lxc-user-nic: support detach-netns

[AkihiroSuda]

(Follow-up to #379)

For lxc-user-nic we have to run a dummy process with the detached netns, as lxc-user-nic does not support creating a veth under a custom netns path.

https://man7.org/linux/man-pages/man1/lxc-user-nic.1.html

  path to network namespace
         A path to open to get a file descriptor for the target
         network namespace.  This is only relevant when an veth
         device is deleted.

[fahedouch]

Hi @AkihiroSuda, I am looking on this ticket and trying to resolve it. as you mentioned, the idea is to run a dummy process using exec.Command with the mounted network ns

cmd := exec.Command("nsenter", "-n", "/path/to/mount/ns", "sleep", "infinity")

but then I can't get the PID of the dummy process to pass it to lxc-user-nic binary.

Could you help please :) ?

[AkihiroSuda]

nsenter --no-fork may work for you?