使用GNAT.Sockets接收IP组播流量失败的问题排查与解决

我最近在做Ada语言的IP组播实验时碰到了个棘手的问题：监听程序完全收不到发往目标组播组的流量，调用Receive_Socket后一直阻塞，但实际组播数据包是存在的。

已完成的排查验证： #

• 用Wireshark抓包确认：本机已发送组播加入请求，且确实有数据发往目标组播组239.255.128.128:8807
• 用Windows命令netsh interfaces ip show joins验证：程序运行时目标组播组的引用计数为1，关闭程序后计数回到0，说明操作系统已正确注册组播成员关系

我的监听程序代码（调用参数：Mcast_IP => "239.255.128.128"、Mcast_Port => "8807"）： #

with GNAT.Sockets;
with Ada.Streams;
with Ada.Text_IO;
procedure Receive_Multicast (Mcast_IP : in String; Mcast_Port : in String) is
   package GS renames GNAT.Sockets;
   package AS renames Ada.Streams;
   package Tio renames Ada.Text_IO;
   use GS;
   use type Ada.Streams.Stream_Element_Offset;
   Socket : GS.Socket_Type;
   Address : GS.Sock_Addr_Type;
   Data : AS.Stream_Element_Array (1 .. 2**16);
   Offset : AS.Stream_Element_Offset;
   Sender : GS.Sock_Addr_Type;
begin
   Address.Addr := Any_Inet_Addr;
   Address.Port := Port_Type'Value (Mcast_Port);
   Create_Socket (Socket => Socket, Family => Family_Inet, Mode => Socket_Datagram);
   Bind_Socket (Socket, Address);
   -- Set socket options
   Set_Socket_Option (Socket, Socket_Level, (Reuse_Address, True));
   Set_Socket_Option (Socket, IP_Protocol_For_IP_Level, (Multicast_TTL, 1));
   Set_Socket_Option (Socket, IP_Protocol_For_IP_Level, (Multicast_Loop, True));
   Set_Socket_Option (Socket, IP_Protocol_For_IP_Level, (Add_Membership, Inet_Addr (Mcast_IP), Any_Inet_Addr));
   Tio.Put_Line ("Listening for MULTICASTS on port " & Address.Port'Img);
   -- Receive the packet from the socket.
   loop
      Tio.Put_Line ("Waiting for incoming packets...");
      Receive_Socket (Socket => Socket, Item => Data, Last => Offset, From => Sender);
      Tio.Put_Line ("Received " & Offset'Img & " bytes.");
   end loop;
end Receive_Multicast;

程序启动后会卡在Receive_Socket调用处持续阻塞，明明Wireshark能看到组播包抵达本机，但程序就是接收不到。

最终解决办法： #

折腾半天发现是卡巴斯基杀毒软件拦截了本机的组播回环流量，卸载卡巴斯基之后，程序立刻就能正常接收组播数据包了！另外要说明的是，这段代码本身是完全合规的，公认的组播监听实现方案也能完美适配这个场景。

内容的提问来源于stack exchange，提问作者chrillof