make CoreDNS DoH Server (#1619)

* WIP: make CoreDNS DoH Server

* It works

* Fix tests

* Review from Tom - on diff. PR

* correct mime type

* Cleanups and use the pkg/nonwriter

* rename and updates

* implement get

* implement GET

* Code review comments

* correct context

* tweaks

* code review

core/dnsserver/https.go

@@ -0,0 +1,56 @@
+package dnsserver
+
+import (
+	"encoding/base64"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+
+	"github.com/miekg/dns"
+)
+
+// mimeTypeDOH is the DoH mimetype that should be used.
+const mimeTypeDOH = "application/dns-message"
+
+// pathDOH is the URL path that should be used.
+const pathDOH = "/dns-query"
+
+// postRequestToMsg extracts the dns message from the request body.
+func postRequestToMsg(req *http.Request) (*dns.Msg, error) {
+	defer req.Body.Close()
+
+	buf, err := ioutil.ReadAll(req.Body)
+	if err != nil {
+		return nil, err
+	}
+	m := new(dns.Msg)
+	err = m.Unpack(buf)
+	return m, err
+}
+
+// getRequestToMsg extract the dns message from the GET request.
+func getRequestToMsg(req *http.Request) (*dns.Msg, error) {
+	values := req.URL.Query()
+	b64, ok := values["dns"]
+	if !ok {
+		return nil, fmt.Errorf("no 'dns' query parameter found")
+	}
+	if len(b64) != 1 {
+		return nil, fmt.Errorf("multiple 'dns' query values found")
+	}
+	return base64ToMsg(b64[0])
+}
+
+func base64ToMsg(b64 string) (*dns.Msg, error) {
+	buf, err := b64Enc.DecodeString(b64)
+	if err != nil {
+		return nil, err
+	}
+
+	m := new(dns.Msg)
+	err = m.Unpack(buf)
+
+	return m, err
+}
+
+var b64Enc = base64.RawURLEncoding