diff --git a/src/Pure/General/ssh.scala b/src/Pure/General/ssh.scala
--- a/src/Pure/General/ssh.scala
+++ b/src/Pure/General/ssh.scala
@@ -1,112 +1,115 @@
 /*  Title:      Pure/General/ssh.scala
     Author:     Makarius
 
 SSH client based on JSch (see also http://www.jcraft.com/jsch/examples).
 */
 
 package isabelle
 
 
 import com.jcraft.jsch.{JSch, Logger => JSch_Logger, Session => JSch_Session,
   OpenSSHConfig, UserInfo, ChannelExec, ChannelSftp}
 
 
 object SSH
 {
   /* init */
 
   def init(config_dir: Path = Path.explode("~/.ssh"),
     config_file: Path = Path.explode("~/.ssh/config"),
     identity_files: List[Path] =
       List("~/.ssh/id_dsa", "~/.ssh/id_ecdsa", "~/.ssh/id_rsa").map(Path.explode(_))): SSH =
   {
     if (!config_dir.is_dir) error("Bad ssh config directory: " + config_dir)
 
     val jsch = new JSch
 
     if (config_file.is_file)
       jsch.setConfigRepository(OpenSSHConfig.parseFile(File.platform_path(config_file)))
 
     val known_hosts = config_dir + Path.explode("known_hosts")
     if (!known_hosts.is_file) known_hosts.file.createNewFile
     jsch.setKnownHosts(File.platform_path(known_hosts))
 
     for (identity_file <- identity_files if identity_file.is_file)
       jsch.addIdentity(File.platform_path(identity_file))
 
     new SSH(jsch)
   }
 
 
   /* logging */
 
   def logging(verbose: Boolean = true, debug: Boolean = false)
   {
     JSch.setLogger(if (verbose) new Logger(debug) else null)
   }
 
   private class Logger(debug: Boolean) extends JSch_Logger
   {
     def isEnabled(level: Int): Boolean = level != JSch_Logger.DEBUG || debug
 
     def log(level: Int, msg: String)
     {
       level match {
         case JSch_Logger.ERROR | JSch_Logger.FATAL => Output.error_message(msg)
         case JSch_Logger.WARN => Output.warning(msg)
         case _ => Output.writeln(msg)
       }
     }
   }
 
 
   /* session */
 
   class Session private[SSH](val session: JSch_Session)
   {
     override def toString: String =
       (if (session.getUserName == null) "" else session.getUserName + "@") +
       (if (session.getHost == null) "" else session.getHost) +
       (if (session.getPort == 22) "" else ":" + session.getPort) +
       (if (session.isConnected) "" else " (disconnected)")
 
     def close { session.disconnect }
 
     def channel_exec: ChannelExec =
       session.openChannel("exec").asInstanceOf[ChannelExec]
 
     def channel_sftp: ChannelSftp =
       session.openChannel("sftp").asInstanceOf[ChannelSftp]
   }
 
   object No_User_Info extends UserInfo
   {
     def getPassphrase: String = null
     def getPassword: String = null
     def promptPassword(msg: String): Boolean = false
     def promptPassphrase(msg: String): Boolean = false
     def promptYesNo(msg: String): Boolean = false
     def showMessage(msg: String): Unit = Output.writeln(msg)
   }
 }
 
 class SSH private(val jsch: JSch)
 {
   def open_session(host: String, port: Int = 22, user: String = null,
+    connect_timeout: Time = Time.seconds(60),
     compression: Boolean = true): SSH.Session =
   {
     val session = jsch.getSession(user, host, port)
 
     session.setUserInfo(SSH.No_User_Info)
     session.setConfig("MaxAuthTries", "3")
 
     if (compression) {
       session.setConfig("compression.s2c", "zlib@openssh.com,zlib,none")
       session.setConfig("compression.c2s", "zlib@openssh.com,zlib,none")
       session.setConfig("compression_level", "9")
     }
 
-    session.connect
+    if (connect_timeout.is_zero) session.connect
+    else session.connect(connect_timeout.ms.toInt)
+
     new SSH.Session(session)
   }
 }