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    How do I make Winsock use a specific network interface
    Ebook. On a machine with multiple network interfaces (a modem for dialup Internet and a LAN card).


Visual Basic Winsock

    On a machine with multiple network interfaces (a modem for dialup Internet and a LAN card, for example), it can sometimes be useful to force Winsock to use a specific interface. Before I go into how, keep in mind that the routing layer of the stack exists to handle this for you. If your setup isn't working the way you want, maybe you just need to change the routing tables. (This is done with the "route" and "netstat" command-line programs on Microsoft stacks.)
    There are two common reasons why you might want to force Winsock to use a particular network interface. The first is when you only want your server program to handle incoming connections on a particular interface. For example, if you have an NT machine set up as an Internet gateway, and it also runs a server that you only want internal LAN users to be able to access, you will want to set it to only listen on the LAN interface. The other reason is that you have two or more possible outgoing routes, and you want your client program to connect using a particular one without the routing layer getting in the way.
    You can do both of these things with the bind() function. Using one of the "get my IP addresses" examples, you can present your user with a list of possible addresses. Then they can pick the appropriate address to use, which your program will use in the bind() call. Obviously, this is only feasible for programs intended for advanced users.
    Incidentally, this is how virtual hosting on the Internet works. A single server is set up with a single network card but several IP addresses. Windows NT derivatives can do this, but Win95 derivatives cannot. To set this up in NT, go into the TCP/IP area of the Network control panel, and then click the Advanced button. IIRC, you can enter up to five network addresses per interface in NT Workstation, perhaps more in NT Server.
    Note that this information does not apply to the Win9x multihomed computer Dialup Networking bug. This problem cannot be fixed by bind()ing to the LAN interface in an effort to force the OS to use it exclusively. The problem is due to a bug in the OS's name resolver. See the DUN bug FAQ item for workarounds.

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Visual Basic Winsock

    Keywords: - Visual, Basic, eBook, Winsock

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