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Port forwarding

PortForwardingEventListener

Informs and allows tracking of port forwarding events as described in RFC 4254 - section 7 as well as the (simple) SOCKS protocol (versions 4, 5). In this context, one can create a PortForwardingTracker that can be used in a try-with-resource block so that the set up forwarding is automatically torn down when the tracker is close()-d:

client.addPortForwardingEventListener(new MySuperDuperListener());

try (ClientSession session = client.connect(user, host, port).verify(...timeout...).getSession()) {
    session.addPasswordIdentity(password);
    session.auth().verify(...timeout...);

    try (PortForwardingTracker tracker = session.createLocal/RemotePortForwardingTracker(...)) {
        ...do something that requires the tunnel...
    }

    // Tunnel is torn down when code reaches this point
}

Standard port forwarding

Port forwarding as specified in RFC 4254 - section 7 is fully supported by the client and server. From the client side, this capability is exposed via the start/stopLocal/RemotePortForwarding method. The key player in this capability is the configured ForwardingFilter that controls this feature - on both sides - client and server. By default, this capability is disabled - i.e., the user must provide an implementation and call the appropriate setForwardingFilter method on the client/server.

SshClient client = ...create/obtain an instance...
client.setForwardingFilter(...filter instance...);

SshServer server = ...create/obtain an instance...
server.setForwardingFilter(...filter instance...);

The code contains 2 simple implementations - an AcceptAllForwardingFilter and a RejectAllForwardingFilter one that can be used for these trivial policies. Note: setting a null filter is equivalent to rejecting all such attempts.

In order to help with the forwarding policy, the filter is actually made up of 3 "groups" of forwarding:

  • AgentForwardingFilter
  • X11ForwardingFilter
  • TcpForwardingFilter

It is possible to implement each and every one separately and then combine them via ForwardingFilter#asForwardingFilter. In this context, one does not have to implement all 3 - any implementation not provided is assumed to be disabled. Furthermore, there are reasonable default implementations for all 3, so one can override only a specific group policy and provide defaults for the rest.

SOCKS

The code implements a SOCKS proxy for versions 4 and 5. The proxy capability is invoked via the start/stopDynamicPortForwarding methods.

Proxy agent

The code provides to some extent an SSH proxy agent via the available SshAgentFactory implementations. As of latest version both Secure Shell Authentication Agent Protocol Draft 02 and its OpenSSH equivalent are supported. Note: in order to support this feature the Apache Portable Runtime Library needs to be added to the Maven dependencies:

    <dependency>
        <groupId>tomcat</groupId>
        <artifactId>tomcat-apr</artifactId>
    </dependency>

Note: Since the portable runtime library uses native code, one needs to also make sure that the appropriate .dll/.so library is available in the LD_LIBRARY_PATH.