The `unix` listener configures Boundary to listen on a Unix domain socket.

It is only available for listeners for `api` and `cluster` purpose.
Unix sockets cannot currently be used for `"proxy"` purpose on a worker.

```hcl
listener "unix" {
  purpose = "api"
  address = "/var/run/boundary-api.sock"
}
```

The `listener` stanza may be specified more than once to make Boundary listen on
multiple interfaces; however, only one listener marked for `cluster` purpose is
allowed.