[Bug 2141715] Re: FAN bridges broken inside containers on Noble HWE kernels

It's interesting to note that the `IFLA_VXLAN_FAN_MAP` mismatch is also
the root cause for https://github.com/actions/runner-images/issues/13669

** Bug watch added: github.com/actions/runner-images/issues #13669
https://github.com/actions/runner-images/issues/13669

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https://bugs.launchpad.net/bugs/2141715

Title:
FAN bridges broken inside containers on Noble HWE kernels

Status in linux package in Ubuntu:
New

Bug description:
It seems the `sysctl` used when creating FAN bridges are not visible
inside containers. This bug might be related to
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2141703.

In the 24.04 host:

```
# uname -a
Linux v4 6.14.0-1017-azure #17~24.04.1-Ubuntu SMP Mon Dec 1 20:10:50 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux

# lxc network create lxdfan0 bridge.mode=fan
Network lxdfan0 created

# lxc network delete lxdfan0
Network lxdfan0 deleted
# sysctl net.fan.IFLA_VXLAN_FAN_MAP net.fan.vxlan
net.fan.IFLA_VXLAN_FAN_MAP = 34
net.fan.vxlan = 4
```

In a 24.04 container on that host:

```
# lxc exec m1 -- lxc network create lxdfan0 bridge.mode=fan
Error: Failed starting network: Failed adding link: Failed to run: ip link add name lxdfan0-fan type vxlan id 15728640 dev eth0 local 10.176.231.58 dstport 0 fan-map 240.0.0.0/8:10.176.231.0/24: exit status 2 (info: Using default fan map value (33)
Error: binary attribute size out of range.)


# lxc exec m1 -- sysctl net.fan.IFLA_VXLAN_FAN_MAP net.fan.vxlan
sysctl: cannot stat /proc/sys/net/fan/IFLA_VXLAN_FAN_MAP: No such file or directory
sysctl: cannot stat /proc/sys/net/fan/vxlan: No such file or directory
```

So IFLA_VXLAN_FAN_MAP seems to have the following values based on the
kernel version:

* 6.8: 33
* 6.11: 33
* 6.14: 34
* 6.17: 35

It looks as if when that sysctl key is not readable, `iproute2`
assumes the value of 33 causing this problem.

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