conn: try harder to have v4 and v6 ports agree

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

2 files changed, 14 insertions, 0 deletions

diff --git a/conn/conn_default.go b/conn/conn_default.go
index 4f6b6d1..22248af 100644
--- a/conn/conn_default.go
+++ b/conn/conn_default.go
@@ -99,7 +99,9 @@ func extractErrno(err error) error {
 func createBind(uport uint16) (Bind, uint16, error) {
 	var err error
 	var bind nativeBind
+	var tries int
 
+again:
 	port := int(uport)
 
 	bind.ipv4, port, err = listenNet("udp4", port)
@@ -108,6 +110,10 @@ func createBind(uport uint16) (Bind, uint16, error) {
 	}
 
 	bind.ipv6, port, err = listenNet("udp6", port)
+	if uport == 0 && err != nil && extractErrno(err) == syscall.EADDRINUSE && tries < 100 {
+		tries++
+		goto again
+	}
 	if err != nil && extractErrno(err) != syscall.EAFNOSUPPORT {
 		bind.ipv4.Close()
 		bind.ipv4 = nil
diff --git a/conn/conn_linux.go b/conn/conn_linux.go
index f6638dd..716028f 100644
--- a/conn/conn_linux.go
+++ b/conn/conn_linux.go
@@ -104,7 +104,11 @@ func createBind(port uint16) (Bind, uint16, error) {
 	var err error
 	var bind nativeBind
 	var newPort uint16
+	var tries int
+	originalPort := port
 
+again:
+	port = originalPort
 	// Attempt ipv6 bind, update port if successful.
 	bind.sock6, newPort, err = create6(port)
 	if err != nil {
@@ -118,6 +122,10 @@ func createBind(port uint16) (Bind, uint16, error) {
 	// Attempt ipv4 bind, update port if successful.
 	bind.sock4, newPort, err = create4(port)
 	if err != nil {
+		if originalPort == 0 && err == syscall.EADDRINUSE && tries < 100 {
+			tries++
+			goto again
+		}
 		if err != syscall.EAFNOSUPPORT {
 			unix.Close(bind.sock6)
 			return nil, 0, err