Migrating to LDAP PAM Pass Through Auth
The Research Computing authentication path is more complex than I'd like.
We start with
pam_ssswhich, of course, authenticates against sssd.Because we have users from multiple home institutions, both internal and external, sssd is configured with multiple domains.
Two of our configured domains authenticate against Duo and Active Directory. To support this we run two discrete instances of the Duo authentication proxy, one for each domain.
The Duo authentication proxy can present either an LDAP or RADIUS interface. We went with RADIUS. So sssd is configured with
auth_provider = proxy, with a discrete pam stack for each domain. This pam stack usespam_radiusto authenticate against the correct Duo authentication proxy.The relevant Duo authentication proxy then performs AD authentication to the relevant authoritative domain and, on success, performs Duo authentication for second factor.
All of this technically works, and has been working for some
time. However, we've increasingly seen a certain bug in sssd's
proxy authentication provider, which manifests as an incorrect
monitoring or management of authentication threads.
The problem
[sssd[be[rc.colorado.edu]]] [dp_attach_req] (0x0400): Number of active DP request: 32
sssd maintains a number of pre-forked children for performing this
proxy authentication. This default to 10 threads, and is configurable
per-domain as proxy_max_children. Somewhere in sssd a bug exists
that either prevents threads from being closed properly or fails to
decrement the active thread count when they are closed. When the
"Number of active DP request" exceeds proxy_max_children sssd will
no longer perform authentication for the affected domain.
We have reported this issue to Red Hat, but 8 months on and we
still don't have a fix. Meanwhile, I'm interested in simplifying our
authentication path, hopefully removing the proxy authentication
provider from our configuration in the process, and making sssd
optional for authentication in our environment.
Our solution
We use 389 Directory Server as our local LDAP server. 389 has with it the capability to proxy authentication via PAM. A previous generation RC LDAP used this to perform authentication; but only in a way that supported a single authentication path. However, with some research and experimentation, we have managed to configure our instance with different proxy authentication paths for each of our child domains.
First we simply activate the PAM Pass Through Auth plugin by
setting nsslapd-pluginEnabled: on in the existing LDAP entry.
dn: cn=PAM Pass Through Auth,cn=plugins,cn=config objectClass: top objectClass: nsSlapdPlugin objectClass: extensibleObject objectClass: pamConfig cn: PAM Pass Through Auth nsslapd-pluginPath: libpam-passthru-plugin nsslapd-pluginInitfunc: pam_passthruauth_init nsslapd-pluginType: betxnpreoperation nsslapd-pluginEnabled: on nsslapd-pluginloadglobal: true nsslapd-plugin-depends-on-type: database pamMissingSuffix: ALLOW pamExcludeSuffix: cn=config pamIDMapMethod: RDN pamIDAttr: uid pamFallback: FALSE pamSecure: TRUE pamService: ldapserver nsslapd-pluginId: pam_passthruauth nsslapd-pluginVersion: 1.3.4.0 nsslapd-pluginVendor: 389 Project nsslapd-pluginDescription: PAM pass through authentication plugin
The specifics of authentication can be specified at this level as well, if we're able to express our desired behavior in a single configuration. However, the plugin supports multiple simultaneous configurations expressed as nested LDAP entries.
dn: cn=colorado.edu PAM,cn=PAM Pass Through Auth,cn=plugins,cn=config objectClass: pamConfig objectClass: top cn: colorado.edu PAM pamMissingSuffix: ALLOW pamExcludeSuffix: cn=config pamIDMapMethod: RDN ENTRY pamIDAttr: uid pamFallback: FALSE pamSecure: TRUE pamService: curc-twofactor-duo pamFilter: (&(objectClass=posixAccount)(!(homeDirectory=/home/*@*))) dn: cn=colostate.edu PAM,cn=PAM Pass Through Auth,cn=plugins,cn=config objectClass: pamConfig objectClass: top cn: colostate.edu PAM pamMissingSuffix: ALLOW pamExcludeSuffix: cn=config pamIDMapMethod: RDN ENTRY pamIDAttr: uid pamFallback: FALSE pamSecure: TRUE pamService: csu pamFilter: (&(objectClass=posixAccount)(homeDirectory=/home/*@colostate.edu))
Our two sets of users are authenticated using different PAM stacks, as before. Only now this proxy authentication is happening within the LDAP server, rather than within sssd. This may seem like a small difference, but there are multiple benefits:
The proxy configuration exists, and need only be maintained, only within the LDAP server. It does not require all login nodes to run sssd and a complex, multi-tiered PAM stack.
The LDAP "PAM Pass Through Auth" plugin does not have the same bug as the sssd
proxyauthentication method, bypassing our immediate problem.Applications that do not support PAM authentication, such as XDMoD, Foreman, and Grafana, can now be configured with simple LDAP authentication, and need not know anything of the complexity of authenticating our multiple domains.
For now I'm differentiating our different user types based on the name
of their home directory, because it happens to include the relevant
domain suffix. In the future we expect to update usernames in the
directory to match and would then likely update this configuration to
use uid.
Cleaning up a few remaining issues
However, when I first tied this back into sssd, I DOS'd our LDAP server.
[domain/rc.colorado.edu] debug_level = 3 description = CU Boulder Research Computing id_provider = ldap auth_provider = ldap chpass_provider = none min_id=1000 enumerate = false entry_cache_timeout = 300 ldap_id_use_start_tls = True ldap_tls_reqcert = allow ldap_uri = ldap://ldap.rc.int.colorado.edu ldap_search_base = dc=rc,dc=int,dc=colorado,dc=edu ldap_user_search_base = ou=UCB,ou=People,dc=rc,dc=int,dc=colorado,dc=edu ldap_group_search_base = ou=UCB,ou=Groups,dc=rc,dc=int,dc=colorado,dc=edu
This seemed simple enough: when I would try to authenticate using this
configuration, I would enter my password as usual and then respond to
a Duo "push." But the authentication never cleared in sssd, and I
would keep receiving Duo pushes until I stopped sssd. This despite the
fact that I could authenticate with ldapsearch as expected.
$ ldapsearch -LLL -x -ZZ -D uid=[redacted],ou=UCB,ou=People,dc=rc,dc=int,dc=colorado,dc=edu -W '(uid=[redacted])' dn Enter LDAP Password: dn: uid=[redacted],ou=UCB,ou=People,dc=rc,dc=int,dc=colorado,dc=edu
I eventually discovered that sssd has a six-second timeout for "calls to synchronous LDAP APIs," including BIND. This timeout is entirely reasonable--even generous--for operations that do not have a manual intervention component. But when BIND includes time to send a notification to a phone, unlock the phone, and acknowledge the notification in an app, it is easy to exceed this timeout. sssd gives up and tries again, prompting a new push that won't be received until the first is addressed. In this way, the timeouts just extend against each other.
Thankfully, this timeout is also configurable as ldap_opt_timeout
in the relevant sssd domain section. I went with ldap_opt_timeout =
90, which is likely longer than anyone will need.
There is still the matter of the fact that this DOS'd the LDAP server, however. I suspect I had exhausted the number of directory server threads with pending, long-living (due to manual intervention required / timeout) BIND requests.
The number of threads Directory Server uses to handle simultaneous connections affects the performance of the server. For example, if all threads are busy handling time-consuming tasks (such as add operations), new incoming connections are queued until a free thread can process the request.
Red Hat suggests that nsslapd-threadnumber should be 32 for an
eight-CPU system like ours; so for now I simply increased to this
recommendation from 16. If we continue to experience thread exhaustion
in real-world use, we can always increase the number of threads again.