nss-pam-ldapd - NSS and PAM libraries for name lookups and authentication
nss-pam-ldapd started as nss-ldapd which was a fork from nss_ldap which was
originally written by Luke Howard of PADL Software Pty Ltd.
In 2006 Arthur de Jong of West Consuling forked the library to split it
into a thin NSS part and a server part. Most of the code was rewritten.
The software was renamed to nss-pam-ldapd when PAM code contributed by
Howard Chu for the OpenLDAP nssov module was integrated. Solaris
compatibility was developed by Ted C. Cheng of Symas Corporation.
Copyright (C) 1997-2006 Luke Howard
Copyright (C) 2006-2007 West Consulting
Copyright (C) 2006-2018 Arthur de Jong
Copyright (C) 2009 Howard Chu
Copyright (C) 2010 Symas Corporation
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
This is the nss-pam-ldapd library which consists of an NSS module to do name
lookups to an LDAP directory server and a PAM module to do authentication to
an LDAP server. The NSS part of this library was forked from nss_ldap as
provided by Luke Howard of PADL Software Pty Ltd. The PAM module was mostly
provided by Howard Chu of the OpenLDAP project.
The NSS library allows distributing account, group, host and other
configuration information from a central LDAP server. Because LDAP is a
hierarchical directory service, information can be organised in a manner which
reflects an organisational structure. This contrasts with the flat, single
domain policy of NIS. LDAP has many of the advantages of NIS+ (security and
scalability) without the complexity. The system will work alongside your
existing NIS, NIS+, DNS and flat file name services.
The PAM library (module) can be used to perform authentication based on
information inside the LDAP directory.
Both libraries consist of a thin NSS or PAM part that proxies the requests to
a local daemon (nslcd) that handles the LDAP lookups. This simplifies the
software architecture and fixes some scalability and locking problems in the
original design of nss_ldap.
It is also possible to use the thin NSS and PAM modules together with the
nssov overlay in the OpenLDAP server (slapd).
The three parts (NSS module, PAM module, and nslcd server) can be built
separately and are not strongly tied together. This means that for instance
you can still use pam_ldap and use the NSS module from nss-pam-ldapd or use an
alternative implementation of nslcd (for instance with the nssov slapd overlay
or the pynslcd implementation).
improvements over nss_ldap
The fork from nss_ldap was done to implement some major design changes to fix
some structural problems in the library.
One of those problems were host name lookups through LDAP which could cause
deadlocks. Another is that nss_ldap loaded an SSL library into executables
that may not be designed to load it (e.g. problem with suid applications).
A number of refactoring steps were done to simplify the code and improve
maintainability. Legacy code was removed and support for non-Linux operating
systems was initially removed to make the code more readable. Portability was
re-added using compatibility wrappers.
The most practical improvements over nss_ldap are:
- the LDAP library is not loaded for every process doing LDAP lookups
- the number of connections to the LDAP server is limited, because not every
process will open its own connection
- hostname lookups should now be deadlock-free because the LDAP server name is
no longer looked up using the ldap method
- avoid problems with TLS connections in suid binaries and other process-local
- it is easier to debug because logging in nslcd can be enabled without
the need to restart all processes doing name lookups
- unavailability timeouts are global instead of per-process
comparison to pam_ldap
The PAM module that is currently implemented contains functionality for
authentication, account management, password management and session
management. The nslcd daemon currently implements authentication,
authorisation and password modification. The OpenLDAP nssov overlay also
implements session functionality.
supported C libraries (for NSS module)
This library currently supports the GNU C Library, the Solaris C library and
the FreeBSD C library.
supported name databases
Currently the following name databases are supported:
aliases, ethers, group, hosts, netgroup, networks, passwd, protocols, rpc,
services and shadow
When using IPv6 ipHostNumber attributes, the address in LDAP must be in the
preferred form as defined in section 2.2 of RFC1884, specifically the format
as returned by inet_ntop(3). All leading zeros should be omitted and the
longest range of zeroes should be replaced with :: (e.g.
MAC addresses in the macAddress attribute should be in maximal, colon
separated hex notation (e.g. 00:00:92:90:ee:e2).
automounter map lookups (which are also defined in /etc/nsswitch.conf) are
currently not supported because the NSS interface is not used for these. The
common autofs implementation (on GNU/Linux) currently uses its own method for
getting the maps from LDAP.
Although mail aliases are exposed through NSS, most mail servers parse
/etc/aliases themselves (bypassing NSS) and getting aliases from LDAP requires
some configuration in the mail server.
The publickey, bootparams and netmasks are currently unsupported. Some
investigation should be done if these are needed for anything, which
interfaces should be exported and how the LDAP schema part should look like.
supported PAM implementation
The PAM module is currently only regularly tested on Linux PAM but other PAM
implementations should also work.
supported LDAP libraries
The current version of nss-pam-ldapd has been developed with OpenLDAP 2.4 but
other LDAP libraries and older versions of OpenLDAP may also work.
Since nss-pam-ldapd was forked from nss_ldap most of the features that came
with nss_ldap are available. The most important differences:
- the configuration file formats are not fully compatible
- rootbinddn/rootbindpw support is removed and is not likely to return
(the rootpwmoddn and rootpwmodpw work differently but accomplish the same
For the PAM module some functionality is missing. Comparing it to pam_ldap:
- only BIND authentication is supported
- only LDAP password modify EXOP is supported as password changing mechanism
Some things work a little different in nss-pam-ldapd. For instance the
attribute defaults and overrides of nss_ldap are implemented with mapping
expressions and pam_ldap's pam_check_*_attr options can be implemented with
the pam_authz_search option.
The nss-pam-ldapd library uses autoconf and automake for building. Installing
nss-pam-ldapd should be as simple as:
% make install
It is a good idea to first go through the options of configure by running:
% ./configure --help
The last step (make install) should install the libnss_ldap.so.* and
pam_ldap.so files and the daemon (nslcd). The proper location of the NSS and
PAM modules are guessed. The boot process needs to be modified to start the
nslcd daemon at the right time.
It is recommended to create a dedicated user for the nslcd daemon. Configure
this user in /etc/nslcd.conf using the uid and gid options.
After installation, the name service switch configuration file
(/etc/nsswitch.conf) needs to be modified to do name lookups using the new
module. This consist mostly of adding ldap in the list of lookup methods in
the right place. See the nsswitch.conf(5) manual page for details on the
format. As an example the file could look a little like this:
# the following contain normal unix user and group information
passwd: files ldap
group: files ldap
shadow: files ldap
# hostname lookups through ldap before dns should work now
hosts: files ldap dns
networks: files ldap
# normal flat-file definitions
protocols: files ldap
services: files ldap
ethers: files ldap
rpc: files ldap
# whether alias lookups really use NSS depends on the mail server
aliases: files ldap
Configuring PAM differs a little from platform to platform but this is a
minimal set-up for files under /etc/pam.d:
auth sufficient pam_unix.so
auth sufficient pam_ldap.so use_first_pass
auth required pam_deny.so
account required pam_unix.so
account sufficient pam_ldap.so
account required pam_permit.so
session required pam_unix.so
session optional pam_ldap.so
password sufficient pam_unix.so nullok md5 shadow use_authtok
password sufficient pam_ldap.so try_first_pass
password required pam_deny.so
Lastly, a configuration file for nslcd (by default /etc/nslcd.conf) needs to
be made. See the shipped manual page for details on the format and options. It
should at the very least contain something like:
# the location of LDAP server
# search base for all queries.
service discovery through DNS
nss-pam-ldapd supports looking up LDAP server names through DNS SRV records as
specified in RFC 2782. However, Priority and Weight are not considered
separately and a single list of servers in added as if they had been specified
with uri options in the configuration file.
To use this feature specify DNS as an uri in the configuration file and
include something like the following in your zone:
_ldap._tcp SRV 10 0 389 ldapserver
nss-pam-ldapd supports a wide range of possible LDAP schema configurations and
it can be customized heavily. The LDAP schema used is described in RFC 2307.
Groups using the member attribute that hold distinguished names (RFC 2307bis)
are also supported (but see group membership below for more information).
This paragraph describes the mapping between the NSS lookups and the LDAP
database. The mapping may be modified by changing the nslcd.conf configuration
file. See the nslcd.conf(5) manual page for details.
cn - alias name
rfc822MailMember - members of the alias (recipients)
cn - host name
macAddress - ethernet address
cn - group name
userPassword - password (by default mapped to "*")
gidNumber - gid
memberUid - members (user names)
member - members (DN values)
cn - host name (and aliases)
ipHostNumber - addresses
cn - netgroup name
nisNetgroupTriple - triplets describing netgroup entries
memberNisNetgroup - reference to other netgroup
cn - network name
ipNetworkNumber - network address
uid - account name
userPassword - password (by default mapped to "*")
uidNumber - uid
gidNumber - gid
gecos - gecos
homeDirectory - home directory
loginShell - shell
cn - protocol name
ipProtocolNumber - protocol number
cn - rpc name
oncRpcNumber - rpc number
cn - service name
ipServicePort - service port
ipServiceProtocol - service protocol
uid - use name
userPassword - password
shadowLastChange - date of last password change
shadowMin - days before password may be changed again
shadowMax - days after which password must be changed
shadowWarning - days before max password age to present a warning
shadowInactive - days after max password age that account is disabled
shadowExpire - account expiration date
shadowFlag - reserved field
using Microsoft Active Directory
When using Microsoft Active Directory server some changes need to be made to
the nslcd.conf configuration file. The included sample configuration file has
some commented out attribute mappings for such a set-up.
Currently, two ways of specifying group membership are supported. The first,
by using the memberUid attribute, is the simplest and by far the fastest
(takes the least number of lookups). The attribute values are user names (same
as the uid attribute for posixAccount entries) and are returned without
The second method is to use DN values in the member attribute (attribute names
can be changed by using the attribute mapping options as described in the
manual page). This is potentially a lot slower because in the worst case every
DN has to be looked up in the LDAP server to find the proper value for the uid
If the LDAP server supports the deref control (provided by the deref overlay
in OpenLDAP) the DN to uid expansing is performed by the LDAP server.
If the DN value already contains a uid value (e.g. uid=arthur, dc=example,
dc=com) a further lookup is skipped and the uid value from the DN is used.
For other DN values an extra lookup is performed to expand it to a uid. These
lookups are cached and are configurable with the cache dn2uid configuration
The member attribute may also contain the DN of another group entry. These
nested groups are parsed recursively depending on the nss_nested_groups
Currently, the memberOf attribute in posixAccount entries is unsupported.
Most values in NSS databases are considered case-sensitive (e.g. the user
"Foo" is a different user from the user "foo"). Most values in an LDAP
database are however considered case-insensitive. nss-pam-ldapd tries to solve
this problem by adding an extra filtering layer to ensure that when looking
for the user "foo" it will not consider a user "Foo" that is found in LDAP.
For the group, netgroup, passwd, protocols, rpc, services and shadow maps the
matches will be checked case-sensitively and for aliases, ethers, hosts and
networks matches will be case-insensitive (this seems to be what Glibc is
doing currently in flat files). Only searching for groups by user is done
case-insensitive. In all cases the case-use in the LDAP directory is returned.
This behaviour can be disabled with the ignorecase configuration option but
may be a security risk.
Note that having entries that only differ in case is a bad idea and will
likely get you in trouble. One example of such a problem is that the DN
uid=test,dc=example,dc=com is considered the same in LDAP as
If you find any bugs or missing features please send email to
If you are using a packaged version of nss-pam-ldapd you are encouraged to use
the distributor's bug tracking system. Please include as much information as
possible (platform, output of configure if compilation fails, error messages,
output of nslcd -d, etc). Patches are more than welcome (also see the file