- Preserve existing spacing and inline comments when rewriting `/etc/nsswitch.conf` `hosts:` lines.
- Preserve indentation, comment prefix, inline comments, and field separators when rewriting `/etc/hosts` rows.
- Add tests for the `nsswitch.conf` spacing/comment behavior.
ⓘ Treat Linux active virtual interfaces as secondary IP addresses instead of independent links, fixing alias parsing, hiding invalid status controls, rejecting down-state creation, and removing existing aliases with ip addr del when needed.
Reproduce path:
Example repro before this fix:
1. Go to **Network Configuration → Network Interfaces → Active Now**.
2. Click **Add a new interface**.
3. Enter:
```text
Name: enp0s5:1
IPv4 address: 10.211.55.21
Netmask: 255.255.255.0
Status: Down
```
4. Click **Create**.
Before the fix, Webmin could still create the alias or handle it inconsistently, because `enp0s5:1` is not a real link that can be “down”. It is just an extra IP address on `enp0s5`.
Expected after the fix:
- The UI should not offer `Status` for active virtual aliases.
- If someone submits `up=0` manually anyway, Webmin rejects it with:
`Virtual interfaces cannot be created with down status`
- If an existing active virtual alias is saved as down through lower-level code, Webmin removes the IP using something like:
```bash
ip addr del 10.211.55.21/24 dev enp0s5
```
ⓘ Prefer Netplan when Debian has Netplan YAML config, otherwise select the existing NetworkManager backend for Debian systems with saved NM connection profiles, with regression tests for backend selection.
https://github.com/webmin/webmin/issues/2559
* Note: Preserve existing Netplan indentation when updating DNS settings, validate with netplan generate before applying, and surface apply errors to avoid network loss from malformed YAML.
