cmd/tailscaled: respect $PORT on all platforms, not just Linux

Updates #5114

Change-Id: I6c6e28c493d6a026a03088157d08f9fd182ef373
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/tailscaled.go

@@ -26,6 +26,7 @@ import (
 	"os/signal"
 	"path/filepath"
 	"runtime"
+	"strconv"
 	"strings"
 	"syscall"
 	"time"
@@ -97,6 +98,20 @@ func defaultTunName() string {
 	return "tailscale0"
 }
 
+// defaultPort returns the default UDP port to listen on for disco+wireguard.
+// By default it returns 0, to pick one randomly from the kernel.
+// If the environment variable PORT is set, that's used instead.
+// The PORT environment variable is chosen to match what the Linux systemd
+// unit uses, to make documentation more consistent.
+func defaultPort() uint16 {
+	if s := envknob.String("PORT"); s != "" {
+		if p, err := strconv.ParseUint(s, 10, 16); err == nil {
+			return uint16(p)
+		}
+	}
+	return 0
+}
+
 var args struct {
 	// tunname is a /dev/net/tun tunnel name ("tailscale0"), the
 	// string "userspace-networking", "tap:TAPNAME[:BRIDGENAME]"
@@ -142,7 +157,7 @@ func main() {
 	flag.StringVar(&args.socksAddr, "socks5-server", "", `optional [ip]:port to run a SOCK5 server (e.g. "localhost:1080")`)
 	flag.StringVar(&args.httpProxyAddr, "outbound-http-proxy-listen", "", `optional [ip]:port to run an outbound HTTP proxy (e.g. "localhost:8080")`)
 	flag.StringVar(&args.tunname, "tun", defaultTunName(), `tunnel interface name; use "userspace-networking" (beta) to not use TUN`)
-	flag.Var(flagtype.PortValue(&args.port, 0), "port", "UDP port to listen on for WireGuard and peer-to-peer traffic; 0 means automatically select")
+	flag.Var(flagtype.PortValue(&args.port, defaultPort()), "port", "UDP port to listen on for WireGuard and peer-to-peer traffic; 0 means automatically select")
 	flag.StringVar(&args.statepath, "state", "", "absolute path of state file; use 'kube:<secret-name>' to use Kubernetes secrets or 'arn:aws:ssm:...' to store in AWS SSM; use 'mem:' to not store state and register as an emphemeral node. If empty and --statedir is provided, the default is <statedir>/tailscaled.state. Default: "+paths.DefaultTailscaledStateFile())
 	flag.StringVar(&args.statedir, "statedir", "", "path to directory for storage of config state, TLS certs, temporary incoming Taildrop files, etc. If empty, it's derived from --state when possible.")
 	flag.StringVar(&args.socketpath, "socket", paths.DefaultTailscaledSocket(), "path of the service unix socket")