Added support to auto discovery with SRV DNS registry (#60)

* Added support to auto discovery witih SRV

The yaml configuration has been modified to suport srv parameters.
Now the name is taken form the router identity to allow dynamic
discovery of devices based on SRV registry.

* Corrected format

* feat(collector): Added support to use custom DNS

* feat(collector): get default dns server from resolv.conf

* Remove getIdentity on the static config devices

collector/collector.go

@@ -8,11 +8,15 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"os"
+	"strconv"
+	"strings"
 	"sync"
 	"time"
 
 	"mikrotik-exporter/config"
 
+	"github.com/miekg/dns"
 	"github.com/prometheus/client_golang/prometheus"
 	log "github.com/sirupsen/logrus"
 	routeros "gopkg.in/routeros.v2"
@@ -22,6 +26,7 @@ const (
 	namespace  = "mikrotik"
 	apiPort    = "8728"
 	apiPortTLS = "8729"
+	dnsPort    = 53
 
 	// DefaultTimeout defines the default timeout when connecting to a router
 	DefaultTimeout = 5 * time.Second
@@ -194,9 +199,52 @@ func (c *collector) Describe(ch chan<- *prometheus.Desc) {
 // Collect implements the prometheus.Collector interface.
 func (c *collector) Collect(ch chan<- prometheus.Metric) {
 	wg := sync.WaitGroup{}
-	wg.Add(len(c.devices))
+
+	var realDevices []config.Device
 
 	for _, dev := range c.devices {
+		if (config.SrvRecord{}) != dev.Srv {
+			log.WithFields(log.Fields{
+				"SRV": dev.Srv.Record,
+			}).Info("SRV configuration detected")
+      conf, _ := dns.ClientConfigFromFile("/etc/resolv.conf")
+			dnsServer := net.JoinHostPort(conf.Servers[0], strconv.Itoa(dnsPort))
+			if (config.DnsServer{}) != dev.Srv.Dns {
+				dnsServer = net.JoinHostPort(dev.Srv.Dns.Address, strconv.Itoa(dev.Srv.Dns.Port))
+				log.WithFields(log.Fields{
+					"DnsServer": dnsServer,
+				}).Info("Custom DNS config detected")
+			}
+			dnsMsg := new(dns.Msg)
+			dnsCli := new(dns.Client)
+
+			dnsMsg.RecursionDesired = true
+			dnsMsg.SetQuestion(dns.Fqdn(dev.Srv.Record), dns.TypeSRV)
+			r, _, err := dnsCli.Exchange(dnsMsg, dnsServer)
+
+			if err != nil {
+				os.Exit(1)
+			}
+
+			for _, k := range r.Answer {
+				if s, ok := k.(*dns.SRV); ok {
+					d := config.Device{}
+					d.Name = strings.TrimRight(s.Target, ".")
+					d.Address = strings.TrimRight(s.Target, ".")
+					d.User = dev.User
+					d.Password = dev.Password
+					c.getIdentity(&d)
+					realDevices = append(realDevices, d)
+				}
+			}
+		} else {
+			realDevices = append(realDevices, dev)
+		}
+	}
+
+	wg.Add(len(realDevices))
+
+	for _, dev := range realDevices {
 		go func(d config.Device) {
 			c.collectForDevice(d, ch)
 			wg.Done()
@@ -206,6 +254,30 @@ func (c *collector) Collect(ch chan<- prometheus.Metric) {
 	wg.Wait()
 }
 
+func (c *collector) getIdentity(d *config.Device) error {
+	cl, err := c.connect(d)
+	if err != nil {
+		log.WithFields(log.Fields{
+			"device": d.Name,
+			"error":  err,
+		}).Error("error dialing device")
+		return err
+	}
+	defer cl.Close()
+	reply, err := cl.Run("/system/identity/print")
+	if err != nil {
+		log.WithFields(log.Fields{
+			"device": d.Name,
+			"error":  err,
+		}).Error("error fetching ethernet interfaces")
+		return err
+	}
+	for _, id := range reply.Re {
+		d.Name = id.Map["name"]
+	}
+	return nil
+}
+
 func (c *collector) collectForDevice(d config.Device, ch chan<- prometheus.Metric) {
 	begin := time.Now()