发表于：2005-12-29 08:41:00
3楼
F O U N D A T I O N
Using OPC via DCOM
with
Microsoft Windows XP Service Pack 2
Karl-Heinz Deiretsbacher, Siemens AG
Jim Luth, ICONICS, Inc.
OPC Foundation Technical Director
Rashesh Mody, Invensys/Wonderware
OPC Foundation Chief Architect
Using OPC via DCOM with
Microsoft Windows XP
Service Pack 2
F O U N D A T I O N
Released
© 2004 OPC Foundation, Inc. -1- All Rights Reserved
Abstract
The major goal of Windows XP Service Pack 2 is to reduce common available scenarios
for malicious attack on Windows XP. The Service Pack will reduce the effect of most
common attacks in four ways:
1. Improvement in shielding Windows XP from the network
a. RPC and DCOM communication enhancements
b. Enhancements to the internal Windows firewall
2. Enhanced memory protection
3. Safer handling of e-mail
4. Internet Explorer security enhancements.
Most OPC Clients and Servers use DCOM to communicate over a network and thus will
be impacted due to the changes in Service Pack 2. When Service Pack 2 is installed with
its default configuration settings, OPC communication via DCOM will cease to work.
This paper describes the settings necessary to restore OPC communication when using
XP Service Pack 2 (SP2).
SP2 includes many changes and security enhancements, two of which directly impact
OPC via DCOM. First new DCOM limit settings have been added. Secondly the software
firewall included with XP has been greatly enhanced and is turned on by default.
Since the callback mechanism used by OPC essentially turns the OPC Client into a
DCOM Server and the OPC Server into a DCOM Client, the instructions provided here
must be followed on all nodes that contain either OPC Servers or OPC Clients.
Note: OPC communication that is confined to a single machine (using COM, but not
DCOM) will continue to work properly after installing XP SP2 without following the
instructions in this white paper.
Windows Firewall
The Windows Firewall allows traffic across the network interface when initiated locally,
but by default stops any incoming “unsolicited” traffic. However, this firewall is
“exception” based, meaning that the administrator can specify applications and ports that
are exceptions to the rule and can respond to unsolicited requests.
The firewall exceptions can be specified at two main levels, the application level and the
port and protocol level. The application level is where you specify which applications are
able to respond to unsolicited requests and the port and protocol level is where you can
specify the firewall to allow or disallow traffic on a specific port for either TCP or UDP
traffic. To make any OPC client/server application work via DCOM, changes need to be
made on both levels.
Using OPC via DCOM with
Microsoft Windows XP
Service Pack 2
F O U N D A T I O N
Released
© 2004 OPC Foundation, Inc. -2- All Rights Reserved
Note: Developers of OPC Products may want to automatically make the necessary
firewall settings programmatically. Microsoft supplies the Windows Firewall API to
support this:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/ics/ics/inetfwauthorizedapplication_name.asp
Configuring the Firewall
1. By default the windows firewall is set to “On”. This setting is recommended by
Microsoft and by OPC to give your machine the highest possible protection. For trouble
shooting, you may wish to temporarily turn off the firewall to prove or disprove that the
firewall configuration is the source of any communication failure.
Note: It may be appropriate to permanently turn off the firewall if the machine is
sufficiently protected behind a corporate firewall. When turned off, the individual
firewall settings outlined here need not be performed to allow OPC communication.
Using OPC via DCOM with
Microsoft Windows XP
Service Pack 2
F O U N D A T I O N
Released
© 2004 OPC Foundation, Inc. -3- All Rights Reserved
2. Select the “Exceptions” tab and add all OPC Clients and Servers to the exception list.
Also add Microsoft Management Console (used by the DCOM configuration utility in the
next section) and the OPC utility OPCEnum.exe found in the Windows\System32
directory.
In the Add a Program dialog, there is a listing of most applications on the machine, but
note that not all of them show up on this list. Use the “Browse” button to find other
executables installed on the computer.
Note: Only EXE files are added to the exceptions list. For in-process OPC Servers and
Clients (DLLs and OCXs) you will need to add the EXE applications that call them to the
list instead.
Using OPC via DCOM with
Microsoft Windows XP
Service Pack 2
F O U N D A T I O N
Released
© 2004 OPC Foundation, Inc. -4- All Rights Reserved
3. Add TCP port 135 as it is needed to initiate DCOM communications, and allow for
incoming echo requests. In the Exceptions tab of the Windows Firewall, click on Add
Port.
In the Add a Port dialog, fill out the fields as follows:
Name: DCOM
Port number: 135
Choose the TCP radio button
Using OPC via DCOM with
Microsoft Windows XP
Service Pack 2
F O U N D A T I O N
Released
© 2004 OPC Foundation, Inc. -5- All Rights Reserved
DCOM Enhancements
Service Pack 2 for Windows XP has also made some security enhancements to DCOM;
two in particular need to be taken into consideration when using OPC on a network: First,
the default Launch and Access permissions dialogs have been modified to allow the user
to configure “limits” on the permissions given to applications using DCOM. Secondly,
for each user now defined in the Launch and Access permissions, both local and remote
access can be explicitly defined.
A brief background on default Launch and Access permissions in DCOM: Launch
permissions define who can launch a COM based application (such as an OPC server)
both over the network or locally. Access permissions define who can access that
application once it has been launched. Applications can get their Launch and Access
permissions from one of three places: they can use explicitly defined setting for their
application, they can use the default permissions or they can set their own permissions
programmatically. Because an application could set its own permissions
programmatically, the explicitly defined or default settings, although set properly, may
not be used and therefore the user is not able to explicitly have control over these settings.
To overcome this security flaw, Microsoft has added “limits” to the DCOM security
settings from Launch and Access to limit the permissions that an application can use.
This limit prevents the application from using permissions beyond what is specified in
the DCOM configuration settings. By default the limits set by Service Pack 2 will not
allow for OPC communications over the network.
In addition to the new permissions limits, one must now specify if the user or group
specified has permissions locally or remotely (or both). In order for OPC applications to
work over the network with DCOM, the permissions must be set such that remote users
can launch and/or access the OPC servers and clients on the machine.
Configuring DCOM
Follow these steps to configure DCOM for OPC Communications using Windows XP
Service Pack 2:
1. Go to Start -> Run and type DCOMCnfg and click on OK.
Using OPC via DCOM with
Microsoft Windows XP
Service Pack 2
F O U N D A T I O N
Released
© 2004 OPC Foundation, Inc. -6- All Rights Reserved
2. Click on Component Services under the Console Root to expand it.
3. Click on Computers under Component Services to expand it.
4. Right click on My Computer in the pane on the right and select Properties
5. Go to the COM Security tab and note these are the four permission configurations
that we will have to edit:
Using OPC via DCOM with
Microsoft Windows XP
Service Pack 2
F O U N D A T I O N
Released
© 2004 OPC Foundation, Inc. -7- All Rights Reserved
6. Edit the Limits for Access and Launch
a. Access Permissions – Edit Limits...
You need to check the Remote Access box for the user labeled
ANONYMOUS LOGIN in this dialog.
Note: This setting is necessary for OPCEnum.exe to function and for some OPC Servers
and Clients that set their DCOM 'Authentication Level' to 'None' in order to allow
anonymous connections. If you do not use OPCEnum you may not need to enable remote
access for anonymous users.
b. Launch and Activation Permissions – Edit Limits...
You need to check the remote boxes for the user labeled Everyone in this
dialog.
Note: Since Everyone includes all authenticated users, it is often desirable to add these
permissions to a smaller subset of users. One suggested way to accomplish this is to
create a group named “OPC Users” and add all user accounts to this group that will
execute any OPC Server or Client. Then substitute “OPC Users” everywhere that
Everyone appears in these configuration dialogs.
Using OPC via DCOM with
Microsoft Windows XP
Service Pack 2
F O U N D A T I O N
Released
© 2004 OPC Foundation, Inc. -8- All Rights Reserved
7. Edit Default Permissions for Access and Launch
For each user (or group) that participates in OPC communication (e.g. “OPC Users”),
make sure that both the Local Allow and Remote Allow checkboxes are both checked.
Access Permissions per user:
Launch and Activation permissions per user:
Using OPC via DCOM with
Microsoft Windows XP
Service Pack 2
F O U N D A T I O N
Released
© 2004 OPC Foundation, Inc. -9- All Rights Reserved
Disclaimer
The material presented in this paper is based on a pre-release version of Service Pack 2.
Although the paper is based on “best practices” as judged by the authors, the OPC
Foundation and the authors assume no responsibility for its accuracy or suitability for
application by its readers.
References
1. MS White paper: Windows XP Service Pack 2 Overview
Published: February 2004
For the latest information, please see http://msdn.microsoft.com/security
2. Windows XP Service Pack 2 - Security Information for Developers
http://msdn.microsoft.com/security/productinfo/XPSP2/default.aspx
3. Changes to Functionality in Microsoft Windows XP Service Pack 2
http://www.microsoft.com/technet/prodtechnol/winxppro/maintain/sp2chngs.mspx
[1608]
客服
小程序
公众号
工控网智造工程师好文精选
