Transform queries via an initial edns mangling plugin

Yet another thing that was utterly broken in dnscrypt-proxy v1.x

dnscrypt-proxy/main.go

@@ -138,6 +138,8 @@ func (proxy *Proxy) processIncomingQuery(serverInfo *ServerInfo, serverProto str
 	if len(query) < MinDNSPacketSize {
 		return
 	}
+	pluginsState := NewPluginsState()
+	query, _ = pluginsState.ApplyQueryPlugins(query)
 	encryptedQuery, clientNonce, err := proxy.Encrypt(serverInfo, query, serverProto)
 	if err != nil {
 		return

dnscrypt-proxy/plugins.go

@@ -0,0 +1,63 @@
+package main
+
+import (
+	"github.com/miekg/dns"
+)
+
+type PluginsAction int
+
+const (
+	PluginsActionNone    = 0
+	PluginsActionForward = 1
+)
+
+type PluginsState struct {
+	sessionData            map[string]interface{}
+	action                 PluginsAction
+	originalMaxPayloadSize int
+	maxPayloadSize         int
+	proto                  string
+}
+
+func NewPluginsState() PluginsState {
+	return PluginsState{action: PluginsActionForward, maxPayloadSize: MaxDNSUDPPacketSize - ResponseOverhead}
+}
+
+func (pluginsState *PluginsState) ApplyQueryPlugins(packet []byte) ([]byte, error) {
+	msg := dns.Msg{}
+	if err := msg.Unpack(packet); err != nil {
+		return packet, err
+	}
+
+	if ret := pluginsState.BuiltinPluginsGetSetPayloadSize(&msg); ret != nil {
+		return packet, ret
+	}
+
+	packet2, err := msg.PackBuffer(packet)
+	if err != nil {
+		return packet, err
+	}
+	return packet2, nil
+}
+
+func (pluginsState *PluginsState) BuiltinPluginsGetSetPayloadSize(msg *dns.Msg) error {
+	pluginsState.originalMaxPayloadSize = 512 - ResponseOverhead
+	opt := msg.IsEdns0()
+	dnssec := false
+	if opt != nil {
+		pluginsState.originalMaxPayloadSize = Min(int(opt.UDPSize())-ResponseOverhead, pluginsState.originalMaxPayloadSize)
+		dnssec = opt.Do()
+	}
+	pluginsState.maxPayloadSize = Min(MaxDNSUDPPacketSize-ResponseOverhead, Max(pluginsState.originalMaxPayloadSize, pluginsState.maxPayloadSize))
+	if pluginsState.maxPayloadSize > 512 {
+		extra2 := []dns.RR{}
+		for _, extra := range msg.Extra {
+			if extra.Header().Rrtype != dns.TypeOPT {
+				extra2 = append(extra2, extra)
+			}
+		}
+		msg.Extra = extra2
+		msg.SetEdns0(uint16(pluginsState.maxPayloadSize), dnssec)
+	}
+	return nil
+}