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Configuring LDAP Authentication

This page describes the steps needed to get user names, groups and other information that is usually stored in flat files in /etc or NIS from an LDAP server. This information is exposed through NSS (Name Services Switch) as configured in /etc/nsswitch.conf.

The following databases can be served from LDAP: aliases (mail aliases, ignored by most mail daemons), ethers (ethernet numbers), group (groups of users), hosts (host names and numbers), netgroup (host and user groups used for access controls), networks (network names and numbers), passwd (users), protocols (network protocols), rpc (remote procedure call names and numbers), services (network service names and numbers) and shadow (shadow user passwords).

There are currently two packages available to configure NSS lookups through LDAP: the libnss-ldap package and the libnss-ldapd package. Which one to choose depends on the needs. In general libnss-ldapd is simpler but newer and libnss-ldap is more mature but more complex. Also libnss-ldap has some known issues with serving host information and lookups during boot which should be addressed in libnss-ldapd. In addition, libnss-ldap breaks setuid programs (su, sudo) when using LDAP+SSL (see http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=579647).

On running nscd

For debugging it is recommended to not to run nscd (the Name Service Caching Daemon) because nscd can mask problems by serving entries from it's cache. Either don't install the nscd package until it is clear that everything is functional or stop nscd with

  # /etc/init.d/nscd stop

However, running nscd is required when using libnss-ldap and permissions of /etc/libnss-ldap.conf do not allow normal users to read the file (e.g. when using the bindpw option). Certain versions of libnss-ldap have been known to set restrictive permissions on this file. Note that not all NSS lookups will go through nscd (only passwd, group and host) so this may not work in all circumstances.

For production use it is recommended to run nscd as it saves on doing lookups to the LDAP server. You may consider tuning the time-to-live values of the cache in /etc/nscd.conf if you need to pick up changes in the LDAP directory quickly (through the defaults are fine in most circumstances).

NSS Setup with libnss-ldap

Install the package:

  # apt-get install libnss-ldap

Be sure to read the docs that are installed in /usr/share/doc/libnss-ldap/

If you plan to do hostname lookups through LDAP you should add the hostname of your LDAP server in /etc/hosts (even if you use an IP address to configure the connection to the server). Without this nasty things happen on bootup as things attempt to use LDAP which recurses on itself looking up the hostname.

Edit /etc/libnss-ldap.conf to include al least the following (replace the values with options that are specific to your environment):

  # Your LDAP server. Must be resolvable without using LDAP.
  uri ldap://10.0.0.1

  # The distinguished name of the search base.
  base dc=<your>,dc=<domain>

If you specified rootbinddn you need to put the LDAP admin password in /etc/ldap.secret with mode 600 (rw-------).

Edit /etc/nsswitch.conf to use add LDAP to the services you want to have enabled (be careful to put LDAP *after* "files").

  passwd:         files ldap
  group:          files ldap
  shadow:         files ldap

  hosts:          files dns ldap
  networks:       files ldap

  protocols:      db files
  services:       db files
  ethers:         db files
  rpc:            db files

  netgroup:       nis

NSS Setup with libnss-ldapd

An alternative is to use libnss-ldapd. This software has been developed to fix some of the shortcomings of libnss-ldap, see the nss-ldapd homepage for more details. Install the package with:

  # apt-get install libnss-ldapd

Most of the configuration for common setups is performed during installation. The following questions are generally asked:

libnss-ldapd provides reasonable defaults for most values (looking at environment and possibly existing configurations). This should be enough to enable NSS lookups through LDAP in most common cases.

If you have a more unusual setup or require more configuration (e.g. SSL/TLS certificates, SASL/Kerberos configuration, etc) see the nslcd.conf manual page and documentation in /usr/share/doc/libnss-ldapd.

The configuration file can be found at /etc/nslcd.conf. nslcd should be restarted if any changes are made to it.

Verify that NSS is operational

Check that NSS is seeing things from LDAP using getent:

  # getent passwd

should show you accounts from LDAP that are not in the /etc/passwd file. Similar tests can be done with the group, shadow and other in /etc/nsswitch.conf configured databases.

Be sure to also run some tests as non-root users. Also try rebooting to see if NSS lookups are performed correctly during boot.

Note that getent shadow should only return data for root users. Also, passwords are generally not returned unless the LDAP server has been configured to return this data and are in a supported format. If pam_ldap is used (see LDAP/PAM) it is not needed to expose passwords from the LDAP server.

When using the libnss-ldapd package debugging can be done by starting nslcd (the connection daemon) in debugging mode (remember to stop nscd when debugging):

  # /etc/init.d/nscd stop
  # /etc/init.d/nslcd stop
  # nslcd -d

Further debugging can be done with the ldapsearch utility from the ldap-utils package. You can search for all the information that is available for a single user:

  % ldapsearch -h <ldapserver> -b dc=<your>,dc=<domain> -x uid=<username>

Specify the -D and -W options to log in if the binddn or rootbinddn options are used.

Offline caching of NSS with nscd

While continuous LDAP connectivity can be assumed for workstations and servers in a LAN, laptop users often do not have network connectivity. From a system administrators point of view it is tempting to create local users on the laptop but this causes trouble when these laptops have to access domain resources like network shares (NFS, sshfs, Samba, etc.) back in the office (with a stable network connection). Many of these network shares rely on a central name service database like LDAP because of user and group information and permissions on the share.

NSCD is often used to cache NSS information, so that the LDAP server does not have to be queried for every request (which has also an impact on the speed of the answer). NSCD can also be used to serve these requests while there is no network connectivity. In short: NSCD is configured to cache the information much longer than the default values from Debian (Lenny)

Recipe:

NSCD has a configuration file /etc/nscd.conf

There are two configuration options which have to be altered in order to use the pseudo-offline capability:

reload-count            unlimited
positive-time-to-live   <service>          #number of second

The positive-time-to-live has to be configured for at least the passwd and group service. To cache user and group information for 30 days, you would use:

positive-time-to-live   passwd          2592000
positive-time-to-live   group           2592000

Caveats:

When configured for offline mode, NSCS's NSS information is not updated for a long period of time. This can be troublesome depending on the time-to-live setting, because changes in the LDAP database are not known to the system because it uses the cached information. For now, I have only one workaround with which I am not too happy: manually cleaning the cache when online again with:

# nscd -i passwd
# nscd -i group

I guess this could be automated in some way (when the LDAP server is reachable again) but I am not sure if this is the right way

Other possibilities:

See Petter Reinholdtsens Blog on caching password, user and group on a roaming Debian laptop for more information.

Hint: PAM can also be configured for offline caching of credentials, see LDAP/PAM