feat: Add DOH support

pkg/resolvers/doh.go

@@ -1 +1,72 @@
 package resolvers
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"time"
+
+	"github.com/miekg/dns"
+)
+
+// DOHResolver represents the config options for setting up a DOH based resolver.
+type DOHResolver struct {
+	client  *http.Client
+	servers []string
+}
+
+// NewDOHResolver accepts a list of nameservers and configures a DOH based resolver.
+func NewDOHResolver(servers []string) (Resolver, error) {
+	httpClient := &http.Client{
+		Timeout: 10 * time.Second,
+	}
+	return &DOHResolver{
+		client:  httpClient,
+		servers: servers,
+	}, nil
+}
+
+func (r *DOHResolver) Lookup(questions []dns.Question) error {
+	var messages = make([]dns.Msg, 0, len(questions))
+	for _, q := range questions {
+		msg := dns.Msg{}
+		msg.Id = dns.Id()
+		msg.RecursionDesired = true
+		// It's recommended to only send 1 question for 1 DNS message.
+		msg.Question = []dns.Question{q}
+		messages = append(messages, msg)
+	}
+	for _, m := range messages {
+		b, err := m.Pack()
+		if err != nil {
+			return err
+		}
+		for _, srv := range r.servers {
+			resp, err := r.client.Post(srv, "application/dns-message", bytes.NewBuffer(b))
+			if err != nil {
+				return err
+			}
+			if resp.StatusCode != http.StatusOK {
+				return err
+			}
+
+			body, err := ioutil.ReadAll(resp.Body)
+			if err != nil {
+				return err
+			}
+
+			r := &dns.Msg{}
+			err = r.Unpack(body)
+			if err != nil {
+				return err
+			}
+			for _, ans := range r.Answer {
+				if t, ok := ans.(*dns.A); ok {
+					fmt.Println(t.String())
+				}
+			}
+		}
+	}
+	return nil
+}