From 43f3e64bd9db491bd4c158a6136a8b3bc9762ad6 Mon Sep 17 00:00:00 2001
From: Frank Denis <github@pureftpd.org>
Date: Mon, 5 Feb 2018 11:30:10 +0100
Subject: [PATCH] DoH: fallback to GET on servers that don't support POST

---
 dnscrypt-proxy/proxy.go       |  2 +-
 dnscrypt-proxy/serversInfo.go | 14 ++++++++++----
 dnscrypt-proxy/sources.go     |  2 +-
 dnscrypt-proxy/stamps.go      |  4 +++-
 dnscrypt-proxy/xtransport.go  | 15 ++++++++++++++-
 5 files changed, 29 insertions(+), 8 deletions(-)

diff --git a/dnscrypt-proxy/proxy.go b/dnscrypt-proxy/proxy.go
index 775043f7..d62a2776 100644
--- a/dnscrypt-proxy/proxy.go
+++ b/dnscrypt-proxy/proxy.go
@@ -284,7 +284,7 @@ func (proxy *Proxy) processIncomingQuery(serverInfo *ServerInfo, clientProto str
 		} else if serverInfo.Proto == StampProtoTypeDoH {
 			tid := TransactionID(query)
 			SetTransactionID(query, 0)
-			resp, _, err := proxy.xTransport.Post(serverInfo.URL, "application/dns-udpwireformat", "application/dns-udpwireformat", query, proxy.timeout)
+			resp, _, err := proxy.xTransport.DoHQuery(serverInfo.useGet, serverInfo.URL, query, proxy.timeout)
 			SetTransactionID(query, tid)
 			if err != nil {
 				return
diff --git a/dnscrypt-proxy/serversInfo.go b/dnscrypt-proxy/serversInfo.go
index 9db894a6..93d5cb54 100644
--- a/dnscrypt-proxy/serversInfo.go
+++ b/dnscrypt-proxy/serversInfo.go
@@ -54,6 +54,7 @@ type ServerInfo struct {
 	lastActionTS       time.Time
 	rtt                ewma.MovingAverage
 	initialRtt         int
+	useGet             bool
 }
 
 type LBStrategy int
@@ -243,11 +244,15 @@ func (serversInfo *ServersInfo) fetchDoHServerInfo(proxy *Proxy, name string, st
 	body := []byte{
 		0xca, 0xfe, 0x01, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x02, 0x00, 0x01, 0x00, 0x00, 0x29, 0x10, 0x00, 0x00, 0x00, 0x80, 0x00, 0x00, 0x00,
 	}
-	_, _, err := proxy.xTransport.Post(url, "application/dns-udpwireformat", "application/dns-udpwireformat", body, proxy.timeout)
-	if err != nil {
-		return ServerInfo{}, err
+	useGet := false
+	if _, _, err := proxy.xTransport.DoHQuery(useGet, url, body, proxy.timeout); err != nil {
+		useGet = true
+		if _, _, err := proxy.xTransport.DoHQuery(useGet, url, body, proxy.timeout); err != nil {
+			return ServerInfo{}, err
+		}
+		dlog.Debugf("Server [%s] doesn't appear to support POST; falling back to GET requests", name)
 	}
-	resp, rtt, err := proxy.xTransport.Post(url, "application/dns-udpwireformat", "application/dns-udpwireformat", body, proxy.timeout)
+	resp, rtt, err := proxy.xTransport.DoHQuery(useGet, url, body, proxy.timeout)
 	if err != nil {
 		return ServerInfo{}, err
 	}
@@ -297,6 +302,7 @@ func (serversInfo *ServersInfo) fetchDoHServerInfo(proxy *Proxy, name string, st
 		URL:        url,
 		HostName:   stamp.providerName,
 		initialRtt: int(rtt.Nanoseconds() / 1000000),
+		useGet:     useGet,
 	}
 	return serverInfo, nil
 }
diff --git a/dnscrypt-proxy/sources.go b/dnscrypt-proxy/sources.go
index c95e280f..5ad4c38e 100644
--- a/dnscrypt-proxy/sources.go
+++ b/dnscrypt-proxy/sources.go
@@ -80,7 +80,7 @@ func fetchWithCache(xTransport *XTransport, urlStr string, cacheFile string) (in
 	if err != nil {
 		return
 	}
-	resp, _, err = xTransport.Get(url, 30*time.Second)
+	resp, _, err = xTransport.Get(url, "", 30*time.Second)
 	if err == nil && resp != nil && (resp.StatusCode < 200 || resp.StatusCode > 299) {
 		err = fmt.Errorf("Webserver returned code %d", resp.StatusCode)
 		return
diff --git a/dnscrypt-proxy/stamps.go b/dnscrypt-proxy/stamps.go
index 21c12af5..9392ea1f 100644
--- a/dnscrypt-proxy/stamps.go
+++ b/dnscrypt-proxy/stamps.go
@@ -154,7 +154,9 @@ func newDoHServerStamp(bin []byte) (ServerStamp, error) {
 			return stamp, errors.New("Invalid stamp")
 		}
 		pos++
-		stamp.hashes = append(stamp.hashes, bin[pos:pos+len])
+		if len > 0 {
+			stamp.hashes = append(stamp.hashes, bin[pos:pos+len])
+		}
 		pos += len
 		if vlen&0x80 != 0x80 {
 			break
diff --git a/dnscrypt-proxy/xtransport.go b/dnscrypt-proxy/xtransport.go
index 733aa1e5..1fad6e07 100644
--- a/dnscrypt-proxy/xtransport.go
+++ b/dnscrypt-proxy/xtransport.go
@@ -3,6 +3,7 @@ package main
 import (
 	"bytes"
 	"context"
+	"encoding/base64"
 	"errors"
 	"fmt"
 	"io"
@@ -152,7 +153,7 @@ func (xTransport *XTransport) Fetch(method string, url *url.URL, accept string,
 	return resp, rtt, err
 }
 
-func (xTransport *XTransport) Get(url *url.URL, timeout time.Duration) (*http.Response, time.Duration, error) {
+func (xTransport *XTransport) Get(url *url.URL, accept string, timeout time.Duration) (*http.Response, time.Duration, error) {
 	return xTransport.Fetch("GET", url, "", "", nil, timeout)
 }
 
@@ -160,3 +161,15 @@ func (xTransport *XTransport) Post(url *url.URL, accept string, contentType stri
 	bc := ioutil.NopCloser(bytes.NewReader(body))
 	return xTransport.Fetch("POST", url, accept, contentType, &bc, timeout)
 }
+func (xTransport *XTransport) DoHQuery(useGet bool, url *url.URL, body []byte, timeout time.Duration) (*http.Response, time.Duration, error) {
+	dataType := "application/dns-udpwireformat"
+	if useGet {
+		qs := url.Query()
+		qs.Add("ct", "")
+		qs.Add("body", base64.RawURLEncoding.EncodeToString(body))
+		url2 := *url
+		url2.RawQuery = qs.Encode()
+		return xTransport.Get(&url2, dataType, timeout)
+	}
+	return xTransport.Post(url, dataType, dataType, body, timeout)
+}