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Example 3 - Client with PAM + SSSD for Kerberos Auth , LDAP user information and Kerberos Password

Overview

In this model, starting from example one, we will see how to make a more secure authentication in the system using the best of Kerberos and Ldap technologies.

For this example, in the Client we will see how the System-Auth works with these two technologies, and we will perform a series of checks to make sure it works correctly.

Docker Images used for this example:

Fast Deployment

I Made this example for show how to configure your Ldap Server , anyway if you only want check how to it works , maybe be interesed in this another explanation.Automated Build and How it Works

Features

  • Openldap Server for use to backup user information
  • Secure GSSAPI Authentification for LDAP client utilities
  • Secure connection between client and server using StartTLS
  • Fastest operations with LDAP Client Utilities
  • Secure authentification through Kerberos

Requisites

Instalation

Hostnames and our ips

  • LDAP Server: ldap.edt.org 172.18.0.2
  • Kerberos Server: kserver.edt.org 172.18.0.3
  • Client: client.edt.org 172.18.0.8

Starting

Starting from the base that we already have an openldap server running without these technologies.

To properly configure the servers and the client, we have to be careful in 3 essential things.

  • Need to have working the GSSAPI Auth On LDAP Server , if you dont have it please see Example 1
  • Configure sssd service for Kerberos Auth , Take information from LDAP, and finally use kerberos password instead ldap
  • Configure auth-config for enable sssd Service
  • Check if Pam is properly configured
  • Try in client to login with an user with principal created and another one without

In our case for this example we will use some docker containers that I created for the occasion. In these containers are already made the modifications, are just an example, you can do it on your own server If you dont want use my examples , go directly to Configure.

Note: My Containers can communicate between them , because i configurated ldap for do ip resolution Here , if u don't using my containers , you should put all container entries in /etc/hosts for each container , like this.

172.18.0.2 ldap.edt.org
172.18.0.3 kserver.edt.org
172.18.0.8 client.edt.org

Create Docker Network

This is for a very important reason, we need to always have the same container IP for the proper Ip distribution through LDAP and NSLCD. In the default Bridge Network , we can't assign ips for containers

# docker network create --subnet 172.18.0.0/16 -d bridge ldap

Run Docker LDAP

# docker run --name ldap --hostname ldap.edt.org --net ldap --ip 172.18.0.2  --detach antagme/ldap_sssd

Run Docker Kerberos (TGT)

# docker run --name kerberos --hostname kserver.edt.org --net ldap --ip 172.18.0.3  --detach antagme/kerberos:supervisord

Run Docker Client

# docker run --name client --hostname client.edt.org --net ldap --ip 172.18.0.8 --detach antagme/client:pam_tls

These dockers containers are not interactive, to access you have to do the following order:

docker exec --interactive --tty [Docker Name] bash

Automated Script

If you preffer to use an Automated Builds , can take the script i created for this.HERE

Configure

Configure sssd service for Kerberos Auth , Take information from LDAP, and finally use kerberos password instead ldap

We gonna configure the sssd , the official daemon offered by Fedora for Control Authentification in the System and configure the Backends and use it in authconfig.

The first step you need is install the Packets of SSSD with this command: dnf install -y sssd sssd-client authconfig Note:You should install authconfig packet too for configure system-auth

Installed this , we gonna configure the sssd.conf file. The key parts is configure our kerberos and ldap servers for the properly authentification to system.

Lets see the file:

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

[domain/default]
id_provider = ldap
ldap_uri = ldap://172.18.0.2
ldap_search_base = dc=edt,dc=org
auth_provider = krb5
chpass_provider = krb5
krb5_realm = EDT.ORG
krb5_server = 172.18.0.3
krb5_kpasswd = 172.18.0.3
cache_credentials = True
krb5_store_password_if_offline = True

[nss]
filter_groups = root
filter_users = root
reconnection_retries = 3

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

This is my configuration , maybe this can be better , anyway sssd is a pussy daemon. Now , we gonna profundize through the key parts.

  • [domain/default]

    • id_provider = ldap Define what is our id provider. (if the entry doesnt exist on ldap , can't enter to the system)
    • ldap_uri = ldap://172.18.0.2 Our ldap uri (Some problem on Docker with DNS i resolve this putting Ip instead FQDN)
    • ldap_search_base = dc=edt,dc=org Base of the LDAP Backend
    • auth_provider = krb5 Define who is the authentification provider , in our case we wants to auth through kerberos for PAM
    • chpass_provider = krb5 Define who is the Password PAM provider , in our case is kerberos
    • krb5_realm = EDT.ORG Define who is our kerberos5 REALM
    • krb5_server = 172.18.0.3 Define what is the kerberos5 server (Some problem on Docker with DNS i resolve this putting ip instead FQDN)
    • krb5_kpasswd = 172.18.0.3 Define what is the kerberos5 password server ( This enable the possibility to passwd and change our password
    • cache_credentials = True Define if you want keep in cache the credentials (This is needed for work without ethernet connection)
    • krb5_store_password_if_offline = True Same the last entry but for password

  • [nss]

    • filter_groups = root Filter Group root when do nss resoluion
    • filter_users = root Filter user root when do nss resolution
    • reconnection_retries = 3 Try 3 times reconnect and if fail , dont start this process

  • [pam]

    • reconnection_retries = 3 Try connect 3 times when use PAM and if fail dont start this process
    • offline_credentials_expiration = 2 Days to expire the credentials offline
    • offline_failed_login_attempts = 3 Chances to fail if offline
    • offline_failed_login_delay = 5 Delay for fail login offline

Now we have sssd.conf file propery configurated for Auth:Kerberos User:LDAP Password:Kerberos but we need configure authconfig and check PAM before start all.

Configure authconfig for enable sssd

I do not know exactly what is the best configuration about authconfig , this tool is a headcache at all , so i will put you my configuration file and if you want to do for you , you are welcome to try it and help me to improve.

The file is:

IPADOMAINJOINED=no
PASSWDALGORITHM=sha512
USESSSD=yes
USEMKHOMEDIR=no
USEDB=no
USEPASSWDQC=no
USESSSDAUTH=yes
USEHESIOD=no
CACHECREDENTIALS=yes
USESYSNETAUTH=no
USEECRYPTFS=no
USESMARTCARD=no
USEWINBINDAUTH=no
USEIPAV2=no
USELDAP=yes
WINBINDKRB5=no
USELOCAUTHORIZE=yes
USEKERBEROS=yes
USELDAPAUTH=no
USEPAMACCESS=no
USEWINBIND=no
FORCELEGACY=no
USEPWQUALITY=yes
USENIS=no
FORCESMARTCARD=no
USESHADOW=yes
USEFPRINTD=no
IPAV2NONTP=no

You should copy this to /etc/sysconfig/ and perform this order  authconfig --update

Now my recommendation is check your /etc/krb5.conf, /etc/nsswitch/ and /etc/openldap/ldap.conf for make sure this files not changed for authconfig.

Pam Configuration

Now for reach our objetive only need if our pam configuration is the needed and if not , configure this.

We need to configure su and system-authPAM files for properly system authentification .

Lets see my files(the location is /etc/pam.d/):

su PAM file:

 #%PAM-1.0
auth		substack	system-auth
auth		include		postlogin
account		sufficient	pam_succeed_if.so uid = 0 use_uid quiet
account		include		system-auth
password	include		system-auth
session		include		system-auth
session		include		postlogin
session optional pam_xauth.so

Note: This check all from system-auth PAM file

system-auth PAM FILE:

#%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        [default=1 success=ok] pam_localuser.so
auth        [success=done ignore=ignore default=die] pam_unix.so nullok try_first_pass
auth        requisite     pam_succeed_if.so uid >= 1000 quiet_success
auth        sufficient    pam_sss.so use_authtok
auth        sufficient    pam_krb5.so use_first_pass
auth        required      pam_deny.so

account     required      pam_unix.so broken_shadow
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     [default=bad success=ok user_unknown=ignore] pam_krb5.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 user_authtok
password    sufficient    pam_krb5.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     required      pam_unix.so
session     optional      pam_sss.so
session optional pam_krb5.so

The system authentification process will check if this account is UNIX , if not , check if kerberos account.

Note:I know this PAM file is not the perfect configuration , fell free to help me to improve

Client Authentification

Now is time to check if our configuration is working well!!! We need to start sssd daemon in the client machine. In our case , we will supervisorctl start sssd and we gonna try if the client can start properly with a kerberos user.

We gonna try with our user01 he have kuser01 password.

su user01

now check with id and who the identity of the user.

and finally check who we are in Ldap Database.

ldapwhoami -ZZ

We configured the system authentification for perform Kerberos authentification , account information from Ldap and finally the password will be retrieved from Kerberos

Bibliography