post-quantum key exchange defaults and OpenSSH 10 match conditions before using strict allowlists or banner-based compatibility checks.

Cloud and virtualization quick reference

Validate container host compatibility

Ubuntu 24.10 containers require cgroup v2 and an LXD build with systemd-credential support. Older hosts and affected LXD 5.0 installations need the documented kernel, channel, or nesting workaround; very old containers cannot run on that cgroup-v2 host.

Docker inspection consumers must use DNSNames, not Aliases, to find the short container ID. IPv6 iptables is enabled by default for Linux bridge networks.

Preserve virtual-machine compatibility

Before a QEMU upgrade, inventory machine types, serial arguments, 9pfs backends, block migration, compression, host bitness, and confidential-guest firmware. Before a libvirt upgrade, inventory split driver packages, default URI overrides, NVRAM format, migration storage, IOMMU scale, and CPU-model versioning.

Desktop quick reference

Ubuntu Desktop 25.10 is Wayland-only. Its default image viewer and terminal are Loupe and Ptyxis, and Ubuntu Insights replaces Ubuntu Report without carrying forward prior consent. Test extensions against GNOME 50 on Ubuntu Desktop 26.04.

For graphics, distinguish hardware enablement from installation conditions: VA-API may need va-driver-all, supported NVIDIA laptops enable Dynamic Boost only on AC power under load, and offline NVIDIA selection installs Nouveau. GTK4 rendering can regress under VirtualBox or VMware with 3D acceleration.

Working method

1. Identify the exact Ubuntu release, architecture, image type, and upgrade path.
2. Inventory package providers, removed options, custom service configuration, initramfs tooling, network readiness rules, and cloud-init datasource setup.
3. Read every matching topic reference before editing automation or images.
4. Test boot, networking, authentication, storage, and service migrations on a disposable instance that matches the target architecture and image seed.
5. After an upgrade, review snap channels, time sources, package transitions, crash-dump capacity, cloud-init state, and services intentionally left stopped.