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Lpic2.210.2

PAM authentication (210.2)

The candidate should be able to configure PAM to support authentication using various available methods.

Key Knowledge Areas

  • PAM configuration files, terms and utilities

  • passwd and shadow passwords

  • basic SSSD functionality for LDAP authentication

Terms and Utilities

  • /etc/pam.d

  • pam.conf

  • nsswitch.conf

  • pam_unix, pam_cracklib, pam_limits, pam_listfile

What is PAM?

PAM is the acronym for Pluggable Authentication Modules. PAM consists of a set of libraries and an API (Application Programming Interface) that can be used to perform authentication tasks. Privilege granting programs, such as login and su, use the API to perform standard authentication tasks.

How does it work?

account

  • Provide account verification types of service: has the user's password expired? Is this user permitted access to the requested service?

authentication

  • Establish if the user really is whom he claims to be. This can be done, for example, by asking a password or, given the right module, by reading a chip-card or by performing a retinal or fingerprint scan.

password

  • This group's responsibility is the task of updating authentication mechanisms. Typically, such services are strongly coupled to those of the authentication group. Some authentication mechanisms lend themselves well to being updated. The user might be presented with a question like "Please enter the new password".

session

  • This group of tasks covers things that should be done prior to a service being offered and after it is withdrawn. Such tasks include the maintenance of audit trails and the mounting of the user's home directory. The session management group is important as it provides both an opening and closing hook for modules that affect the services available to a user.

PAM can be configured using the file /etc/pam.conf ConfiguringPAM PAMpam.conf which has the following format:

    service   type   control   module-path   module-arguments

service

  • This is the name of the application involved, for example: PAMlogin PAMssh PAMpasswd login, ssh or passwd.

type

  • This is the type of group the task to be performed belongs to: account, auth (the authentication group), password or session.

control

  • This field indicates what the PAM-API should do in case authentication fails for any module.

    re-quisite

    • Upon failure, the authentication process will be PAMrequisite terminated immediately.

    required

    • This will return failure after the remaining modules PAMrequired for this service and type have been invoked.

    sufficient

    • Upon success, the authentication process will be PAMsufficient satisfied, unless a prior required module has failed the authentication.

    optional

    • The success or failure of this module is only important PAMoptional if this is the only module associated with this service and this type.

module-path

  • This is the filename, including the full path, of the PAM that is to be used by the application.

module-arguments

  • These are module specific arguments, separated by spaces, that are to be passed to the module. Refer to the specific module's documentation for further details.

Configuration is also possible using individual configuration files, which is recommended. These files should all be located in the /etc/pam.d directory. If this directory exists, the file /etc/pam.conf will be ignored. The filenames should all be lowercase and be identical to the name of the service, such as login. The format of these files is identical to /etc/pam.conf with the exception that there is no service field.

Modules

pam_unix

This module configures authentication via /etc/passwd and /etc/shadow.

account

  • The type "account" does not authenticate the user but checks PAMaccount other things such as the expiration date of the password and might force the user to change his password based on the contents of the files /etc/passwd and /etc/shadow.

    debug

    • Log information using syslog.

    audit

    • Also logs information, even more than debug does.

auth

  • The type "auth" checks the user's password against the password PAMauth database(s). This component is configured in the file /etc/nsswitch.conf. Please consult the man page (man nsswitch.conf) for further details.

    audit

    • Log information using syslog.

    debug

    • Also logs information using syslog but less than audit.

    nodelay

    • This argument sets the delay-on-failure, which has a default of a second, to nodelay.

    nullok

    • Allows empty passwords. Normally authentication fails if PAMnullok the password is blank.

    try_first_pass

    • Use the password from the previous stacked auth module and PAM try_first_pass prompt for a new password if the retrieved password is blank or incorrect.

    use_first_pass

    • Use the result from the previous stacked auth module, never PAM use_first_pass prompt the user for a password and fails if the result was a fail.

password

  • The type "password" changes the user's password. PAM password

    audit

    • Log information using syslog.

    bigcrypt

    • Use the DEC "C2" extension to crypt().

    debug

    • Also logs information using syslog but less than audit.

    md5

    • Use md5 encryption instead of crypt().

    nis

    • Use NIS (Network Information Service) passwords.

    not_set_pass

    • Don't use the passwords from other stacked modules and do not give the new password to other stacked modules.

    nullok

    • Allows empty passwords. Normally authentication fails if the password is blank.

    remember

    • Remember the last n passwords to prevent the user from using one of the last n passwords again.

    try_first_pass

    • Use the password from the previous stacked auth module, and prompt for a new password if the retrieved password is blank or incorrect.

    use_authtok

    • Set the new password to the one provided by a previous module.

    use_first_pass

    • Use the result from the previous stacked auth module, never prompt the user for a password and fails if the result was a fail.

session

  • The type "session" uses syslog to log the user's name and session PAMsession type at the start and end of a session.

    The "session" type does not support any options.

For each service that requires authentication a file with the name of that service must be created in /etc/pam.d. Examples of those services are: login, ssh, ppp, su.

For example purposes the file /etc/pam.d/login will be used:

    # Perform password authentication and allow accounts without a password
    auth       required   pam_unix.so nullok

    # Check password validity and continue processing other PAM's even if
    # this test fails. Access will only be granted if a 'sufficient' PAM,
    # that follows this 'required' one, succeeds.
    account    required   pam_unix.so

    # Log the user name and session type to syslog at both the start and the end
    # of the session.
    session    required   pam_unix.so

    # Allow the user to change empty passwords (nullok), perform some additional
    # checks (obscure) before a password change is accepted and enforce that a
    # password has a minimum (min=4) length of 4 and a maximum (max=8) length of
    # 8 characters.
    password   required   pam_unix.so nullok obscure min=4 max=8

pam_nis

This module configures authentication via NIS. ConfiguringNIS Authentication To be able to authenticate via NIS, the module pam_nis.so is needed. This module can be PAMpam_nis.so found at PAM NIS Authorisation Module.

To set up things in such a way that NIS authentication is sufficient (and if that is not the case try pam_unix.so), the lines that do the trick in /etc/pam.d/login are:

    auth    sufficient pam_nis.so item=user \
    sense=allow map=users.byname value=compsci
    auth    required   pam_unix.so try_first_pass

    account sufficient pam_ldap.so \
    item=user sense=deny map=cancelled.byname error=expired
    account required   pam_unix.so

pam_ldap

This module configures authentication via LDAP. To be able to authenticatie via LDAP, the module ConfiguringLDAP Authentication pam_ldap.so is needed. This module can be found at PADL Software Pty Ltd.

To set up things in such a way that LDAP authentication is sufficient, (and if that is not the case try pam_unix.so), the PAMpam_ldap.so lines that do the trick in /etc/pam.d/login are:

    auth    sufficient pam_ldap.so
    auth    required   pam_unix.so try_first_pass

    account sufficient pam_ldap.so
    account required   pam_unix.so

pam_cracklib

This plugin provides strength-checking for passwords. This is done by performing a number of checks to ensure passwords are not too weak. It checks the password against dictonaries, the previous password(s) and rules about the use of numbers, upper and lowercase and other characters.

    #%PAM-1.0
    #
    # These lines allow a md5 systems to support passwords of at least 14
    # bytes with extra credit of 2 for digits and 2 for others the new
    # password must have at least three bytes that are not present in the
    # old password
    #
    password  required pam_cracklib.so \
    difok=3 minlen=15 dcredit= 2 ocredit=2
    password  required pam_unix.so use_authtok nullok md5

pam_limits

The pam_limits PAM module sets limits on the system resources that can be obtained in a user-session. Users of uid=0 are affected by this limits, too. By default limits are taken from the /etc/security/limits.conf config file. Then individual files from the /etc/security/limits.d/ directory are read. The files are parsed one after another in the order of \"C\" locale. The effect of the individual files is the same as if all the files were concatenated together in the order of parsing. If a config file is explicitely specified with a module option then the files in the above directory are not parsed. The module must not be called by a multithreaded application.

pam_listfile

This module allows or denies an action based on the presence of the item in a listfile. A listfile is a textfile containing a list of usernames, one username per line. The type of item can be set via the configuration parameter item and can have the value of user, tty, rhost, ruser, group, or shell. The sense configuration parameter determines whether the entries in the list are allowed. Possible values are allow and deny.

SSSD

Configure SSSD for LDAP authentication

The following steps describe the configuration of SSSD to use LDAP for authentication:

1. The following packages need to be installed:

    sssd-client
    sssd-common
    sssd-common-pac
    sssd-ldap
    sssd-proxy
    python-sssdconfig
    authconfig
    authconfig-gtk

Use your package manager to install these packages.

2. Check the current settings for sssd, if any:

    # authconfig test

This will show you the current settings which are already in place. Also check for an existing /etc/sssd/sssd.conf file. On a fresh installation you can expect all settings to be disabled and that the sssd.conf file will not be present.

3. Now configure sssd:

    # authconfig \
    --enablesssd \
    --enablesssdauth \
    --enablelocauthorize \
    --enableldap \
    --enableldapauth \
    --ldapserver=ldap://ldap.example.com:389 \
    --disableldaptls \
    --ldapbasedn=dc=example,dc=com \
    --enablerfc2307bis \
    --enablemkhomedir \
    --enablecachecreds \
    --update

4. Check the configuration in /etc/sssd/sssd.conf.

In case you're using TLS make sure that the ldap_tls_cacertdir and ldap_tls_cacert parameters are configured correctly and point to your certificates. Also change ldap_id_use_start_tls to "True".

To effect the changes, run:

    # systemctl restart sssd

Verify that all changes are effective by running:

    # authconfig test

5. Update /etc/openldap.conf to use the same ldap settings. Your ldap.conf file will look like this:

    SASL_NOCANON on
    URI ldaps://ldap.example.com:389
    BASE dc=example,dc=com
    TLS_REQUIRE never
    TLS_CACERTDIR /etc/pki/tls/cacerts
    TLS_CACERT /etc/pki/tls/certs/mybundle.pem

Please note that TLS_REQUIRE is set to never. This is done in order to avoid issues with application stacks like PHP, which have difficulties with LDAPS and TLS.

6. Make sure that sssd is up and running and that it will be started after a system reboot. Run systemctl status sssd to check this. To start sssd, run systemctl start sssd and to make sssd persistent across reboots, run systemctl enable sssd.