What is external authentication?
External authentication uses the Central Authentication Service (CAS), which enables Single Sign-On (SSO), and allows a user to authenticate with a CAS, instead of directly with Alfresco Content Services. For example, this might be using the mod_cas Apache module. For more information about mod_cas, see CAS Apache Module.
CAS is usually used with a proxy, for example, the Apache mod_proxy module.
External authentication is set with the authentication.chain parameter in your alfresco-global.properties file to use the external authentication subsystem.
To provide SSO, an external authentication system (or CAS) can be integrated Alfresco Content Services. For example, the identity of the logged-in user is extracted by the CAS, passed to Alfresco Content Services servlets and extracted using the HttpServletRequest.getRemoteUser() method. As a result, when a user connects to Share they are shown their user dashboard, but will not see the Share login page.
The subsystem also allows a proxy user to be configured, so that requests made through this proxy user are made in the name of an alternative user, whose name is carried in a configured HTTP request header. This allows, for example, the Alfresco Share application and other Surf applications to act as a client to an SSO-protected Alfresco Content Services application and assert the user name in a secure manner.A disabled user can still log in to Alfresco using external authentication. If the external control synchronization is configured appropriately, a user's status of disabled can be synchronized via the LDAP directory. In summary, if an administrator wants to prevent a user from authenticating to Alfresco, then the user should be disabled in Alfresco either directly, or in the LDAP directory that is referenced by the ldap.synchronization.userAccountStatusProperty property.
If troubleshooting a user log in issue, first check Alfresco to see if the user account is enabled, and then step through the authentication chain to see if the user can successfully authenticate using one of the members of the chain.
For example, if external authentication is the only authentication system in the chain and auto-create missing people is enabled, then the users will be able to authenticate automatically. In other words, users that are not already synchronized to Alfresco will be auto-created and enabled, by default. If the user is subsequently set to disabled (either directly via APIs or via LDAP synchronization), then the user will no longer be able to access Alfresco. The user will also appear as disabled in Share > Admin Tools > Users.
Here are two scenarios where external authentication is configured with Alfresco Content Services and Share. In both scenarios, an HTTP or HTTPS request is sent to an authentication proxy. If authentication is OK, the proxy passes the request to Share using the AJP protocol.
external.authentication.proxyUserName=Alfresco Content Services trusts the header (defined by external.authentication.proxyHeader) sent by Share. This scenario is typically used if you want to prohibit direct access to Alfresco Content Services and enforce using the proxy, for example, by using firewall rules to the proxy.
external.authentication.proxyUserName=alfresco-systemApache uses the certificate to check that the request is coming from Share with the correct user (that is, the value of external.authentication.proxyUserName) and forwards the request to Alfresco Content Services. This scenario is typically used to allow direct access to Share, using HTTPS and the originator (the proxy) sends a client certificate when establishing the SSL tunnel.
- The complexity of authentication moves to an external software layer (a proxy). Alfresco Content Services listens to the authenticated user name that it receives using a custom HTTP header, or it reads the CGI REMOTE_USER variable that can be passed using the AJP protocol.
- Configuring external authentication (using the Admin Console) provides more information on configuring the external authentication subsystem.
- Most of the responsibility for authentication is not controlled by Alfresco Content Services, but controlled by the external software layer. Unless there is a problem when the authenticated user name is transmitted, the issue is located in the external software layer. In these cases, work with your proxy vendor or implementer of the authentication proxy to resolve the issue.
How is Single Sign-On (SSO) related to external authentication?
- If you are using Kerberos, you can use either the kerberos authentication subsystem, or the external authentication subsystem with a proxy that handles Kerberos authentication.
- If you are using CAS, you must use the external authentication subsystem with a proxy that handles CAS authentication.
In summary, external authentication and SSO are not interdependent: you can set up external authentication that is not SSO (for example, using an Apache proxy with a mod_auth_basic setting), and you can set up an SSO system that is not using the external authentication subsystem (for example, using the kerberos authentication subsystem).
See Authentication subsystem types for a listing of the authentication subsystems and the features that they support.
How is Alfresco Office Services related to external authentication?
There are some limitations when using Microsoft SharePoint support, as provided by Alfresco Office Services, with the Alfresco external authentication subsystem. External authentication can work well when using a web browser client, but not when using the MS Office client. This is because no authentication information is sent with the file URL, and MS Office does not store authentication information, so starts a new authentication process.
An example of this is when using CAS. CAS authenticates using an HTML form and a web browser that follows an HTTP redirect. The web authentication works correctly, but MS Office authentication will not work because it does not permit completion of the form. This problem is caused by the limited set of authentication protocols that MS Office supports.
- HTTP Basic
- HTTP Digest
For more information about Alfresco Office Services limitations, see Considerations when using Alfresco Office Services.