How do I configure sssd to authenticate against LDAP using client certificates / SASL EXTERNAL

I have a need to configure various Ubuntu Trusty machines using sssd against a 389ds server that expects to be bound to using a binddn selected automatically via a client certificate mapping.

I have successfully set up 389ds with a certmap as follows:

# By default, we trust any valid certificate that has an ou attribute that
# matches an entry (currently ou=Servers) in the DIT
certmap default     default
default:DNComps
default:FilterComps ou
default:verifycert  off

In addition, I have disabled anonymous binds and forced SASL external binds as follows:

# disable anonymous binds
dn: cn=config
changetype: modify
replace: nsslapd-allow-anonymous-access
nsslapd-allow-anonymous-access: off

# force sasl external binds to use cert
dn: cn=config
changetype: modify
replace: nsslapd-force-sasl-external
nsslapd-force-sasl-external: on

On the sssd side, I have /etc/sssd/sssd.conf that looks as follows:

[sssd]
config_file_version = 2
domains = LDAP
services = nss, pam

[nss]
filter_groups = root
filter_users = root
reconnection_retries = 3
entry_cache_timeout = 300
entry_cache_nowait_percentage = 75

[pam]
reconnection_retries = 3
offline_credentials_expiration = 2
offline_failed_login_attempts = 3
offline_failed_login_delay = 5

# A native LDAP domain
[domain/LDAP]
enumerate = true
cache_credentials = TRUE
debug_level = 9

id_provider = ldap
auth_provider = ldap
chpass_provider = ldap

ldap_uri = ldaps://ldap.example.com:636
ldap_user_search_base = dc=example,dc=com
tls_reqcert = demand
ldap_tls_cacert = /etc/ssl/certs/root-ca.crt
ldap_tls_cert = /etc/ssl/certs/my.crt
ldap_tls_key = /etc/ssl/private/my.key
ldap_sasl_mech = EXTERNAL

When I start up sssd, sssd attempts to bind to 389ds, first by attempting to bind anonymously (which then fails), and then by using the SASL EXTERNAL mechanism (which also fails):

(Thu Mar 17 16:08:34 2016) [sssd[be[LDAP]]] [sdap_process_message] (0x4000): Message type: [LDAP_RES_SEARCH_RESULT]
(Thu Mar 17 16:08:34 2016) [sssd[be[LDAP]]] [sdap_get_generic_ext_done] (0x0400): Search result: Inappropriate authentication(48), Anonymous access is not allowed.
(Thu Mar 17 16:08:34 2016) [sssd[be[LDAP]]] [sdap_get_generic_ext_done] (0x0040): Unexpected result from ldap: Inappropriate authentication(48), Anonymous access is not allowed.
(Thu Mar 17 16:08:34 2016) [sssd[be[LDAP]]] [sdap_get_generic_done] (0x0100): sdap_get_generic_ext_recv failed [5]: Input/output error
(Thu Mar 17 16:08:34 2016) [sssd[be[LDAP]]] [sdap_get_server_opts_from_rootdse] (0x0200): No known USN scheme is supported by this server!
(Thu Mar 17 16:08:34 2016) [sssd[be[LDAP]]] [sdap_cli_auth_step] (0x0100): expire timeout is 900
(Thu Mar 17 16:08:34 2016) [sssd[be[LDAP]]] [sdap_cli_auth_step] (0x1000): the connection will expire at 1458231814
(Thu Mar 17 16:08:34 2016) [sssd[be[LDAP]]] [sasl_bind_send] (0x0100): Executing sasl bind mech: EXTERNAL, user: (null)
(Thu Mar 17 16:08:34 2016) [sssd[be[LDAP]]] [sasl_bind_send] (0x0020): ldap_sasl_bind failed (-6)[Unknown authentication method]
(Thu Mar 17 16:08:34 2016) [sssd[be[LDAP]]] [sasl_bind_send] (0x0080): Extended failure message: [SASL(-4): no mechanism available: ]

Using ssldump, it appears that the client side sent a client certificate, however the ssldump -A option is buggy and it refuses to tell me anything about this cert:

1 3  0.0283 (0.0218)  C>SV3.3(7)  Handshake
      Certificate

The questions I have is:

• Why is sssd’s attempt to bind anonymously failing? In theory the “nsslapd-force-sasl-external: on” should cause all bind attempts to be ignored in favour of the client certificate.

• Why is ssd’s attempt to bind using SASL/EXTERNAL failing?

• Is there any kind of guide or howto that describes sssd+ldap along with client certificates?

For the avoidance of doubt, in this scenario simple binds are not an option.

Update:

When I try use openssl s_client to connect to the 389ds server using the correct client certificate, the following correctly appears in the 389ds log indicating that the client certificate triggered a successful bind:

[17/Mar/2016:16:35:02 +0000] conn=130 SSL 128-bit AES-GCM; client CN=stuff,OU=Servers,O=Example,DC=example,DC=com; issuer CN=morestuff,OU=Example Signing CA,O=Example,DC=example,DC=com
[17/Mar/2016:16:35:02 +0000] conn=130 SSL client bound as ou=Servers,dc=example,dc=com

It appears in this case that sssd is not attempting to bind with the certificate and key provided. Anyone know why?

# force sasl external binds to use cert
dn: cn=config
changetype: modify
replace: nsslapd-force-sasl-external
nsslapd-force-sasl-external: rootdse