Q129065: PRB: Getsockname() Returns IP Address 0.0.0.0 for UDP

Article: Q129065
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.11,4.0; :3.1,3.5
Operating System(s): 
Keyword(s): kbnetwork kbIP kbOSWinNT310 kbOSWinNT350 kbSDKPlatform kbOSWin95 kbWinsock kbOSWin311 k
Last Modified: 17-OCT-1999

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The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) versions 3.11, 4.0 
- Microsoft Win32 Software Development Kit (SDK) for Windows NT, versions 3.1, 3.5 
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SYMPTOMS
========

By following the steps listed below, you might think you should get back the
interface address over which the connection was made. However, it actually
returns the address 0.0.0.0.

1. Open a UDP socket.

2. Bind it to INADDR_ANY.

3. Call connect() to make a UDP connection.

4. Call getsockname() on your socket.

However, if it was a TCP socket, you would get back the IP address of the
interface.

CAUSE
=====

UDP
---

This is the behaviour expected from some flavors of UNIX, notably those derived
from BSD. When an application calls connect() on a UDP socket that is bound to
INADDR_ANY, the operating system associates the remote address with the local
socket. This saves the programmer from having to specify the remote IP address
in each sendto() or recvfrom(). Instead they may use send() and recv(). Note
that this is just a convenience provided by the operating system; there is no
network traffic associated with this call. At this point, the underlying IP
software determines the interface over which packets will be sent. As described
earlier, under BSD UNIX, calling getsockname() will return the IP address of the
interface to the application.

This however, is not expected behaviour under Windows NT, Windows 95, or
Microsoft TCP IP/32 for Windows for Workgroups version 3.11. Calling
getsockname() will return the IP address 0.0.0.0 (INADDR_ANY). Applications
should not assume that they can get the IP address of the interface.

TCP
---

The behaviour is different if it was a TCP socket. In this case, calling
getsockname() on a connected socket that was bound to INADDR_ANY will return the
IP address of the interface over which the connection was made. The state of the
connection can also be observed by typing 'netstat' at a command prompt.

NOTE: To enumerate all the IP addresses on an IP host, the application should
call gethostname(), call gethostbyname(), and then iterate through the
h_addr_list[] member of the hostent struct returned by gethostbyname() as in
this example:

     char     Hostname[100];
     HOSTENT *pHostEnt;
     int      nAdapter = 0;

     gethostname( Hostname, sizeof( Hostname ));
     pHostEnt = gethostbyname( Hostname );

     while ( pHostEnt->h_addr_list[nAdapter] )
     {
        // pHostEnt->h_addr_list[nAdapter] -the current address in host order
        nAdapter++;
     }

STATUS
======

This behavior is by design.

Additional query words:

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Keywords          : kbnetwork kbIP kbOSWinNT310 kbOSWinNT350 kbSDKPlatform kbOSWin95 kbWinsock kbOSWin311 kbGrpDSNet 
Technology        : kbWinNT400xsearch kbWin32SDKSearch kbAudDeveloper kbWin3xSearch kbSDKSearch kbWin32sSearch kbWin32SDKNT310 kbWin32SDKNT350 kbWinSDKSearch kbWinSDK311 kbWinSDK400
Version           : WINDOWS:3.11,4.0; :3.1,3.5
Issue type        : kbprb

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