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<!--
SPDX-FileCopyrightText: 2025 Tomasz Kramkowski <tomasz@kramkow.ski>
SPDX-License-Identifier: CC-BY-SA-4.0
-->
# MQTTR
MQTTR (named for "MQTT Router") is a MQTT router (duh) which lets you subscribe
to MQTT topics and execute arbitrary programs when a message matching that topic
arrives.
It is the successor of [MQTTT](https://the-tk.com/cgit/mqttt/about/) which was
the result of realising that using a filesystem hierarchy to represent the
configuration did not appear to have any particular benefits over just storing
the information in the already existing config file.
## Building and Installation
Use `cargo build` as normal.
The `SYSCONFDIR` environment variable controls the location of where `mqttr.toml` is searched. The default is `/usr/local/etc`.
Once built, the `mqttr` program (found in `target/{debug,release}/mqttr`) should
be located in some sensible place (e.g. `/usr/local/bin/mqttr`) and configured
as a system service.
`mqttr` does not have a way to double-fork, if you need this behaviour, use
`daemon` or some equivalent utility.
## Configuration
Create a `mqttr.toml` in the appropriate location. The file should contain at
minimum a set of routes. The following is the default configuration:
```toml
host = "localhost" # MQTT server host
port = 1883 # MQTT server port
# [credentials] # Uncomment to specify MQTT connection credentials
# username = "username"
# password = "password"
id = "mqttr" # MQTT client identifier
```
The routes follow the following format:
```toml
[routes]
"zigbee2mqtt/+/light" = [
["/path/to/program"],
["/path/to/another/program", "with", "args"],
]
"foo/bar/baz" = [ ... ]
```
On startup, `mqttr` will read the config file and subscribe to all the topics.
When a message is received, `mqttr` will match the message topic against the
routes and execute every program which is part of a matching route. Each program
will have the message topic as well as the DUP, QoS, RETAIN and Packet
Identifier (when applicable) appended as an additional arguments (in that order)
and will receive the message on stdin.
An example program is this script:
```bash
#!/usr/bin/env bash
topic=$1
dup=$2
qos=$3
retain=$4
((qos == 1 || qos == 2)) && pkid=$5
action=$(jq --raw-output .action)
[[ $action == "toggle" ]] || exit 0
mosquitto_pub --topic "zigbee2mqtt/light/set" --message '{"state":"TOGGLE"}'
```
This can be bound to a topic such as `zigbee2mqtt/light` which would result in
it being ran every time a new MQTT message is published to this topic.
## Missing Features
* Configurable timeouts (eventually configurable per process)
* Configurable QoS for each subscription (default is 0 (at most once))
* Configurable logging
* Ability to configure programs with non-UTF-8 in paths
* Maybe config reloading on SIGHUP
* TLS
* mTLS
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