Service Pack 2 for Windows XP/ Windows 2003 Server Service Pack 1 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 (such as thorough CoInitializeSecurity,marked by me ). 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 / Service Pack 1 will not allow for OPC communications over the network.
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 (such as thorough CoInitializeSecurity,marked by me ). 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 / Service Pack 1 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.
commented by me: limits's priority take precedence over any other dcom settings including server-wide default and applicaiton-wide settings.I have done experiments which confirm my understanding.
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