Configuring sssd’s Active Directory provider

Following up on the previous post, here’s how we get sssd to actually provide access to our Samba-driven Active Directory.

I started with the instructions in the Samba wiki but these actually go beyond the minimum that is necessary. Let me also add some context to the individual components and settings involved.

How sssd’s components work together

sssd is quite modular: if you read the sssd.conf man page, you’ll learn about services and domains. You will also learn about different providers such as the already mentioned Active Directory provider that we are going to use. Do not be fooled however: providers are not mutually-exclusive. For example, our Active Directory provider works together with the LDAP and the Kerberos providers as shown here:

Individual sssd components working together
Individual sssd components working together

As a consequence, we’ll have to consider not only sssd-ad configuration directives but also some of those of sssd-ldap and sssd-krb5. And, because sssd-krb5 uses the Kerberos library we’ll also have to consider /etc/krb5.conf.

Configuration explained

Without further ado, here’s an example for a minimal /etc/sssd.conf that takes advantage of autodiscovery:


[sssd]
config_file_version=2
services=nss, pam
domains=ad.mydomain.foo.bar

[domain/ad.mydomain.foo.bar]
id_provider=ad
access_provider=ad
dyndns_update=false
enumerate=true
ldap_id_mapping=true
krb5_realm=AD.MYDOMAIN.FOO.BAR
krb5_keytab=/etc/sssd/ad.mydomain.foo.bar.keytab

Setting id_provider and access_provider activates sssd-ad as identity provider (ie. the source for user and group information) and access provider (eg. checks if a user is allowed access). However it also activates it as authentication provider (ie. checks passwords) and chpass provider (ie. changes passwords), because id_provider‘s value is a default for auth_provider, which in turn is the default for chpass_provider.

We do not specify ad_domain because the default is to use the configuration section’s name (minus the “domain/” part, of course). We do not specify ad_server either because Samba’s DNS server has automagically set up SRV records for us that sssd-ad/ can use for service discovery.

I disabled dyndns_update for now because it gave me problems. Setting enumerate to true is debatable and recommended for small setups only, but you might want it for playing around with nested groups and see how they work. The default is false.

There’s no need to specify any of ldap_uri, ldap_search_base, ldap_sasl_mech or ldap_sasl_authid, ldap_user_* and ldap_group_*sssd-ad will have taken care of these parameters for you.

ldap_id_mapping is set to true so that sssd itself takes care of mapping Windows SIDs to Unix UIDs. Otherwise the Active Directory must be able to provide POSIX extensions. If yours does, you can omit this option, of course.

It is obligatory to specify krb5_realm, which, by convention, is always upper-case and in most cases will the DNS name of the Active directory. krb5_keytab specifies the keytab file sssd will use to connected to Samba’s KDC (see also: What is a keytab file). The keytab file can be exported on the Samba server as per the Samba Wiki instructions.

Again, krb5_server, krb5_kpasswd will have already been provided by sssd-ad for you.

As you can see the net result is a much simpler configuration than with sssd-ldap and sssd-krb5 alone. Provided you’ve followed the other necessary steps, eg. PAM and NSS configuration (again, see eg. the Samba Wiki instructions), you can now try eg. getent passwd and should be able to see your Active Directory users.

Debugging hints

I had a hard time to get everything working, receiving “GSSAPI Error: Miscellaneous failure (Server not found in Kerberos database)” error messages all the time. Some hints:

  1. Start sssd manually and in debug mode with sssd -i -d 5, choosing the debug level as appropriate.
  2. One can not stress enough: get your DNS working properly. In my case, the DHCP service advertising the DNS servers to use ran on a separate machine, and of course I forgot to specify the IP address of the Samba server there…
  3. The Kerberos client library will do one thing you might not immediately be aware of: reverse DNS lookups. If, for example, you decided to use a separate DNS subdomain for Active Directory (which you definitely do want to do) but let your hostname sambahost.ad.foo.bar point at 1.2.3.4 and 1.2.3.4 in turn resolves to sambahost.foo.bar, things won’t work unless you specify rdns = false in your /etc/krb5.conf.

    By the way, the only way to really debug such issues is either to run Wireshark or to look at Samba’s logfiles — the client itself won’t even tell you it does such things (reverse lookups and their results) if you strangle it.

Update 08.02.2014: It seems that krb5_realm is not strictly necessary. The krb5_ad provider will take care of that for you:

(Sat Feb  8 23:55:13 2014) [sssd[be[ad.mydomain.foo.bar]]] [ad_set_ad_id_options] (0x0100): Option krb5_realm set to AD.MYDOMAIN.FOO.BAR

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