Initial commit: ArtNet packet sniffer and validator

Sniffs ArtNet packets on UDP port 6454 and validates against the Art-Net 4 spec.
Currently validates ArtPoll and ArtPollReply packets.

Features:
- Structured logging with slog
- Name filter regex (-name flag)
- Verbose mode (-v flag)
- Universe display as net:subnet:universe (portaddr)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

main.go

@@ -0,0 +1,132 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"log/slog"
+	"net"
+	"os"
+	"regexp"
+)
+
+const (
+	DefaultArtNetPort = 6454 // 0x1936
+)
+
+var (
+	verbose    bool
+	nameFilter *regexp.Regexp
+)
+
+func main() {
+	port := flag.Int("port", DefaultArtNetPort, "UDP port to listen on (default: 6454/0x1936)")
+	bindAddr := flag.String("bind", "0.0.0.0", "IP address to bind to")
+	namePattern := flag.String("name", "", "Filter by node name (regex)")
+	flag.BoolVar(&verbose, "v", false, "Verbose output (show all field details)")
+	flag.Parse()
+
+	if *namePattern != "" {
+		var err error
+		nameFilter, err = regexp.Compile(*namePattern)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "Invalid name regex: %v\n", err)
+			os.Exit(1)
+		}
+	}
+
+	ip := net.ParseIP(*bindAddr)
+	if ip == nil {
+		fmt.Fprintf(os.Stderr, "Invalid bind address: %s\n", *bindAddr)
+		os.Exit(1)
+	}
+
+	addr := net.UDPAddr{
+		Port: *port,
+		IP:   ip,
+	}
+
+	conn, err := net.ListenUDP("udp4", &addr)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Failed to listen on UDP port %d: %v\n", *port, err)
+		fmt.Fprintf(os.Stderr, "You may need to run with sudo or use setcap cap_net_bind_service=+ep\n")
+		os.Exit(1)
+	}
+	defer conn.Close()
+
+	slog.Info("listening", "addr", fmt.Sprintf("%s:%d", *bindAddr, *port))
+	if *port != DefaultArtNetPort {
+		slog.Warn("non-standard port", "expected", DefaultArtNetPort)
+	}
+
+	buf := make([]byte, 65535)
+	for {
+		n, remoteAddr, err := conn.ReadFromUDP(buf)
+		if err != nil {
+			slog.Error("read failed", "error", err)
+			continue
+		}
+
+		packet := make([]byte, n)
+		copy(packet, buf[:n])
+
+		processPacket(packet, remoteAddr)
+	}
+}
+
+func processPacket(data []byte, src *net.UDPAddr) {
+	result := ValidatePacket(data, src)
+
+	// Apply name filter
+	if nameFilter != nil {
+		name := result.getField("PortName")
+		if !nameFilter.MatchString(name) {
+			return
+		}
+	}
+
+	// Build log attributes
+	attrs := []any{
+		"src", src.IP.String(),
+		"size", len(data),
+		"type", result.PacketType,
+	}
+
+	// Add summary fields based on packet type
+	for _, kv := range result.SummaryAttrs() {
+		attrs = append(attrs, kv.Key, kv.Value)
+	}
+
+	// Add error/warning counts if any
+	if len(result.Errors) > 0 {
+		attrs = append(attrs, "errors", len(result.Errors))
+	}
+	if len(result.Warnings) > 0 {
+		attrs = append(attrs, "warnings", len(result.Warnings))
+	}
+
+	// Log the packet
+	if len(result.Errors) > 0 {
+		slog.Error("packet", attrs...)
+	} else if len(result.Warnings) > 0 {
+		slog.Warn("packet", attrs...)
+	} else {
+		slog.Info("packet", attrs...)
+	}
+
+	// Always log individual warnings
+	for _, w := range result.Warnings {
+		slog.Warn("validation", "src", src.IP.String(), "warning", w)
+	}
+
+	// Always log individual errors
+	for _, e := range result.Errors {
+		slog.Error("validation", "src", src.IP.String(), "error", e)
+	}
+
+	// Verbose: log all fields
+	if verbose && len(result.Fields) > 0 {
+		for _, f := range result.Fields {
+			slog.Debug("field", "src", src.IP.String(), "name", f.Name, "value", f.Value)
+		}
+	}
+}