Bind Basics

In Windows NT, the system typically assigns network-based TCP/IP services to a well-known port, in accordance with Request for Comments (RFC) 1700. POP3 servers, the topic of this continuing series of articles, run on TCP port 110. The first step in getting your mail server to answer requests on that port is to bind the socket.

To understand how to bind a TCP socket to a port, you need to look at the arguments for the bind() function. One of these arguments (the second) is a pointer to a sockaddr structure. For IP applications, that pointer is typically a sockaddr_in structure that contains the numeric IP address and port that you want to bind to locally. If you can't easily identify what interfaces are available, you can simply bind to all available local interfaces by specifying INADDR_ANY as the address.

One security risk that you need to be aware of is that users can bind two sockets to the same port using a socket option known as SO_REUSEADDR. In other words, two different applications can answer connections on the same port. So, in the case of a POP3 server, one application might be delivering mail, while another is stealing passwords. Working with NT 4.0 and earlier requires a lot of Registry reads to determine which IP addresses are available. As a result, many programmers simply specify INADDR_ANY to get all the interfaces. Under Windows 2000, you can have new interfaces popping up and down on the fly. Unless you’re prepared to write code to detect these changes, your service might not behave properly if you don’t bind to all the interfaces.

You're probably thinking, "Why not just specify INADDR_ANY all the time and avoid these problems?" Here's the catch: If someone does bind to a specific IP address, the OS prefers the specifically bound socket to one that binds to all available interfaces. To handle this problem, Microsoft implemented the SO_EXCLUSIVEADDRUSE socket option in NT 4.0 Service Pack 4 (SP4) and Win2K. . . .