feat: implement DNS server fingerprinting (HAN-384)

- Add internal/fingerprint package with Fingerprinter type
  - Sends version.bind CHAOS TXT query to each server
  - Caches results per IP to avoid redundant queries
  - Concurrent batch fingerprinting via FingerprintAll
  - Gracefully handles non-responding servers (returns empty string)
  - Injectable exchange function for testability

- Wire fingerprinting into RunTraversal
  - Triggered when both ShowVersions and ShowServers are enabled
  - Collects unique server IPs from traversal results
  - Stores results in output.Config.Fingerprints before WriteSummary

- Display versions in text output (writeServers)
  - Appends version string after IP when ShowVersions is true
  - No output change when version is unknown

- Include version in JSON output (jsonServer.Version field)
  - omitempty: field absent when version is unknown

- Add comprehensive fingerprint tests (11 test cases)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: multica-agent <github@multica.ai>

internal/output/formatter.go

@@ -30,6 +30,10 @@ type Config struct {
 	Quiet              bool
 	Color              bool
 	Debug              int
+
+	// Fingerprints maps server IP strings to their version.bind version strings.
+	// Populated by RunTraversal when ShowVersions and ShowServers are both true.
+	Fingerprints map[string]string
 }
 
 func DefaultConfig() *Config {

internal/output/json.go

@@ -44,8 +44,9 @@ type jsonResult struct {
 }
 
 type jsonServer struct {
-	Name string   `json:"name"`
-	IPs  []string `json:"ips"`
+	Name    string   `json:"name"`
+	IPs     []string `json:"ips"`
+	Version string   `json:"version,omitempty"`
 }
 
 type jsonSummary struct {
@@ -104,10 +105,16 @@ func (f *jsonFormatter) WriteSummary(results []traverse.TraversalResult) error {
 	if f.cfg.ShowServers {
 		servers := collectServers(results)
 		for name, ips := range servers {
-			f.payload.Servers = append(f.payload.Servers, jsonServer{
-				Name: name,
-				IPs:  ips,
-			})
+			srv := jsonServer{Name: name, IPs: ips}
+			if f.cfg.ShowVersions && f.cfg.Fingerprints != nil {
+				for _, ip := range ips {
+					if v := f.cfg.Fingerprints[ip]; v != "" {
+						srv.Version = v
+						break
+					}
+				}
+			}
+			f.payload.Servers = append(f.payload.Servers, srv)
 		}
 	}
 

internal/output/runner.go

@@ -3,7 +3,9 @@ package output
 import (
 	"context"
 	"fmt"
+	"net"
 
+	"github.com/hits/ExploreDNS/internal/fingerprint"
 	"github.com/hits/ExploreDNS/internal/traverse"
 )
 
@@ -25,6 +27,13 @@ func RunTraversal(ctx context.Context, traverser *traverse.Traverser, cfg *Confi
 		return results, err
 	}
 
+	// Fingerprint servers when both ShowVersions and ShowServers are enabled.
+	// Gating on ShowServers avoids unnecessary network calls when versions
+	// would not be displayed anyway.
+	if cfg.ShowVersions && cfg.ShowServers {
+		cfg.Fingerprints = fingerprint.New().FingerprintAll(ctx, collectUniqueServerIPs(results))
+	}
+
 	if err := formatter.WriteSummary(results); err != nil {
 		return results, err
 	}
@@ -34,3 +43,20 @@ func RunTraversal(ctx context.Context, traverser *traverse.Traverser, cfg *Confi
 
 	return results, nil
 }
+
+// collectUniqueServerIPs returns the set of unique server IPs seen in results.
+func collectUniqueServerIPs(results []traverse.TraversalResult) []net.IP {
+	seen := make(map[string]bool)
+	var ips []net.IP
+	for _, r := range results {
+		if r.Response == nil || r.Response.Server == nil {
+			continue
+		}
+		key := r.Response.Server.String()
+		if !seen[key] {
+			seen[key] = true
+			ips = append(ips, r.Response.Server)
+		}
+	}
+	return ips
+}

internal/output/text.go

@@ -99,7 +99,13 @@ func (f *textFormatter) writeServers(results []traverse.TraversalResult) error {
 
 	for _, name := range names {
 		for _, ip := range servers[name] {
-			if _, err := fmt.Fprintf(f.w, "%*s: %-15s\n", width, name, ip); err != nil {
+			line := fmt.Sprintf("%*s: %-15s", width, name, ip)
+			if f.cfg.ShowVersions {
+				if version, ok := f.cfg.Fingerprints[ip]; ok && version != "" {
+					line += "  " + version
+				}
+			}
+			if _, err := fmt.Fprintln(f.w, line); err != nil {
 				return err
 			}
 		}