UDP search traverse NAT

Change the CMD_SEARCH message, adding a reply-to-sender-port flag
to allow for replies to traverse a NAT.

The meaning of that flag is that the recipient should ignore the
replyPort field, and instead send a reply to the apparent port
which sent the request.

A forwarder should overwrite the replyPort field, and clear this
new MustReply flag.

Author: mdavidsaver

src/client.cpp

@@ -1044,8 +1044,12 @@ void ContextImpl::tickSearch(SearchKind kind, bool poked)
         // flags and reserved.
         // initially flags[7] is cleared (bcast)
         auto pflags = M.save();
-        to_wire(M, uint8_t(kind == SearchKind::discover ?
-                           pva_search_flags::MustReply : 0u)); // must-reply to discovery, ignore regular negative search
+        to_wire(M, uint8_t(
+                // must-reply to discovery, ignore regular negative search
+                (kind == SearchKind::discover ? pva_search_flags::MustReply : 0u)
+                | pva_search_flags::ReplyPort
+                )
+        );
         to_wire(M, uint8_t(0u));
         to_wire(M, uint16_t(0u));
 

src/pvaproto.h

@@ -579,7 +579,7 @@ void from_wire(Buffer& buf, shared_array<const void>& varr)
 
 struct pva_version {
     enum {
-        client = 2,
+        client = 3,
         server = 2,
     };
 };
@@ -640,6 +640,7 @@ enum pva_app_msg_t : uint8_t {
 struct pva_search_flags {
     enum type_t : uint8_t {
         MustReply = 0x01,
+        ReplyPort = 0x02,
         Unicast   = 0x80,
     };
 };

src/udp_collector.cpp

@@ -317,6 +317,13 @@ void UDPCollector::process_one(const SockAddr &dest, const uint8_t *buf, size_t
                 return;
             }
         }
+        if((origin==Broadcast || origin==Forwarding)
+                && peerVersion >= 3
+                && (flags&pva_search_flags::ReplyPort))
+        {
+            // peer requests reply to (apparent) sender port
+            port = src.port();
+        }
         server.setPort(port);
 
         if(!M.good() || !(flags&pva_search_flags::Unicast) || dest.family()!=AF_INET) {
@@ -325,11 +332,12 @@ void UDPCollector::process_one(const SockAddr &dest, const uint8_t *buf, size_t
         } else if(dest.compare(lo_mcast_addr.addr,false)!=0) {
             assert(buf==&this->buf[cmd_origin_tag_size]);
             // clear unicast flag in forwarded message
-            *save_flags &= ~pva_search_flags::Unicast;
+            *save_flags &= ~(pva_search_flags::Unicast | pva_search_flags::ReplyPort);
             // recipient of forwarded message must use, and trust, replyAddr in body :(
             {
-                FixedBuf R(M.be, save_replyAddr, 16u);
+                FixedBuf R(M.be, save_replyAddr, 16u + 2u);
                 to_wire(R, server);
+                to_wire(R, port);
                 assert(R.good());
             }
             forwardM(dest, buf, nrx);