Integrate Dovecot IMAP with (Free)IPA using Kerberos SSO

Dovecot can make use of Kerberos authentication and enjoying Single-Sign-On when checking emails via IMAP. This post shows you how you enable this feature. With IPA its rather simple to do so.

First enroll your mail server to the IPA domain with ipa-client-install as described in various previously posted articles.

Creating a Kerberos Service Priciple

Ensure you have a Kerberos ticket as admin user

ipa1:~# kinit admin
Password for admin@EXAMPLE.COM: 
ipa1:~#
ipa1:~# ipa service-add imap/mail.example.com
---------------------------------------------
Added service "imap/mail.example.com@EXAMPLE.COM"
---------------------------------------------
  Principal name: imap/mail.example.com@EXAMPLE.CCOM
  Principal alias: imap/mail.example.com@EXAMPLE.COM
  Managed by: mail.example.com
ipa1:~# 

Fetch and install the Kerberos Keytab for Dovecot

Log in to your mailserver and get a Kerberos ticket as well:

mail:~# kinit admin
Password for admin@EXAMPLE.COM: 
mail:~#

Fetch the Keytab:

mail:~# ipa-getkeytab -s ipa1.example.com -p imap/mail.example.com -k /etc/dovecot/dovecot-krb5.keytab
Keytab successfully retrieved and stored in: /etc/dovecot/dovecot-krb5.keytab
mail:~# 

A common mistake is to have the wrong ownership and access rights on the keytab file.

mail:~# chown dovecot:dovecot /etc/dovecot/dovecot-krb5.keytab
mail:~# chmod 600 /etc/dovecot/dovecot-krb5.keytab

Edit the following lines in /etc/dovecot/conf.d/10-auth.conf

auth_krb5_keytab = /etc/dovecot/dovecot-krb5.keytab
auth_mechanisms = plain gssapi login
auth_gssapi_hostname = mail.example.com
auth_realms = EXAMPLE.COM
auth_default_realm = EXAMPLE.COM

A note about auth_mechanisms: Usually you dont want to use Kerberos only authentication but plain (over TLS/SSL) as well.

Testing

In /var/log/maillog check if you see messages similar to this:

Feb 19 11:43:25 mail dovecot: imap-login: Login: user=, method=GSSAPI, rip=10.10.10.10, lip=192.168.0.10, mpid=5195, TLS, session=<asdfasdfasdf>

How about LDAP?

Since identity lookup is done with sssd, LDAP integration is not needed in such a case, there is not benefit using LDAP.

FreeIPA and Selective 2FA with Kerberos Authentication Indicators

One of the major new features in FreeIPA 4.4 is the introduction of Authentication Indicators in Kerberos tickets. This allows you to selectively enforce 2FA.

Usecases

Usually a Linux environment consists on a lot of different services. Some of them are security sensitive such as payroll systems while others are more relaxed such as simple Intranet Webservers.

Some services do not nicely play with 2FA, see https://blog.delouw.ch/2015/04/09/2fa-with-free-ipa-the-good-the-bad-and-the-ugly/. With Authentication Indicators you can allow users accessing this services without 2FA while deploying 2FA on all other services.

One of the obstacles for 2FA is user acceptance. With selective 2FA you can enforce it on the critical servers and/or services only.

Limitations

At the moment, selective 2FA with Authentication Indicators is only working with Fedora 24 and 25. There is no support (yet) for RHEL and its EL clones such as CentOS. Support for Authentication was added in SSSD 1.14, please also see the Release notes for SSSD 1.14.

At the moment, users on RHEL clients always need to provide the second factor. This probably will change for RHEL 7.3. Please also see Bugzilla #1290381. It is already included in public Beta.

Testing the new release

FreeIPA 4.4.2 is available in Fedora 25 Beta. SSSD 1.14 is available on Fedora 24 and newer and in RHEL 7.3 Beta.

Installing FreeIPA 4.4

Get Fedora 25 Beta and install four servers with it. (two replicas, two clients). Fedora 25 Beta can be downloaded here.

[root@ipa1 ~]# dnf -y install freeipa-server freeipa-server-dns

Dependencies will be resolved automatically.

Configure FreeIPA

For tests only, you can disable firewalld to avoid connectivity problems.

[root@ipa2 ~]# systemctl stop firewalld
[root@ipa2 ~]# systemctl disable firewalld

Note: the –allow-zone-overlap is only needed if you make tests with existing DNS domains such as example.com. Usually you should not use this parameter to not violate the highlander principle.

[root@ipa1 ~]# ipa-server-install --subject="O=EXAMPLE.COM 2016101501" --allow-zone-overlap --setup-dns --forwarder=8.8.8.8 --forwarder=8.8.4.4 

Install a replica

The second replica is first set up as a normal IPA Client and will then be promoted to be a replica.

Be sure you point your DNS to the first replica to allow detection of SRV DNS entries to correctly setup the client.

[root@ipa2 ~]# dnf -y install freeipa-server freeipa-server-dns 

Now setup the replica as a client

[root@ipa2 ~]# ipa-client-install

Get a Kerberos Ticket as admin user

[root@ipa2 ~]# kinit admin

Promote to be a replica

[root@ipa2 ~]# ipa-replica-install --setup-dns --setup-ca --forwarder=8.8.8.8 --forwarder=8.8.4.4

Enroll two or more clients

For our tests we need some clients, enroll some

Enable 2FA Authentication

As a default, 2FA is not enabled, lets change that

[root@ipa2 ~]# ipa config-mod --user-auth-type={password,otp}

Add some users

Add one or more users and set a password. Log in and set a new valid password

To be able to authenticate with both, Password only and 2FA, we need to provide that information when creating a new user. You also need to set an initial password.

[root@ipa2 ~]# ipa user-add --user-auth-type={password,otp} --first Joe --last Doe --shell=/bin/bash jdoe
[root@ipa2 ~]# ipa passwd jdoe

Get a Kerberos Ticket for jdoe, you will be promted to set a new password.

[root@ipa2 ~]# kinit jdoe
Password for jdoe@EXAMPLE.COM: 
Password expired.  You must change it now.
Enter new password: 
Enter it again: 
[root@ipa2 ~]# 

Add a 2FA Soft token

You can assign yourself a soft token with the CLI or WebUI.

[root@ipa2 ~]# ipa otptoken-add jdoe
----------------------
Added OTP token "jdoe"
----------------------
  Unique ID: jdoe
  Type: TOTP
  Owner: jdoe
  Manager: jdoe
  Algorithm: sha1
  Digits: 6
  Clock interval: 30
  URI: otpauth://totp/jdoe@EXAMPLE.COM:jdoe?digits=6&secret=NOBAETXGLCVEW7BSINC6II4XLSPTFPDK&period=30&algorithm=SHA1&issuer=jdoe%40EXAMPLE.COM


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[root@ipa2 ~]# 

You can add this QR code with the FreeOTP or Google Authenticator

Enforcing 2FA on a host principal

To enforce 2FA on a host, alter the host configuration as follows:

[root@ipa2 ~]# ipa host-mod  --auth-ind=otp ipaclient44-fedora24.example.com

You now can try to log in with one and two factors on that host and on some other hosts to see the difference.

Enforcing 2FA on a service

Enforcing of 2FA can also be done on a single (Kerberized) service.

[root@ipa2 ~]# ipa service-mod --auth-ind=otp http/ipaclient.example.com

Further reading

There are plenty of documents available on the internet, here is a choice:

Have fun 🙂

Integrate IPA in your Web application i.e. WordPress

Tired of log in to your favorite Web application? Integrate it with IPA, kerberize it! This blog post will guide you trough the kerberization of WordPress running on RHEL7 or Fedora. The magic is done by mod_intercept_form_submit and mod_auth_gssapi

Assumptions

  • You have a running IPA or FreeIPA infrastructure
  • Your Kerberos REALM is EXAMPLE.COM
  • The hostname where your WordPress instance is running is wptest.example.com
  • WordPress is installed in /var/www/html and ready to run
  • You are using a Linux Workstation with Kerberos, other client OS may or may not work
    • Result


      You can log in to your WordPress instance with a Kerberos enabled Web Browser and username/password with Browser not enabled. The access is restricted by HBAC (Host Base Access Control)

      Install the required software

      root@wptest ~]# yum -y install ipa-client mod_intercept_form_submit mod_auth_gssapi
      root@wptest ~]# setsebool -P allow_httpd_mod_auth_pam 1
      root@wptest ~]# wget https://downloads.wordpress.org/plugin/http-authentication.4.5.zip
      

      Ensure the modules are loaded by Apache, please check the files in /etc/httpd/conf.modules.d/. The WordPress plugin is needed to enable WordPress to make use of the REMOTE_USER server environment. Unzip and place it in /var/www/html/wp-content/plugins/ and run restorecon to relabel the content to be able to use SELinux in enforcing mode.

      root@wptest ~]# unzip http-authentication.4.5.zip
      root@wptest ~]# mv http-authentication /var/www/html/wp-content/plugins/
      root@wptest ~]# restorecon -v -R /var/www/html/
      

      Enroll your Linux server

      Of course your server must be enrolled with IPA.

      root@wptest ~]# ipa-client-install
      

      Get your Kerberos Keytab

      First the Kerberos service principal must be created. Go to one of your IPA servers and add the principal.

      root@ipa1 ~]# kinit admin
      root@ipa1 ~]# ipa service-add HTTP/wptest.example.com
      

      Go to the client, fetch the Kerberos Keytab and make it available for Apache

      root@wptest ~]# kinit admin
      root@wptest ~]# ipa-getkeytab -s ipa1.example.com -p HTTP/wptest.example.com -k /etc/httpd/conf/http.keytab
      root@wptest ~]# chown apache /etc/httpd/conf/http.keytab
      root@wptest ~]# chmod 600 /etc/httpd/conf/http.keytab
      

      Configure PAM

      We are going to create a PAM config file called “wordpress” as a requirement for the authentication method used. Remember, you need to use SSSD because want to make use of HBAC.

      cat << EOF >> /etc/pam.d/wordpress
      auth    required   pam_sss.so
      account required   pam_sss.so
      EOF
      

      Hacking WordPress

      After the configuration of Apache, non-kerberized logins would not work anymore. This is probably not a scenario you want. You will still be able to log in with your IPA credentials, HBAC still works.

      The trick is that if Kerberos authentication fails, you will be redirected to a non-kerberized login page.

      [root@wptest ~]# cp /var/www/html/wp-login.php /var/www/html/wp-login2.php
      [root@wptest ~]# sed -i s/wp-login.php/wp-login.php /var/www/html/wp-login2.php
      [root@wptest ~]# restorecon -v /var/www/html/wp-login2.php
      

      Configure Apache

      The two different modules must be configured to get ready to use.

      Important to know: the parameters InterceptFormLogin and InterceptFormPassword are containing the name of the user and password input field name of the form, as defined in the HTML code. I.e. <input type=”text” name=”log” id=”user_login” …

      cat << EOF >> /etc/httpd/conf.d/intercept_form_submit.conf
      <LocationMatch ^/wp-login.php>
        InterceptFormPAMService wordpress
        InterceptFormLogin log
        InterceptFormPassword pwd
      </LocationMatch>
      
      <LocationMatch ^/wp-login2.php>
        InterceptFormPAMService wordpress
        InterceptFormLogin log
        InterceptFormPassword pwd
      </LocationMatch>
      EOF
      

      The next configuration files is for the mod_auth_gssapi

      The interesting part is the ErrorDocument where you tell the browser to redirect to the previously created wp-login2.php form. Since that form is only protected by mod_intercept_form_submit there will be no Kerbros authentification and prompting the user for the username and password.

      cat << EOF >> /etc/httpd/conf.d/auth_gssapi.conf
      <Location "/wp-login.php">
        AuthType GSSAPI
        AuthName "Kerberos Login"
        GssapiCredStore keytab:/etc/httpd/conf/http.keytab
        GssapiCredStore client_keytab:/etc/httpd/conf/http.keytab
        GssapiDelegCcacheDir /var/run/httpd/clientcaches
        GssapiUseS4U2Proxy on
        Require pam-account wordpress
        ErrorDocument 401 '<html><meta http-equiv="refresh" content="5; URL=/wp-login2.php"><body><h1>Kerberos authentication did not pass</h1><h2>Make sure your browser is configured correctly and you got a Kerberos Ticket.</h2></body></html>'
      </Location>
      EOF
      

      Configure the WordPress plugin

      Point your Browser to https://wptest.example.com/wp-admin/options-general.php?page=http-authentication%2Fhttp-authentication.php and set the following Parameters:

      • Allow WordPress authentication -> Enabled
      • Automatically create accounts -> Enabled
      • Email address domain -> Whatever the users domain is

      Now point your browser to http://wptest.example.com/wp-admin/options-general.php And select the default role of the users being created when logging in the first time.

      Create a HBAC rule

      Usually you want to create a HBAC rule to i.e. allow only certain users or group(s) of users to log in.
      Some time ago I wrote an article on how to do so.

      Restarting Apache

      Now it is time to restart your Apache Server to activate the settings.

      [root@wptest ~]# systemctl restart httpd.service
      

      Configure Firefox

      To be able to use Kerberos with Firefox, you need to set a few options. Please point your browser to one of your IPA repliacas, i.e. http://ipa1.example.com/ipa/config/browserconfig.html”

      Useful Links

      Have fun 🙂 Feedback welcome..

Identity Management und 2FA mit (Free)IPA @Chemnitzer Linuxtage 2015

My first post in German, publishing the Slide Deck (in German) for my presentation about IPA and 2FA held at Chemnitzer Linux days 2015.

Mein erster Post in Deutsch. Hier die Slides von meinem Vortrag an den Chemnitzer Linux Tagen 2015.

Abstract:
IPA ist ein Identity Management System für Linux und Unix, das stetig an Bedeutung gewinnt. Mittlerweile ist es des öfteren in Behörden, Banken, Versicherungen, aber auch in KMUs im Einsatz. IPA kann man sich als «Active Directory» für Linux vorstellen. IPA verheiratet LDAP und Kerberos zu einem Opensource Produkt das leicht zu installieren und zu unterhalten ist. Mit IPA kann dank Kerberos Single-Sign-On realiert werden (Authentifizierung). Regelsätze legen fest, welche Benutzer von welchen Benutzergruppen auf welche Services und Hosts zugreifen dürfen.

Seit einiger Zeit lassen sich mit IPA auch sehr einfach 2FA-Lösungen (Zwei-Faktor-Autentifizierung) realisieren, um die Sicherheit weiter zu erhöhen.

Das Slide Deck gibt es hier:

Slides vom Vortrag

Die Slides habe ich übringens bei einem Spontanvortrag bei der Berliner Linux User Group am 2015-04-08 wiederverwendet. Aufgrund des Feedbacks wird in den nächsten Wochen ein ca. 4h Workshop an einem Samstag organisiert.

Ich hoffe es hat allen anwesenden Spass gemacht und konnte Euch etwas Wissen vermitteln. Feedback zu beiden Anlässen willkommen.