feat: Add support for multiple resolvers

pkg/resolvers/utils.go

@@ -1,6 +1,13 @@
 package resolvers
 
-import "github.com/miekg/dns"
+import (
+	"errors"
+	"fmt"
+	"net"
+	"runtime"
+
+	"github.com/miekg/dns"
+)
 
 // prepareMessages takes a slice fo `dns.Question`
 // and initialises `dns.Messages` for each question
@@ -16,3 +23,26 @@ func prepareMessages(questions []dns.Question) []dns.Msg {
 	}
 	return messages
 }
+
+func getDefaultServers() ([]string, error) {
+	if runtime.GOOS == "windows" {
+		// TODO: Add a method for reading system default nameserver in windows.
+		return nil, errors.New(`unable to read default nameservers in this machine`)
+	}
+	// if no nameserver is provided, take it from `resolv.conf`
+	cfg, err := dns.ClientConfigFromFile(DefaultResolvConfPath)
+	if err != nil {
+		return nil, err
+	}
+	servers := make([]string, 0, len(cfg.Servers))
+	for _, s := range cfg.Servers {
+		ip := net.ParseIP(s)
+		// handle IPv6
+		if ip != nil && ip.To4() != nil {
+			servers = append(servers, fmt.Sprintf("%s:%s", s, cfg.Port))
+		} else {
+			servers = append(servers, fmt.Sprintf("[%s]:%s", s, cfg.Port))
+		}
+	}
+	return servers, nil
+}