I work for an organization that follows the common model of assigning people systematically generated user ids. Like most technically inclined employees of this organization, I have local accounts on my workstation that don’t bear any relation to the generated account ids. For the most part this isn’t a problem, except that our organization uses Kerberos to authenticate access to a variety of resources (such as the mailserver and a variety of web applications).
In the past, I’ve gotten along by running an explicit
kinit lkellogg@EXAMPLE.COM on the command line once in a while, and that
works, but it’s not particularly graceful.
I’m running Fedora, which of course ships with SSSD. Two of the neat features available through SSSD are (a) you can have it acquire a token for you automatically when you authenticate and (b) renew that token periodically, assuming that you have a renewable token.
There are two problems that were preventing me from taking advantage of this service.
Combining Kerberos with local accounts
The first problem is that there is a general assumption that if you’re
using Kerberos for authentication, you are also using some sort of
enterprise-wide identity service like LDAP. The practical evidence of
this in SSSD is that you can’t use Kerberos as an
you are using the
id_provider. If you attempt a naive
configuration that includes the following:
[domain/local] id_provider = local auth_provider = krb5
(Thu Jul 16 22:19:44:802460 2015) [sssd] [confdb_get_domain_internal] (0x0010): Local ID provider does not support [krb5] as an AUTH provider.
It turns out that you can work around this limitation with a “proxy” identity provider. With this method, SSSD proxies identity requests to an existing NSS library. This can, for example, be used to get SSSD to interoperate with a legacy NIS environment, as in this example:
[domain/PROXY_KRB5] auth_provider = krb5 krb5_server = 192.168.1.1 krb5_realm = EXAMPLE.COM id_provider = proxy proxy_lib_name = nis enumerate = true cache_credentials = true
proxy_lib_name setting identifies the particular NSS provider to
use for identity information. This would make use of the
libnss_nis.so.2) for identity information while using
Kerberos for authentication.
For my own use case I want to use my local accounts for identity
information, which means I need to use the
files NSS provider:
[domain/example.com] id_provider = proxy proxy_lib_name = files auth_provider = krb5
Mapping a local username to a Kerberos principal
The second problem I had been struggling with was how to map my local
lars) to the organizational Kerberos principal
lkellogg@EXAMPLE.COM). I had originally been looking at solutions
kinit, but despite promising verbage in the
k5identity(5 man page, I wasn’t meeting with much success.
It turns out that SSSD has the
krb5_map_user option for exactly this
purpose; the syntax looks like:
krb5_map_user = <local name>:<principal name>
So, for me:
krb5_map_user = lars:lkellogg
Automatic ticket renewal
SSSD is able to automatically renew your Kerberos tickets for you,
provided that you’re able to acquire a renewable ticket. You can
check for this by running
klist and seeing if your ticket has a
renew until date in the future, as in the following example:
Ticket cache: KEYRING:persistent:1000:krb_ccache_rOS6mR8 Default principal: lkellogg@REDHAT.COM Valid starting Expires Service principal 07/17/2015 11:02:31 07/17/2015 21:02:31 krbtgt/REDHAT.COM@REDHAT.COM renew until 07/24/2015 11:02:31
If you meet this criteria, then you can add the following configuration options to your domain configuration:
krb5_renewable_lifetime = 7d krb5_renew_interval = 30m
The first (
krb5_renewable_lifetime) specifies the renewable lifetime
to request when requesting a ticket, and the second (
krb5_renew_interval) indicates how often SSSD should check to see if the ticket should be renewed.
An example configuration
This is approximately (names of been changed to protect the innocent) configuration that I am currently using with SSSD:
[domain/default] cache_credentials = True [sssd] config_file_version = 2 reconnection_retries = 3 sbus_timeout = 30 services = nss, pam domains = example.com [nss] filter_groups = root filter_users = root reconnection_retries = 3 [pam] reconnection_retries = 3 [domain/example.com] id_provider = proxy proxy_lib_name = files enumerate = True auth_provider = krb5 krb5_server = kerberos.example.com krb5_realm = EXAMPLE.COM cache_credentials = True krb5_store_password_if_offline = True krb5_map_user = lars:lkellogg chpass_provider = krb5 krb5_kpasswd = kerberos.example.com offline_credentials_expiration = 0 krb5_renewable_lifetime = 7d krb5_renew_interval = 30m
You’re not done yet! Once you have SSSD configured correctly, you
need to configure your system to make use of it for authentication.
First, you’ll want to ensure that your
/etc/nsswitch.conf file is
configured to use SSSD. You’ll want at least the
group databases configured to use SSSD:
passwd: files sss shadow: files sss group: files sss
Next, you’ll want configure PAM. On my system, I need to change two configuration files:
/etc/pam.d/system-auth, which is the default for many services, and
/etc/pam.d/password-auth, which provides defaults for other services, including
In my case, both files actually end up having identical content, which looks like this (largely cribbed from the Fedora documentation):
#%PAM-1.0 # This file is auto-generated. # User changes will be destroyed the next time authconfig is run. auth required pam_env.so auth sufficient pam_unix.so nullok try_first_pass auth requisite pam_succeed_if.so uid >= 1000 quiet_success auth sufficient pam_sss.so use_first_pass auth required pam_deny.so account required pam_unix.so account sufficient pam_localuser.so account sufficient pam_succeed_if.so uid < 1000 quiet account [default=bad success=ok user_unknown=ignore] pam_sss.so account required pam_permit.so password requisite pam_pwquality.so try_first_pass local_users_only retry=3 authtok_type= password sufficient pam_unix.so sha512 shadow nullok try_first_pass use_authtok password sufficient pam_sss.so use_authtok password required pam_deny.so session optional pam_keyinit.so revoke session required pam_limits.so -session optional pam_systemd.so session [success=1 default=ignore] pam_succeed_if.so service in crond quiet use_uid session optional pam_sss.so session required pam_unix.so
Note the entries for
pam_sss.so in each stanza.
The proof in the pudding
I start on my local system with no Kerberos tickets:
$ klist klist: Credentials cache keyring 'persistent:1000:krb_ccache_Pzo4C6u' not found
Then I lock my screen and unlock it using my Kerberos password, and now:
$ klist Ticket cache: KEYRING:persistent:1000:krb_ccache_rOS6mR8 Default principal: lkellogg@EXAMPLE.COM Valid starting Expires Service principal 07/16/2015 22:45:43 07/17/2015 08:45:43 krbtgt/EXAMPLE.COM@EXAMPLE.COM renew until 07/23/2015 22:45:43
I found the easiest way to troubleshoot SSSD was to stop the service:
# systemctl stop sssd
And then run
sssd on the command line in debug mode:
# sssd -d 5 -i
This generates logs on
stderr and helped me identity problems in my
Thanks to Jakub Hrozek for suggesting the use of the a proxy identity
provider to overcome the limitation on combining Kerberos with the