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 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 Content Services. For example, the identity of the logged-in user is extracted by the CAS, passed to Content Services servlets and extracted using the HttpServletRequest.getRemoteUser() method. As a result, when a user connects to Share they’re shown their user dashboard, but won’t 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 Content Services application and assert the user name in a secure manner.
A disabled user can still login 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 login 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 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.
In the first scenario, the Share endpoint-url (http://localhost:8080/alfresco/wcs) sends the request directly to Content Services using HTTP and a User Header. No certificate is used and the external.authentication.proxyUserName is blank:
```bash external.authentication.proxyUserName= ```
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 Content Services and enforce using the proxy, for example, by using firewall rules to the proxy.
In the second scenario, the Share endpoint-url (http://your.server.com/alfresco/wcs) sends the request back to Apache, using HTTP and a User Header (defined by external.authentication.proxyHeader), and a certificate. external.authentication.proxyUserName is set:
```bash external.authentication.proxyUserName=alfresco-system ```
Apache 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 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.
Using the external authentication subsystem means that:
- The complexity of authentication moves to an external software layer (a proxy). 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.
- Most of the responsibility for authentication is not controlled by 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.