Microsoft KB Archive/175396

= Windows Socket Connection from a Multiple-Homed Computer =

Article ID: 175396

Article Last Modified on 1/22/2007

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APPLIES TO


 * Microsoft Windows 2000 Server
 * Microsoft Windows 2000 Advanced Server
 * Microsoft Windows 2000 Professional Edition
 * Microsoft Windows NT Workstation 3.5
 * Microsoft Windows NT Workstation 3.51
 * Microsoft Windows NT Workstation 4.0 Developer Edition
 * Microsoft Windows NT Server 3.5
 * Microsoft Windows NT Server 3.51
 * Microsoft Windows NT Server 4.0 Standard Edition
 * Microsoft Windows 98 Second Edition
 * Microsoft Windows 95

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This article was previously published under Q175396



SUMMARY
This article describes how a network adapter is chosen for an outbound Internet protocol (IP) datagram or stream of datagrams, and how a local source IP address is chosen for those datagrams on a multiple-homed computer.



MORE INFORMATION
Because of the method that is used to determine this behavior, multiple-homed computers may send packets through one network adapter but use the source IP address of another network adapter in the computer. Some hardware or software firewall products may identify these packets as "spoofed," and therefore generate an IP spoofing error.

This article applies specifically to programs that use the Windows Sockets interface to the TCP/IP stack.

For additional information about how an outbound network adapter is chosen forprograms that use NetBIOS over TCP/IP (such as file and print sharing), click the article number below to view the article in the Microsoft Knowledge Base:

166159 NetBIOS Connections from Multi-homed Computer

The TCP/IP component of all Microsoft Windows operating systems is modeled on a "Weak End System" or a "Weak E/S" model. This model gives program developers the greatest amount of leeway when they design programs that use the network and are compatible with Microsoft products. This model also puts the responsibility of the behavior of the networking program on the developers, because the developers specify how the program accesses the TCP/IP stack and responds to incoming and outgoing frames.

When a Windows Sockets program binds to a socket, one of the parameters that is passed in the bind call is the local (source) IP address that should be used for outbound packets. Most programs do not have any knowledge of network topology, so they specify IPADDR_ANY instead of a specific IP address in their bind call. IPADDR_ANY tells the stack that the program is going to let the stack choose the best local IP address to use; the program does not specify the local IP address.

On a computer that has one network adapter, the IP address that is chosen is the IP address of the network adaptor in the computer. However, on a multiple-homed computer, the stack must make a choice. The stack cannot make an intelligent choice until it knows the target IP address for a Transmission Control Protocol (TCP) connection or a User Datagram Protocol (UDP) datagram.

When the program sends a connect call to a target IP address, or sends a send call to a UDP datagram, the stack references the target IP address, and then examines the IP route table so that it can choose the best network adapter over which to send the packet. After this network adapter has been chosen, the stack reads the source IP address associated with that network adapter and uses that IP address as the source IP address for the outbound packets.

If the program specifies a source IP address to use in the bind call, that IP address is used as the source IP address for TCP connections or UDP datagrams sourced from that socket. However, the route table is still used to route the outbound IP datagrams, based on the target IP address. As a result of this behavior, the source IP address may not be the one associated with the network adapter that is chosen to send the packets.

