[Bug 43249] New: connect(2) changes INADDR_ANY meaning for UDP sockets

[bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r@public.gmane.org]

https://bugzilla.kernel.org/show_bug.cgi?id=43249

           Summary: connect(2) changes INADDR_ANY meaning for UDP sockets
           Product: Documentation
           Version: unspecified
    Kernel Version: 3.0.0
          Platform: All
        OS/Version: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: man-pages
        AssignedTo: documentation_man-pages-ztI5WcYan/vQLgFONoPN62D2FQJk+8+b@public.gmane.org
        ReportedBy: sebastian.lauwers-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
        Regression: No


Created an attachment (id=73297)
 --> (https://bugzilla.kernel.org/attachment.cgi?id=73297)
Testcase which demonstrates the bug scenario

The POSIX man page for connect(2) [1] specifies that:

> If the socket has not already been bound to a local address, connect()
> shall bind it to an address which, unless the socket's address family
> is AF_UNIX, is an unused local address.

The Linux man page for connect(2) does not provide this information, and this
may be confusing, regarding the following behaviour:

After creating a socket of type SOCK_DGRAM through socket(2), and calling
bind(2) on it to have it "listen" on all interfaces (through the use of
INADDR_ANY as value of sockaddr_in.sin_addr.s_addr), it is possible to call
connect(2) on said socket. This behaviour is correctly explained by the
connect(2) Linux man page [2]:

> If the socket sockfd is of type SOCK_DGRAM then addr is the address to 
> which datagrams are sent by default, and the only address from which data‐
> grams are received.

However, upon doing so, the socket is no longer "bound" to any interface (as
could be assumed through the usage of INADDR_ANY), but one in particular (and
it is apparently not possible to know which one, before calling connect(2)).
This seems to contradict the information provided by the ip(7) man page [3]:

> When a process wants to receive new incoming packets or connections, it
> should bind a socket to a local interface address using bind(2). Only
> one IP socket may be bound to any given local (address, port) pair. When
> INADDR_ANY is specified in the bind call, the socket will be bound to
> all local interfaces.

I have attached a test-case which demonstrates this apparently conflicting
behaviour.

I would suggest adding a paragraph or comment that would explain this subtlety,
as it can cause difficult to diagnose issues when using connected UDP sockets.

[1]: http://www.unix.com/man-page/posix/3posix/connect/
[2]: http://linux.die.net/man/2/connect
[3]: http://linux.die.net/man/7/ip

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