catch EPERM sendmsg errors for the very first packet on Linux (#4111)

sys_conn_helper_linux.go

@@ -97,3 +97,14 @@ func isGSOError(err error) bool {
 	}
 	return false
 }
+
+// The first sendmsg call on a new UDP socket sometimes errors on Linux.
+// It's not clear why this happens.
+// See https://github.com/golang/go/issues/63322.
+func isPermissionError(err error) bool {
+	var serr *os.SyscallError
+	if errors.As(err, &serr) {
+		return serr.Syscall == "sendmsg" && serr.Err == unix.EPERM
+	}
+	return false
+}