# Mosquitto {#module-services-mosquitto}

Mosquitto is a MQTT broker often used for IoT or home automation data transport.

## Quickstart {#module-services-mosquitto-quickstart}

A minimal configuration for Mosquitto is

```nix
{
  services.mosquitto = {
    enable = true;
    listeners = [
      {
        acl = [ "pattern readwrite #" ];
        omitPasswordAuth = true;
        settings.allow_anonymous = true;
      }
    ];
  };
}
```

This will start a broker on port 1883, listening on all interfaces of the machine, allowing
read/write access to all topics to any user without password requirements.

User authentication can be configured with the `users` key of listeners. A config that gives
full read access to a user `monitor` and restricted write access to a user `service` could look
like

```nix
{
  services.mosquitto = {
    enable = true;
    listeners = [
      {
        users = {
          monitor = {
            acl = [ "read #" ];
            password = "monitor";
          };
          service = {
            acl = [ "write service/#" ];
            password = "service";
          };
        };
      }
    ];
  };
}
```

TLS authentication is configured by setting TLS-related options of the listener:

```nix
{
  services.mosquitto = {
    enable = true;
    listeners = [
      {
        port = 8883; # port change is not required, but helpful to avoid mistakes
        # ...
        settings = {
          cafile = "/path/to/mqtt.ca.pem";
          certfile = "/path/to/mqtt.pem";
          keyfile = "/path/to/mqtt.key";
        };
      }
    ];
  };
}
```

## Configuration {#module-services-mosquitto-config}

The Mosquitto configuration has four distinct types of settings:
the global settings of the daemon, listeners, plugins, and bridges.
Bridges and listeners are part of the global configuration, plugins are part of listeners.
Users of the broker are configured as parts of listeners rather than globally, allowing
configurations in which a given user is only allowed to log in to the broker using specific
listeners (eg to configure an admin user with full access to all topics, but restricted to
localhost).

Almost all options of Mosquitto are available for configuration at their appropriate levels, some
as NixOS options written in camel case, the remainders under `settings` with their exact names in
the Mosquitto config file. The exceptions are `per_listener_settings` (which is always set to
`true`) and the per-listener access control, which is always configured via instances of the
`acl-file` and `password-file` plugins generated from the `acl` and `users` attributes of each
listener.

### Password authentication {#module-services-mosquitto-config-passwords}

Mosquitto can be run in two modes, with a password file or without. Each listener has its own
password file, and different listeners may use different password files. Password file generation
can be disabled by setting `omitPasswordAuth = true` for a listener; in this case it is necessary
to either set `settings.allow_anonymous = true` to allow all logins, or to configure other
authentication methods like TLS client certificates with `settings.use_identity_as_username = true`.

The default is to generate a password file for each listener from the users configured to that
listener. Users with no configured password will not be added to the password file and thus
will not be able to use the broker.

### ACL format {#module-services-mosquitto-config-acl}

Every listener has an instance of the Mosquitto `acl-file` plugin attached to it. This ACL is
configured via two attributes of the config:

  * the `acl` attribute of the listener configures pattern ACL entries and topic ACL entries
    for anonymous users. Each entry must be prefixed with `pattern` or `topic` to distinguish
    between these two cases.
  * the `acl` attribute of every user configures in the listener configured the ACL for that
    given user. Only topic ACLs are supported by Mosquitto in this setting, so no prefix is
    required or allowed.

The default ACL for a listener is empty, disallowing all accesses from all clients. To configure
a completely open ACL, set `acl = [ "pattern readwrite #" ]` in the listener.
