[dns] validate query_class and query_type config values

This allows to catch simple config errors earlier.

config/config.go

@@ -23,6 +23,7 @@ import (
 
 	yaml "gopkg.in/yaml.v3"
 
+	"github.com/miekg/dns"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/common/config"
 )
@@ -233,6 +234,17 @@ func (s *DNSProbe) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	if s.QueryName == "" {
 		return errors.New("query name must be set for DNS module")
 	}
+	if s.QueryClass != "" {
+		if _, ok := dns.StringToClass[s.QueryClass]; !ok {
+			return fmt.Errorf("query class '%s' is not valid", s.QueryClass)
+		}
+	}
+	if s.QueryType != "" {
+		if _, ok := dns.StringToType[s.QueryType]; !ok {
+			return fmt.Errorf("query type '%s' is not valid", s.QueryType)
+		}
+	}
+
 	return nil
 }
 

config/config_test.go

@@ -51,6 +51,14 @@ func TestLoadBadConfigs(t *testing.T) {
 			ConfigFile:    "testdata/invalid-dns-module.yml",
 			ExpectedError: "error parsing config file: query name must be set for DNS module",
 		},
+		{
+			ConfigFile:    "testdata/invalid-dns-class.yml",
+			ExpectedError: "error parsing config file: query class 'X' is not valid",
+		},
+		{
+			ConfigFile:    "testdata/invalid-dns-type.yml",
+			ExpectedError: "error parsing config file: query type 'X' is not valid",
+		},
 		{
 			ConfigFile:    "testdata/invalid-http-header-match.yml",
 			ExpectedError: "error parsing config file: regexp must be set for HTTP header matchers",

config/testdata/invalid-dns-class.yml

@@ -0,0 +1,8 @@
+modules:
+  dns_test:
+    prober: dns
+    timeout: 5s
+    dns:
+      query_name: example.com
+      query_class: X
+      query_type: A

config/testdata/invalid-dns-type.yml

@@ -0,0 +1,8 @@
+modules:
+  dns_test:
+    prober: dns
+    timeout: 5s
+    dns:
+      query_name: example.com
+      query_class: CH
+      query_type: X