Re: [PATCH proxmox] fix #7329: proxmox-network-api: add missing `Auto` network config method

["Shan Shaji" <s.shaji@proxmox.com>]

On Thu Mar 12, 2026 at 3:57 PM CET, Christian Ebner wrote:
> Thanks for the patch, gave this a quick spin but ran into an issue.
>
> If I do have a network config containing e.g.:
> ```
> auto nic1
> iface nic1 inet static
> 	address 172.16.0.18/24
>
> iface nic1 inet6 auto
> ```
> and I go about to edit this NIC via the WebUI, e.g. adding a comment, 
> the suggested diff would drop this, suggesting the following diff:
>
> ``` auto nic1
>   iface nic1 inet static
>   	address 172.16.0.18/24
> -
> -iface nic1 inet6 auto
> +#comment
> ```

Hi Chris, Thank you very much for catching that.

After digging into this a bit more, it looks like this happens in the interface
update handler on the API side. If the inet6 address family's cidr6/gateway6 values are
not set, interface.method6 value is mutated with `NetworkConfigMethod::Manual`:

```
if interface.cidr6.is_some() || interface.gateway6.is_some() {
        interface.method6 = Some(NetworkConfigMethod::Static);
    } else {
        interface.method6 = Some(NetworkConfigMethod::Manual);
}
```

Later, when writing the config back, if the method6 value is `NetworkConfigMethod::Manual`
and there are no IPv6 specific attributes present, the inet6 stanza gets skipped:  

```
fn write_iface(iface: &Interface, w: &mut dyn Write) -> Result<(), Error> {

[...]

    if let Some(method6) = iface.method6 {
        let mut skip_v6 = false; // avoid empty inet6 manual entry
        if iface.method.is_some()
            && method6 == NetworkConfigMethod::Manual
            && iface.comments6.is_none()
            && iface.options6.is_empty()
        {
            skip_v6 = true;
        }

        if !skip_v6 {
            writeln!(w, "iface {} inet6 {}", iface.name, method_to_str(method6))?;
            write_iface_attributes_v6(iface, w, method6)?;
            if iface.method.is_none() {
                // only write common attributes once
                write_iface_attributes(iface, w)?;
            }
            writeln!(w)?;
        }
    }

    Ok(())
}

```

> Can this easily be avoided?

I was able to avoid this by changing the API-side update logic so that
method6 is only set to `NetworkConfigMethod::Manual` if interface.method6 is
not already set. I still want to do a bit more testing to make sure i am
not missing anything.