IPv6-enabled EC2 subnets always have dhcp enabled

[ubuntu-server-builder]

This bug was originally filed in Launchpad as LP: #1976526

Launchpad details

affected_projects = ['cloud-init (Fedora)']
assignee = None
assignee_name = None
date_closed = None
date_created = 2022-06-01T15:26:26.024707+00:00
date_fix_committed = None
date_fix_released = None
id = 1976526
importance = high
is_complete = False
lp_url = https://bugs.launchpad.net/cloud-init/+bug/1976526
milestone = None
owner = rackerhacker
owner_name = Major Hayden
private = False
status = triaged
submitter = rackerhacker
submitter_name = Major Hayden
tags = []
duplicates = []

Launchpad user Major Hayden(rackerhacker) wrote on 2022-06-01T15:26:26.024707+00:00

A Fedora user opened an issue[0] about an issue with cloud-init on EC2. When they launch an instance on a IPv6-enabled subnet, cloud-init sets the ipv6.method to dhcp in NetworkManager.

However, AWS uses router advertisements to let instances know about the nearest router. Using the ipv6.method: dhcp setting prevents router advertisements from working. The instance ends up with an IPv6 address assigned, but it cannot route traffic.

A workaround is to run NetworkManager commands to fix the issue:

nmcli con edit CONNECTION_UUID
nmcli> set ipv6.method auto 
nmcli> save
nmcli> activate

The instance immediately routes IPv6 traffic after making that change. The dhcp setting appears to be applied automatically for EC2 subnets with IPv6 addresses assigned[1].

I would expect ipv6.method to be auto in these situations.

Thank you!

[0] https://pagure.io/cloud-sig/issue/382
[1]

cloud-init/cloudinit/sources/DataSourceEc2.py

Lines 861 to 863 in 53a995e

	nic_metadata = macs_metadata.get(mac)
	if nic_metadata.get("ipv6s"): # Any IPv6 addresses configured
	dev_config["dhcp6"] = True

[ubuntu-server-builder]

Launchpad user mhayden(mhayden-redhat-bugs) wrote on 2022-06-01T15:28:42+00:00

A Fedora user opened an issue[0] about an issue with cloud-init on EC2. When they launch an instance on a IPv6-enabled subnet, cloud-init sets the ipv6.method to dhcp in NetworkManager.

However, AWS uses router advertisements to let instances know about the nearest router. Using the ipv6.method: dhcp setting prevents router advertisements from working. The instance ends up with an IPv6 address assigned, but it cannot route traffic.

A workaround is to run NetworkManager commands to fix the issue:

nmcli con edit CONNECTION_UUID
nmcli> set ipv6.method auto
nmcli> save
nmcli> activate

The instance immediately routes IPv6 traffic after making that change. The dhcp setting appears to be applied automatically for EC2 subnets with IPv6 addresses assigned[1].

I would expect ipv6.method to be auto in these situations.

Thank you!

[0] https://pagure.io/cloud-sig/issue/382
[1]

cloud-init/cloudinit/sources/DataSourceEc2.py

Lines 861 to 863 in 53a995e

	nic_metadata = macs_metadata.get(mac)
	if nic_metadata.get("ipv6s"): # Any IPv6 addresses configured
	dev_config["dhcp6"] = True

[aciba90]

On systems without NetworkManager (directly used, or as a Netplan backend), the switch to dhcp6=false may be considered a change of behavior.

As shown in #5131, Ubuntu systems upgrading cloud-init to 24.1.x in bionic, focal, jammy and mantic.

Do we want to patch this fix out for b, f, j and m? @holmanb.