Recon network batch 2: fleet relay scan and UDP hints.

POST /api/v1/recon/relay-scan probes from the dashboard when reachable, otherwise dispatches recon_relay_scan to a same-/24 online agent. Agents reuse probePortsFn for TCP and optionally UDP 53/51820 with dns/wireguard Path Tracer tags.

agent/deploy/relay_scan.go

@@ -0,0 +1,94 @@
+package deploy
+
+import (
+	"encoding/json"
+	"net"
+	"strconv"
+	"strings"
+	"time"
+)
+
+var RelayScanUDPPorts = []int{53, 51820}
+
+type RelayScanResult struct {
+	Host      string         `json:"host"`
+	OpenPorts []int          `json:"open_ports"`
+	UDPHints  []RelayUDPHint `json:"udp_hints,omitempty"`
+}
+
+type RelayUDPHint struct {
+	Port    int    `json:"port"`
+	Open    bool   `json:"open"`
+	Service string `json:"service"`
+}
+
+var udpGuessPortsFn func(host string, ports []int) []RelayUDPHint
+
+func SetRelayUDPGuessHook(fn func(host string, ports []int) []RelayUDPHint) { udpGuessPortsFn = fn }
+
+func RunRelayScan(host string, udpGuess bool) RelayScanResult {
+	host = strings.TrimSpace(host)
+	open := probePorts(host, SubnetReconPorts)
+	result := RelayScanResult{Host: host, OpenPorts: append([]int(nil), open...)}
+	if udpGuess {
+		result.UDPHints = guessRelayUDPPorts(host, RelayScanUDPPorts)
+	}
+	return result
+}
+
+func RunRelayScanJSON(host string, udpGuess bool) (string, error) {
+	b, err := json.Marshal(RunRelayScan(host, udpGuess))
+	if err != nil {
+		return "", err
+	}
+	return string(b), nil
+}
+
+func guessRelayUDPPorts(host string, ports []int) []RelayUDPHint {
+	if udpGuessPortsFn != nil {
+		return udpGuessPortsFn(host, ports)
+	}
+	var out []RelayUDPHint
+	for _, port := range ports {
+		out = append(out, RelayUDPHint{Port: port, Open: probeRelayUDPQuick(host, port), Service: relayUDPServiceLabel(port)})
+	}
+	return out
+}
+
+func relayUDPServiceLabel(port int) string {
+	switch port {
+	case 53:
+		return "dns"
+	case 51820:
+		return "wireguard"
+	default:
+		return ""
+	}
+}
+
+func probeRelayUDPQuick(host string, port int) bool {
+	conn, err := net.DialTimeout("udp", net.JoinHostPort(host, strconv.Itoa(port)), 800*time.Millisecond)
+	if err != nil {
+		return false
+	}
+	defer conn.Close()
+	_ = conn.SetDeadline(time.Now().Add(800 * time.Millisecond))
+	if b := relayUDPProbePayload(port); len(b) > 0 {
+		_, _ = conn.Write(b)
+	}
+	buf := make([]byte, 512)
+	n, err := conn.Read(buf)
+	return err == nil && n > 0
+}
+
+func relayUDPProbePayload(port int) []byte {
+	switch port {
+	case 53:
+		return []byte{0x00, 0x01, 0x01, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			0x07, 'v', 'e', 'r', 's', 'i', 'o', 'n', 0x04, 'b', 'i', 'n', 'd', 0x00, 0x00, 0x10, 0x00, 0x03}
+	case 51820:
+		return []byte{0x01, 0x00, 0x00, 0x00}
+	default:
+		return nil
+	}
+}