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Q166159: NetBIOS Connections from a Multihomed Computer

Article: Q166159
Product(s): Microsoft Windows NT
Version(s): 4.0
Operating System(s): 
Keyword(s): kbnetworkkbbuglist kbfixlist
Last Modified: 09-AUG-2001

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The information in this article applies to:

- Microsoft Windows NT Workstation version 4.0 
- Microsoft Windows NT Server version 4.0 
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SYMPTOMS
========

When you connect from a multihomed computer running Windows NT to another
computer, the local source IP address used may not be optimal. If the multihomed
system is attached to two different (disjoint) networks, and the target system
is on only one of the networks, there may be a delay in establishing the
connection.

This behavior only occurs on NetBIOS over TCP/IP sessions (such as file and print
sharing); therefore, this information does not apply to Windows Sockets
applications such as Web browsers.


CAUSE
=====

The Windows NT 4.0 (and earlier) redirector used the following logic to
establish a NetBIOS session:

- Place a call to the destination name on all bound transports, in the order
  they are bound.

- Wait for the "primary" (first-bound) transport to complete, and if it was
  successful in reaching the target, set up a session on it and disconnect
  (cancel) the other calls.

In the case of a computer that is multihomed on two connected (by a router)
networks, if the target system is on only one of those networks, there are two
paths to the target system. In this case, the call succeeds on both paths, and
the "primary" transport (for example, the local source IP address associated
with that binding of NetBT) is the chosen one. The other call is cancelled. This
means that the less optimal path (through the router) may win out over the more
optimal (direct) path. In this case, packets sent from the multihomed computer
are internally routed to the directly connected subnet (because the route table
shows a direct route to it), but packets sent in response from the target system
traverse the router. This scenario can be confusing to troubleshoot.

RESOLUTION
==========

A Windows NT redirector has been produced that allows a new configurable
behavior, accepting the first transport to complete a call, and immediately
canceling the calls on the others. A new version of NetBT that supports better
handling of multiple calls and cancels is also available.

To take advantage of this improvement, a registry parameter must be added as
follows:

WARNING: Using Registry Editor incorrectly can cause serious, system-wide
problems that may require you to reinstall Windows NT to correct them. Microsoft
cannot guarantee that any problems resulting from the use of Registry Editor can
be solved. Use this tool at your own risk.

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Rdr\Parameters

  Value Name: IgnoreBindingOrder
  Value type: REG_DWORD
  Default: 0

  Setting this value to "1" causes the redirector to accept the first
  transport to complete a call, rather than waiting for success or failure
  on the "primary" transport.

STATUS
======

Microsoft has confirmed this to be a problem in Windows NT version 4.0. This
problem was corrected in the latest Microsoft Windows NT 4.0 U.S. Service Pack.
For information on obtaining the service pack, query on the following word in
the Microsoft Knowledge Base (without the spaces):

  S E R V P A C K


MORE INFORMATION
================

This problem and hotfix do not apply to Windows Sockets-based connections. When
a Windows Sockets application places a call from a multi-homed host the best
local source address is chosen automatically (using the route table), unless the
application specifies otherwise by choosing a local IP address to use in the
bind() call.

Additional query words:

======================================================================
Keywords          : kbnetwork kbbuglist kbfixlist
Technology        : kbWinNTsearch kbWinNTWsearch kbWinNTW400 kbWinNTW400search kbWinNT400search kbWinNTSsearch kbWinNTS400search kbWinNTS400
Version           : :4.0
Issue type        : kbbug

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