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

internal/config/config.go

@@ -28,15 +28,18 @@ type Config struct {
 	AllRootServers bool
 	RootAAAA       bool
 	FollowAAAA     bool
-	UDPSize        int
-	AllowTCP       bool
-	AlwaysTCP      bool
-	MaxDepth       int
-	Retries        int
-	Fast           bool
-	Verbose        bool
-	Debug          int
-	Quiet          bool
+	// DNSUpstream is the upstream resolver used for root server discovery.
+	// Format: "host:port" (e.g. "8.8.8.8:53"). Empty means use the system resolver.
+	DNSUpstream string
+	UDPSize     int
+	AllowTCP    bool
+	AlwaysTCP   bool
+	MaxDepth    int
+	Retries     int
+	Fast        bool
+	Verbose     bool
+	Debug       int
+	Quiet       bool
 
 	ShowProgress       bool
 	ShowResolves       bool
@@ -127,6 +130,12 @@ func (c *Config) Validate() error {
 		return ErrAlwaysTCPRequiresTCP
 	}
 
+	if c.DNSUpstream != "" {
+		if _, _, err := net.SplitHostPort(c.DNSUpstream); err != nil {
+			return fmt.Errorf("--dns-upstream %q is not a valid host:port address", c.DNSUpstream)
+		}
+	}
+
 	return nil
 }
 
@@ -173,6 +182,7 @@ func PrintUsage() {
 			{"--all-root-servers", "Use all 13 root servers"},
 			{"--root-aaaa", "Include IPv6 root addresses"},
 			{"--follow-aaaa", "Only follow AAAA for referrals"},
+			{"--dns-upstream", "Upstream resolver for root discovery (default: system)"},
 		},
 		"Transport Options": {
 			{"--udp-size", "EDNS0 buffer size (default 2048)"},
@@ -220,6 +230,7 @@ func DefaultConfig() *Config {
 		AllRootServers:     false,
 		RootAAAA:           false,
 		FollowAAAA:         false,
+		DNSUpstream:        "",
 		UDPSize:            2048,
 		AllowTCP:           true,
 		AlwaysTCP:          false,