Single Sign-On for Kerberos

This guide will discuss how to set up single sign-on using Microsoft Active Directory.


SSO can only be configured on self-hosted installations.

Login screen with SSO button for one-click login.

As of Zammad 3.5, enabling SSO adds a new button to the sign-in page.

Conceptual Overview

Like every other web application out there, Zammad has its own logic for signing users up, storing their passwords, authenticating them, and managing their sessions.

If your IT department keeps its own user identity store (like Active Directory), Zammad’s SSO support allows you to leverage that existing auth system so that anyone with an account on your local intranet will 1) automatically have an account in Zammad and 2) be able to log in with a single click.


If you don’t have this IT infrastructure but still want one-click login, see Third-Party Authentication for alternatives.

How does it work?

Once enabled, single sign-on activates an endpoint at When the Zammad server receives a GET request at this endpoint with a valid username in any one of the following:

  • an X-Forwarded-User request header

  • a REMOTE_USER web server environment variable

  • an HTTP_REMOTE_USER web server environment variable

it creates a new session for that user.


😬 Wait. SSO allows you to sign in with only a username?

In principle, yes.

How is that okay?

In this guide, we configure our web server (Apache) to intercept all requests to the /auth/sso endpoint. Instead of forwarding them to Zammad, Apache initiates a three-sided login process (Kerberos authentication) between the itself, the user, and the Active Directory server.

If Active Directory doesn’t recognize the user or their password, Zammad never sees the request, and the session is never created.

What does this all mean?

It means there are many ways you could set up SSO—you don’t need to follow this guide or even use Active Directory or Kerberos—but if you don’t know what you’re doing, you’re going to end up with a massive security hole.

Getting Started


😵 Too busy to handle it on your own?

We’ve got you covered. Our experts offer custom-tailored workshops to get your team up and running fast and with confidence. Just drop us a line!

You will need:

  • a Microsoft Active Directory environment with

    • root access

    • support for AES 256-bit encryption

  • a Zammad host with

    • root access

    • a fully-qualified domain name (FQDN)

  • some familiarity with system administration (e.g., Apache configuration)

For best results, set up LDAP integration to make sure your Active Directory and Zammad user accounts are always in sync.

Step 1: Configure Active Directory

In the Kerberos authentication scheme, the authentication server (Active Directory) needs to maintain shared secrets with the service (Zammad). To make this possible, we need to register a service principal name (SPN) for Zammad on Active Directory.


These directions have been confirmed on Windows Server 2016.

1a. Create a service account

You may use an existing service account if you have one. Admin privileges are not required; a normal user account will do.

Login screen with SSO button for one-click login.

Select “This account supports Kerberos AES 256 bit encryption” under Properties > Account > Account options.

1b. Register an SPN for Zammad

Replace the following placeholders in the command below:


Zammad FQDN


Service account logon name


Password of the service account (Option /pass * did prove to not work)


Windows domain


Master domain controller IP/FQD

Below command will prompt for the users password:

$ setspn -s HTTP/<zammad-host> <service-acct>
$ ktpass /princ HTTP/<zammad-host>@<DOMAIN> \
         /mapuser <service-acct> \
         /crypto AES256-SHA1 \
         /ptype KRB5_NT_PRINCIPAL \
         /pass <password> -SetPass +DumpSalt \
         /target <master-domain-controller> \
         /out zammad.keytab

1c. Note the secret key and version number

The output of the command above contains important data for Step 2e below:

Using legacy password setting method
Failed to set property 'servicePrincipalName' to 'HTTP/<zammad-host>' on Dn 'CN=Zammad Service,DC=<domain>,DC=<tld>': 0x13.
WARNING: Unable to set SPN mapping data.
If <service-acct> already has an SPN mapping installed for HTTP/<zammad-host>, this is no cause for concern.
Building salt with principalname HTTP/<zammad-host> and domain <domain> (encryption type 18)...
Hashing password with salt "<domain><service-acct>".
Key created.
Output keytab to zammad.keytab:
Keytab version: 0x502
keysize 67 <service-acct>@<domain> ptype 1 (KRB5_NT_PRINCIPAL) vno 3 etype 0x12 (AES256-SHA1) keylength 32 (0x5ee827c30c736dd4095c9cbe146eabc216415b1ddb134db6aabd61be8fdf7fb1)

On the last line, take note of:

the secret key

in parentheses at the end (0x5ee827…)

the secret key version number

preceded by vno (3)

Step 2: Remove NGINX, Set up Apache + Kerberos

Next, the Zammad host must be configured to support Kerberos (and to accept auth credentials provided by the Active Directory server).

In most cases, you would have to recompile NGINX from source with an extra module to enable Kerberos support. To get around this, we will use Apache, which offers Kerberos support through a plug-in module instead.


All commands in this section must be run as root (or with sudo).

2a. Turn off NGINX


This will take your Zammad instance offline until Apache is fully configured and running.

$ systemctl stop nginx     # turn off nginx
$ systemctl disable nginx  # keep it off after reboot

If you wish to minimize downtime, you can save this step for last; just bear in mind that Apache will not start if the port it wants to listen on is being used by NGINX.

If for any reason you can’t complete this tutorial, simply turn off Apache and restore NGINX:

$ systemctl stop apache2
$ systemctl disable apache2
$ systemctl enable nginx
$ systemctl start nginx

2b. Pre-Configure Apache

This documentation expects a already working Apache configuration. Please see Configure the webserver before continuing.

2c. Install further Apache dependencies

$ apt update
$ apt install krb5-user libapache2-mod-auth-gssapi

2d. Enable Apache modules

SSO requires modules that are not enabled by default. By default you can use a2enmod to do so.

$ a2enmod auth_gssapi rewrite
$ systemctl restart apache2

2e. Configure Kerberos

Kerberos realm configuration is how you tell the Zammad server how to reach the domain controller (Active Directory server).

Replace the following placeholders in the sample config below:


Windows domain


Domain controller IP/FQDN(s)


Master domain controller IP/FQDN

(must not be read-only, but can be the same as <domain-controller>)

# /etc/krb5.conf

   default_realm = <DOMAIN>

   default_tkt_enctypes = aes256-cts-hmac-sha1-96
   default_tgs_enctypes = aes256-cts-hmac-sha1-96
   permitted_enctypes = aes256-cts-hmac-sha1-96

   kdc_timesync = 1
   ccache_type = 4
   forwardable = false
   proxiable = false
   fcc-mit-ticketflags = false

         # multiple KDCs ok (one `kdc = ...` definition per line)
         <DOMAIN> = {
                  kdc = <domain-controller>
                  admin_server = <master-domain-controller>
                  default_domain = <domain>

                  # below is only for GSSAPI
                  auth_to_local = RULE:[1:$1@$0](.*@<domain>)s/@<domain>$//
                  auth_to_local = DEFAULT

         .<domain> = <DOMAIN>
         <domain> = <DOMAIN>

2f. Generate keytab

Apache needs a Kerberos keytab (key table) to manage its shared secrets with the domain controller.

Replace the following placeholders in the commands below:


Zammad FQDN


Windows domain


Secret key (omit the leading 0x)


Secret key version number

The secret key and version number were found in Step 1: Configure Active Directory above.

$ ktutil

ktutil: addent -key -p HTTP/<zammad-host>@<DOMAIN> -k <vno> -e aes256-cts
Key for HTTP/<zammad-host>@<domain> (hex): <secret-key>

ktutil: list  # confirm the entry was added successfully
slot KVNO Principal
---- ---- ---------------------------------------------------------------
   1    3 HTTP/<zammad-host>@<DOMAIN>

ktutil: wkt /root/zammad.keytab  # write keytab to disk

ktutil: quit

Then, place the keytab in the Apache config directory and set the appropriate permissions:

$ mv /root/zammad.keytab /etc/apache2/
$ chown root:www-data /etc/apache2/zammad.keytab
$ chmod 640 /etc/apache2/zammad.keytab

2g. Configure Apache

Add the following directive to the end of the virtual host configuration file to create your Kerberos SSO endpoint at /auth/sso:

Replace the following placeholders in the command below:


Zammad FQDN


Windows domain

The configuration for CentOS and OpenSUSE below contains two Krb5KeyTab lines! Keep only the one you need.

# /etc/apache2/sites-available/zammad.conf

<LocationMatch "/auth/sso">
   AuthType GSSAPI
   AuthName "Your Zammad"
   GssapiBasicAuth On
   GssapiCredStore keytab:/etc/apache2/zammad.keytab
   GssapiLocalName On
   require valid-user

   RewriteEngine On
   RewriteCond %{LA-U:REMOTE_USER} (.+)
   RewriteRule . - [E=RU:%1,NS]
   RequestHeader set X-Forwarded-User "%{RU}e" env=RU

2g. Restart Apache to apply changes

$ systemctl restart apache2

Step 3: Enable SSO in Zammad

Next, enable “Authencation via SSO” in Zammad’s Admin Panel under Settings > Security > Third-Party Applications:

“Authentication via SSO” toggle button in the Admin Panel

In Zammad 3.5, this option adds a Sign in using SSO button to the sign-in page.


On older versions of Zammad, visit to sign in.

Step 4: Configure Client System (Windows Only)

For the full SSO experience (i.e., for passwordless, one-click sign-in), Zammad users must:

  1. be on the Active Directory server’s local intranet; and

  2. modify their network settings for the Zammad host to be treated as a local intranet server.

In-browser login prompt for single sign-on

Without this step, users must enter their Active Directory credentials during SSO.


This setting can be centrally managed across the entire intranet using a group policy object (GPO).

  1. Add your Zammad FQDN in Internet Options under Security > Local Intranet > Sites > Advanced.

  2. Select “Require server verification (https:) for all sites in this zone”.

  3. Under Security level for this zone > Custom level… > Settings > User Authentication > Logon, select “Automatic logon only in Intranet Zone”.

Adding Zammad as a single sign-on site in Windows Internet options


  • Are all relevant FQDNs/hostnames reachable from your Active Directory and Zammad servers (including each other’s)?

  • Are the system clocks of your Active Directory and Zammad servers synchronized within five minutes of each other? (Kerberos is a time-sensitive protocol.)

Errors in Apache Logs


Try raising your Apache log level temporarily.

Add LogLevel debug to your virtual host configuration, then restart the service to apply the changes.

“an unsupported mechanism was requested”

Does your Active Directory service account have Kerberos AES 256-bit encryption enabled?

If for some reason your server does not support AES 256-bit encryption, the LDAP Wiki has more information about Kerberos encryption types.

“failed to verify krb5 credentials: Key version is not available”

Did you use the exact version number (vno) provided by ktpass when generating your keytab?

Try generating it again, just to be sure.

“unspecified GSS failure. Minor code may provide more information (, No key table entry found for HTTP/FQDN@DOMAIN)”

Does the service name you provided to setspn exactly match the one you used when generating your keytab?

Try generating it again, just to be sure.

“No key table entry found for HTTP/FQDN@DOMAIN”

Does your virtual host configuration’s KrbServiceName setting exactly match the service name you provided to setspn?

This setting is case-sensitive.

“Warning: received token seems to be NTLM, which isn’t supported by the Kerberos module. Check your IE configuration”

Is your Zammad host accessible at an FQDN? This error may indicate that you configured your Zammad host as a numeric IP address instead.

“Cannot decrypt ticket for HTTP/FQDN@DOMAIN”

Did you make sure to change the password on your Active Directory service account after enabling 256-bit AES encryption?

And did you make sure to register the SPN (with ktpass) and generate your keytab (with ktutil) after changing your password?

Try running kinit -k -t <path to keytab> HTTP/<zammad-host>@<DOMAIN>. If no output is returned, you’re good - if you see “kinit: Preauthentication failed while getting initial credentials” your credentials provided were wrong or you used /pass * during ktpass command.

“failed when verifying KDC” and “failed to verify krb5 credentials: Decrypt integrity check failed”

Ensure KrbServiceName is the correct ServiceName provided via setspn.

Ensure your Active Directory supports the encryption method configured.

If all above is correct and the rest of FAQ also is ensured, make sure your client does not cache the results. klist purge clears the clients cache - a reboot of your client would do too.