Using Samba to Authenticate against AD

FreeRADIUS uses Samba to integrate with Active Directory (AD) for authentication. For authorization such as group membership, FreeRADIUS can query an Active Directory over LDAP.

Configure Authentication with AD

Once the PAP authentication test has been successful, the next step for sites using AD is to configure the system to perform user authentication against AD. The clear-text passwords are unavailable through Active Directory, so we have to use Samba, and the ntlm_auth helper program. In this configuration, we are using AD as an authentication oracle, and not as an LDAP database.

Using ntlm_auth for PAP authentication may not work on recent versions of Samba and Active Directory. If so, just skip to the next section.

Once Samba has been installed on your system, you should edit the smb.conf file, and configure the [global] section to point to your NT server, including hostname and NT domain.

# workgroup = NT-Domain-Name
   workgroup = MYDOMAIN


# Security mode. Most people will want user level security. See
# security_level.txt for details.
   security = ads
# Use password server option only with security = server
   password server = nt-server-hostname.company.com

   realm = realm.company.com

For Samba 4, you also have to set the ntlm auth configuration variable. It should be set to either yes, or to mschapv2-and-ntlmv2-only. This configuration needs to be set all participating Samba members, and also on (Samba4) AD-DC servers.

`ntlm auth = mschapv2-and-ntlmv2-only`

You may also have to edit the /etc/krb5.conf file, to add an entry that points to the Active Directory Server. This is often not necessary, as Samba can just "figure it out" when Active Directory is also the main DNS server.

[realms]

realm.company.com = {
      kdc = nt-server-hostname.company.com
}

Start the Samba and Kerberos servers, and as root join the domain:

$ net join -U Administrator

Enter the administrator password at the prompt.

Next, verify that a user in the domain can be authenticated:

$ wbinfo -a user%password

You should see a number of lines of text, followed by authentication succeeded. The next step is to try the same login with the ntlm_auth program, which is what FreeRADIUS will be using:

$ ntlm_auth --request-nt-key --domain=MYDOMAIN --username=user --password=password

If all goes well, you should see authentication succeeding (NT_STATUS_OK). You may also see the NT_KEY output, which is needed in order for FreeRADIUS to perform MS-CHAP authentication.

Configure FreeRADIUS to use ntlm_auth

Once you have verified that Samba is installed and working correctly, and that the ntlm_auth program works, you can proceed with configuring FreeRADIUS to use ntlm_auth. For initial testing, we will be using the exec module, and will run the exact command line used above.

The module configuration is located in mods-enabled/ntlm_auth. The contents of the file are below, with the fields to edit in bold.

        exec ntlm_auth {
                wait = yes
                program = "/path/to/ntlm_auth --request-nt-key --domain=MYDOMAIN --username=%{mschap:User-Name} --password=%{User-Password}"
        }

This configuration tells the server to run the ntlm_auth program with the user name and password obtained from the Access-Request. You will also have to list ntlm_auth in the authenticate sections of each the sites-enabled/default file, and of the sites-enabled/inner-tunnel file:

authenticate ntlm_auth {
        ...
        ntlm_auth
        ...
}

Add the following text for testing purposes only to the top of the users file. In version 3 and above, the "users" file has moved to mods-config/files/authorize.

DEFAULT Auth-Type = ntlm_auth

This configuration says "for all users, if the authenticate method has not been set, set it to use the ntlm_auth program".

Start the server using radiusd -X, and wait for the debugging text to stop scrolling by. If all goes well, you should see the following text:

Ready to process requests. In another terminal window on the same machine, type the following command:

$ radtest user password localhost 0 testing123

If all goes well, you should see the server returning an Access-Accept message, and the window with radtest should print text similar to the following:

rad_recv: Access-Accept packet from host 127.0.0.1 port 1812, length=20

This text means that authentication succeeded. A few lines above this text, the debug output will also show the exact command line used to run ntlm_auth.

Configure FreeRADIUS to use ntlm_auth for MS-CHAP

Once you have the previous steps working, configuring FreeRADIUS to use ntlm_auth for MS-CHAP is simple. First, delete the testing entry used above from the users file, as leaving it in will break other authentication types. Then, find the mschap module in raddb/modules/mschap file, and look for the line containing `ntlm_auth = `. It is commented out by default, and should be uncommented, and edited to be as follows. As before, update the fields in bold to match your local configuration.

ntlm_auth = "/path/to/ntlm_auth --request-nt-key --allow-mschapv2 --username=%{mschap:User-Name:-None} --domain=%{%{mschap:NT-Domain}:-MYDOMAIN} --challenge=%{mschap:Challenge:-00} --nt-response=%{mschap:NT-Response:-00}"

Start the server and use radtest to send an MS-CHAP authentication request.

$ radtest -t mschap bob hello localhost 0 testing123

If everything goes well, you should see the server returning an Access-Accept message as above.

If it does not work, double-check the password you entered on the supplicant against the password in Active Directory. If it still does not work, it might be a bug in Samba. Change your version of Samba, either by installing a fixed version, or by repeatedly down-grading it (and testing) until it works.

If it does not work, then it is possible to test authentication with just the ntlm_auth command-line. Look at the FreeRADIUS debug output, and see the arguments passed to ntlm_auth. Copy and paste them to a command-line, and then use that command line for testing. This limited test is often simpler and faster than running a complex test with a full RADIUS server. When this limited test passes, then authentication with FreeRADIUS will work, too.