station/hostapd: Resolve station IPs via udhcpd leases and ARP fallback

The hostapd backend never populated IPs: NewDHCPCorrelator was defined
but never instantiated, and even when it was, the parser only handled
ISC dhcpd's text format. On a BusyBox-based router using udhcpd, every
device showed up with an empty IP.

Two fixes:

- Add a udhcpd binary lease parser. The format is documented in
  busybox/networking/udhcp/dhcpd.{h,c}: an 8-byte big-endian unix-time
  header followed by 36-byte dyn_lease records (expires, IP, MAC,
  20-byte hostname, 2-byte pad). ParseLeases auto-detects the format
  by inspecting the header so the same code path handles both udhcpd
  and ISC text leases.

- Wire the DHCPCorrelator into Backend.Initialize and have it merge
  two sources: ARP first (universal IP fallback for any station that
  has been talked to) and DHCP leases on top (authoritative, carries
  the hostname). ARP fills the gap when leases are missing or the
  station uses a static IP; DHCP wins on conflict.

Default DHCPLeasesPath updated to /var/lib/udhcpd/udhcpd.leases — the
common BusyBox path. Configurable as before.

internal/station/hostapd/correlation.go

@@ -1,32 +1,38 @@
 package hostapd
 
 import (
-	"bufio"
 	"log"
-	"os"
 	"strings"
 	"time"
 
-	"github.com/nemunaire/repeater/internal/models"
+	"github.com/nemunaire/repeater/internal/station/arp"
+	"github.com/nemunaire/repeater/internal/station/dhcp"
 )
 
-// DHCPCorrelator helps correlate hostapd stations with DHCP leases to discover IP addresses
+// DHCPCorrelator helps correlate hostapd stations with DHCP leases (and the
+// ARP table as a fallback) so the UI gets an IP and hostname for every
+// station, not just a MAC. Hostapd itself only knows MACs.
 type DHCPCorrelator struct {
 	backend        *Backend
 	dhcpLeasesPath string
+	arpTablePath   string
 	stopChan       chan struct{}
 	running        bool
 }
 
 // NewDHCPCorrelator creates a new DHCP correlator
-func NewDHCPCorrelator(backend *Backend, dhcpLeasesPath string) *DHCPCorrelator {
+func NewDHCPCorrelator(backend *Backend, dhcpLeasesPath, arpTablePath string) *DHCPCorrelator {
 	if dhcpLeasesPath == "" {
-		dhcpLeasesPath = "/var/lib/dhcp/dhcpd.leases"
+		dhcpLeasesPath = "/var/lib/misc/udhcpd.leases"
+	}
+	if arpTablePath == "" {
+		arpTablePath = "/proc/net/arp"
 	}
 
 	return &DHCPCorrelator{
 		backend:        backend,
 		dhcpLeasesPath: dhcpLeasesPath,
+		arpTablePath:   arpTablePath,
 		stopChan:       make(chan struct{}),
 	}
 }
@@ -39,7 +45,7 @@ func (dc *DHCPCorrelator) Start() {
 
 	dc.running = true
 	go dc.correlationLoop()
-	log.Printf("DHCP correlation started for hostapd backend")
+	log.Printf("DHCP/ARP correlation started (leases=%s, arp=%s)", dc.dhcpLeasesPath, dc.arpTablePath)
 }
 
 // Stop stops the correlation loop
@@ -72,65 +78,54 @@ func (dc *DHCPCorrelator) correlationLoop() {
 	}
 }
 
-// correlate performs one correlation cycle
+// correlate performs one correlation cycle. We pull from two sources:
+//
+//   - The DHCP lease file (udhcpd binary or ISC text — auto-detected) is the
+//     authoritative source for hostnames and the IP that was actually
+//     assigned.
+//   - The ARP table is a universal IP fallback. If the lease file is missing
+//     or the station uses a static IP, ARP still gives us its current IPv4.
+//
+// The two are merged: ARP fills gaps, DHCP wins on conflict because it
+// carries the hostname and is less prone to stale entries.
 func (dc *DHCPCorrelator) correlate() {
-	// Parse DHCP leases
-	leases, err := parseDHCPLeases(dc.dhcpLeasesPath)
-	if err != nil {
-		log.Printf("Warning: Failed to parse DHCP leases: %v", err)
-		return
-	}
-
-	// Build MAC -> IP and MAC -> hostname mappings. Hostnames live in the
-	// dhcpd "client-hostname" field and let us label devices instead of
-	// falling back to bare MAC addresses.
 	macToIP := make(map[string]string)
 	macToHostname := make(map[string]string)
-	for _, lease := range leases {
-		mac := strings.ToLower(lease.MAC)
-		macToIP[mac] = lease.IP
-		if lease.Hostname != "" {
-			macToHostname[mac] = lease.Hostname
+
+	// ARP first, so DHCP overrides on conflict.
+	if entries, err := arp.ParseTable(dc.arpTablePath); err == nil {
+		for _, entry := range entries {
+			// Flags 2 (COMPLETE) and 6 (COMPLETE|PERM) — incomplete
+			// entries have a zero MAC and would pollute the mapping.
+			if entry.Flags != 2 && entry.Flags != 6 {
+				continue
+			}
+			mac := strings.ToLower(entry.HWAddress.String())
+			if mac == "" || mac == "00:00:00:00:00:00" {
+				continue
+			}
+			macToIP[mac] = entry.IP.String()
 		}
+	} else {
+		log.Printf("ARP table read failed (%s): %v", dc.arpTablePath, err)
+	}
+
+	if leases, err := dhcp.ParseLeases(dc.dhcpLeasesPath); err == nil {
+		for _, lease := range leases {
+			mac := strings.ToLower(lease.MAC)
+			if mac == "" {
+				continue
+			}
+			if lease.IP != "" {
+				macToIP[mac] = lease.IP
+			}
+			if lease.Hostname != "" {
+				macToHostname[mac] = lease.Hostname
+			}
+		}
+	} else {
+		log.Printf("DHCP lease parse failed (%s): %v", dc.dhcpLeasesPath, err)
 	}
 
 	dc.backend.UpdateLeaseInfo(macToIP, macToHostname)
 }
-
-// parseDHCPLeases reads and parses DHCP lease file
-func parseDHCPLeases(path string) ([]models.DHCPLease, error) {
-	var leases []models.DHCPLease
-
-	file, err := os.Open(path)
-	if err != nil {
-		return leases, err
-	}
-	defer file.Close()
-
-	scanner := bufio.NewScanner(file)
-	var currentLease models.DHCPLease
-
-	for scanner.Scan() {
-		line := strings.TrimSpace(scanner.Text())
-
-		if strings.HasPrefix(line, "lease ") {
-			ip := strings.Fields(line)[1]
-			currentLease = models.DHCPLease{IP: ip}
-		} else if strings.Contains(line, "hardware ethernet") {
-			mac := strings.Fields(line)[2]
-			mac = strings.TrimSuffix(mac, ";")
-			currentLease.MAC = mac
-		} else if strings.Contains(line, "client-hostname") {
-			hostname := strings.Fields(line)[1]
-			hostname = strings.Trim(hostname, `";`)
-			currentLease.Hostname = hostname
-		} else if line == "}" {
-			if currentLease.IP != "" && currentLease.MAC != "" {
-				leases = append(leases, currentLease)
-			}
-			currentLease = models.DHCPLease{}
-		}
-	}
-
-	return leases, scanner.Err()
-}