Kerberos Windows Interoperability

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Introduction

Kerberos allows single sign and can assist with Windows and Linux interoperability. The basic goal is to get systems attached to an AD domain to be able to access servers using pass through authentication. Although I have entitled the page Windows Interop, its really AD integration. Linux clients using the various non server related bits can then join in - if that's your requirement.

For example, start up a browser and point it at an Apache webserver. The web server allows access to the browser user because they have been authenticated in the background via Kerberos.

The setup here is designed to use Samba to manage the Kerberos keytab side of things for servers like Squid, Apache and sshd. As is often the case there are other ways to do it but these notes provide an approach that seems to work with minimal maintenance once set up.

This howto does not cover setting up a Key Distribution Centre (MIT or otherwise) nor does it cover all of the other Samba features that you may be interested in. This is just the Kerberos related stuff. Be prepared to put some effort in - this is not a simple copy and paste effort.

Before you start this you must ensure that the clock on your system agrees with the Windows DC that holds the "PDC Emulator" FSMO role. Realistically any of them will do not just that one but that DC is the time source for the whole AD. You must also make sure that DNS is set up correctly. If you can resolve the name of the AD domain, then that is a good start. If either time or DNS go astray then you will get some tricky errors to debug. Always check them both first.

Kerberos (MIT)

On Gentoo USE = kerberos should pull in app-crypt/mit-krb5. Configuration for a simple setup (set your realm):

File/etc/krb5.confMain Kerberos config file

[libdefaults]
   default_realm = EXAMPLE.CO.UK
   kdc_timesync = 1
   ccache_type = 4
   forwardable = true
   proxiable = true
   fcc-mit-ticketflags = true
   default_keytab_name = FILE:/etc/krb5.keytab

You may want to create a realm to domain mapping section [realms]. This is especially useful for Apache when the system may have a different external domain to its internal one.

Do not create the file /etc/krb5.keytab - Samba will give an error if it can't create the keytab file.

The following command will grab a Kerberos ticket for the currently logged in user. <user> can actually be any valid Kerberized user account, if omitted then the current Unix username is used. If the default realm is already specified in krb5.conf then it can be also be omitted.

user $ kinit <user>@EXAMPLE.CO.UK

If the realm is already set and the ticket is for a username that matches the Unix user then simply run kinit and enter a password.

It is not always possible to use supplementary groups with some daemons eg Squid. The following will add additional ACLs to the Kerberos keytab file file to allow the processes to read the keytab. This assumes that you have ACL support on the system. If not then you will need some other method of allowing the daemons to access the single keytab. Failing that you will have to create separate keytabs for each application and get them set up manually - web search for that. Only do this bit after getting Samba to create the file, otherwise you will get errors.

root # setfacl -m u:squid:r /etc/krb5.keytab
root # setfacl -m u:apache:r /etc/krb5.keytab
root # getfacl /etc/krb5.keytab
file: etc/krb5.keytab
owner: root
group: root 
user::rwx
user:squid:r--
user:apache:r--
group::r--
mask::r--
other::---

Samba

Make sure you have a reasonably recent Samba release installed (eg 3.5 or 3.6+)

  • Set Samba to allow access via both secrets (winbind and local passwd) and Kerberos. There are many more options that you might want to configure when joining an AD domain but here we only consider Kerberos related ones.
File/etc/samba/smb.confPart of Samba config

[global]
   realm = EXAMPLE.CO.UK
   kerberos method = secrets and keytab
   winbind refresh tickets = yes
  • Tell winbind to setup a ticket cache for a logged in user.
File/etc/security/pam_winbind.confPart of winbind's PAM config

[global]
   krb5_auth = yes
   krb5_ccache_type = FILE
  • Join the domain
root # net ads join -U <admin user>
  • Start or restart Samba. Make sure that /etc/conf.d/samba has winbind included in the list of daemons.

Apache

  • Edit the module config or add it into your vhost configuration
File/etc/apache2/modules.d/11_mod_auth_kerb.confExample Apache Kerberos config

<IfDefine AUTH_KERB>
LoadModule auth_kerb_module modules/mod_auth_kerb.so

#LogLevel Debug

<Location "/">
       AuthType Kerberos
       AuthName "Kerberos Login"
       Krb5Keytab /etc/krb5.keytab
       KrbAuthRealms EXAMPLE.CO.UK
       KrbMethodNegotiate On
       KrbMethodK5Passwd Off       
       Require valid-user
</Location>
</IfDefine>

Set KrbMethodK5Passwd on to get prompted for authentication. I suggest you only use this over SSL/TLS (https) for obvious reasons.

  • Use Samba to set the service principle(s) for Apache (HTTP):
root # net ads keytab add HTTP -U <a_domain_admin>
root # net ads keytab add HTTP/<another_SPN>@EXAMPLE.CO.UK -U <a_domain_admin>
root # net ads keytab list

Note the format in the second command. This will get non default Service Principle Names into the keytab, eg for externally facing vhosts. Remember to set the [realms] section in krb5.conf if you need an external domain to map to an internal realm (or AD domain as MS call them!) The second command also seems to need the SPN added to AD using setspn.exe (on a Windows machine with Domain Admin rights and probably an elevated command prompt on server 2008+):

c:\>setspn.exe -A HTTP/<another SPN> <computer account>

Test it with this index.php in the web server's htdocs somewhere (assuming you have PHP installed and configured):

Fileindex.php

<?php
 echo "You have logged in as <b>" . $_SERVER['REMOTE_USER'] . "</b>;";
?>

Errors like this in errors_log means that the keytab can't be read - check the permissions on it for the apache user:

gss_acquire_cred() failed: Unspecified GSS failure.  Minor code may provide more information (, Permission denied)

SSHD

Use Samba to get the service principle for SSHD - "host". Just to spell it out in case of confusion: use the word host in your command. It is not part of an example, only change the bits between < and >.

root # net ads keytab add host -U <a_domain_admin>
File/etc/ssh/sshd_confPart of sshd's configuration file

KerberosAuthentication yes                   
KerberosOrLocalPasswd yes                          
KerberosTicketCleanup yes                     
GSSAPIAuthentication yes 
GSSAPICleanupCredentials yes
UsePAM no

When you turn off PAM you obviously wont be able to get it to make a home directory for a new user. So another method will be needed to do this. Winbindd can do this - see the docs for it if you need this.

Squid

Squid also seems to use "HTTP" as its service name like Apache. This will probably need a Squid 3.1.x release.

File/etc/squid/squid.confPart of the Squid configuration

auth_param negotiate program /usr/libexec/squid/squid_kerb_auth -d
auth_param negotiate children 10
auth_param negotiate keep_alive on 

acl AUTHENTICATED proxy_auth REQUIRED
http_access allow AUTHENTICATED
http_access deny all
File/etc/conf.d/squidPart of Squid RC config file

SQUID_KEYTAB="/etc/krb5.keytab"

NFS

WIP: Gerdesj 19:39, 28 January 2012 (UTC)

Windows Server

Add the file services role to Windows 2008(R2) and include services for NFS. To add NFS support on a folder, right click it and set it up on the NFS tab. NFS shares do not seem to appear in the Server Manager under Share and Storage Management. The Services for Network File System MMC snap in is a bit random as well.

Linux

Client apps

setspn.exe

Check Service Principle Names:

c:\>setspn.exe -l <account_name>

net ads keytab

Check a Samba maintained keytab like this:

root # net ads keytab list

ktutil and klist

Check keytabs like this:

root # ktutil

or

root # klist -ke /etc/krb5.keytab

Firefox

  • about:config
  • Type "negotiate" into the filter
  • Set network.negotiate-auth.trusted-uris to example.co.uk
  • Additional domains (eg short names) can be added separated by commas: eg example.co.uk,host1,host2.
  • (On Linux only) at a user shell prompt type "kinit" and enter your AD password to authenticate and get a ticket from the KDC (This is not needed if you set up /etc/security/pam_winbind.conf)

Chrome(ium)

Start Chrome with --auth-server-whitelist="*.example.co.uk" --auth-negotiate-delegate-whitelist="*.example.co.uk". You can edit /usr/share/applications/chrome.desktop to add this. Not needed on Windows.

See http://www.chromium.org/developers/design-documents/http-authentication and http://www.chromium.org/administrators/policy-list-3

IE

Does single signon out of the box. IE 7+. Don't use <=6!

ssh

For the SSH client on Linux. This can go in the system wide configuration or in a user specific configuration:

File/etc/ssh/ssh_confssh config snippet

host *.example.co.uk
  GSSAPIAuthentication yes

PuTTY

  • Get hold of a development snapshot until it is in the core release.
  • Connection -> Data, select Use system username (your name)
  • Connection -> SSH -> Auth -> GSSAPI, make sure Attempt GSSAPI authentication is selected