Summary

This PR exposes k6's internal DNS resolver to extensions, enabling them to perform DNS lookups that respect the user's k6 configuration including hosts overrides, custom DNS servers, and DNS caching.

Fixes #5161

Changes

1. Added ResolveAddr method to Dialer (lib/netext/dialer.go)

• New public method ResolveAddr(addr string) (net.IP, int, error) that resolves hostnames to IP addresses
• Accepts addresses in both "host:port" and "host" formats (port is optional)
• Returns the resolved IP address, port (0 if not specified), and any error
• Respects all k6 DNS configurations:
  • hosts overrides from options.hosts
  • Custom DNS servers from options.dns
  • IP blacklists
  • DNS caching with TTL support

2. Refactored getDialAddr internal method

• Changed return type from string to *types.Host for better type safety and to avoid redundant string conversions
• Updated all call sites to use .String() when the string representation is needed

3. Added AddrResolver interface (lib/vu_state.go)

• Defines the public API contract for DNS resolution: ResolveAddr(addr string) (net.IP, error)
• Allows extensions to type-assert the Dialer to access DNS resolution functionality

4. Added GetAddrResolver method to State

• Provides safe, convenient access to the DNS resolver from the VU state
• Returns nil if the Dialer doesn't implement AddrResolver (safe type assertion with comma-ok idiom)
• Prevents potential panics when checking for resolver support

Testing

Added comprehensive test coverage:

• TestDialerResolveAddr: 17 test cases covering:

  • IPv4 and IPv6 resolution with and without ports
  • Direct IP addresses (bypass DNS)
  • Host mappings from options.hosts
  • Custom port mappings in hosts
  • IP blacklist enforcement
  • Non-existent hosts (error handling)

• TestGetAddrResolver: 2 test cases verifying:

  • Returns the same Dialer instance when it implements AddrResolver
  • Returns nil when Dialer doesn't implement AddrResolver

All tests pass with the existing test suite.

Usage Example

Extensions can now access the DNS resolver like this:

// In your extension code where you have access to lib.State
if resolver := state.GetAddrResolver(); resolver != nil {
    ip, err := resolver.ResolveAddr("example.com:443")
    if err != nil {
        // Handle error
    }
    // Use resolved IP...
}

Breaking Changes

None. This is a purely additive change that exposes existing functionality.

Documentation

• Inline documentation added for new public APIs
• Method signatures clearly document expected input/output formats