Support using MX records for DNS discovery (#10099)

It's currently possible to use blackbox_exporter to probe MX records
themselves. However it's not possible to do an end-to-end test, like is
possible with SRV records. This makes it possible to use MX records as a
source of hostnames in the same way as SRV records.

Signed-off-by: David Leadbeater <dgl@dgl.cx>

discovery/dns/dns.go

@@ -40,6 +40,8 @@ const (
 	dnsSrvRecordPrefix      = model.MetaLabelPrefix + "dns_srv_record_"
 	dnsSrvRecordTargetLabel = dnsSrvRecordPrefix + "target"
 	dnsSrvRecordPortLabel   = dnsSrvRecordPrefix + "port"
+	dnsMxRecordPrefix       = model.MetaLabelPrefix + "dns_mx_record_"
+	dnsMxRecordTargetLabel  = dnsMxRecordPrefix + "target"
 
 	// Constants for instrumentation.
 	namespace = "prometheus"
@@ -100,7 +102,7 @@ func (c *SDConfig) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	}
 	switch strings.ToUpper(c.Type) {
 	case "SRV":
-	case "A", "AAAA":
+	case "A", "AAAA", "MX":
 		if c.Port == 0 {
 			return errors.New("a port is required in DNS-SD configs for all record types except SRV")
 		}
@@ -136,6 +138,8 @@ func NewDiscovery(conf SDConfig, logger log.Logger) *Discovery {
 		qtype = dns.TypeAAAA
 	case "SRV":
 		qtype = dns.TypeSRV
+	case "MX":
+		qtype = dns.TypeMX
 	}
 	d := &Discovery{
 		names:    conf.Names,
@@ -195,7 +199,7 @@ func (d *Discovery) refreshOne(ctx context.Context, name string, ch chan<- *targ
 	}
 
 	for _, record := range response.Answer {
-		var target, dnsSrvRecordTarget, dnsSrvRecordPort model.LabelValue
+		var target, dnsSrvRecordTarget, dnsSrvRecordPort, dnsMxRecordTarget model.LabelValue
 
 		switch addr := record.(type) {
 		case *dns.SRV:
@@ -206,6 +210,13 @@ func (d *Discovery) refreshOne(ctx context.Context, name string, ch chan<- *targ
 			addr.Target = strings.TrimRight(addr.Target, ".")
 
 			target = hostPort(addr.Target, int(addr.Port))
+		case *dns.MX:
+			dnsMxRecordTarget = model.LabelValue(addr.Mx)
+
+			// Remove the final dot from rooted DNS names to make them look more usual.
+			addr.Mx = strings.TrimRight(addr.Mx, ".")
+
+			target = hostPort(addr.Mx, d.port)
 		case *dns.A:
 			target = hostPort(addr.A.String(), d.port)
 		case *dns.AAAA:
@@ -222,6 +233,7 @@ func (d *Discovery) refreshOne(ctx context.Context, name string, ch chan<- *targ
 			dnsNameLabel:            model.LabelValue(name),
 			dnsSrvRecordTargetLabel: dnsSrvRecordTarget,
 			dnsSrvRecordPortLabel:   dnsSrvRecordPort,
+			dnsMxRecordTargetLabel:  dnsMxRecordTarget,
 		})
 	}
 

discovery/dns/dns_test.go

@@ -80,6 +80,7 @@ func TestDNS(t *testing.T) {
 							"__meta_dns_name":              "web.example.com.",
 							"__meta_dns_srv_record_target": "",
 							"__meta_dns_srv_record_port":   "",
+							"__meta_dns_mx_record_target":  "",
 						},
 					},
 				},
@@ -110,6 +111,7 @@ func TestDNS(t *testing.T) {
 							"__meta_dns_name":              "web.example.com.",
 							"__meta_dns_srv_record_target": "",
 							"__meta_dns_srv_record_port":   "",
+							"__meta_dns_mx_record_target":  "",
 						},
 					},
 				},
@@ -140,12 +142,14 @@ func TestDNS(t *testing.T) {
 							"__meta_dns_name":              "_mysql._tcp.db.example.com.",
 							"__meta_dns_srv_record_target": "db1.example.com.",
 							"__meta_dns_srv_record_port":   "3306",
+							"__meta_dns_mx_record_target":  "",
 						},
 						{
 							"__address__":                  "db2.example.com:3306",
 							"__meta_dns_name":              "_mysql._tcp.db.example.com.",
 							"__meta_dns_srv_record_target": "db2.example.com.",
 							"__meta_dns_srv_record_port":   "3306",
+							"__meta_dns_mx_record_target":  "",
 						},
 					},
 				},
@@ -175,6 +179,7 @@ func TestDNS(t *testing.T) {
 							"__meta_dns_name":              "_mysql._tcp.db.example.com.",
 							"__meta_dns_srv_record_target": "db1.example.com.",
 							"__meta_dns_srv_record_port":   "3306",
+							"__meta_dns_mx_record_target":  "",
 						},
 					},
 				},
@@ -195,6 +200,45 @@ func TestDNS(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "MX record query",
+			config: SDConfig{
+				Names:           []string{"example.com."},
+				Type:            "MX",
+				Port:            25,
+				RefreshInterval: model.Duration(time.Minute),
+			},
+			lookup: func(name string, qtype uint16, logger log.Logger) (*dns.Msg, error) {
+				return &dns.Msg{
+						Answer: []dns.RR{
+							&dns.MX{Preference: 0, Mx: "smtp1.example.com."},
+							&dns.MX{Preference: 10, Mx: "smtp2.example.com."},
+						},
+					},
+					nil
+			},
+			expected: []*targetgroup.Group{
+				{
+					Source: "example.com.",
+					Targets: []model.LabelSet{
+						{
+							"__address__":                  "smtp1.example.com:25",
+							"__meta_dns_name":              "example.com.",
+							"__meta_dns_srv_record_target": "",
+							"__meta_dns_srv_record_port":   "",
+							"__meta_dns_mx_record_target":  "smtp1.example.com.",
+						},
+						{
+							"__address__":                  "smtp2.example.com:25",
+							"__meta_dns_name":              "example.com.",
+							"__meta_dns_srv_record_target": "",
+							"__meta_dns_srv_record_port":   "",
+							"__meta_dns_mx_record_target":  "smtp2.example.com.",
+						},
+					},
+				},
+			},
+		},
 	}
 
 	for _, tc := range testCases {

docs/configuration/configuration.md

@@ -961,8 +961,8 @@ A DNS-based service discovery configuration allows specifying a set of DNS
 domain names which are periodically queried to discover a list of targets. The
 DNS servers to be contacted are read from `/etc/resolv.conf`.
 
-This service discovery method only supports basic DNS A, AAAA and SRV record
+This service discovery method only supports basic DNS A, AAAA, MX and SRV
-queries, but not the advanced DNS-SD approach specified in
+record queries, but not the advanced DNS-SD approach specified in
 [RFC6763](https://tools.ietf.org/html/rfc6763).
 
 The following meta labels are available on targets during [relabeling](#relabel_config):
@@ -970,13 +970,14 @@ The following meta labels are available on targets during [relabeling](#relabel_
 * `__meta_dns_name`: the record name that produced the discovered target.
 * `__meta_dns_srv_record_target`: the target field of the SRV record
 * `__meta_dns_srv_record_port`: the port field of the SRV record
+* `__meta_dns_mx_record_target`: the target field of the MX record
 
 ```yaml
 # A list of DNS domain names to be queried.
 names:
   [ - <string> ]
 
-# The type of DNS query to perform. One of SRV, A, or AAAA.
+# The type of DNS query to perform. One of SRV, A, AAAA or MX.
 [ type: <string> | default = 'SRV' ]
 
 # The port number used if the query type is not SRV.