Chris here again. If you have read the previous five part of the series you are at this point very familiar with the installation and configuration of the OCSP Responder. I covered implementing the OCSP Responder to support a variety of scenarios. One thing I have not covered, however, is the configuration of the OCSP Client.
If you have read my blog series on Implementing and OCSP Responder you will be aware that one of the configuration steps is to specify the OCSP URI on the CA so that it is included in issued certificates. This would definitely help with newly issued certificates, but how about certificates that have already been issued? If you could point clients to an OCSP Responder, you would now be able to use OCSP with previously issued certificates.
After some leg work by my colleague, he was able to determine that this feature already exists as of Service Pack 1. Needless to say, I felt ecstatic and dumb at the same time. Ecstatic that the feature was already implemented, and dumb that I was not aware of it. As of Windows Vista Service Pack 1, you can point clients to a specific OCSP server. You will need Windows 2008 servers or Windows Vista clients with RSAT installed to have the ability to implement this setting as a Group Policy. In other words, there is no requirement to have Windows 2008 domain controllers, only a requirement to manage the group policy with a Windows Vista SP1 /Windows Server 2008 computer.
Directing clients to an OCSP URL for certificates
The first step is to export the Certification Authority certificate from the CA. Logon to the CA and open a command prompt, then type certutil -ca.cert <CA Name>.cer and press Enter.
1. Open up the Group Policy Management Console. Find the GPO for which you would like to make the change and right click on that policy and select Edit.
2. In the Group Policy Editor navigate to Computer Configuration\Windows Settings\Security Settings\Public Key Policies\Trusted Root Certification Authorities if your issuing CA for example is not a Root CA the CA certificate would be located in the Intermediate Certification Authorities container. So, you can import the CA cert to that container in the Group Policy and add the appropriate OCSP URI.
3. This will start the Certificate Import Wizard, click Next
4. Then on the File to Import page of the wizard, click Browse…
5. Then browse to the CA certificate that was previously exported, select the certificate and then select Open
6. Then click Next
7. On the Certificate Store page, verify that Trusted Root Certification Authorities is selected and select Next
8. Then click Finish to close the wizard.
9. When prompted that The import was successful click OK
10. Then right click on the certificate that was just imported and select Properties.
11. Then click on the OCSP Tab, enter the URL for the OCSP server I want clients to query (FCOSP.FourthCoffee.com/ocsp) in the text box, and select Add URL. Also, if you want to disable CRL checking, you can check the Disable Certificate Revocation Lists (CRL) check box. I then Click OK when finished.
After group policy is updated you see two CA certificates for the CA in the Trusted Root Certification Authorities store. This is because the CA certificate is already in that store prior to adding it to Group Policy. The net result of which is that you will have two of the CA certificates in the Trusted Root Certification Authorities store. Regardless, when the chain is built, the OCSP location that was added via the group policy will be incorporated in the revocation checking process. Now clients will check the OCSP URL that you configured for revocation status even if the OCSP URI is not included in certificates.
The option to add the OCSP URI via group policy adds additional flexibility when using the OCSP Client included in Windows Vista. This feature will also be extremely helpful to customers that do have isolated networks as well as those customers that want OCSP support and are not ready to renew their CA hierarchy. It is also useful if you need to change the DNS name of your OCSP Responder which may occur for many reasons, including transitioning to a load balanced array, or adding additional OCSP responders.
Implementing an OCSP responder: Part I Introducing OCSP
Implementing an OCSP responder: Part II Preparing Certificate Authorities
Implementing an OCSP responder: Part III Configuring OCSP for use with Enterprise CAs
Implementing an OCSP responder: Part IV Configuring OCSP for use with Standalone CAs
Implementing an OCSP Responder: Part V High Availability
Implementing an OCSP Responder: Part VI Configuring Custom OCSP URIs via Group Policy
-Chris “TGIOCSP” Delay