Lync Certificate Planning and Assignments

Lync Certificate Planning and Assignments
(Edge, Reverse Proxy, Director, Frontend, Mediation, WAC)

Download the article as PDF: SlideShare Link (here)

The following article is optimized for Lync 2013, but in general valid for Lync 2010 or OCS 2007

First I need to highlight; Lync Server and Client make use of Certificates, therefor the technical principals of certificate deployments are necessary to understand. If on your Clients or Servers an Internet Explorer Setting with a Proxy Server is activated, make sure you have the correct design. The CRL (Certificate Revocation List) check is mostly HTTP based (in AD Environments also possible via FILE or LDAP), if you have setup an internal Proxy, which cannot redirect the request into your LAN, you will run into major issues!

I wrote another article in 2012 which maybe from interest for you too:


Lync Certificate Planning must be separated into three different areas:

1. INTERNAL Deployment
(all internally deployed Lync Servers, e.g. Frontend, Directors, Mediation,..)

Including the internal NIC of the EDGE Server

2. EXTERNAL Deployment
2.1 Edge Server
2.2 Reverse Proxy

Indirectly there is a fourth area, this is if you have Pool Server configuration, due the Virtual Service configured on the Load Balancer. But I will explain this in detail within another blog later.
All Lync Server have one requirement in common, this is the way how they accept authentication based on TLS. Accepting the trust, Lync Server need a matching between the certificates common name and it FQDN. The server or client, initiating the communication with the certificate holder use DNS lookup to refer to this server FQDN. If this reference does not match the common name of the certificate, the authentication will fail.
The common name, notated as CN in X.500 terminology, is what is referenced and must match the DNS record for the server’s FQDN. For details about the specific format
This explains why a dedicated wildcard certificate would not work in Lync Server, because the common name must match exactly to the FQDN of the A record defined for the referenced server or pool. The DNS A record and the certificate subject name/common name (SN/CN) is also referenced to the trusted server list in Active Directory service Global or Configuration settings.

Reference: Microsoft Technet Certificate Guide
Important: You cannot use a wildcard CN/SN (for example, * when you configure certificates for Office Communications Server 2007 R2 and Office Communications Server 2007 (now Lync). If you do so, they will not operate as expected and the problem is very difficult to diagnose. You can use wildcard entries in the subject alternative name, but the common name is specific. Specific issues include the inability to start services because the trusted services in Active Directory Domain Services (AD DS) and the SN and CN do not match, mutual authentication fails, and so on. 

Note at last:
And, as mentioned earlier, public CAs and your internal CA can create wildcard SN/CN certificates, but they are neither reliable nor supported. It is recommended that you do this right the first time and avoid the potential for serious issues in the future by not trying to use a certificate that uses a wildcard SN/CN, such as *, to define the three Edge Server services.


Server Components
(Certificates are requited)

2.1 INTERNAL Deployment:

  • Standard Edition Front End Pool Server
    This server is the consolidated “all-in-one” Server and requires an internal certificate.

  •  Enterprise Edition Front End Pools
    This server is the High-Available Lync Core Component. Beside the local servers themselves, they also provide the consolidated access names and are attached with a Load Balancer. The certificate must contain the Pool and Server name. In certain circumstance it makes sense haven a generic certificate, which contains all Pool Server Names and the Pool Name (SAN certificate).
  • Director Pools
    This server is the “Authentication and Redirection” server. In lager deployment, with multiple site, you need the Director to offload authentication traffic and redirect the user to the homed pool.
  • Mediation Pools
    This server is responsible for Media Conversion
  • Persistent Chat Pools
    This server handles the “Group Chats”

  • Trusted Application Server
    All Server, which need to be trusted by Lync have to be publish that Lync is aware of them. If A certificate is required if the trusted server will us TLS.

  • PSTN Gateway
    The PSTN Gateway object, might be a Lync Gateway, Gateway card or an SIP Trunk. With the PSTN Gateway, this depends on how the setup must or can be done. If you make use of a TLS connection, e.g. to an ISDN card, you will need a certificate stored on the PSTN gateway.

  • Office Web Apps Server
    The WAC/ OWA server requires a certificate, this is OAuth ready.

As described in the section for Front End Pool Server, generally it has to be part of the planning how certificates are requested if a Load Balancer is involved. A Load Balancer can be setup in different way (in-band or out-band), this will discussed in a separate blog. But you need to remember, the Load Balancer is the central point for the IP connection, therefor it needs the FQDN of the POOL in its certificate presenting to the connecting client. Depending on how the Load Balancer is established, you will than understand why the Pool Member Server needs beside the Pool FQDN also its local FQDN in its local certificate!

2.2 EXTERNAL Deployment:


  • Edge Pools
    The Edge Server is the main component used to communicate from and with outside of the organization. (Responsible for PIC, XMPP, Federation, remote access and Web Conferencing)
Edge Pools have one specialty, for best practice and security reason, they make us of 2 NICs, an internal and external.

Edge Server need to have 2x NIC with different subnet, need the primary internal DNS Suffix set, must not be a domain member and will need to certificate, and internal CA issued certificate for the internal directed interface and an official, public certificate (where I will take more later about). Additionally, remember to set the default gateway on the external facing NIC and all internal subnet must be assigned a static route based on the internal facing NIC. 

·         Reverse Proxy
This optional component only needs an external certificate and it’s responsible for Web-Based Services, e.g. Address Book or Dailin Conferencing page.


3 Topologies

Topology represents your entire corporate Lync Server deployment and all involved Lync Systems, with one exception, the Reverse Proxy. Since we want to define the necessary certificates, it is necessary to fully understand the topology and server function which then represents the service making use of. 

3.1 Internet facing Systems

Before we actually start with the topologies, we need a clarification what the external facing system will do, what they are responsible for and what not.

Else which kind of usability scenarios do we have?

  • Remote Users
  • Federated User
  • Public Instant Messaging Connectivity Users
  • Mobile Users

And the type of communication:

  • IM
  • Presence
  • Audio/ Video/ App Sharing
  • Web Conferencing
  • A/V Conferencing


3.1.1 Edge Server:

The Edge Server, the Internet facing system responsible for enabling users to communicate with external partners, connect remotely and establish connectivity with Public IM Services, like Live or Skype.
Also the Audio/ Video and App Sharing runs through the Edge server if a Meeting is in place.
One newer component, called XMPP (Extensible Messaging and Presence Protocol), is established in Edge Server since Lync 2013, it is used for partner federation e.g. Google Talk.
Edge Server is not responsible for any other service as the described services in this section.

3.1.2 Reverse Proxy:

Reverse Proxy as an optional, not Lync Server Topology component, becomes responsible for several areas and will publish internal resources.
It can be separated into two areas, the remote user connectivity and generally spoke “meeting’s”.

Remote User:
Remote user need to connect to Lync server internal service, called “Web Service”, they are responsible for Address Book Synchronization, Distribution List Expansion, Device Updates, Mobility Services.

Access to Meetings, Conference Join Locations (PSTN Dial-In Numbers), Access to personal Dial-In and PIN information, Download Meeting Content.

3.2 Topology and certificate assignment

In sum we will have one primary and two secondary SIP Domains in our example topologies defined.
The third deployment would be a very complex scenario, where we have multiple geographically deployed Edge Server/ Reverse Proxy scenario.
I’m not having a look into Enterprise Voice, it is not required since we want to understand the certificate design. 

Our deployed domains are:
Active Directory Domain:             INTERNAL.AD
SIP Secondary Domain:                 DOMAIN-A.COM and DOMAIN-B.COM


In general, what we have to remember for Lync Topology designs and the related certificates is:

  1. On Edge Server, Wildcard Certificates are not allowed
  2. On Edge Server we need matching CN and 1st SAN entry of access FQDN, e.g. SIP.DOMAIN.COM
  3. On Edge Server we need SAN entries for AV and WebConferencing
  4. On Reverse Proxy, we need a matching CN with the associated Director Pool external Web Service FQDN
  5. On Reverse Proxy, all external Web Service FQDN must be in SAN
  6. On Reverse Proxy all other FQDN can be consolidated in a Wildcard entry


The “SIMPLE TOPOLOGY” is the most common deployment for smaller customers. High availability is mostly not required by Lync due to virtualization. For those customers, VM Host availability and snapshots are sufficient enough.
The simple deployment includes the full feature set of Lync in direction to the internet. This includes login possibility for all Lync Clients, incl. App Store and Mobile clients. Federation is also handled.



The “COMPLEX TOPOLOGY” is the most common deployment for lager, multi pool customers. High availability is required for Lync and due to multi pool deployments, login traffic must be handled by Director Servers.
This deployment includes the full feature set of Lync in direction to the internet. This includes login possibility for all Lync Clients, incl. App Store and Mobile clients. Federation is also handled.



The “GEOGRPHICALLY COMPLEX TOPOLOGY” is the most complex deployment for international customers. High availability is required for Lync this is also extended into a multi-region Edge Access scenario.
This deployment includes the fully feature set of Lync in direction to the internet. This includes login possibility for all Lync Clients, incl. App Store and Mobile clients. Federation is also handled.
The main component for geographically distributed deployments is the GEO-Load Balancer. It handles the Internet based distribution for Edge Access.
Since I’m talking about Certificates, it is important to understand the Certificates distribution.


4 Certificate Template Table

Making it easier for you, I prefilled in the Template with this configuration example:
We have 3 SIP domains in our deployment 1x Enterprise Pool, plus 1x Standard Edition Server in a branch. I also have 1x Director installed. 

4.1 Edge Server

Common Name
Primary SIP domain
First SAN entry must repeat the primary SIP domain
Web Conferencing only for the named primary SIP domain needed
XMPP Federation (if installed) of primary SIP domain
Second SIP domain
Third SIP domain

Table 1 Edge Server external Certificate

4.2 Reverse Proxy Server

Common Name
Just a Common Name
External URL of Director Server. Must be primary SIP domain
External URL of Enterprise Pool Server. Must be primary SIP domain
External URL of Standard Server. Must be primary SIP domain

Table 2 Reverse Proxy Server external Certificate 

4.3 Hybrid Certificate (Summary)

Common Name
Primary SIP domain
This is the Wildcard part for Revers Proxy of
This is the Wildcard part for Revers Proxy of

Table 3 Consolidated, public Certificate


5 Certificate Request Generation

How do I request the Wildcard+SAN certificate?
The following demonstration explains hybrid certificate request in Lync. This has to be done on the Edge Server itself. You have to login to the Edge Server and start the Bootstripper, than you chose the “Request, Install and Assign Certificates”.


In this example, I’m using three domains:
SECONDARY SIP Domains: and

Since this will be our Hybrid Certificate, there is still one point we haven’t spoken about. How do we request this certificate? If you for example initiate the request with DigiCert, you need to buy three (3) wildcard certificates first, than you process with DigiCert manually via email.
So remember you might take one/ two days longer in placing the order.

We need to prepare a CSR file for external, manual requests:

The friendly name can is only for better identification of the certificate in the store:

The first defined SN'S are provided by Lync automatically:

Next, you need to include the addressed SIP domains configured in Lync Topology builder:

As discussed, here we come to the point, where we need to add additional SAN entries as explained and defined the table earlier:

Verify the correct CN and SAN entries:

Finally you defined the Certificate Request. This is your CSR file. Provide this information to your Certificate supplier.
Remember, the Certificate File you will receive will NOT contain the PRIVATE KEY. The Private Key will be generate once you apply this certificate on the Edge Server where you generated the statement !
Only after its process is fully done, you have the Private Key and the Certificate is ready to be exported and imported on the other servers, e.g. Edge and Reverse Proxy

6 Best Practice

Beside the certificate design and planning process, there are some more point to remember.
I have listed all important areas you must consider during your design and planning process.

  • Network Interface Cards:
You have to use two NIC, one for internal and one for external communication. The default gateway has to be set on the external facing NIC, while you must use “persistent static routes” to all you internal networks. The DNS should be pointing to the internal DNS Server, if you are choosing an external DNS or a DNS in a DMZ, make you can resolve the internal Lync Server, if you can’t, you need to provide a hosts file.

  • Edge Server and Reverse Proxy combination
As stated earlier, the full feature set in Lync is only available if you make user of Edge Server, Reverse Proxy and all required external DNS entries (incl SRV Records). If the RevProxy is not deployed, you will miss the following features, e.g. address book download, location information, device update, Lync Web App and NON-DOMAIN Client login)
The non-domain client login requires an authenticated access the Certificate Provisioning Service.!
Also the App Store and Mobile Clients can’t login without the publish autodiscovery services.
This is the same with access to Exchange Web Services (EWS).

  • Director Server Service
The Director Server is an optional component, responsible for offload user authentication and pool redirection. IT also provide an additional layer of protection for external client connections.

  • Revers Proxy Listener
Keep the Web Listener as limited as possible. Us only one (1) Listener per internal destination server each. Make sure the Listener can work with the Hybrid Certificate to minimize costs.



(c)  2013


  1. Great article, but the formatting got a a bit screwed up, so I had to read it on the slideshare link where it was all ok. Anyway, great post.

  2. Hi Thomas
    I am a new one in Lync system and at this time trying to deploy Lync 2013. However I am stuck in Certificate Wizard step. I really do not now how these certificate work and how I can get them.
    Please help me out

  3. Hi Tom,

    first I need to know which certificate you are talking about, Edge internal, external or the Frontend server.

    For you it makes it more easy, if you are using the table I've provided and set the external Domain you are using. than start the Certification Wizard in the same way as provided in this blog, than if you have the Certificate request file, you need to talk to your Certificate Authority Partner of your choice, and explain you need a hybrid certificate, possible they will manually process it for you.
    Easy to talk to is DigiCert !

  4. Hi Thomas

    Do you perhaps know of a guide for using internal PKI signed certificates for client authentication rather than the self signed comms server cert ?

    1. Hi Pieter, sorry but I don't know a guide. Maybe you have a look into the Microsoft Wiki/ Gallery at TechNet

  5. Hi Thomas,

    thanks for your great explanation.

    Summarizing your article, is it right, that I need for every ent pool the external web services FQDN in the ext reverse proxy cert? Even If have addional SIP domains, the amount of the external web services FQDN does not change, they are just related to the ent pools, right?

    1. Hi Robert,
      yes you understood this correctly.
      The Pool EXTERNAL Web Services require a FQDN each. Therefor it must also be published on the RevProxy.
      The Ext Web Services do NOT depend on additional SIP Domains, rather than the Default SIP Domain (Best practice).
      But you are able to assign other, e.g., it must not be any SIP domain, the Web Service FQDN will be told to the client with AutoDiscovery Service.

  6. Hi Thomas,

    If i have the standard server in a local domain, and the external web services published in another public domain, the certificate for the external web services site must have in its certificate de SAN o CN refer to local domain? or only the public domain?


    1. Well, first best practice is having the webservice run externally with the same SIP domain DN.
      So if you have decided not doing so, e.g. SIP dom is DOM.COM and the WS are running e.g. MOD.COM you simply need a SAN entry at the reverse proxy with WS.MOD.COM and CN is not required matching the DOM.COM.
      hope this helps


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