fix(dns): retry configurable resolvers

Treat resolver transport failures as inconclusive checks so they do not open outage alerts.

checks/cdns/config.go

@@ -1,6 +1,9 @@
 package cdns
 
 import (
+	"net"
+	"os"
+	"strings"
 	"time"
 
 	"github.com/miekg/dns"
@@ -33,7 +36,27 @@ var (
 
 func init() {
 	conf = &dns.ClientConfig{
-		Servers: []string{"8.8.8.8", "1.1.1.1", "77.88.8.8"},
+		Servers: resolversFromEnv(),
 		Port:    "53",
 	}
 }
+
+func resolversFromEnv() []string {
+	const defaults = "188.93.17.19,188.93.16.19,82.117.245.122"
+	resolvers := strings.Split(os.Getenv("RSMON_DNS_RESOLVERS"), ",")
+	if strings.TrimSpace(os.Getenv("RSMON_DNS_RESOLVERS")) == "" {
+		resolvers = strings.Split(defaults, ",")
+	}
+
+	valid := make([]string, 0, len(resolvers))
+	for _, resolver := range resolvers {
+		resolver = strings.TrimSpace(resolver)
+		if net.ParseIP(resolver) != nil {
+			valid = append(valid, resolver)
+		}
+	}
+	if len(valid) == 0 {
+		return strings.Split(defaults, ",")
+	}
+	return valid
+}