Netplan replaces /etc/network/interfaces in Ubuntu 22.04: handling leftover entries and custom ifupdown scripts after upgrade

Netplan migration on Ubuntu 22.04 LTS

After upgrading a server from Ubuntu 20.04 to 22.04 LTS, the documented default network configuration path is Netplan YAML under /etc/netplan/ rather than /etc/network/interfaces. The goal is to complete this transition cleanly on a host that has both legacy ifupdown entries and a few custom ifupdown hook scripts, without losing connectivity or ending up with duplicate routes.

Constraints and uncertainty

The documentation indicates that legacy interfaces may be ignored when both configuration styles exist, but it is not clear how the system decides precedence during an in-place upgrade, or whether orphaned entries can still produce duplicate routing in some renderer combinations. The setup also includes a bonded interface with a VLAN on top, which may require an explicit renderer choice rather than relying on defaults. The custom ifupdown scripts apparently need to be rewritten as Netplan-compatible actions, but the exact supported mechanism for hooking pre/post events is uncertain.

Questions

• When both /etc/network/interfaces and Netplan YAML exist after a 20.04-to-22.04 upgrade, which configuration actually wins, and how can leftover entries be detected before they cause duplicate routes?
• For a bond plus VLAN setup, must the renderer (networkd vs. alternatives) be declared explicitly, and what breaks if it is left at the default?
• What is the supported replacement for custom ifupdown hook scripts under Netplan, and can netplan try be relied on to roll back safely if the rewritten configuration fails to bring the interface up?