# SSH Multiplexing

**Connection multiplexing** lets multiple SSH commands and shells reuse a single connection, avoiding the overhead of establishing new connections repeatedly. Configure in `~/.ssh/config`:

```
Host *
    ControlMaster auto
    ControlPath ~/.ssh/socket-%r@%h:%p
    ControlPersist 10m
```

How it works:

- First connection to a host creates a "master" socket at `~/.ssh/socket-user@host:port`
- Subsequent commands detect the socket and reuse the connection
- `ControlPersist 10m` keeps the master alive for 10 minutes after you disconnect

Practical example:

```bash
$ ssh host1          # creates master socket
$ # (in another terminal or background)
$ ssh host1 "ls"     # reuses socket, no new connection
$ scp file host1:/   # reuses socket
$ ssh -O check host1 # check if master exists
```

Per-host configuration:

```
Host work
    HostName work.example.com
    ControlMaster auto
    ControlPath ~/.ssh/socket-work-%r@%h:%p
    ControlPersist 30m
```

Control commands:

```bash
$ ssh -O check host       # check if master is running
$ ssh -O exit host        # close master socket
$ ssh -O forward -L 8000:localhost:3000 host  # add forwarding to existing master
$ ssh -O cancel -L 8000:localhost:3000 host   # remove forwarding
```

Benefits:

- Faster SSH commands (no handshake overhead)
- Multiple terminals in one session
- Shared authentication and agent forwarding

Disable multiplexing for a single command:

```bash
$ ssh -o ControlMaster=no user@host
```

On slow networks, multiplexing is especially noticeable.