Wednesday, November 21, 2012

Testing Kerberos in Ubuntu

In fixing a LightDM bug recently I needed to set up Kerberos authentication for testing. Now, Kerberos comes with quite a reputation for complexity so this was not a task I was looking forward to. And googling around to get some simple Ubuntu instructions only ended up confirming my expectations. But in the end, I was able to get it to work [1] and here is what I did. You should probably not rely on this information for an actual Kerberos implementation.

I start with two machines running Ubuntu, one as the Kerberos server [2] and one as a client. The client is already installed with a user account called test.

Server configuration


Edit /etc/krb5.conf to set the default realm [3]:
 
[libdefaults]
default_realm = TEST

Install the Kerberos server:

$ sudo apt-get install krb5-kdc krb5-admin-server

Create the realm. You will be prompted for a master password for the realm:

$ sudo krb5_newrealm

Add a new user (called a principal in Kerberos language) into the realm with the same username as on the client. You will be prompted for a password for this user [4]:

$ sudo kadmin.local
kadmin.local:  add_principal test


And now the server should be running. You can check things are working by watching the log:

$ tail -f /var/log/auth.log

Client configuration


The client is a lot easier, as the packages do most of the work for you:

$ sudo apt-get install krb5-user

You will be prompted for the following information:
  • Set "Default Kerberos version 5 realm" to TEST
  • Set "Kerberos server for your realm" to address / hostname of your server
  • Set "Administrative server for your Kerberos realm" to address / hostname of your server
Now you can test by getting a ticket [5] from the server. You will be prompted for the password you set when running kadmin.local on the server:

$ kinit
$ kdestroy


If that worked then you're ready to go. Have a look at the auth.log on the sever if it didn't work (the error messages are a bit cryptic though).

The next step is to setup PAM [6] to allow authentication with Kerberos. There's no configuration required, just install it:

$ sudo apt-get install libpam-krb5

Now you can log into your client machine (e.g. from LightDM/Unity Greeter) using the Kerberos password you setup on the server. Remember if something went wrong you can still use the local password to get in [7].

The reason I set all this up was to test Kerberos accounts which need password changes. You can control this feature from the server using the following:

$ sudo kadmin.local
kadmin.local:  modify_principal +needchange test



[1] on Ubuntu 13.04 (server) and 12.04 (client). I don't know which other combinations will work.
[2] Called a Key Distribution Centre in Kerberos jargon.
[3] Kerberos calls different authentication domains realms. I've used the realm TEST though in proper usage this would be a domain name e.g. EXAMPLE.COM to avoid name collision.
[4] You will already have a password set for this user on the client machine. Pick a different password as this allows you log in with either Kerberos or local passwords - both passwords will work.
[5] A ticket is the name for an authentication token provided by the server. In a real implementation this ticket will allow you to access services without re-entering your password.
[6] PAM is the library that does authentication when logging into Ubuntu.
[7] The PAM configuration that the packages setup first tries your password with the Kerberos server, then the local passwords (/etc/shadow) if that fails.

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