feat: implement traversal engine (HAN-380) (#6)

Co-authored-by: Hansen IT Solutions <gary@hansenits.com>
Co-committed-by: Hansen IT Solutions <gary@hansenits.com>

internal/dns/query.go

@@ -125,6 +125,74 @@ func QueryWithExchange(ctx context.Context, server net.IP, name string, qtype ui
 	return nil, fmt.Errorf("query %s %s failed after %d retries: %w", name, QNameType(qtype), cfg.Retries, lastErr)
 }
 
+func IterativeQuery(ctx context.Context, server net.IP, name string, qtype uint16, cfg *QueryConfig) (*dns.Msg, error) {
+	if cfg == nil {
+		cfg = DefaultQueryConfig()
+	}
+	if cfg.UDPSize <= 0 {
+		cfg.UDPSize = DefaultEDNS0UDPSize()
+	}
+	return IterativeQueryWithExchange(ctx, server, name, qtype, cfg, realExchange)
+}
+
+func IterativeQueryWithExchange(ctx context.Context, server net.IP, name string, qtype uint16, cfg *QueryConfig, exchangeFn ExchangeFunc) (*dns.Msg, error) {
+	if cfg == nil {
+		cfg = DefaultQueryConfig()
+	}
+	if cfg.UDPSize <= 0 {
+		cfg.UDPSize = DefaultEDNS0UDPSize()
+	}
+
+	msg := buildQuery(name, qtype, cfg.UDPSize)
+	msg.RecursionDesired = false
+	serverStr := server.String()
+
+	var lastErr error
+
+	for attempt := 0; attempt < cfg.Retries; attempt++ {
+		if attempt > 0 {
+			select {
+			case <-ctx.Done():
+				return nil, fmt.Errorf("query retries cancelled: %w", ctx.Err())
+			case <-time.After(100 * time.Millisecond):
+			}
+		}
+
+		if cfg.UseTCP {
+			resp, err := exchangeFn(ctx, serverStr, msg, true)
+			if err != nil {
+				lastErr = err
+				continue
+			}
+			return resp, nil
+		}
+
+		resp, err := exchangeFn(ctx, serverStr, msg, false)
+		if err != nil {
+			lastErr = err
+			continue
+		}
+
+		if resp == nil {
+			lastErr = fmt.Errorf("nil response")
+			continue
+		}
+
+		if resp.Truncated {
+			resp, err = exchangeFn(ctx, serverStr, msg, true)
+			if err != nil {
+				lastErr = err
+				continue
+			}
+			return resp, nil
+		}
+
+		return resp, nil
+	}
+
+	return nil, fmt.Errorf("iterative query %s %s failed after %d retries: %w", name, QNameType(qtype), cfg.Retries, lastErr)
+}
+
 func buildQuery(name string, qtype uint16, udpSize int) *dns.Msg {
 	m := new(dns.Msg)
 	m.SetQuestion(dns.Fqdn(name), qtype)