Understanding how to troubleshoot Kerberos Authentication on Linux is essential to keeping your system secure. Here’s everything you need to know about it. 

Authentication is a fundamental security objective in any communication system, and several protocols are available in Linux to address the authentication problem. Amongst these protocols, Kerberos is one such that’s widely used.

In this article, we will discuss the various issues that arise during the use of Kerberos, and we will also discuss the steps for troubleshooting these problems in Linux. 

Introduction to Kerberos

Kerberos is a three-phased authentication protocol for client-server applications using cryptographic techniques. It was developed by MIT in 1988 to protect network services provided by Project Athena. The client can authenticate using this protocol over an insecure network. After a client is authenticated, further messages are to ensure integrity and privacy. 

We will now proceed towards the discussions on various problems related to Kerberos authentication in Linux and also provide the steps for troubleshooting. These problems relate to setup and monitoring, client utility management, KDC support, and keytab issues.

How to Troubleshoot Kerberos Authentication on Linux

These are the initial problems associated with setup and monitoring. Following are the few steps that you may employ to address these problems.

Installation of protocols on both machines

To work correctly, a functional Kerberos protocol must be installed on both machines. Please make sure this requirement is met before proceeding to steps ahead.

Timings are the same on both machines

The Kerberos authentication may fail because of clock skew errors. Make sure that the timings on both machines are the same, i.e., the policy server clock and KDC system clock are synchronized. For this purpose, you can use network time synchronization (NTS) such that both machines are within 5 minutes of each other.

Check domain name service (DNS) entries on both machines

Make sure that the DNS entries are the same on both machines. To be precise, ensure that each entry in the host file has the relevant internet protocol (IP) address, hostname, and fully qualified domain name (FQDN). Alternatively, access should have been provided in the /etc/hosts file. A good entry should look like the following:

IP address
FQDN hostname

Note: The order of the DNS entries provided matters if you want the operating system to try each of those nameservers in sequence to find a given domain. Also, multiple entries should be separated by spaces.

Troubleshooting client utility issues of Kerberos

On topics related to managing client utility, the following methods may be tried out.

Try the klist command to visualize tickets

The klist can be used to list down the information of the keytab file. It is an essential command that can also visualize all the tickets in any credentials’ cache or the keys in the key tab file. After having the ticket, the details can be forwarded to complete the authentication process. Following is the sample output of klist:

bash-2.05$ list
Ticket cache: /tmp/krb5cc_1002
Default principal: HTTP/[email protected]
Valid starting            Expires                  Service principal
Mon Dec 12 15:00:03 2022 Mon Dec 12 21:40:03 2012 krbtgt/[email protected]
Kinit

Try kinit command for checking KDC client issues

This command can be used to verify if there are any issues with the KDC host and client. The utility can be used to get and cache a ticket-granting ticket for the service principal and the user. It is possible that the client utility issue arises from the wrong principal name or the wrong user. Following is the kinit syntax for the user principal:

kinit username

When you run the above command, it will prompt you for the password before creating a user principal. While using the kinit command, ensure you provide the user name with a valid entry from the directory.

Now, we will see the use of kinit command for the service principal. Following is the syntax for this command:

kinit –k [-t keytab_file] principal_name

Note: The kinit command for the service principal will not ask for any password as the keytab file is provided, and this file will be used for authentication.

Use kinit command to check the SMP problems

After trying the above two steps, you can also use kinit command to determine if there are any issues related to the SMP host. For this purpose, the following command can be used:

kbd> kinit –S host/mail.company.com#SERVER01.COMPANY.COM

Try ktpass command

One of the possible reasons for issues can be the password problem. To make sure that the problem is not arising because of the password, you can use the ktpass utility version.

Troubleshooting KDC support issues

The Kerberos can cause issues because of KDC encryption support. The following error may arise in this case:

kinit: KDC has no support for encryption type while getting initial credentials.

In such as case, you may try following two steps to resolve the issues. Specifically, make sure that:

• The KDC settings are not blocking or restricting any encryption types
• All encryption types are checked in the server account

Troubleshooting the keytab issues

There may be issues arising because of keytab. In this case, the following steps may be taken:

• There are details related to location and name in krb5.conf file. Similar details should be provided in the key tab file
• The principal names have been provided for both client and servers
• Verify the encryption type before the creation of the tab file
• Confirm the validity of the key tab file. This step can be performed using the following kinit command:

kinit –t khan.keytab HTTP/[email protected]
Kinit host/[email protected] 

This command should run fine without any error if the key tab file is valid. If any error arises, verify the validity of SPN.

Finally, the following are some more suggestions:

• Install any packet tracer to visualize the tokens that are exchanged between the browser and web server
• It is recommended that logs are enabled for the policy server and web agent such that authentication error messages are recorded
• You should log off the workstation host after any change in the encryption type
• Finally, confirm the key version number (kvno). The version number of the service principal should match the value in keytab file

In this article, we have discussed the common issues that arise for Kerberos. The article discusses the steps for troubleshooting these problems.

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