Use EDNS0 padding when using DoH over POST

This mechanism is horrible, slow (requires re-unpacking and re-packing
the query), should be done at transport layer and not at content layer, and
of course, it is incompatible with some resolvers.

However, in spite of https://go-review.googlesource.com/c/go/+/114316/2/src/net/http/transfer.go ,
we may still end up sending the header and the content in distinct packets.

So, use that horror for POST queries only. For GET, this is not needed.

dnscrypt-proxy/dnsutils.go

@@ -208,3 +208,31 @@ func updateTTL(msg *dns.Msg, expiration time.Time) {
 		}
 	}
 }
+
+func addPaddingIfNoneFound(packet *[]byte, paddingLen int) *[]byte {
+	msg := dns.Msg{Compress: true}
+	if msg.Unpack(*packet) != nil {
+		return packet
+	}
+	edns0 := msg.IsEdns0()
+	if edns0 == nil {
+		msg.SetEdns0(uint16(MaxDNSPacketSize), false)
+		edns0 = msg.IsEdns0()
+		if edns0 == nil {
+			return packet
+		}
+	}
+	for _, option := range edns0.Option {
+		if option.Option() == dns.EDNS0PADDING {
+			return packet
+		}
+	}
+	ext := new(dns.EDNS0_PADDING)
+	ext.Padding = make([]byte, paddingLen)
+	edns0.Option = append(edns0.Option, ext)
+	paddedPacket, err := msg.Pack()
+	if err != nil {
+		return packet
+	}
+	return &paddedPacket
+}

dnscrypt-proxy/xtransport.go

@@ -313,9 +313,6 @@ func (xTransport *XTransport) Fetch(method string, url *url.URL, accept string,
 	if len(contentType) > 0 {
 		header["Content-Type"] = []string{contentType}
 	}
-	if padding != nil {
-		header["X-Pad"] = []string{*padding}
-	}
 	if body != nil {
 		h := sha512.Sum512(*body)
 		qs := url.Query()
@@ -323,6 +320,9 @@ func (xTransport *XTransport) Fetch(method string, url *url.URL, accept string,
 		url2 := *url
 		url2.RawQuery = qs.Encode()
 		url = &url2
+		if padding != nil {
+			body = addPaddingIfNoneFound(body, len(*padding))
+		}
 	}
 	host, _ := ExtractHostAndPort(url.Host, 0)
 	if xTransport.proxyDialer == nil && strings.HasSuffix(host, ".onion") {