How to use OpenSSH Multiplexer To Speed Up OpenSSH Connections on Linux

This article will help you understand how to multiplex SSH sessions by setting up a master session and using a multiplexer to speed up SSH connections on Linux.

What is SSH Multiplexing?

SSH Multiplexing allows multiple SSH sessions to share a single TCP/IP connection. Instead of establishing separate connections for each SSH session, subsequent connections reuse the existing master connection, reducing server load and improving connection speed.

Advantages of SSH Multiplexing

• Reuses existing connections − Uses the established Unix socket to connect.

• No additional TCP overhead − Eliminates the need for new TCP/IP connections.

• Skips key exchanges − Subsequent connections bypass cryptographic handshakes.

• No re-authentication − Authentication is handled by the master connection.

Configuring SSH Multiplexing

Create or edit the SSH configuration file in your home directory. If ~/.ssh/config doesn't exist, create it with permissions 600:

chmod 600 ~/.ssh/config

Add the following configuration to ~/.ssh/config:

Host *
    ControlPath ~/.ssh/master-%r@%h:%p.socket
    ControlMaster auto
    ControlPersist 30m

Configuration Options Explained

• Host * − Applies configuration to all SSH connections.

• ControlPath − Specifies the path to the control Unix socket. Variables: %r (remote username), %h (remote host), %p (remote port).

• ControlMaster auto − Automatically creates a master connection or reuses existing ones.

• ControlPersist 30m − Keeps the master connection open for 30 minutes after the last session closes.

Testing SSH Multiplexing

Making a Connection

Connect to your server as usual. The first connection becomes the master:

$ ssh root@192.168.2.225
The authenticity of host '192.168.2.225 (192.168.2.225)' can't be established.
RSA key fingerprint is f7:c8:62:c9:6f:02:50:8e:14:cd:3a:95:ad:b1:67:af.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '192.168.2.225' (RSA) to the list of known hosts.
root@192.168.2.225's password:
Last login: Fri Apr 22 13:26:56 2016 from 192.168.1.84

Verifying Multiplexing is Active

Check for active master connections using lsof:

# lsof -U | grep master
ssh     69518     root    4u    unix0xffff8801378f7580    0t0 607468 /root/.ssh/master-root@192.168.2.225.socket

Alternatively, check the master connection status:

$ ssh -O check 192.168.2.225
Master running (pid=69518)

Managing Master Connections

You can control master connections using SSH's -O option:

# Stop the master connection
ssh -O stop 192.168.2.225

# Exit the master connection
ssh -O exit 192.168.2.225

Conclusion

SSH multiplexing significantly improves connection speed by sharing a single TCP connection among multiple SSH sessions. This is especially beneficial when running multiple terminals or scripts that frequently connect to the same server, reducing authentication overhead and network latency.