dhcp4d: optionally publish DHCP leases to MQTT

Enable using:

  mkdir -p /perm/dhcp4d
  echo 'tcp://10.0.0.54:1883' > /perm/dhcp4d/mqtt-broker.txt

cmd/dhcp4d/dhcp4d.go

@@ -234,6 +234,8 @@ type srv struct {
 }
 
 func newSrv(permDir string) (*srv, error) {
+	mayqtt := MQTT()
+
 	http.Handle("/metrics", promhttp.Handler())
 	if err := updateListeners(); err != nil {
 		return nil, err
@@ -399,6 +401,30 @@ func newSrv(permDir string) (*srv, error) {
 		if err := notify.Process("/user/dnsd", syscall.SIGUSR1); err != nil {
 			log.Printf("notifying dnsd: %v", err)
 		}
+
+		// Publish the DHCP lease as JSON to MQTT, if configured:
+		leaseVal := struct {
+			Addr         string    `json:"addr"`
+			HardwareAddr string    `json:"hardware_addr"`
+			Expiration   time.Time `json:"expiration"`
+		}{
+			Addr:         latest.Addr.String(),
+			HardwareAddr: latest.HardwareAddr,
+			Expiration:   latest.Expiry.In(time.UTC),
+		}
+		leaseJSON, err := json.Marshal(leaseVal)
+		if err != nil {
+			log.Fatal(err)
+		}
+		identifier := latest.Hostname
+		if identifier == "" {
+			identifier = latest.HardwareAddr
+		}
+		mayqtt <- PublishRequest{
+			Topic:    "router7/dhcp4d/lease/" + identifier,
+			Retained: true,
+			Payload:  leaseJSON,
+		}
 	}
 	conn, err := conn.NewUDP4BoundListener(*iface, ":67")
 	if err != nil {

cmd/dhcp4d/mayqtt.go

@@ -0,0 +1,53 @@
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"strings"
+
+	mqtt "github.com/eclipse/paho.mqtt.golang"
+)
+
+type PublishRequest struct {
+	Topic    string
+	Qos      byte
+	Retained bool
+	Payload  interface{}
+}
+
+func publisherLoop(requests <-chan PublishRequest) error {
+	const configFn = "/perm/dhcp4d/mqtt-broker.txt"
+	b, err := ioutil.ReadFile(configFn)
+	if err != nil {
+		// discard requests:
+		for range requests {
+		}
+		return nil
+	}
+	// e.g. tcp://10.0.0.54:1883, which is a static DHCP lease for the dr.lan
+	// Raspberry Pi, which is running an MQTT broker in my network.
+	broker := strings.TrimSpace(string(b))
+	log.Printf("Connecting to MQTT broker %q (configured in %s)", broker, configFn)
+	opts := mqtt.NewClientOptions().AddBroker(broker)
+	opts.SetClientID("dhcp4d")
+	mqttClient := mqtt.NewClient(opts)
+	if token := mqttClient.Connect(); token.Wait() && token.Error() != nil {
+		return fmt.Errorf("MQTT connection failed: %v", token.Error())
+	}
+
+	for r := range requests {
+		// discard Token, MQTT publishing is best-effort
+		_ = mqttClient.Publish(r.Topic, r.Qos, r.Retained, r.Payload)
+	}
+	return nil
+}
+
+func MQTT() chan<- PublishRequest {
+	result := make(chan PublishRequest)
+	go func() {
+		if err := publisherLoop(result); err != nil {
+			log.Print(err)
+		}
+	}()
+	return result
+}

go.mod

@@ -7,6 +7,7 @@ require (
 	github.com/digineo/go-ping v1.0.0
 	github.com/gokrazy/breakglass v0.0.0-20200527163858-efff2172eebe // indirect
 	github.com/gokrazy/gdns v0.0.0-20200218203540-6b3b6244ea39 // indirect
+	github.com/eclipse/paho.mqtt.golang v1.2.0
 	github.com/gokrazy/gokrazy v0.0.0-20201006151115-caded4667633
 	github.com/gokrazy/internal v0.0.0-20200713084155-ab6fc6e02a03 // indirect
 	github.com/gokrazy/timestamps v0.0.0-20200713073712-54fdc319126e // indirect

go.sum

@@ -70,6 +70,7 @@ github.com/dustin/go-humanize v1.0.0/go.mod h1:HtrtbFcZ19U5GC7JDqmcUSB87Iq5E25Kn
 github.com/eapache/go-resiliency v1.1.0/go.mod h1:kFI+JgMyC7bLPUVY133qvEBtVayf5mFgVsvEsIPBvNs=
 github.com/eapache/go-xerial-snappy v0.0.0-20180814174437-776d5712da21/go.mod h1:+020luEh2TKB4/GOp8oxxtq0Daoen/Cii55CzbTV6DU=
 github.com/eapache/queue v1.1.0/go.mod h1:6eCeP0CKFpHLu8blIFXhExK/dRa7WDZfr6jVFPTqq+I=
+github.com/eclipse/paho.mqtt.golang v1.2.0 h1:1F8mhG9+aO5/xpdtFkW4SxOJB67ukuDC3t2y2qayIX0=
 github.com/eclipse/paho.mqtt.golang v1.2.0/go.mod h1:H9keYFcgq3Qr5OUJm/JZI/i6U7joQ8SYLhZwfeOo6Ts=
 github.com/edsrzf/mmap-go v1.0.0/go.mod h1:YO35OhQPt3KJa3ryjFM5Bs14WD66h8eGKpfaBNrHW5M=
 github.com/envoyproxy/go-control-plane v0.6.9/go.mod h1:SBwIajubJHhxtWwsL9s8ss4safvEdbitLhGGK48rN6g=