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UDP/DTLS sockets communication pattern is broken in Cygwin

Hi All

I found a non-standard behavior of UDP sockets in Cygwin. Normally, people = do not experience it, but the communication pattern that I am going to desc= ribe here is often used in DTLS (actually, this is virtually the only way t= o make OpenSSL working with DTLS on the server side), so I suppose that wit= h the growing DTLS popularity people will experience the problem often.

So this is how to reproduce the problem in "plain" UDP (without actually using DTLS):

1)      Server application: open a UDP socket (socket A);

2)      Server application: bind socket A to a local server address (say,;

3)      Server application: wait for a packet from a client application;

4)      Client application: open a UDP socket (socket C);

5)      Client application: bind socket C it to a local client address (say,;

6)      Client application: send a UDP packet P1 from socket C to server socket A (to;

7)      Server application: socket A receives the packet P1 from socket C;

8)      Server application: create another UDP socket B;

9)      Server application: bind socket B TO THE SAME LOCAL ADDRESS as socket A (;

10)   Server application: connect socket B to the remote address of socket C ( by calling connect() on the datagram socket B.

11)   Server application: send packet P2 from socket B to socket C (to

12)   Client application: on socket C, receive packet P2 from socket B (from

13)   Client application: from socket C, send packet P3 to the server address

14)   Server application: socket A receives the packet P3 from the client socket. ERROR !!!

Step 14 is wrong: the packet P3 must be delivered to socket B, because socket B is "connected" 
to the remote address, but socket A  is "unconnected". 
Both sockets (A and B) are "bound" to the same server ad= dress ( but the connected one 
(socket B) must be obtaining packets from the remote address that it is connected to.

This is a very essential functionality for anybody who wants to implement the server-side DTLS communications.

This patterns works in any OS that I tried (all FreeBSD versions, all Linux  versions and Solaris) but Cygwin fails, unfortunately.

I am trying to migrate (port) our server application to Cygwin, and it stops us completely. It works everywhere else.

Please take a look if this is something that can be fixed quickly.

Oleg Moskalenko

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