Embracing SSSD in Linux

When RHEL6 came around and I saw that sssd was a new way to sync up to the LDAP server, I cringed in horror. The PADL tool set that was shoehorned into RHEL5 was painful, and it took a lot to get it working correctly.  The last thing I wanted to do was to re-open that wound.

Caching and why you want to do it
With the advent of centralized services, the IT world took a great step forward.  It wasn’t perfect, though. It still required the client to be able to talk to the service (ldap, hosts, automount maps, etc) in real time.  If the centralized service were down, clients would break as well.  Introducing redundancy in to the environment (active/passive LDAP pairs) sort of solved this problem but not entirely.  Those with laptops taking their machines off the wire either to go home or to wander to a new location, would lose access.  Individual services might cache credentials locally, but those eventually have to phone home, fail, and then lock the user out.

The goal, really, is centralized management. Make it easier to manage the complex linux environment.  Everything from user accounts and permissions, to domain name resolution.

Single Sign-On SSO
As we get closer to a Single-Sign-On (SSO) environment for Linux, funneling all requests through a master intermediary which would handle the connection to the various backend solutions and cache the data locally looked to be a decent option. The PADL tools were the beginnings of that, but their implementation was clunky and hack-ish.  Despite that, the results were worth the headache.

With enough griping, when RHEL5 hit, it was all re-wrapped in to nslcd.  Slightly updated from previous incarnations, those familiar with the configuration files of old were able to easily implement nslcd.  Combined with nscd, you had a workable solution yet still filled with headaches of its own.

System Security Services Daemon (SSSD)
RHEL6 came in to place and introduced sssd. Aside from the awful name, it was yet another hideous beast fighting to control. Like many others, I had reluctantly embraced nslcd because it was close to the old ldap.conf file, the configurations were generally the same, and it took a short amount of time to get implemented.  SSSD was a completely different beast that requires some time to learn and understand before diving fully in.

Unlike other people, I didn’t hold an angry grudge against the PADL tools.  It was a means to an end, and, when all was said and done, it delivered what it promised.  I had a bunch of integration to OpenLDAP through PAM with caching with nslcd. Things worked.

Why implement SSSD?
Despite what the administrator in you feels (“do it once, do it right” creedo), the Admin/Engineer’s world is about doing things better. SSSD combines the functionality of nslcd and nscd without the array of bugs, without the odd “third wheel” product support, and it expands the scope of what can be managed easily.

Admit it: the LDAP world is changing, and Single Sign On (SSO) is continually evolving. The tools to support that have grown in functionality and scope, and new options are out there. For example, one of the key centralized backend tools, OpenLDAP, is admittedly unfriendly on a good day.  Alternatives have come along, such as the Fedora Directory Server (FDS) – now known as 389-ds -, and have matured significantly.  When bundled with SSSD and IPA, you have the makings of the Windows Active Directory equivalent in Linux.  389-ds is the supported RedHat tool, and it is actively being developed in the Fedora world, too.  IPA is a RedHat only option, but there is always FreeIPA for the rest of us.

In other words: you have gone as far as you can go with nslcd/nscd, and implementing sssd prepares you for the future.

BEWARE of documentation caveats
SSSD is still growing and evolving. Changes to the content and format of the sssd.conf file are happening quite frequently, and here are a few items to be aware of:

  • SSSD fires off a separate daemon handler for each service. If you want to debug a service, you cannot turn debugging on in the [sssd] stanza, but you must define it within the service stanza. For example, “debug_level = 9” would go under “[autofs]” to debug autofs.
  • The RedHat documentation is good but it gets confusing with regard to setting autofs attributes. To clarify, those settings must be made at the [domain/default] stanza otherwise they will be quietly ignored.
  • Debug settings are no longer numbers from 0-10.  They are now a crazy scheme of 0x4000 type of numbering.
  • SSSD’s debugging is a bit painful. It doesn’t always log what you want where you want it to. If you get close to the end of your rope, it is very helpful to run sssd in the foreground in one window while testing in another to watch the output live.
    sssd -d 9 -c /etc/sssd/sssd.conf -i

The actual migration
Migrating from nslcd to sssd can be a bit of a pain as it is likely you already have a number of dependencies in place.  For me, my edge hosts only needed to talk back to the LDAP servers for local authentication and automount tables.  Documentation from sites such as RedHat are very complete yet are still lacking in some key areas.

Migrating basic authentication is rather easy:

  1. yum -y install sssd
  2. Build your /etc/sssd/sssd.conf:
    [sssd]
    config_file_version = 2
    reconnection_retries = 3
    services = nss, pam, autofs, sudo
    # SSSD will not start if you do not configure any domains.
    # Add new domain configurations as [domain/] sections, and
    # then add the list of domains (in the order you want them to be
    # queried) to the "domains" attribute below and uncomment it.
    domains = LDAP
    #debug_level = 10
    
    [nss]
    filter_users = root,ldap,named,avahi,haldaemon,dbus,radiusd,news,nscd
    reconnection_retries = 3
    
    [pam]
    reconnection_retries = 3
    
    [autofs]
    
    [sudo]
    
    [domain/LDAP]
    ldap_tls_reqcert = never
    # Note that enabling enumeration will have a moderate performance impact.
    # Consequently, the default value for enumeration is FALSE.
    # Refer to the sssd.conf man page for full details.
    enumerate = true
    auth_provider = ldap
    # ldap_schema can be set to "rfc2307", which stores group member names in the
    # "memberuid" attribute, or to "rfc2307bis", which stores group member DNs in
    # the "member" attribute. If you do not know this value, ask your LDAP
    # administrator.
    #ldap_schema = rfc2307bis
    ldap_schema = rfc2307
    ldap_search_base = dc=ourdomain,dc=com
    ldap_group_member = uniquemember
    id_provider = ldap
    ldap_id_use_start_tls = False
    chpass_provider = ldap
    ldap_uri = ldap://ldap1.ourdomain.com/,ldap://ldap2.ourdomain.com/
    ldap_chpass_uri = ldap://ldap.ourdomain.com/
    # Allow offline logins by locally storing password hashes (default: false).
    cache_credentials = True
    ldap_tls_cacertdir = /etc/openldap/cacerts
    entry_cache_timeout = 600
    ldap_network_timeout = 3
    autofs_provider = ldap
    ldap_autofs_search_base = dc=ourdomain,dc=com
    ldap_autofs_map_object_class = automountMap
    ldap_autofs_map_name = ou
    ldap_autofs_entry_object_class = automount
    ldap_autofs_entry_key = cn
    ldap_autofs_entry_value = automountInformation
    sudo_provider = ldap
    ldap_sudo_search_base = ou=Sudoers,dc=ourdomain,dc=com
    ldap_sudo_full_refresh_interval=86400
    ldap_sudo_smart_refresh_interval=3600
    # Enable group mapping otherwise only the user's primary group will map correctly. Without this
    # defined group membership won't work
    ldap_group_object_class = posixGroup
    ldap_group_search_base = ou=group,dc=ourdomain,dc=com
    ldap_group_name = cn
    ldap_group_member = memberUid
    
    # An example Active Directory domain. Please note that this configuration
    # works for AD 2003R2 and AD 2008, because they use pretty much RFC2307bis
    # compliant attribute names. To support UNIX clients with AD 2003 or older,
    # you must install Microsoft Services For Unix and map LDAP attributes onto
    # msSFU30* attribute names.
    ; [domain/AD]
    ; id_provider = ldap
    ; auth_provider = krb5
    ; chpass_provider = krb5
    ;
    ; ldap_uri = ldap://your.ad.example.com
    ; ldap_search_base = dc=example,dc=com
    ; ldap_schema = rfc2307bis
    ; ldap_sasl_mech = GSSAPI
    ; ldap_user_object_class = user
    ; ldap_group_object_class = group
    ; ldap_user_home_directory = unixHomeDirectory
    ; ldap_user_principal = userPrincipalName
    ; ldap_account_expire_policy = ad
    ; ldap_force_upper_case_realm = true
    ;
    ; krb5_server = your.ad.example.com
    ; krb5_realm = EXAMPLE.COM
    
  3. Start sssd
    systemctl enable sssd; systemctl start sssd
  4. Update your nsswitch.conf file by replacing all references to ldap with sss
  5. RESTART your services.  I cannot stress this one enough. Most of the services do some form of internal connection caching on their own and will still be attempting to communicate to the back-end service as it was previously defined in nsswitch.conf.
  6. Stop and disable nscd:
    systemctl stop nscd; systemctl disable nscd
  7. Test each service
    users accounts: getent passwd
    user authentication: ssh atestuser@localhost
    POP3/IMAP access via dovecot: fire up a mail client (your phone, a web client) to see if you can access your email
    Sendmail authentication via SASL: make sure you can SEND an email
    Email filtering via MailScanner: this should work because it is not connected to users, but it won't hurt it to restart
    Web authentication via LDAP or tools such as owncloud, roundcube, etc: try logging in
    Filesystem mounts via NFS: cd /home/atestuser
  8. Stop and disable nslcd:
    systemctl stop nslcd; systemctl disable nslcd

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