[client] Implement DNS query caching in DNSForwarder

[hakansa]

Describe your changes

[client] Implement DNS query caching in DNSForwarder

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)

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[Copilot]

Pull Request Overview

This PR implements DNS query caching in the DNSForwarder to improve performance and provide fallback responses when upstream DNS servers fail. The cache stores successful DNS query results and serves them when upstream resolution fails.

• Added a new cache component to store DNS query results by domain and query type
• Integrated cache storage after successful DNS resolutions
• Enhanced error handling to serve cached responses as fallback when upstream DNS fails

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
client/internal/dnsfwd/cache.go	New cache implementation with thread-safe operations for storing and retrieving DNS records
client/internal/dnsfwd/forwarder.go	Integrated cache into DNSForwarder and enhanced error handling to use cached responses as fallback

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[lixmal]

• Currently the cache is unbounded and there is no way to manually clear it, can we do something about it?
• NXDOMAIN is not cached, but it should be
• UpdateDomains might remove some domains in which case we should remove those from the cache as well

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

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

See analysis details on SonarQube Cloud

[hakansa]

@lixmal added cache for NXDOMAIN, also added a method to remove stale cache entries on UpdateDomains.

Regarding to the clearing cache, maybe we can expire not used cache items after some time but i think it can be implemented later /cc @mlsmaycon

[solariz]

Thanks for implementing that, can help quite a lot.
Not a go coder but if I may ask some things I did not figured out looking at the code:

• is it only cached for the time netbird is running?
• On long running instances records could potentially get quite big
• so for later improvements cache should respect ttl
• for now it is only implemented as fallback if the initial lookup failed, would be great to have it in future -maybe first optional- to answer from cache first if the record is fresh. But for sure this needs a garbage collection and reliant ttl cleanup

Again thanks for implementing this fallback.