Multiplexing

SSH connection multiplexing allows multiple SSH sessions to be established over a single, shared connection, reducing the overhead of setting up new connections and improving efficiency when connecting to the same host multiple times. The features ControlMaster and ControlPath in SSH configuration enable this multiplexing, allowing users to open new SSH sessions instantly without repeating the authentication process.

Overview of ControlMaster and ControlPath

ControlMaster is a feature that allows the first SSH connection to a host to act as a "master" connection. Subsequent connections to the same host reuse the existing session rather than establishing a new one, eliminating the need to re-authenticate or renegotiate the connection parameters. This can greatly improve performance, especially when multiple SSH sessions to the same host are required.

ControlPath specifies the path to a Unix socket used for the multiplexed connections. This socket allows new SSH sessions to communicate with the master session.

Configuring Multiplexing

To enable SSH connection multiplexing, two key options, ControlMaster and ControlPath, must be configured in the SSH client configuration file (~/.ssh/config).

Example Configuration:

Host remote-server
    ControlMaster auto
    ControlPath ~/.ssh/controlmasters/%r@%h:%p
    ControlPersist 10m

• ControlMaster auto: This enables the master connection to be automatically established when the first session to remote-server is initiated.
• ControlPath: This sets the path for the Unix socket, allowing future sessions to connect to the master session. %r represents the remote username, %h represents the hostname, and %p represents the port number.
• ControlPersist: This option keeps the master connection alive even after the initial session is closed. In the example, the master connection will persist for 10 minutes, allowing new connections to be made during that time without starting a new session.

Advantages of Connection Multiplexing

Faster connection setup
By reusing an existing connection, multiplexing eliminates the need to authenticate and negotiate encryption parameters for each new session. This drastically reduces the time required to open new SSH connections.

Reduced resource usage
Connection multiplexing reduces the overhead on both the client and the server. Instead of creating multiple SSH tunnels, a single connection is maintained, lowering resource usage and improving efficiency.

Verifying Multiplexing

To verify whether a master connection is in use, the following command can be run:

ssh -O check user@remote-server

This command checks if a multiplexed connection is active. If the connection is active, the response will confirm that the master session is running.

If the session needs to be closed, the following command can be used to terminate the master connection:

ssh -O exit user@remote-server

Security Considerations

While multiplexing provides efficiency benefits, it also introduces some security considerations. Since multiple sessions share a single connection, if the master session is compromised, it could affect all subsequent connections. Proper use of ControlPersist to limit the lifespan of the master session and ensuring that multiplexing is only used on trusted servers can help mitigate potential risks.

In addition, the socket path specified in ControlPath should be located in a secure, private directory, ensuring that only the intended user has access to the Unix socket.