After some coffee, some 'what have I done to deserve this' moments and a bit of reading - it's actually not quite as painful as you might imagine. There's a good amount of documentation on how to achieve this with Microsoft ISA / TMG but not much for other vendors. Here's how I set it up with a Blue Coat Proxy SG running SGOS 188.8.131.52.
1. To make things a bit easier, you'll need all of your certificates in PEM format (Base 64 encoded - the ones if you open in a text edit start with ===BEGIN CERTIFICATE/KEY===). If you generate them from your Microsoft enterprise CA you'll most likely be given them in PFX or P12 unless you want to generate CSRs from the Blue Coat device. You'll need to use openssl to convert them into a PEM file containing the unencrypted private key and the certificate.
You will need the certificates for your enterprise CA, exchange server if not issued by your CA and one for the Blue Coat device itself to identify to the clients (issued by your enterprise CA).
2. Add the certificate for the Blue Coat device to your proxy as a keyring (configuration -> SSL -> Keyrings). This should be the only certificate with a private key contained in the PEM file.
3. Add the other CA certificates to the proxy in CA certificates section (configuration -> SSL -> CA certificates). This will ensure the proxy trusts any certificates issued by your enterprise CA (clients and the Exchange server). It's quite common for Exchange to use a self-signed certificate - if this is the case you'll need to export this certificate and add this to the Blue Coat as well (otherwise you'll need to add policy later on to ignore the fact it's issuer isn't trusted.
4. Create a new CA Certificate list and add in the CA certificates you created above. We'll use this shortly on our proxy service listener.
5. Under Configuration -> Services -> Proxy Services, create a new service group and named it 'Reverse proxy'. (this is optional but keeps things more organised). Create a new and fill in the details as below.
Name: Something meaningful to you
Service group: Group you created above
Proxy: HTTPS Reverse Proxy
Keyring: This is the keyring for the Blue Coat appliance you created previously (the one with the private key).
CCL: This is the CCL you created in step 4. This will make sure the proxy only accepts certificates which were issued by your enterprise CA.
SSL versions: Enable whatever you require here - but make sure you tick Verify Client and don't enable Forward Client Cert (it's impossible to forward the actual client certificate so the Blue Coat forwards some information from the certificate in the HTTP headers. This doesn't work for our goal which is why we're using KCD).
ADN: unticked - it's not relevant here.
Listeners: This is the address that you will NAT from your firewall (or a public address if your proxy is deployed in that way). This will be where clients connect. You can use an IP that isn't in use by the proxy so long as you add it as a Virtual IP.
6. Join the proxy to your domain (you'll need a domain administrator account for this). Under Authentication -> Windows Domain create a new domain and then join it. Make a note of the name you join to the domain with as you'll need this for the KCD settings. If you have problems joining - make sure the DNS on your proxy is pointing toward your Active Directory server. Once joined, go to Authentication -> IWA -> IWA servers and make sure under Client Authentication Modes - 'Kerberos credentials' is ticked.
7. Create a certificate authentication realm under Authentication -> Certificate. Then under Certificate Main, configure the extractors so the proxy is able to obtain the username from the clients certificate.
Your certificates might vary so you might need to try a few options to find what works for you. Our AD / CA setup is mostly standard so far as I know so these settings should work. You'll get errors in the event and access logs saying 'unable to extract username' if these settings aren't relevant to your environment. The options on the other tabs can be left at default.
8. Over to your AD setup for a moment now to enable your Blue Coat device to perform KCD for authentication to your Exchange server. In the AD 'Users And Computers' view - find the new device representing the proxy (the name you chose in step 6)and edit the properties. Under the delgation tab, configure your options as in the screenshot.
9. Back to the proxy now. Go to configuration -> Forwarding -> Forwarding Hosts and create a new forwarding host. This needs to have the below settings.
Host: IP address of your Exchange server
Others left unticked as per the default.
10. Open up the VPM via Configuration -> Policy -> Visual Policy Manager.
For the purpose of this guide - I'll be working with a totally blank policy to keep things simple. The policy layers should be setup as below.
Layer 1 - Web Authentication Layer.
Destination: Request URL object. Simple match - value should be the DNS name clients will be accessing for your reverse proxy. Eg, activesync.mydomain.co.uk
Action: Combination object containing an Authenticate Object (Realm should be your certificate authentication realm from step 7. Mode: Auto) and a Kerberos Constrained Delegation object. (Authentication type should be origin, IWA Realm should be the IWA direct realm from step 6. Service Principal Name should be 'host/' then the exact name of your Exchange server without the domain).
Layer 2 - Web Access Layer.
Source: Authenticated user (no configuration for this object)
Destination: Request URL object (Advanced match - scheme any, Path "/Microsoft-Server-ActiveSync"). This will ensure only EAS connections are allowed to this reverse proxy connection.
Layer 3 - SSL Access Layer
NOTE: This layer isn't strictly necessary but it's useful to troubleshoot SSL problems whilst you're setting up. The destination is the IP of your Exchange server and the Action is an SSL certificate validation action to ignore untrusted issuer.
Layer 4 - Forwarding Layer
Source: Authenticated User (no options again)
Destination: Server URL object (simple match again with the DNS name of your reverse proxy connection).
Action: Select Forwarding Object. Move the object name you created in step 9 into the right hand column and then ok.
11. We're done! Install the VPM policy and then you're ready to test. Make sure the client certificates you deploy contain the full certificate chain or you'll get SSL errors when your devices try connecting to the proxy. If you use Active Directory to issue user certificates you'll probably find they're in PFX format. In this case, they should contain the full chain by default.