Office Online Server/Office Web App Server Pool Certificates

I’m working on a deployment that’s using a farm of OOS servers behind a load balancer (actually, behind a high-availability pair of load balancers!) for high-availability. If you’re just using one server, this is a great guide┬áto what you need to implement. If you’re using a farm with 2 or more servers behind a load balancer, there are a few more considerations.

To start with, the subject name on the certificate needs to be the URL that you’re defining in the SfB topology. You might use oos.example.com, for example. Now the fun part – the first SAN listed on your certificate also needs to be oos.example.com.

Everything will work at this point, but you don’t have a very good high-availability story. The load balancer will need to be configured to monitor each server in the farm to determine if the server is functional. Otherwise, a server could stop functioning and the load balancer would continue to send traffic to it – that’s no good. The load balancer will┬ámost likely monitor the servers by trying to access https://oosnode1.example.com/hosting/discovery (and the same thing for oosnode2, etc.), and watching for a 200 OK to be returned.

Most load balancers have setup wizards that will set the monitoring up for you as part of the configuration process for OOS, SfB, Exchange, SharePoint, and more. Check the vendor’s website.

In order for the HTTPS request to oosnode1.example.com to success, oosnode1.example.com needs to be a SAN on the certificate. The same holds true for the other servers in the farm.

You could cheat and monitor your OOS servers by using HTTP and thus not require a certificate. This is a bad thing to do. You should be monitoring the actual URL that will be accessed on the server for a true indication of the server status.

To summarize, your OOS certificate needs to have:

Subject Name (SN) of the URL you will use in the SfB topology, such as oos.example.com

The first Subject Alternate Name (SAN) needs to be the same as the SN.

Then, you’ll need a SAN entry for each server in the farm, such as oosnode1.example.com and oosnode2.example.com.