Replace Dial with ListenUDP, add unit test

cmd/cloudflared/tunnel/configuration.go

@@ -489,19 +489,17 @@ func parseConfigBindAddress(ipstr string) (net.IP, error) {
 }
 
 func testIPBindable(ip net.IP) error {
-	var network, address string
-	if ip.To4() != nil {
-		network, address = "udp4", "127.0.0.1:4"
-	} else {
-		network, address = "udp6", "[::1]:4"
+	// "Unspecified" = let OS choose, so always bindable
+	if ip == nil {
+		return nil
 	}
 
-	dialer := net.Dialer{LocalAddr: &net.UDPAddr{IP: ip}}
-	conn, err := dialer.Dial(network, address)
+	addr := &net.UDPAddr{IP: ip, Port: 0}
+	listener, err := net.ListenUDP("udp", addr)
 	if err != nil {
 		return err
 	}
-	conn.Close()
+	listener.Close()
 	return nil
 }
 

cmd/cloudflared/tunnel/configuration_test.go

@@ -9,6 +9,7 @@ import (
 	"crypto/x509"
 	"crypto/x509/pkix"
 	"encoding/asn1"
+	"net"
 	"os"
 	"testing"
 
@@ -214,3 +215,23 @@ func getCertPoolSubjects(certPool *x509.CertPool) ([]*pkix.Name, error) {
 func isUnrecoverableError(err error) bool {
 	return err != nil && err.Error() != "crypto/x509: system root pool is not available on Windows"
 }
+
+func TestTestIPBindable(t *testing.T) {
+	assert.Nil(t, testIPBindable(nil))
+
+	// Public services - if one of these IPs is on the machine, the test environment is too weird
+	assert.NotNil(t, testIPBindable(net.ParseIP("8.8.8.8")))
+	assert.NotNil(t, testIPBindable(net.ParseIP("1.1.1.1")))
+
+	addrs, err := net.InterfaceAddrs()
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, addr := range addrs {
+		if i >= 3 {
+			break
+		}
+		ip := addr.(*net.IPNet).IP
+		assert.Nil(t, testIPBindable(ip))
+	}
+}