feat: DNS host selection - fix code review issues (HAN-400) (#19)

internal/dns/roots.go

@@ -26,7 +26,13 @@ func (rs *RootServer) AllIPs(includeAAAA bool) []net.IP {
 }
 
 type RootDiscoveryConfig struct {
-	Server      string
+	// Server overrides which root server to use as the traversal starting point.
+	// When empty, a root server is discovered via the upstream resolver.
+	Server string
+	// Resolver is the upstream DNS resolver used to resolve root server names.
+	// When empty, the system resolver from /etc/resolv.conf is used.
+	// Format: "host:port" (e.g. "8.8.8.8:53" or "1.1.1.1:53").
+	Resolver    string
 	AllRoots    bool
 	IncludeAAAA bool
 }
@@ -43,20 +49,44 @@ func DiscoverRoots(ctx context.Context, cfg *RootDiscoveryConfig) ([]RootServer,
 		cfg = DefaultRootDiscoveryConfig()
 	}
 
+	resolver := resolverFromConfig(cfg)
+
 	if cfg.Server != "" {
-		return discoverRootOverride(ctx, cfg.Server, cfg.IncludeAAAA)
+		return discoverRootOverride(ctx, resolver, cfg.Server, cfg.IncludeAAAA)
 	}
 
 	if cfg.AllRoots {
-		return discoverAllRoots(ctx, cfg.IncludeAAAA)
+		servers, err := discoverAllRoots(ctx, resolver, cfg.IncludeAAAA)
+		if err != nil {
+			return filterHints(RootHints, cfg.IncludeAAAA), nil
+		}
+		return servers, nil
 	}
 
-	return discoverSingleRoot(ctx, cfg.IncludeAAAA)
+	servers, err := discoverSingleRoot(ctx, resolver, cfg.IncludeAAAA)
+	if err != nil {
+		hints := filterHints(RootHints, cfg.IncludeAAAA)
+		if len(hints) > 0 {
+			return hints[:1], nil
+		}
+		return nil, err
+	}
+	return servers, nil
 }
 
-func discoverRootOverride(ctx context.Context, server string, includeAAAA bool) ([]RootServer, error) {
-	resolver := systemResolver()
+// filterHints returns a copy of hints with IPv6 addresses stripped when includeAAAA is false.
+func filterHints(hints []RootServer, includeAAAA bool) []RootServer {
+	out := make([]RootServer, len(hints))
+	for i, h := range hints {
+		out[i] = RootServer{Name: h.Name, IPv4: h.IPv4}
+		if includeAAAA {
+			out[i].IPv6 = h.IPv6
+		}
+	}
+	return out
+}
 
+func discoverRootOverride(ctx context.Context, resolver, server string, includeAAAA bool) ([]RootServer, error) {
 	nsMsg, err := queryResolver(ctx, resolver, ".", dns.TypeNS)
 	if err != nil {
 		return nil, fmt.Errorf("query root NS records: %w", err)
@@ -70,16 +100,14 @@ func discoverRootOverride(ctx context.Context, server string, includeAAAA bool)
 	normalized := normalizeServerName(server)
 	for _, name := range nsSet {
 		if normalizeServerName(name) == normalized {
-			return resolveRootServer(ctx, name, includeAAAA)
+			return resolveRootServer(ctx, resolver, name, includeAAAA)
 		}
 	}
 
-	return resolveRootServer(ctx, server, includeAAAA)
+	return resolveRootServer(ctx, resolver, server, includeAAAA)
 }
 
-func discoverSingleRoot(ctx context.Context, includeAAAA bool) ([]RootServer, error) {
-	resolver := systemResolver()
-
+func discoverSingleRoot(ctx context.Context, resolver string, includeAAAA bool) ([]RootServer, error) {
 	nsMsg, err := queryResolver(ctx, resolver, ".", dns.TypeNS)
 	if err != nil {
 		return nil, fmt.Errorf("query root NS records: %w", err)
@@ -95,12 +123,10 @@ func discoverSingleRoot(ctx context.Context, includeAAAA bool) ([]RootServer, er
 	}
 
 	pick := nsSet[0]
-	return resolveRootServer(ctx, pick, includeAAAA)
+	return resolveRootServer(ctx, resolver, pick, includeAAAA)
 }
 
-func discoverAllRoots(ctx context.Context, includeAAAA bool) ([]RootServer, error) {
-	resolver := systemResolver()
-
+func discoverAllRoots(ctx context.Context, resolver string, includeAAAA bool) ([]RootServer, error) {
 	nsMsg, err := queryResolver(ctx, resolver, ".", dns.TypeNS)
 	if err != nil {
 		return nil, fmt.Errorf("query root NS records: %w", err)
@@ -117,7 +143,7 @@ func discoverAllRoots(ctx context.Context, includeAAAA bool) ([]RootServer, erro
 
 	var servers []RootServer
 	for _, name := range nsSet {
-		resolved, err := resolveRootServer(ctx, name, includeAAAA)
+		resolved, err := resolveRootServer(ctx, resolver, name, includeAAAA)
 		if err != nil {
 			servers = append(servers, RootServer{Name: name})
 			continue
@@ -132,9 +158,7 @@ func discoverAllRoots(ctx context.Context, includeAAAA bool) ([]RootServer, erro
 	return servers, nil
 }
 
-func resolveRootServer(ctx context.Context, name string, includeAAAA bool) ([]RootServer, error) {
-	resolver := systemResolver()
-
+func resolveRootServer(ctx context.Context, resolver, name string, includeAAAA bool) ([]RootServer, error) {
 	var ipv4 []net.IP
 	aMsg, err := queryResolver(ctx, resolver, name, dns.TypeA)
 	if err == nil {
@@ -156,15 +180,32 @@ func resolveRootServer(ctx context.Context, name string, includeAAAA bool) ([]Ro
 	return []RootServer{{Name: name, IPv4: ipv4, IPv6: ipv6}}, nil
 }
 
+// resolverFromConfig returns the upstream DNS resolver address to use.
+// If cfg.Resolver is set, it is used directly. Otherwise the system resolver
+// is read from /etc/resolv.conf. Falls back to 127.0.0.1:53 if neither is available.
+func resolverFromConfig(cfg *RootDiscoveryConfig) string {
+	if cfg != nil && cfg.Resolver != "" {
+		return cfg.Resolver
+	}
+	return systemResolver()
+}
+
+// systemResolver returns the first nameserver from the system DNS configuration.
+// This is Unix-only: it reads /etc/resolv.conf, which does not exist on Windows.
+// On Windows (or any system without /etc/resolv.conf) the fallback 127.0.0.1:53 applies.
 func systemResolver() string {
-	return "127.0.0.1:53"
+	cc, err := dns.ClientConfigFromFile("/etc/resolv.conf")
+	if err != nil || len(cc.Servers) == 0 {
+		return "127.0.0.1:53"
+	}
+	return net.JoinHostPort(cc.Servers[0], cc.Port)
 }
 
 func queryResolver(ctx context.Context, resolverAddr, name string, qtype uint16) (*dns.Msg, error) {
 	c := &dns.Client{
 		Net:          "udp",
-		ReadTimeout:  5,
-		WriteTimeout: 5,
+		ReadTimeout:  5 * time.Second,
+		WriteTimeout: 5 * time.Second,
 	}
 	if deadline, ok := ctx.Deadline(); ok {
 		c.ReadTimeout = time.Until(deadline)