ClearPass auth failure diagnostic

Here’s a “learn from my recent experience” type post

The problem: Clients are unable to authenticate from a new Wi-Fi network that has been added


  • ClearPass appears to be working fine
  • Clients are successfully authenticating from the existing network using EAP-TLS
  • A Policy Manager service has been configured for the new network and incoming requests are correctly categorised
  • Authentication attempts from the new network are rejected, seen in Access Tracker
  • Failing auths are showing as an outer type of EAP, not EAP-TLS
  • No certificate content is shown in the computed attributes of the failed auths
  • Apple Mac clients are able to authenticate to the new network successfully, managed windows clients are not. The same clients work fine on the existing network.

The obvious conclusion is this the new network is incorrectly configured, and this turned out to be the case, but it what’s wrong… The last point in the observations was particularly interesting and threw a spanner at the “network config error” idea, because if the network config is wrong why can a Mac authenticate… is it the client? What’s the difference?

Connections from the new network were proxied via another RADIUS server. This is because the solution uses RADSEC and the new network Wi-Fi controllers don’t support RADSEC.

The information provided by Access Tracker appeared to show insufficient information for the auth to be successful. Crucially there was no client certificate information and the outer method showing as just EAP was… odd.

Looking at the logs for an auth showed an error early in the process:

rlm_eap: Identity does not match User-Name, setting from EAP Identity.

Ultimately here’s what the problem was… The proxy forwarding these authentications had a default to strip information from the username. Windows clients which presented the username as host\<hostname>.<domain> had this stripped back to just the hostname. So the TLS tunnel outer username presented to ClearPass became hostname$. The Mac clients didn’t present the FQDN as the username so nothing was stripped.

ClearPass performs a check on the Outer Identity of the TLS tunnel and the Inner Identity. If the outer identity is valid and the inner identity differs the auth will fail. In the case of EAP-TLS the error above will be displayed in the logs and the auth will fail.

The Outer and Inner identity either must match or the Outer Identity can be set to Anonymous.

Note this applies to EAP-PEAP and EAP-TLS. EAP-TTLS may also have issues with mismatches… basically make sure it all matches, don’t have anything strip data from the outer identity unless it’s being set to Anonymous.

The solution to this was to disable the domain stripping on the proxy.

For anyone who’s found this post after running into this issue, take heart that information presented by Access Tracker is not at all helpful in understanding why the auth has failed. It appears the certificate hasn’t been presented at all when in fact that data just isn’t presented to you.

The error message tells you exactly what’s wrong, once you understand how it works.

ClearPass, Intune and MAC randomisation

As more organisations have moved to Microsoft Azure AD and Intune to manage their devices a common request is how to integrate this with Aruba ClearPass, which handles the RADIUS requests for Network Access Control. The most common deployment pushes certificates to the clients which use these for EAP-TLS authentication with ClearPass. Note this post doesn’t cover the basic Intune integration setup which is documented in an Aruba guide.

But you probably want to know whether a bit more about the client than whether it has a valid certificate, so ClearPass has an Intune extension which will download information from an Azure tenant’s Intune to the Endpoints Repository database.

This allows you to make policy decisions based on Intune attributes, such as compliance state, allowing you to place clients in different roles/vlans depending on what the client is, whether it’s compliance with policy, department, etc etc.

However… the Endpoints Repository uses MAC addresses so problems start if clients are using MAC randomisation. The MAC address presented by the client won’t match what’s recorded by Intune so it won’t be possible to match against Intune attributes in the Endpoint.

The first thing to do is stop using the MAC address as the UID. Much better to use the Intune ID and have this written into the client certificate, either as the CN or a SAN though you can use any variable that ends up computed from the authentication.

You can then either query Intune directly using the Intune Extension HTTP method or use the Endpoints Repository in a slightly different way.

I prefer the latter option most of the time because it provides a level of resilience as well as improving performance. Querying the Intune API via the extension works very well but if Intune is down (which has been known to happen) that won’t work. It will also take longer than querying a local database.

ClearPass assumes you’re using the presented MAC address as the UID of the Endpoint and it isn’t possible to change this. Instead you can query Endpoints as an SQL database with a filter that pulls out the attributes you require based on the presented certificate CN.

ClearPass databases can be accessed externally using the username ‘appexternal’ and the password which is set in cluster-wide parameters under the Database tab.

Next create a new Generic SQL DB Authentication source pointing to the local tipsdb and set a filter that pulls out the attributes you want for the auth session variable presented.

Server name: <server IP>
Databse Name: tipsdb
Username: appexternal
Password: <password you set>
ODBC Driver: PostgreSQL
Password Type: Cleartext

A few things to note here. You can’t use localhost as the server name. If you have a VRRP address you can use this, otherwise you must use the actual IP of the server. This can cause complications in an environment with multiple ClearPass servers and no VRRP. There are ways around this but that’s for another blog.

The filter is the SQL query that pulls out the attributes you want based on an attribute presented. In this case we’re selecting ‘Intune User Principle Name’, ‘Intune Compliance State’ and ‘Intune Device Registration State’ for a record where the ‘Intune ID’ matches Subject-CN of the client certificate

select attributes->>'Intune User Principal Name' as "Intune User Principal Name",attributes->>'Intune Compliance State' as "Intune Compliance State",attributes->>'Intune Device Registration State' as "Intune Device Registration State" FROM tips_endpoints WHERE attributes->>'Intune ID' = LOWER('%{Certificate:Subject-CN}');

You then specify for each of these how you want the system to use the data it gets back, essentially either as an attribute or directly set as a role.

Add your custom authentication source into the service, and you’re good to go. You will now be able to make policy decisions based on the Intune ID lookup rather than the MAC address.

Three more things to note about this.

If you’re limited as to what can go into the certificate CN or SAN you can use the same method to pull out other details, for example with a query like this:

select attributes->>'Intune User Principal Name' as "Intune User Principal Name",attributes->>'Intune Compliance State' as "Intune Compliance State",attributes->>'Intune Device Registration State' as "Intune Device Registration State" FROM tips_endpoints WHERE attributes->>'Intune Device Name' = '%{Authentication:Full-Username}';

This method requires the device to be in downloaded to Endpoints by the Intune Extension. If that doesn’t happen it won’t be there to match on. In some circumstances Intune no longer records MAC addresses for devices – notably self-registered personal Android devices – and because the Endpoints Repository is based around MAC address these devices will be missing.

This is one of the reasons it’s worth using the Intune ID as your device’s UID in the certificate – if you need to query the Intune extension via HTTP you’ll need to present it with the Intune ID, nothing else will work. It’s worth noting devices will also have an Azure AD ID which looks similar, and will likely be the same for devices not managed by Azure AD, but the Intune API only understands the Intune ID when querying for device attributes.