
ISA 2004, forms-based authentication and radius
by
rastix
on Wed 10 Aug 2005 12:23 AM CEST
At the moment, I am busy with an Exchange 2003 migration project. The project includes secure access to Outlook Web Access, Outlook Mobile Access and Server ActiveSync. We use load-balanced ISA 2004 servers, load-balanced front-end servers and clustered Exchange 2003 back-end servers.
The ISA 2004 servers are installed in a workgroup and are connected to an external DMZ. To provide user authentication, ISA server can be configured with forms-based authentication in combination with RADIUS. A RADIUS server is required and we use Microsoft’s IAS.
This solution requires ISA 2004 with SP1 as discussed in KB article 884560. The user can then connect to https://serverFQDN/exchange. ISA will present the FBA form (which by the way, can be customized) and the user types the username and password on the AD domain. ISA will then use RADIUS to authenticate the user. You should use IPSEC or something similar to encrypt this traffic, because this solution only supports PAP, SPAP and CHAP.
After ISA has verified your credentials, access to the front-end servers is granted. If you configure the publishing rule to forward authentication credentials, no additional authentication dialogs will be shown to the user. This works really well and it allows you to use ISA servers not part of the domain and still use AD authentication.
Of course, you can hardly call the above solution secure because it depends on the user’s password. Most companies will require two-factor authentication.
ISA 2003 natively supports RSA SecurID but many other solutions are used. One such solution is Aladdin’s eToken with OTP (one-time password). To authenticate, RADIUS is used again but instead the user supplies the username and OTP from the token. This is checked against a replicated user account in a shadow domain that contains all the information about the eToken, seed values etc… The advantage of using a shadow LDAP directory (can be AD or ADAM) is that the production forest does not need schema extensions but that you can still use AD management tools in the production domain to configure users for OTP etc…
But now we have a problem. The user can specify the username and OTP in the ISA form, but the response from the RADIUS server is not recognized because it is not what ISA server expects. This will soon be solved by Aladdin (we hope). Another question is whether or not this solution will support forwarding the user’s credentials to the front-end server. I expect not so we will have to see if this will be a good solution.
Anyone with a good solution? Other products and technologies to recommend? Anyone??? 