[client] Always register NetBird with plain Linux DNS and use original servers as upstream

[lixmal]

Describe your changes

• Removes the restriction that only all-domains resolvers can be registered on Linux with resolvconf and file managers
• Registers NetBird as the sole nameserver
• Registers original nameservers as upstream with low priority: NetBird-assigned "all domains" resolvers can take precedence
• Replace DNS-related server IP strings with netip.Addr
• Use resolvconf as a fallback instead of openresolv
• Return RcodeRefused if no handler was found in the chain

Issue ticket number and link

Stack

Checklist

• Is it a bug fix
• Is a typo/documentation fix
• Is a feature enhancement
• It is a refactor
• Created tests that fail without the change (if possible)
• Extended the README / documentation, if necessary

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[Copilot]

Pull Request Overview

This PR updates the DNS configuration logic to register NetBird as the sole nameserver while passing the original nameservers as low-priority upstream handlers.

• Removes the resolvconf restriction to allow non-all-domain resolvers on Linux
• Adds fallback support to register original nameservers with a lower priority
• Refactors error handling and prioritization logic in the DNS server and file configurator

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
client/internal/dns/server.go	Added fallback registration, refactored error handling, and updated priority overlap handling
client/internal/dns/resolvconf_unix.go	Added getter for original nameservers and simplified DNS config application
client/internal/dns/host.go	Removed unused error constant
client/internal/dns/handler_chain.go	Introduced PriorityFallback constant with a new negative priority value
client/internal/dns/file_unix.go	Updated backup and restoration process, added retrieval of original nameservers
client/internal/dns/file_parser_unix.go	Removed options timeout generation logic

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
1 Accepted issue

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud