Make use of 'ephemeral unicast udp port' for minmdns optional

src/lib/dnssd/minimal_mdns/BUILD.gn

@@ -14,6 +14,31 @@
 
 import("//build_overrides/chip.gni")
 
+declare_args() {
+  # Makes unicast queries use a separate UDP endpoint that have an ephemeral port
+  #
+  # In practice this means unicast replies will be received on a dedicated port
+  # and will work even if competing mdns servers (including other chip apps)
+  # run on the same machine.
+  #
+  # Downside is that replies sent to a port other than 5353 are considered
+  # LEGACY by mDNS and will include the query section in them (i.e. larger
+  # payloads) and clients need to allocate more resources for this (need one
+  # more UDP socket and corresponding code for the unicast query sending)
+  chip_use_ephemeral_port_for_mdns_unicast_query =
+      current_os == "mac" || current_os == "linux" || current_os == "android"
+}
+
+config("config") {
+  defines = []
+
+  if (chip_use_ephemeral_port_for_mdns_unicast_query) {
+    defines += [ "CHIP_MINMDNS_USE_EPHEMERAL_UNICAST_PORT=1" ]
+  } else {
+    defines += [ "CHIP_MINMDNS_USE_EPHEMERAL_UNICAST_PORT=0" ]
+  }
+}
+
 static_library("minimal_mdns") {
   sources = [
     "ActiveResolveAttempts.cpp",
@@ -39,4 +64,6 @@ static_library("minimal_mdns") {
     "${chip_root}/src/lib/dnssd/minimal_mdns/responders",
     "${chip_root}/src/platform",
   ]
+
+  public_configs = [ ":config" ]
 }

src/lib/dnssd/minimal_mdns/Server.cpp

@@ -57,6 +57,8 @@ class ListenSocketPickerDelegate : public ServerBase::BroadcastSendDelegate
     chip::Inet::UDPEndPoint * Accept(ServerBase::EndpointInfo * info) override { return info->listen_udp; }
 };
 
+#if CHIP_MINMDNS_USE_EPHEMERAL_UNICAST_PORT
+
 /**
  * Extracts the Querying UDP Endpoint from an underlying ServerBase::EndpointInfo
  */
@@ -66,6 +68,12 @@ class QuerySocketPickerDelegate : public ServerBase::BroadcastSendDelegate
     chip::Inet::UDPEndPoint * Accept(ServerBase::EndpointInfo * info) override { return info->unicast_query_udp; }
 };
 
+#else
+
+using QuerySocketPickerDelegate = ListenSocketPickerDelegate;
+
+#endif
+
 /**
  * Validates that an endpoint belongs to a specific interface/ip address type before forwarding the
  * endpoint accept logic to another BroadcastSendDelegate.
@@ -180,11 +188,13 @@ void ShutdownEndpoint(mdns::Minimal::ServerBase::EndpointInfo & aEndpoint)
         aEndpoint.listen_udp = nullptr;
     }
 
+#if CHIP_MINMDNS_USE_EPHEMERAL_UNICAST_PORT
     if (aEndpoint.unicast_query_udp != nullptr)
     {
         aEndpoint.unicast_query_udp->Free();
         aEndpoint.unicast_query_udp = nullptr;
     }
+#endif
 }
 
 } // namespace
@@ -254,13 +264,15 @@ CHIP_ERROR ServerBase::Listen(chip::Inet::InetLayer * inetLayer, ListenIterator
             endpointIndex++;
         }
 
+#if CHIP_MINMDNS_USE_EPHEMERAL_UNICAST_PORT
         // Separate UDP endpoint for unicast queries, bound to 0 (i.e. pick random ephemeral port)
         //   - helps in not having conflicts on port 5353, will receive unicast replies directly
         //   - has a *DRAWBACK* of unicast queries being considered LEGACY by mdns since they do
         //     not originate from 5353 and the answers will include a query section.
         ReturnErrorOnFailure(inetLayer->NewUDPEndPoint(&info->unicast_query_udp));
         ReturnErrorOnFailure(info->unicast_query_udp->Bind(addressType, chip::Inet::IPAddress::Any, 0, interfaceId));
         ReturnErrorOnFailure(info->unicast_query_udp->Listen(OnUdpPacketReceived, nullptr /*OnReceiveError*/, this));
+#endif
     }
 
     return autoShutdown.ReturnSuccess();

src/lib/dnssd/minimal_mdns/Server.h

@@ -79,8 +79,10 @@ class ServerBase
     {
         chip::Inet::InterfaceId interfaceId = chip::Inet::InterfaceId::Null();
         chip::Inet::IPAddressType addressType;
-        chip::Inet::UDPEndPoint * listen_udp        = nullptr;
+        chip::Inet::UDPEndPoint * listen_udp = nullptr;
+#if CHIP_MINMDNS_USE_EPHEMERAL_UNICAST_PORT
         chip::Inet::UDPEndPoint * unicast_query_udp = nullptr;
+#endif
     };
 
     /**
@@ -103,8 +105,10 @@ class ServerBase
     {
         for (size_t i = 0; i < mEndpointCount; i++)
         {
-            mEndpoints[i].listen_udp        = nullptr;
+            mEndpoints[i].listen_udp = nullptr;
+#if CHIP_MINMDNS_USE_EPHEMERAL_UNICAST_PORT
             mEndpoints[i].unicast_query_udp = nullptr;
+#endif
         }
 
         BroadcastIpAddresses::GetIpv6Into(mIpv6BroadcastAddress);