I recently started the upgrade to Exchange 2016 at work. A huge benefit over Exchange 2010, is that REST based client connections are truly stateless. In effect this means that if a server goes down, clients shouldn’t really notice any issues as long as something redirects them to a working server. In my system, this something is HAProxy.
The guys at HAProxy have their own excellent walkthroughs for setting up their load balancer for Exchange 2013, which can pretty much be lifted verbatim to Exchange 2016, but I want to add a few key points to think about:
Service health checks
Each web service has a virtual file to tell its state, called HealthCheck.htm. Let HAProxy use the contents of this file for the server health check. That way it’ll know to redirect clients if one of the services is down, even though the Exchange server in question may still be listening on port 443.
option httpchk GET /owa/HealthCheck.htm http-check expect string 200\ OK server Exchange1 192.168.1.13:443 maxconn 10000 ssl cafile cacert.pem weight 20 check
This example shows a test of the Outlook Web Access service state. Naturally the config can be set to test each of the REST services each Exchange server presents.
Exchange server default firewall rules
Our design puts the load balancer in a DMZ outside of our server networks. Clients connecting through the load balancer will be dropped by Windows firewall rules generated by Exchange; specifically the edge traversal rules for the POP3 and IMAP protocols. Make sure you allow edge traversal for these protocols, letting the network firewall take care of limiting external client connections to them. Also take note there are multiple firewall rules for IMAP and POP3 traffic. Only the ones concerned with client traffic are relevant for this change. There’s no point in kicking open holes in your firewall for no good reason.
Exchange and Outlook suck at IMAP
We use IMAP for an internal order management system. Outlook and Exchange aren’t the best tools for this protocol, but unfortunately we have to live with those due to sins committed long ago. I spent quite some time troubleshooting our IMAP connections:
No matter how I configured Outlook I couldn’t get it to open an IMAP connection to the Exchange servers. Error messages varied depending on the client settings, but essentially I couldn’t log on, couldn’t establish a secure connection, or couldn’t synchronize my folders.
I would get the regular banner when telnetting from the client machine, so I knew traffic was getting through all the way from Exchange via the load balancer.
Mozilla Thunderbird could connect perfectly well and sync accounts, both using STARTTLS on port 143 and over a TLS encrypted connection on port 993. After mulling it over, I turned on debug logging in Outlook and quickly saw that the client was trying and failing to perform an NTLM logon to Exchange. Using the error messages as search terms, I found others who had experienced the same issue. Their solution had been to turn off NTLM authentication for the IMAP protocol on the Exchange server. This seems to be a regression in Exchange Server 2016 from an earlier bug in Exchange 2013.
The command in the Exchange Management Shell:
Set-IMAPSettings -EnableGSSAPIAndNTLMAuth $false
After this, Outlook still is incapable of logging on using TLS over port 993, but at least it consistently manages to run STARTTLS over port 143, which is good enough for my use case.
All in all, the most complicated part here wasn’t to make HAProxy do its magic, but to get Exchange and Outlook do what they should.