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Certificate mapping and matching rules for all providers

Problem statement

Currently Smartcard authentication with rule based mapping and matching of user and certificate is only available for the IPA provider. For all other providers Smartcard authentication is only possible if the full certificate is added to a local override for the given user.

To offer more flexibility and easier configuration the support of mapping and matching rules should be added to the other providers as well. This includes the files provider which which mean that Smartcard authentication for local users will become more flexible as well.

Use cases

Active Directory

In Active Directory environments, where Smartcard authentication for SSH is not needed, Smartcard authentication should be enabled for all AD users with a simple configuration and the mapping similar to the one users by Active Directory itself.

Local users

SSSD should be used as a replacement for pam_pkcs11 to authenticate users defined in /etc/passwd with the help of a Smartcards. The configuration options should be similarly flexible as the ones of pam_pkcs11.

Overview of the solution

In general mapping and matching certificates and users is already discussed in Matching and Mapping Certificates. This page already contains some discussion about how the rules can be added to the SSSD configuration file sssd.conf in section Storing matching and mapping configuration.

To make it easy to drop rules even as config snippet in /etc/sssd/conf.d/ the rules contain the domain name of the related SSSD domain in the section name. Additionally the list of applicable domain in the rule can be used to make sure the right rules are uses in a multi domain configuration (AD provider).

Since for local user only a user name and not an LDAP search filter is needed to find them, rules for local user and remote user should be handled differently. Rules for remote users can be added as [certmap/domain/rule_name] sections while rules for local user can be added as [certmap/domain/user_name].

Implementation details

Certificate mapping and matching rules for remote user can be added as:

[certmap/domain/rule_name]
matchrule = <ISSUER>^CN=My-CA,DC=MY,DC=DOMAIN$
maprule = (userCertificate;binary={cert!bin})
domains = my.domain, your.domain
priority = 10

For a local user the rule would look like:

[certmap/files/username]
matchrule = <SUBJECT>^CN=User.Name,DC=MY,DC=DOMAIN$

where it is assumed that the SSSD domain with the files provider for local users is called ‘files’. The ‘domains’ option is ignored for local users. It is possible to add a rule which can map multiple users with a suitable map rule:

[certmap/files/email]
matchrule = &&<ISSUER>^CN=My-CA,DC=MY,DC=DOMAIN$<SAN:rfc822Name>.*@email\.domain
maprule = ({subject_rfc822_name.short_name})

which would use the name part of an email addresses from the ‘email.domain’ domain stored in the Subject Alternative Names section of the certificate as user name.

Internally the AD, LDAP and files provider will read the rule data from confdb and store the rules in the cache of the corresponding domain as it is currently already done by the IPA provider. The other SSSD components can then use the data from the cache as that already do it for the IPA provider.

Configuration changes

There is a new configuration section type [certmap/domain/name] which is processed by the backend of the SSSD domain ‘domain’. The IPA provider will continue to use the certificate mapping and matching rules defined on the IPA server.

The options for the new section correspond to the options available for the ipa certmaprule-add, namely matchrule, maprule, priority and domains.

How To Test

  • Make sure all general requirements for Smartcards authentication with SSSD are met:

    • the pscs-lite package is installed and the service and socket are enabled and running.
    • a suitable PKCS#11 module is installed, typically this mead that the opensc package is installed
    • pam_cert_auth = True is set in the [pam] section of sssd.conf
    • suitable CA certificates to verify the certificate on the Smartcard are added to /etc/pki/nssdb or /etc/sssd/pki/sssd_auth_ca_db.pem depending of a NSS or openSSL build of SSSD is used.

Active Directory

Assuming that the client system is already joined to AD as SSSD is configured to lookup and authenticate AD user you only need to add suitable mapping and matching rules with:

[certmap/ad.domain.name/rule_name]
matchrule = ....
maprule = ....

where ad.domain.name must match the domain name given in the [domain/...] section defining the AD domain.

Local users

If a sssd.conf file exist and a domain for local users with the files provider is already configured only suitable mapping and matching rules must be added with the name of the domain for the local users.

If SSSD is running without a sssd.conf file to just handle lookups of local users a minimal sssd.conf must be created to enable Smartcard authentication for local users:

[sssd]
services = nss, pam

[pam]
pam_cert_auth = True

[certmap/implicit_files/local_user]
matchrule = _matching_rule_for_local_user_ 

The services option is needed to enable SSSD’s pam responder. Since the domain for local users is called implicit_files by default any certificate mapping and matching rule for local users should use this name as well as long as there is no other domain explicitly configured for local users with a different name (see above).

Migrating from pam_pkcs11.conf

pam_pkcs11 offers different mappers as well. In the following some examples will illustrate how to rewrite an existing pam_pkcs11 configuration for SSSD. You can find all option available for SSSD’s mapping and matching rules in the sss-certmap man page.

LDAP mapper

mapper ldap {
        debug = false;
        module = /usr/$LIB/pam_pkcs11/ldap_mapper.so;
        # where base directory resides
        basedir = /etc/pam_pkcs11/mapdir;
        # hostname of ldap server
        ldaphost = "localhost";
        # Port on ldap server to connect
        ldapport = 389;
        # Scope of search: 0 = x, 1 = y, 2 = z
        scope = 2;
        # DN to bind with. Must have read-access for user entries under "base"
        binddn = "cn=pam,o=example,c=com";
        # Password for above DN
        passwd = "test";
        # Searchbase for user entries
        base = "ou=People,o=example,c=com";
        # Attribute of user entry which contains the certificate
        attribute = "userCertificate";
        # Searchfilter for user entry. Must only let pass user entry for the login user.
        filter = "(&(objectClass=posixAccount)(uid=%s))"
}

Most of the option needed by the pam_pkcs11 LDAP mapper are already set in the related [domain/...] section in sssd.conf. As can be see with the configuration above pam_pkcs11 would search the certificate in the LDAP server by using the full certificate and trying to find it in the userCertificate attribute. The matching certmap rule would look like:

[certmap/ldap.domain/ldap]
maprule = (userCertificate;binary={cert!bin})

cn/pwent mapper

mapper cn {
        debug = false;
        module = internal;
        # module = /usr/$LIB/pam_pkcs11/cn_mapper.so;
        mapfile = file:///etc/pam_pkcs11/cn_map;
}

Here the cn is read from the subject of the certificate and lookup in the cn_map file where the matching local user name can be found as well. For each local user name in the cn_map file create a rule like:

[certmap/files/local_user_name]
matchrule = <SUBJECT>^CN=cn_from_cn_map,.*

mail mapper

mapper mail {
        debug = false;
        module = internal;
        # module = /usr/$LIB/pam_pkcs11/mail_mapper.so;
        # Declare mapfile or
        # leave empty "" or "none" to use no map
        mapfile = file:///etc/pam_pkcs11/mail_mapping;
}

Similar as for the cn/pwent mapper for each entry in mail_mapping create a rule like:

[certmap/files/local_user_name]
matchrule = <SAN:rfc822Name>^email@from.mail.mapping$

If the name part of the email address exactly matches the user name you can use a single rule like:

[certmap/files/email]
matchrule = <SAN:rfc822Name>.*@expected.email.domain
maprule = ({subject_rfc822_name.short_name})

ms mapper

mapper ms {
        debug = false;
        module = internal;
        # module = /usr/$LIB/pam_pkcs11/ms_mapper.so;
        ignorecase = false;
        ignoredomain = false;
        domain = "domain.com";
}

Similar as for the email case a rule like:

[certmap/files/ms]
matchrule = <SAN:ntPrincipalName>.*@domain.com
maprule =({subject_nt_principal.short_name})

would lead to a similar mapping.

How To Debug

For Smartcard authentication 3 SSSD component are used, the PAM responder, p11_child and the configured backend. To enable debugging output in the log files the debug_level option must be set in the [pam] and [domain/... sections of sssd.conf. Since p11_child is called by the PAM responder it will inherit the debug_level set in the [pam] section. In the debug logs the following step should be visible.

If certificate based authentication is enabled (pam_cert_auth = True) the PAM responder will first in the SSS_PAM_PREAUTH step call p11_child if Smartcard authentication is allowed for the given PAM service (see pam_p11_allowed_services in the sssd.conf man page for details). If p11_child can return a valid certificate the PAM responder first does a user lookup by certificate and then a user lookup by name. If both lookups will return the same cache entry the SSS_PAM_PREAUTH step will return the needed details to the pam_sss PAM module and Smartcard authentication can proceed.

After the user entered the PIN for the Smartcard the PAM responder will in the SSS_PAM_AUTHENTICATE step first send the authentication request to the backend to handle server-side authentication like e.g. PKINIT. If the backend is offline or cannot handle Smartcard authentication it will return PAM_AUTHINFO_UNAVAIL and the PAM responder will try local Smartcard authentication which involves the same steps done during the SSS_PAM_PREAUTH steps only that p11_child is called in auth mode instead of preauth and that it receives the PIN as well.

Authors

  • Sumit Bose <sbose@redhat.com>