Description of problem:

Launching a CentOS Stream/RHEL machine in AWS into an IPv6-Only subnet results in an unusable instance due to missing ipv4 routes.

Support was added in cloud-init by this commit: https://github.com/canonical/cloud-init/pull/1160/

I rebased the patch against EL8 here: https://git.rockylinux.org/sig/cloud/patch/cloud-init/-/blob/r8/ROCKY/_supporting/9998-Add-Ec2-IPV6-IMDS.patch

There were three total chunks which failed to apply cleanly; two of them were in tests and trivial to fix. One patch for cloudinit/url_helper.py needed to be rebased slightly to match the EL8 source.

Version-Release number of selected component (if applicable): 22.1-5.el8

How reproducible:
Always

Steps to Reproduce:

1) Create a VPC with and IPv6 CIDR block (using either your own or Amazon's IPv6 address space)
2) Create an IPv6 only subnet by creating a new subnet and checking the "IPv6 Only" box
3) Create a Rocky Linux instance and associate it with the IPv6 capable VPC and the IPv6-only subnet.
4) After approximately 10 minutes, the instance will complete the boot process, but will have "1/2 checks passed" in the "Status Check" column, and "Instance reachability check failed" in the "Status Check" tab of the instance details section.
5) The box will not be connected to the network

Actual results:

System reports the following during boot, and is unreachable once cloud-init times out due the failure.

[ 12.865186] cloud-init[899]: 2022-09-12 20:05:50,230 - url_helper.py[WARNING]: Calling 'http://169.254.169.254/latest/api/token' failed [0/120s]: request error [HTTPConnectionPool(host='169.254.169.254', port=80): Max retries exceeded with url: /latest/api/token (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7fe4404f8128>: Failed to establish a new connection: [Errno 101] Network is unreachable',))]
[ 13.648505] cloud-init[899]: 2022-09-12 20:05:51,234 - url_helper.py[WARNING]: Calling 'http://169.254.169.254/latest/api/token' failed [1/120s]: request error [HTTPConnectionPool(host='169.254.169.254', port=80): Max retries exceeded with url: /latest/api/token (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7fe4404f8a20>: Failed to establish a new connection: [Errno 101] Network is unreachable',))]

These messages repeat many times before the final error messages:

[ 131.835862] cloud-init[899]: 2022-09-12 20:07:49,433 - url_helper.py[WARNING]: Calling 'http://169.254.169.254/latest/api/token' failed [119/120s]: unexpected error [Attempted to set connect timeout to 0.0, but the timeout cannot be set to a value less than or equal to 0.]
[ 138.843417] cloud-init[899]: 2022-09-12 20:07:56,440 - DataSourceEc2.py[WARNING]: IMDS's HTTP endpoint is probably disabled

Expected results:

Cloud-Init connects successfully to the EC2 metadata service and

Additional info:

Originally filed at https://bugs.rockylinux.org/view.php?id=279 - Verified on Rocky, Alma, CentOS Stream, and RHEL AMIs.