How to generate a certificate with subject alternative names (SAN)

How to generate a certificate with subject alternative names (SAN)

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When publishing services like Outlook Anywhere, OWA and Active Sync for exchange in ISA/TMG, we sometimes need certificates with subject alternative names (SAN). This enables us to publish multiple DNS names using one SSL Web Listener. Requesting SAN certificates is something we can perform directly through a Microsoft internal CA. However there are some steps to follow before gathering a new certificate that makes use of SANs.

This article explains how to request a new certificate with a SAN from the Microsoft Management Console on the TMG computer.

The following security best practices apply when adding subject alternate names to certificates:

· In general, the use of user-defined SANs can increase the risk of impersonation attacks because it allows a user to specify arbitrary names in a certificate request. Because user input can be abused by persons with malicious intent, precautions should be taken to mitigate the risks associated with the use of user-defined SANs and protect the integrity of your public key infrastructure (PKI).

· Certificate requests that contain SANs should be held in a pending state until they can be reviewed by a certificate manager. For information about configuring certificate templates to require certificate manager approval, see Issuance Requirements.

· Implement administrative procedures for reviewing pending certificate requests and verifying the requester is authorized to use the requested subject names.

· Implement separation of duties and role-based administration to ensure that individuals who can request certificates with SANs cannot also issue them. See Implement Role-Based Administration.

· Restrict usage of SANs to only those individuals that require it, such as administrators who install Web server certificates. For information about configuring certificate template security, see Issuing Certificates Based on Certificate Templates.

· If you must use SAN attributes because your server that requires a certificate with a SAN is running Windows Server 2003, consider completing certificate enrollment procedures on a computer that is running Windows Server 2008 R2, Windows Server 2008, Windows 7, or Windows Vista.

Hence we definitely suggest to make use of the mmc console to generate your SANs certificates.

Generating a certificate using the certificate console on TMG/ISA

To generate a SANs certificate use the Microsoft Management console on the TMG/ISA server as explained in the following paragraphs.

However before running the mmc certificate console we need to duplicate the template “web server” on the CA and make it visible under mmc-> certificate itself.

To accomplish this step we must open the certification authority as enterprise administrator and under certificate templates, right click on “manage”. We will open the certification template console where we have to duplicate the template “web server”. You have to configure it with the permission to be “visible” under the mmc certificate console, by selecting “enroll” in the security tab for the “authenticated users”. Be careful that the template version must be V2 and NOT V3 (2003 and not 2008), as TMG doesn’t support CNG certificates (http://technet.microsoft.com/de-de/library/ee796231.aspx#dfg9o9i8uuy6tre).

Once we have opened it and we have expanded local computer –> personal, we have to right click and chose “all task” and “request new certificate”.

image

After pressing “next” twice and choosing the template model “web server”, which had been duplicated previously, we select details and we must click on “properties”.

At this stage we have the two fields under the subject tab which are of interest:

Subject name -> type-> CN

Alternative name -> type-> DNS

image

Then press “ok”, “enroll” and if everything goes fine, we should get something like this:

image

At this point the certificate has been correctly created and installed under the certificate store.

However from TMG/ISA there is something to do before enrolling the certificate. By design TMG/ISA blocks DCOM and you’ll see an error similar to this:

image

With certutil from the command line you’ll see something like this:

C:\Windows\system32>certutil -ping -config "WIN2008DC.via.santagiulia\via-WIN2008DC-CA"

Connecting to WIN2008DC.via.santagiulia\via-WIN2008DC-CA ...

Server could not be reached: The RPC server is unavailable. 0x800706ba (WIN32: 1722)

In TMG/ISA we need to disable the Strict RPC compliance using “edit system policies” -> “authentication services” -> “enforce strict RPC compliance” and apply it. This is because TMG/ISA blocks DCOM by design and DCOM is required to acquire a certificate.

You can also use the following command from the command line to check the CA availability on any server:

C:\Windows\system32>certutil -ping -config "FQDN\CA_Name"

if everything is fine we should get:

C:\Windows\system32>certutil -ping -config "WIN2008DC.via.santagiulia\via-WIN2008DC-CA"

Connecting to WIN2008DC.via.santagiulia\via-WIN2008DC-CA ...

Server "via-WIN2008DC-CA" ICertRequest2 interface is alive

CertUtil: -ping command completed successfully.

That’s all. We have successfully created and imported our new SANs certificate.

Please be aware that you need to make sure that this certificate is installed on each node if you’re running an Array of ISA/TMG server, in order to be able to create a SSL listener on each one of the nodes.

Author
Andrea Vescovo
Support Engineer
Microsoft CSS Forefront Edge Team

Technical Reviewer
Philipp Sand
Support Escalation Engineer
Microsoft CSS Forefront Edge Team

Carsten Kinder
Principal Consultant
Microsoft Consulting Services

Comments
  • Very good and practical articles. :)

  • Thx for your article, I also wrote one on SAN-certificats, but in german: blog.forefront-tmg.de

    Greets, Jens Mander...

  • Thank you very much. It was very helpful for me

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