Implement DNS over HTTPS client

2 files changed, 158 insertions, 0 deletions

diff --git a/dns/http/client.go b/dns/http/client.go
new file mode 100644
index 0000000..d26223a
--- /dev/null
+++ b/dns/http/client.go
@@ -0,0 +1,74 @@
+package http
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"net/url"
+	"time"
+
+	"github.com/miekg/dns"
+)
+
+// RFC8484 (https://tools.ietf.org/html/rfc8484) claims that application/dns-message should be used, as does
+// https://developers.cloudflare.com/1.1.1.1/dns-over-https/wireformat/
+//
+// However, Cloudflares service only accept this media type from one of the older RFC drafts
+// (https://tools.ietf.org/html/draft-ietf-doh-dns-over-https-05).
+const mimeType = "application/dns-udpwireformat"
+
+// Client is a DNS-over-HTTPS client.
+type Client struct {
+	httpClient *http.Client
+}
+
+// NewClient creates a new DNS-over-HTTPS client.
+func NewClient(timeout time.Duration) *Client {
+	return &Client{httpClient: &http.Client{Timeout: timeout}}
+}
+
+// Exchange sends the DNS message msg to the DNS-over-HTTPS endpoint addr and returns the response.
+func (c *Client) Exchange(msg *dns.Msg, addr string) (*dns.Msg, time.Duration, error) {
+	u, err := url.Parse(addr)
+	if err != nil {
+		return nil, 0, fmt.Errorf("invalid url: %w", err)
+	}
+
+	p, err := msg.Pack()
+	if err != nil {
+		return nil, 0, err
+	}
+
+	r, err := http.NewRequest(http.MethodPost, u.String(), bytes.NewReader(p))
+	if err != nil {
+		return nil, 0, err
+	}
+	r.Header.Set("Content-Type", mimeType)
+	r.Header.Set("Accept", mimeType)
+
+	t := time.Now()
+	resp, err := c.httpClient.Do(r)
+	if err != nil {
+		return nil, 0, err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, 0, fmt.Errorf("server returned HTTP %d error: %q", resp.StatusCode, resp.Status)
+	}
+	if contentType := resp.Header.Get("Content-Type"); contentType != mimeType {
+		return nil, 0, fmt.Errorf("server returned unexpected ContentType %q, want %q", contentType, mimeType)
+	}
+
+	p, err = ioutil.ReadAll(resp.Body)
+	if err != nil {
+		return nil, 0, err
+	}
+	rtt := time.Since(t)
+	reply := dns.Msg{}
+	if err := reply.Unpack(p); err != nil {
+		return nil, 0, err
+	}
+	return &reply, rtt, nil
+}
diff --git a/dns/http/client_test.go b/dns/http/client_test.go
new file mode 100644
index 0000000..30c8a00
--- /dev/null
+++ b/dns/http/client_test.go
@@ -0,0 +1,84 @@
+package http
+
+import (
+	"encoding/hex"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/miekg/dns"
+)
+
+// Reponse and request data from https://developers.cloudflare.com/1.1.1.1/dns-over-https/wireformat/.
+const response = `
+00 00 81 80 00 01 00 01  00 00 00 00 03 77 77 77
+07 65 78 61 6d 70 6c 65  03 63 6f 6d 00 00 01 00
+01 03 77 77 77 07 65 78  61 6d 70 6c 65 03 63 6f
+6d 00 00 01 00 01 00 00  00 80 00 04 C0 00 02 01
+`
+
+const request = `
+00 00 01 00 00 01 00 00  00 00 00 00 03 77 77 77
+07 65 78 61 6d 70 6c 65  03 63 6f 6d 00 00 01 00
+01
+`
+
+func hexDecode(s string) []byte {
+	replacer := strings.NewReplacer(" ", "", "\n", "")
+	b, err := hex.DecodeString(replacer.Replace(s))
+	if err != nil {
+		panic(err)
+	}
+	return b
+}
+
+func handler(w http.ResponseWriter, r *http.Request) {
+	contentType := r.Header.Get("Content-Type")
+	accept := r.Header.Get("Accept")
+	const mimeType = "application/dns-udpwireformat"
+
+	if contentType != mimeType {
+		w.WriteHeader(http.StatusUnsupportedMediaType)
+		io.WriteString(w, "invalid value for header \"Content-Type\"")
+		return
+	}
+	if accept != mimeType {
+		w.WriteHeader(http.StatusUnsupportedMediaType)
+		io.WriteString(w, "invalid value for header \"Accept\"")
+		return
+	}
+	w.Header().Set("Content-Type", mimeType)
+	w.Write(hexDecode(response))
+}
+
+func TestExchange(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(handler))
+	defer srv.Close()
+
+	msg := dns.Msg{}
+	if err := msg.Unpack(hexDecode(request)); err != nil {
+		t.Fatal(err)
+	}
+
+	client := NewClient(10 * time.Second)
+	reply, _, err := client.Exchange(&msg, srv.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	want := `;; opcode: QUERY, status: NOERROR, id: 0
+;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
+
+;; QUESTION SECTION:
+;www.example.com.	IN	 A
+
+;; ANSWER SECTION:
+www.example.com.	128	IN	A	192.0.2.1
+`
+	if got := reply.String(); got != want {
+		t.Errorf("got %s, want %s", got, want)
+	}
+}