diff -r 35a279a2d246 -r a6bdf4b889ca src/Pure/System/isabelle_system.scala
--- a/src/Pure/System/isabelle_system.scala	Tue Sep 13 23:01:42 2022 +0200
+++ b/src/Pure/System/isabelle_system.scala	Tue Sep 13 23:06:52 2022 +0200
@@ -9,6 +9,7 @@
 
 import java.util.{Map => JMap, HashMap}
 import java.io.{File => JFile, IOException}
+import java.net.ServerSocket
 import java.nio.file.{Path => JPath, Files, SimpleFileVisitor, FileVisitResult,
   StandardCopyOption, FileSystemException}
 import java.nio.file.attribute.BasicFileAttributes
@@ -349,6 +350,15 @@
   }
 
 
+  /* TCP/IP ports */
+
+  def local_port(): Int = {
+    val socket = new ServerSocket(0)
+    val port = socket.getLocalPort
+    socket.close()
+    port
+  }
+
 
   /** external processes **/