Client Authentication:Ubuntu via sssd/ldap

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Warning.png Warning:
This how-to should be validated by Daniel before you use it!!.


Warning.png Warning:
This is based upon limited testing and a small number of users.

Introduction

This how-to shows how to configure a SME-server (>=8b6) and a client Ubuntu for a LDAP based SSSD authentication of the client machine on the configured user accounts of the SME.

The main advantage in comparaison to nss_ldap is that the authentication information stays in the cache and the authentication can therefore still work even in offline mode (when the server not available).

Nevertheless, the creation of a local user with the admin rights is recommended for the emergency case.

These lines are a translation of the method given by Daniel: https://wikit.firewall-services.com/doku.php/tuto/ipasserelle/authentification/ubuntu_sssd_on_sme. Many thanks to him for it.

Backup

Warning.png Warning:
This process may lock you out of the client machine. Make sure that on the client machine you have a local only user account with a name that is different to any users on the server e.g. 'localadminuser'

It is worth taking a backup of the following files /etc/pam.d/common-account /etc/pam.d/common-auth /etc/pam.d/common-password /etc/pam.d/common-session /etc/pam.d/common-session-noninteractive


cp /etc/pam.d/common* /home/myhome/backup

Assumptions

In this how-to we assume that:

the host name of the SME is "sme-server" and the domain is "domain.tld".

Configuration of the SME-server

There is little configuration required in SME server.

  • The only thing to do is to create a user (named "auth" in this how-to) via the server-manager and to give them a valid password ("something_very_secret" in the how-to).

It is not required to make "auth" member of any group.

  • In addition, it is recommended to install and configure PHPki in order to make the managing of the self-created certificates easier.

Configuration of the Ubuntu client

Installation of the required packages

sudo apt-get install sssd libnss-sss libpam-sss auth-client-config

Create a symbolic link

There seems to be a bug in the version of sssd from Ubuntu and therefore the following links must be created:

ln -s /usr/lib /usr/modules

Without it, sssd can't manage membership to the groups in LDAP (source http://us.generation-nt.com/bug-599644-sssd-unable-resolve-ldap-group-memberships-help-200739341.html)

Managing the CA on SME

PHPKi

After having installed PHPki, go to https://www.domain.tld/phpki and download the certificate of authority (ca-certificates.crt) to the client machine.

Place a copy of it or of another CA into /etc/ssl/certs/ and give the 644 permissions:

cp ~/Downloads/ca-certificates.crt /etc/ssl/certs/
chmod 644 /etc/ssl/certs/ca-certificates.crt

Letsencrypt

If you use Letsencypt for your certificates then your client machine should already have the ca-certificate for letsencrypt installed

You should be able to set the following in sssd.conf

ldap_tls_cacert = /etc/ssl/certs/ca-certificates.crt

Configure SSSD

The configuration of sssd is achieved in a standard way (as per Ubuntu or Fedora for example) and is made by the file /ets/sssd/sssd.conf.

  • At the beginning of this file, the used domain has to be set. In sssd, a domain can be taken as a source of content. It is possible to set several domains in order of priority.
  • And deeper in the file, we will add the configuration of the domain

If the file doesn't exist by default it has to be created and it needs to get the permissions 600 to allow the daemon to start. On Ubuntu clients using sudo you may need to get a root shell first:

sudo -i

Now we can copy and paste this into the terminal:

cat <<'_EOF' > /etc/sssd/sssd.conf
[sssd]
config_file_version = 2
services = nss, pam
domains = LDAP

[nss]

[pam]

[domain/LDAP]
id_provider = ldap
auth_provider = ldap
ldap_schema = rfc2307
ldap_uri = ldap://sme-server.domain.tld
ldap_default_bind_dn = uid=auth,ou=Users,dc=domain,dc=tld
ldap_default_authtok = something_very_secret
ldap_default_authtok_type = password
ldap_search_base = dc=domain,dc=tld
ldap_user_search_base = ou=Users,dc=domain,dc=tld
ldap_group_search_base = ou=Groups,dc=domain,dc=tld
ldap_user_object_class = inetOrgPerson
ldap_user_gecos = cn
ldap_tls_reqcert = hard
ldap_tls_cacert = /etc/ssl/certs/ca-certificates.crt
ldap_id_use_start_tls = true
# uncomment below if the SME is a “iPasserelle”
#ldap_user_shell = desktopLoginShell
# comment below if the SME is a “iPasserelle”
override_shell = /bin/bash
cache_credentials = true
enumerate = true
# It is possible to filter the logins via a LDAP-filer
# by commenting the both lines below.
# In this exemple, only the users member of the group netusers
# will be valid on this host.
# posixMemberOF is a parameter only for a iPasserelle
#access_provider = ldap
#ldap_access_filter = (|(posixMemberOf=admins)(uid=backup))
_EOF

Now we need to set the correct permissions on the file:

chmod 600 /etc/sssd/sssd.conf


Information.png Tip:
Make sure that the file /etc/ssl/certs/ca-certificates.crt contains the CA that has signed the certificate of the SME (if PHPki is used, a version > 0,82-13 is required).

Configure the system to use SSSD as a source of authentication:

Setup to use the tool auth-client-config.


Information.png Tip:
If you intend to automatically mount shares please see the Mount Shares section below and add the relevant sections to pam_auth and pam_session here first. You may also want the section in System Permissions

We can copy and paste in a terminal to add following lines:

cat <<'_EOF' > /etc/auth-client-config/profile.d/sss
[sss]
nss_passwd=     passwd:         compat sss
nss_group=      group:          compat sss
nss_shadow=     shadow:         compat
nss_netgroup=   netgroup:       nis
 
pam_auth=       auth           [success=3 default=ignore]      pam_unix.so nullok_secure try_first_pass
                auth           requisite                       pam_succeed_if.so uid >= 500 quiet
                auth           [success=1 default=ignore]      pam_sss.so use_first_pass
                auth           requisite                       pam_deny.so
                auth           required                        pam_permit.so
 
pam_account=   account         required                        pam_unix.so
               account         sufficient                      pam_localuser.so
               account         sufficient                      pam_succeed_if.so uid < 500 quiet
               account         [default=bad success=ok user_unknown=ignore]    pam_sss.so
               account         required                        pam_permit.so
 
pam_password=  password        sufficient                      pam_unix.so obscure sha512
               password        sufficient                      pam_sss.so use_authtok
               password        required                        pam_deny.so

pam_session=   session         required                        pam_mkhomedir.so skel=/etc/skel/ umask=0077
               session         optional                        pam_keyinit.so  revoke
               session         required                        pam_limits.so
               session         [success=1 default=ignore]      pam_sss.so
               session         required                        pam_unix.so
_EOF

And enable this:

sudo auth-client-config -a -p sss

Now you should be able to reboot and login as a LDAP member.

We should be able to restore the original pam config files with

sudo auth-client-config -a -p sss -r


Desktop Setup

Warning.png Warning:
This seems to work on my Xubuntu Trusty 14.04 but YMMV!

Sudoers

Create a 'cliadmins' group on the server. This will be used to identify domain users to the desktop machine.

So that domain users have sudo rights we need to add this group to /etc/sudoers

NOTE - use visudo so you do not break this file ! :

sudo visudo

Add this:

%cliadmins ALL=(ALL) ALL


System Permissions & PolicyKit

I also found to enable shutdown/restart, network indicator etc I had to add this to /etc/auth-client-config/profile.d/sss

pam_session=
               session         optional                        pam_systemd.so

Check if you run Policykit (most likely):

pgrep -lf polkit

To allow admin access on the desktop we need to edit the following file:

/var/lib/polkit-1/localauthority/10-vendor.d/com.ubuntu.desktop.pkla

Add the following to sections as required:

Identity=unix-group:admin;unix-group:sudo;unix-group:cliadmins

Sections:

[Mounting, checking, etc. of internal drives]
[Setting the clock]
[Adding or changing system-wide NetworkManager connections]
[Update already installed software]
[usb-creator]
[Printer administration]
[Modify error reporting settings]

LightDM Login Box

If you want to have a simple login box with manual login only you can do the following:

create /etc/lightdm/lightdm.conf.d/50-unity-greeter.conf

Add the following:

[SeatDefaults]
greeter-show-manual-login=true
greeter-hide-users=true

Mount Shares

Important.png Note:
The following page is worth a read https://wiki.contribs.org/Smeserver-tw-logonscript#Linux_client_integration

It is possible to create a simple local pam_mount.conf.xml file and then load a per user config from the server


If you can successfully login with a domain account you can now try and automatically mounts shares.

You will require at least cif-utils and libpam-mount

sudo apt-get install libpam-mount cifs-utils

In the above file /etc/auth-client-config/profile.d/sss

We need to add the following to the sections for:

pam_auth=
               auth           optional                        pam_mount.so    enable_pam_password
pam_session=
               session           optional                        pam_mount.so    enable_pam_password


We now need to setup global mounts for all users with /etc/security/pam_mount.conf.xml

Note: you can exclude local users from mounting directories with sgrp setting. You MAY need need nounix in mntoptions (needs testing)

Add the following:

cat <<'_EOF' >/etc/security/pam_mount.conf.xml
<pam_mount>
<debug        enable  = "0" />
<mntoptions   allow   = "nosuid,nodev,loop,encryption,fsck,nonempty,allow_root,allow_other,noexec" />
<mntoptions   require = "nosuid,nodev,noexec" />
<logout       wait    = "5" hup    = "0"    term="yes" kill="0" />
<mkmountpoint enable  = "1" remove = "true" />
<volume fstype     = "cifs"
       server     = "sme.server.com"
       path       = "%(USER)"
       mountpoint = "/home/e-smith/files/users/%(USER)/Partages/Personnel"
       options    = "uid=%(USER),nosuid,nodev,noexec"
       user       = "*"
       sgrp       = "admins"/>
<volume fstype     = "cifs"
       server     = "sme.server.com"
       path       = "artwork"
       mountpoint = "/home/e-smith/files/users/%(USER)/Mounts/artwork"
       options    = "uid=%(USER),nosuid,nodev,noexec"
       user       = "*"
       sgrp       = "admins"/>
</pam_mount>
_EOF


Now when you login as a domain user your shares should mount and you should have full sudo access.

Miscellaneous Notes

Local password required for sudo

One irritation that I have seen is that when you run a program requiring sudo e.g. Synaptic it may ask you for the password of a LOCAL user, not the domain user.

I believe adding your new group to the following file will then present you with a list of users who can authenticate:

/etc/polkit-1/localauthority.conf.d/51-ubuntu-admin.conf

[Configuration]
AdminIdentities=unix-group:sudo;unix-group:admin;unix-group:cliadmins

It will present an 'Authenticate' box with a list of users - I have not yet found how to just accept the password for the current logged in user (as per normal case for standalone user). Most likely it requires a modification to lightdm similar to above.

pam_winbind

You may get the following error:

PAM unable to dlopen(pam_winbind.so): /lib/security/pam_winbind.so: cannot open shared object file: No such file or directory

This is due to a file location issue. You can resolve this error by doing the following:

cd /lib;ln -s /lib/x86_64-linux-gnu/security security

pam_kwallet

If you do not use kwallet and get annoyed by this message:

PAM unable to dlopen(pam_kwallet.so): /lib/security/pam_kwallet.so: cannot open shared object file: No such file or directory

edit /etc/pam.d/lightdm and lightdm-greeter and comment lines containing the following:

pam_kwallet.so


Wish I knew all this a week ago !