Misconfigured Skype for Business Edge Server Breaks Office 365 Hybrid Federation

We’ve been moving more customers to Office 365 recently. Not only are they seeing the business case stacking up from a cost point of view but they are also after the cloud only features which are now more frequently appearing. A troubling development with these migrations are the number of broken Skype for Business Edge servers that we are seeing.

Now these aren’t totally broken but just broken enough that when we try to integrate their on-premises Skype for Business environment with Office 365 services things go wrong.

How will you detect this?

This will often show up when trying to get voicemail configured to use hosted voicemail in Exchange Online since this is often the first hybrid service being deployed. When the call is redirected to the Exchange Online server it fails. Looking at the event logs on the front end server it says that the dial plan wasn’t configured correctly.


Attempts to route to servers in an Exchange UM Dialplan failed

No server in the dialplan [Hosted__exap.um.outlook.com__tenant.onmicrosoft.com] accepted the call with id [XXXXXXXXXXXXXXXXXXXXXXXXX].

Cause: Dialplan is not configured properly.

Resolution:

Check the configuration of the dialplan on Exchange UM Servers.


All the configuration looked fine and so we needed to dig into the SIP traffic a little more. We did this using snooper. We could see the message being handed off to the edge server from the front-end server but then the edge server connection timed out.


Response Data
504  Server time-out
ms-diagnostics:  1008;reason=”Unable to resolve DNS SRV"

This was a little strange as the edge server was working fine for other federation partners, and DNS lookups were working on the edge server.

What was happening?

One thing that didn’t look right though was that the internal interface was configured to use the internal DNS server. Referring to the Edge server deployment guide confirmed that this wasn’t correct.

https://docs.microsoft.com/en-us/skypeforbusiness/deploy/deploy-edge-server/deploy-edge-servers

Interface configuration without DNS servers in the perimeter network
1. Install two network adapters for each Edge Server, one for the internal-facing interface, and one for the external-facing interface.

Note
The internal and external subnets must not be routable to each other.


2. On your external interface, you’ll configure one of the following:


a. Three static IP addresses on the external perimeter network subnet. You’ll also need to configure the default gateway on the external interface, for example, defining the internet-facing router or the external firewall as the default gateway. Configure the adapter DNS settings to point to an external DNS server, ideally a pair of external DNS servers.


b. One static IP address on the external perimeter network subnet. You’ll also need to configure the default gateway on the external interface, for example, defining the internet-facing router or the external firewall as the default gateway. Configure the adapter DNS settings to point to an external DNS server, or ideally a pair of external DNS servers. This configuration is ONLY acceptable if you have previously configured your topology to have non-standard values in the port assignments, which is covered in the Create your Edge topology for Skype for Business Server article.


3. On your internal interface, configure one static IP on the internal perimeter network subnet, and don’t set a default gateway. Also leave the adapter DNS settings empty.

4. Create persistent static routes on the internal interface to all internal networks where clients, Skype for Business Server, and Exchange Unified Messaging (UM) servers reside.

5. Edit the HOST file on each Edge Server to contain a record for the next hop server or virtual IP (VIP). This record will be the Director, Standard Edition server or Front End pool you configured as the Edge Server next hop address in Topology Builder. If you’re using DNS load balancing, include a line for each member of the next hop pool.

How to fix it?

The edge servers were changed to meet this guidance by creating a hosts file with all servers in the topology using both short names and FQDNs, as well as setting the external adapter to be the only adapter with DNS settings which were external to the organisation.

Voicemail started working once this change was made.

Why this happens

So why did this happen? Part of the setup for voicemail located in Office 365 is configuring a hosting provider in Skype for Business.


New-CsHostingProvider -Identity 'Exchange Online' -Enabled $True -EnabledSharedAddressSpace $True -HostsOCSUsers $False -ProxyFqdn "exap.um.outlook.com" -IsLocal $False -VerificationLevel UseSourceVerification

This provider has shared address space enabled. This means that endpoints with the same SIP domain name can be located either on-premises or in the cloud. In our case the endpoint is the Exchange Online UM service.

When a call is routed to Exchange Online UM it looks up the local directory to see that the user isn’t located on-premises. The call is passed to the Edge server which performs a lookup of the _sipfederationtls._tcp.domain.com DNS record. Why is it doing this? Well basically it’s trying to make a federation request with it’s own domain and this is the start of that process. But the _sipfederationtls._tcp.domain.com record only exists externally so that lookup is failing. Since it can’t federate with itself it doesn’t go to the next step which is establishing a connection to Exchange Online.

This can also be fixed by adding the DNS record to your internal DNS but the edge server would still not being configured correctly. It’s possible that using the internal DNS server would result in something else not working later on. Far better to fix it properly.

Just as a matter of interest if you tried to configure a hybrid mode with Skype Online you would also experience issues where your on-premises users couldn’t see presence or send messages to cloud users. This is the same reason as the Exchange UM issue with shared address space also enabled on this hosted provider


New-CSHostingProvider -Identity SkypeforBusinessOnline -ProxyFqdn "sipfed.online.lync.com" -Enabled $true -EnabledSharedAddressSpace $true -HostsOCSUsers $true -VerificationLevel UseSourceVerification -IsLocal $false -AutodiscoverUrl https://webdir.online.lync.com/Autodiscover/AutodiscoverService.svc/root

Both Exchange Online and Skype for Business have hybrid relationships with Skype for Business On-Premises. The only difference, apart from the provider endpoint address, is that the Skype Online provider is configured to host users while the Exchange Online provider hosts services.

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