[dns] Add serial number metric for dns soa queries (#333)

Signed-off-by: Ask Bjørn Hansen <ask@develooper.com>

example.yml

@@ -111,6 +111,11 @@ modules:
       validate_additional_rrs:
         fail_if_matches_regexp:
         - ".*127.0.0.1"
+  dns_soa:
+    prober: dns
+    dns:
+      query_name: "prometheus.io"
+      query_type: "SOA"
   dns_tcp_example:
     prober: dns
     dns:

prober/dns.go

@@ -107,6 +107,17 @@ func ProbeDNS(ctx context.Context, target string, module config.Module, registry
 	registry.MustRegister(probeDNSAuthorityRRSGauge)
 	registry.MustRegister(probeDNSAdditionalRRSGauge)
 
+	qt := dns.TypeANY
+	if module.DNS.QueryType != "" {
+		var ok bool
+		qt, ok = dns.StringToType[module.DNS.QueryType]
+		if !ok {
+			level.Error(logger).Log("msg", "Invalid query type", "Type seen", module.DNS.QueryType, "Existing types", dns.TypeToString)
+			return false
+		}
+	}
+	var probeDNSSOAGauge prometheus.Gauge
+
 	var ip *net.IPAddr
 	if module.DNS.TransportProtocol == "" {
 		module.DNS.TransportProtocol = "udp"
@@ -154,15 +165,6 @@ func ProbeDNS(ctx context.Context, target string, module config.Module, registry
 		}
 	}
 
-	qt := dns.TypeANY
-	if module.DNS.QueryType != "" {
-		var ok bool
-		qt, ok = dns.StringToType[module.DNS.QueryType]
-		if !ok {
-			level.Error(logger).Log("msg", "Invalid query type", "Type seen", module.DNS.QueryType, "Existing types", dns.TypeToString)
-			return false
-		}
-	}
 	msg := new(dns.Msg)
 	msg.SetQuestion(dns.Fqdn(module.DNS.QueryName), qt)
 
@@ -180,6 +182,20 @@ func ProbeDNS(ctx context.Context, target string, module config.Module, registry
 	probeDNSAuthorityRRSGauge.Set(float64(len(response.Ns)))
 	probeDNSAdditionalRRSGauge.Set(float64(len(response.Extra)))
 
+	if qt == dns.TypeSOA {
+		probeDNSSOAGauge = prometheus.NewGauge(prometheus.GaugeOpts{
+			Name: "probe_dns_serial",
+			Help: "Returns the serial number of the zone",
+		})
+		registry.MustRegister(probeDNSSOAGauge)
+
+		for _, a := range response.Answer {
+			if soa, ok := a.(*dns.SOA); ok {
+				probeDNSSOAGauge.Set(float64(soa.Serial))
+			}
+		}
+	}
+
 	if !validRcode(response.Rcode, module.DNS.ValidRcodes, logger) {
 		return false
 	}

prober/dns_test.go

@@ -165,11 +165,20 @@ func authoritativeDNSHandler(w dns.ResponseWriter, r *dns.Msg) {
 	m := new(dns.Msg)
 	m.SetReply(r)
 
-	a, err := dns.NewRR("example.com. 3600 IN A 127.0.0.1")
-	if err != nil {
-		panic(err)
+	if r.Question[0].Qtype == dns.TypeSOA {
+		a, err := dns.NewRR("example.com. 3600 IN SOA ns.example.com. noc.example.com. 1000 7200 3600 1209600 3600")
+		if err != nil {
+			panic(err)
+		}
+		m.Answer = append(m.Answer, a)
+
+	} else {
+		a, err := dns.NewRR("example.com. 3600 IN A 127.0.0.1")
+		if err != nil {
+			panic(err)
+		}
+		m.Answer = append(m.Answer, a)
 	}
-	m.Answer = append(m.Answer, a)
 
 	authority := []string{
 		"example.com. 7200 IN NS ns1.isp.net.",
@@ -213,6 +222,12 @@ func TestAuthoritativeDNSResponse(t *testing.T) {
 			}, true,
 		},
 		{
+			config.DNSProbe{
+				PreferredIPProtocol: "ipv4",
+				QueryName:           "example.com",
+				QueryType:           "SOA",
+			}, true,
+		}, {
 			config.DNSProbe{
 				PreferredIPProtocol: "ipv4",
 				QueryName:           "example.com",
@@ -290,6 +305,10 @@ func TestAuthoritativeDNSResponse(t *testing.T) {
 				"probe_dns_authority_rrs":  2,
 				"probe_dns_additional_rrs": 3,
 			}
+			if test.Probe.QueryType == "SOA" {
+				expectedResults["probe_dns_serial"] = 1000
+			}
+
 			checkRegistryResults(expectedResults, mfs, t)
 		}
 	}