Single Sign-On for Kerberos on-premise only

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

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.

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If you don't have this IT infrastructure but still want one-click login, you can use alternatives like Github, Google, Facebook and more.

How Does it Work?

Once enabled, single sign-on activates an endpoint at https://your.zammad.host/auth/sso. When the Zammad server receives a GET request at this endpoint with a valid username in any one of the following entities, it creates a new session for that user:

• an X-Forwarded-User request header
• a REMOTE_USER web server environment variable
• an HTTP_REMOTE_USER web server environment variable

INFO

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

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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 the LDAP integration to make sure your Active Directory and Zammad user accounts are always in sync. You can find it in Zammad's admin interface under Settings > Security > Third-party Applications.

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.

INFO

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.

1b. Reset Password

Reset service account password after enabling the "This account supports Kerberos AES 256 bit encryption".

1c. Register a SPN for Zammad

Replace the following placeholders in the commands below:

• <zammad-host>: Zammad FQDN
• <service-acct>: Service account logon name
• <password>: Password of the service account (Option /pass * did prove to not work)
• <domain>: Windows domain
• <master-domain-controller>: Master domain controller IP/FQD

The commands below 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

1d. 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) // [!code focus]

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.

INFO

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

2a. Turn Off NGINX

WARNING

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

Turn off nginx:

systemctl stop nginx

Keep it off after reboot:

systemctl disable nginx

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:

Details

systemctl stop apache2

systemctl disable apache2

systemctl enable nginx

systemctl start nginx

2b. Pre-Configure Apache

This documentation expects a already working Apache configuration. You should have a look at the webserver configuration guide 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:

• <domain>: Windows domain
• <domain-controller>: Domain controller IP/FQDN(s)
• <master-domain-controller>: Master domain controller IP/FQDN (must not be read-only, but can be the same as <domain-controller>)

# /etc/krb5.conf

[libdefaults]
   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

[realms]
         # 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_realm]
         .<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-host>: Zammad FQDN
• <domain>: Windows domain
• <secret-key>: Secret key (omit the leading 0x)
• <vno>: Secret key version number

The secret key and version number were found in sso-register-spn (Step 1d) above.

Enter ktutil:

ktutil

Add keytab:

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

Confirm the entry was added successfully:

ktutil: list
slot KVNO Principal
---- ---- ---------------------------------------------------------------
   1    3 HTTP/<zammad-host>@<DOMAIN>

Write keytab to disk:

ktutil: wkt /root/zammad.keytab

Leave ktutil:

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-host>: Zammad FQDN
• <domain>: Windows domain

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

<LocationMatch "/auth/sso">
   SSLRequireSSL
   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
</LocationMatch>

2g. Restart Apache to Apply Changes

systemctl restart apache2

Step 3: Enable SSO in Zammad

Next, enable "Authentication via SSO" in Zammad's Admin Panel under Settings > Security > Third-Party Applications

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On older versions of Zammad, visit https://your.zammad.host/auth/sso 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.

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

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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".

Troubleshooting

• 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

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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 <sso-generate-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 <sso-generate-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.