wgengine/magicsock: remove DefaultPort const

Assume it'll stay at 0 forever, so hard-code it
and delete code conditional on it being non-0.

Signed-off-by: Josh Bleecher Snyder <josharian@gmail.com>

cmd/tailscaled/depaware.txt

@@ -144,7 +144,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/version/distro                                 from tailscale.com/control/controlclient+
         tailscale.com/wgengine                                       from tailscale.com/cmd/tailscaled+
         tailscale.com/wgengine/filter                                from tailscale.com/control/controlclient+
-        tailscale.com/wgengine/magicsock                             from tailscale.com/cmd/tailscaled+
+        tailscale.com/wgengine/magicsock                             from tailscale.com/wgengine+
         tailscale.com/wgengine/monitor                               from tailscale.com/wgengine+
         tailscale.com/wgengine/netstack                              from tailscale.com/cmd/tailscaled
         tailscale.com/wgengine/router                                from tailscale.com/cmd/tailscaled+

cmd/tailscaled/tailscaled.go

@@ -44,7 +44,6 @@ import (
 	"tailscale.com/version"
 	"tailscale.com/version/distro"
 	"tailscale.com/wgengine"
-	"tailscale.com/wgengine/magicsock"
 	"tailscale.com/wgengine/monitor"
 	"tailscale.com/wgengine/netstack"
 	"tailscale.com/wgengine/router"
@@ -117,7 +116,7 @@ func main() {
 	flag.StringVar(&args.debug, "debug", "", "listen address ([ip]:port) of optional debug server")
 	flag.StringVar(&args.socksAddr, "socks5-server", "", `optional [ip]:port to run a SOCK5 server (e.g. "localhost:1080")`)
 	flag.StringVar(&args.tunname, "tun", defaultTunName(), `tunnel interface name; use "userspace-networking" (beta) to not use TUN`)
-	flag.Var(flagtype.PortValue(&args.port, magicsock.DefaultPort), "port", "UDP port to listen on for WireGuard and peer-to-peer traffic; 0 means automatically select")
+	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.StringVar(&args.statepath, "state", paths.DefaultTailscaledStateFile(), "path of state file")
 	flag.StringVar(&args.socketpath, "socket", paths.DefaultTailscaledSocket(), "path of the service unix socket")
 	flag.BoolVar(&printVersion, "version", false, "print version information and exit")

wgengine/magicsock/magicsock.go

@@ -365,10 +365,6 @@ type activeDerp struct {
 	createTime time.Time
 }
 
-// DefaultPort is the default port to listen on.
-// The current default (zero) means to auto-select a random free port.
-const DefaultPort = 0
-
 // Options contains options for Listen.
 type Options struct {
 	// Logf optionally provides a log function to use.
@@ -2617,17 +2613,7 @@ func (c *Conn) bind1(ruc **RebindingUDPConn, which string) error {
 			host = "::1"
 		}
 	}
-	var pc net.PacketConn
+	pc, err := c.listenPacket(which, host, c.port)
-	var err error
-	if c.port == 0 && DefaultPort != 0 {
-		pc, err = c.listenPacket(which, host, DefaultPort)
-		if err != nil {
-			c.logf("magicsock: bind: default port %s/%v unavailable; picking random", which, DefaultPort)
-		}
-	}
-	if pc == nil {
-		pc, err = c.listenPacket(which, host, c.port)
-	}
 	if err != nil {
 		c.logf("magicsock: bind(%s/%v): %v", which, c.port, err)
 		return fmt.Errorf("magicsock: bind: %s/%d: %v", which, c.port, err)