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UDP/DTLS sockets communication pattern is broken in Cygwin
- From: Oleg Moskalenko <oleg dot moskalenko at citrix dot com>
- To: "'cygwin at cygwin dot com'" <cygwin at cygwin dot com>
- Date: Thu, 11 Apr 2013 14:19:25 -0700
- Subject: UDP/DTLS sockets communication pattern is broken in Cygwin
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, 172.17.17.107:3478);
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, 220.127.116.11:12345);
6) Client application: send a UDP packet P1 from socket C to server socket A (to 172.17.17.107:3478);
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 (172.17.17.107:3478);
10) Server application: connect socket B to the remote address of socket C (18.104.22.168:12345) by calling connect() on the datagram socket B.
11) Server application: send packet P2 from socket B to socket C (to 22.214.171.124:12345).
12) Client application: on socket C, receive packet P2 from socket B (from 172.17.17.107:3478).
13) Client application: from socket C, send packet P3 to the server address 172.17.17.107:3478.
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 126.96.36.199:12345, but socket A is "unconnected".
Both sockets (A and B) are "bound" to the same server ad= dress (172.17.17.107:3478) 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.
Problem reports: http://cygwin.com/problems.html
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