Don't Break On Duplicate Mac Addresses

This bug was originally filed in Launchpad as [LP: #1996789](https://bugs.launchpad.net/cloud-init/+bug/1996789)

Launchpad details

affected_projects = []

assignee = None

assignee_name = None

date_closed = None

date_created = 2022-11-16T17:15:59.577005+00:00

date_fix_committed = None

date_fix_released = None

id = 1996789

importance = medium

is_complete = False

lp_url = https://bugs.launchpad.net/cloud-init/+bug/1996789

milestone = None

owner = holmanb

owner_name = Brett Holman

private = False

status = triaged

submitter = holmanb

submitter_name = Brett Holman

tags = ['sts']

duplicates = []

_Launchpad user **Brett Holman(holmanb)** wrote on 2022-11-16T17:15:59.577005+00:00_

Currently when duplicate mac addresses are detected, cloud-init dies.

While duplicate macs are typically corner cases, there are cases when they can be valid[1].

Consider this example[2]. After bonding two interfaces, the interfaces were left with duplicate mac addresses. Using cloud-init on this system fails at the time that these devices are detected.

If no network config is given, or if a config is given configuring a single address, we have the opportunity to do something intelligent to allow cloud-init to boot by using the "fallback interface" (in cloud-init this is the first interface), rather than throwing an exception and dying.

Netplan's mac matching assumes 1:1 mapping between mac addresses and interfaces, so in the case of multiple interfaces configured with matches, we still can't do anything intelligent.

[1] Until these have unique addresses, these interfaces will not be usable on the same broadcast domain, but they should still be able to work individually on different networks.
[2] https://stackoverflow.com/questions/74459180/deleted-bond-interface-left-me-with-duplicate-mac-on-two-interfaces