LDAP Authentication and Authorization
Cumulus Linux uses Pluggable Authentication Modules (PAM) and Name Service Switch (NSS) for user authentication.
NSS specifies the order of information sources used to resolve names for each service. Using this with authentication and authorization, it provides the order and location used for user lookup and group mapping on the system. PAM handles the interaction between the user and the system, providing login handling, session setup, authentication of users, and authorization of user actions.
NSS enables PAM to use LDAP to provide user authentication, group mapping, and information for other services on the system.
Configure LDAP Authentication
There are 3 common ways to configure LDAP authentication on Linux:
This chapter describes using
libnss-ldapd only. From internal testing,
this library worked best with Cumulus Linux and is the easiest to
configure, automate, and troubleshoot.
libpam-ldapd package depends on
nslcd. To install
ldap-utils, run the following command:
cumulus@switch:~$ sudo apt-get install libnss-ldapd libpam-ldapd ldap-utils nslcd
Follow the interactive prompts to answer questions about the LDAP URI, search base distinguished name (DN), and services that must have LDAP lookups enabled. This creates a very basic LDAP configuration using anonymous bind and initiates user search under the base DN specified.
Alternatively, you can pre-seed these parameters using the
debconf-utils. To use this method, run
apt-get install debconf-utils
and create the pre-seeded parameters using
the appropriate answers. Run
debconf-show <pkg> to check the settings.
Here is an example of how to pre-seed answers to the installer questions
root# debconf-set-selections <<'zzzEndOfFilezzz' # LDAP database user. Leave blank will be populated later! nslcd nslcd/ldap-binddn string # LDAP user password. Leave blank! nslcd nslcd/ldap-bindpw password # LDAP server search base: nslcd nslcd/ldap-base string ou=support,dc=rtp,dc=example,dc=test # LDAP server URI. Using ldap over ssl. nslcd nslcd/ldap-uris string ldaps://myadserver.rtp.example.test # New to 0.9. restart cron, exim and others libraries without asking nslcd libraries/restart-without-asking: boolean true # LDAP authentication to use: # Choices: none, simple, SASL # Using simple because its easy to configure. Security comes by using LDAP over SSL # keep /etc/nslcd.conf 'rw' to root for basic security of bindDN password nslcd nslcd/ldap-auth-type select simple # Don't set starttls to true nslcd nslcd/ldap-starttls boolean false # Check server's SSL certificate: # Choices: never, allow, try, demand nslcd nslcd/ldap-reqcert select never # Choices: Ccreds credential caching - password saving, Unix authentication, LDAP Authentication , Create home directory on first time login, Ccreds credential caching - password checking # This is where "mkhomedir" pam config is activated that allows automatic creation of home directory libpam-runtime libpam-runtime/profiles multiselect ccreds-save, unix, ldap, mkhomedir , ccreds-check # for internal use; can be preseeded man-db man-db/auto-update boolean true # Name services to configure: # Choices: aliases, ethers, group, hosts, netgroup, networks, passwd, protocols, rpc, services, shadow libnss-ldapd libnss-ldapd/nsswitch multiselect group, passwd, shadow libnss-ldapd libnss-ldapd/clean_nsswitch boolean false ## define platform specific libnss-ldapd debconf questions/answers. ## For demo used amd64. libnss-ldapd:amd64 libnss-ldapd/nsswitch multiselect group, passwd, shadow libnss-ldapd:amd64 libnss-ldapd/clean_nsswitch boolean false # libnss-ldapd:powerpc libnss-ldapd/nsswitch multiselect group, passwd, shadow # libnss-ldapd:powerpc libnss-ldapd/clean_nsswitch boolean false zzzEndOfFilezzz
After the install is complete, the name service LDAP caching daemon
nslcd) runs. This service handles all of the LDAP protocol
interactions and caches information returned from the LDAP server. In
ldap is appended and is the
secondary information source for passwd, group, and shadow. The
local files (
/etc/shadow) are used
first, as specified by the
passwd: compat ldap group: compat ldap shadow: compat ldap
compat as the first source in NSS for passwd, group, and shadow. This prevents you from getting locked out of the system.
After installation, you need to update the main configuration file
/etc/nslcd.conf) to accommodate the expected LDAP server settings.
The nslcd.conf man page details
all the available configuration options. Some of the more important
options relate to security and how the queries are handled.
After first editing the
/etc/nslcd.conf file, and/or enabling LDAP in
/etc/nsswitch.conf file, you need to restart
netd with the
sudo systemctl restart netd command. Also, restart
netd if you disable LDAP.
The LDAP client starts a session by connecting to the LDAP server on TCP and UDP port 389, or on port 636 for LDAPS. Depending on the configuration, this connection might be unauthenticated (anonymous bind); otherwise, the client must provide a bind user and password. The variables used to define the connection to the LDAP server are the URI and bind credentials.
The URI is mandatory and specifies the LDAP server location using the
FQDN or IP address. The URI also designates whether to use
clear text transport, or
ldaps:// for SSL/TLS encrypted transport. You
can also specify an alternate port in the URI. Typically, in production
environments, it is best to utilize the LDAPS protocol; otherwise, all
communications are clear text and not secure.
After the connection to the server is complete, the BIND operation authenticates the session. The BIND credentials are optional, and if not specified, an anonymous bind is assumed. This is typically not allowed in most production environments. Configure authenticated (Simple) BIND by specifying the user (binddn) and password (bindpw) in the configuration. Another option is to use SASL (Simple Authentication and Security Layer) BIND, which provides authentication services using other mechanisms, like Kerberos. Contact your LDAP server administrator for this information as it depends on the configuration of the LDAP server and the credentials that are created for the client device.
# The location at which the LDAP server(s) should be reachable. uri ldaps://ldap.example.com # The DN to bind with for normal lookups. binddn cn=CLswitch,ou=infra,dc=example,dc=com bindpw CuMuLuS
When an LDAP client requests information about a resource, it must connect and bind to the server. Then, it performs one or more resource queries depending on the lookup. All search queries sent to the LDAP server are created using the configured search base, filter, and the desired entry (uid=myuser) being searched. If the LDAP directory is large, this search might take a significant amount of time. It is a good idea to define a more specific search base for the common maps (passwd and group).
# The search base that will be used for all queries. base dc=example,dc=com # Mapped search bases to speed up common queries. base passwd ou=people,dc=example,dc=com base group ou=groups,dc=example,dc=com
It is also common to use search filters to specify criteria used when searching for objects within the directory. This is used to limit the search scope when authenticating users. The default filters applied are:
filter passwd (objectClass=posixAccount) filter group (objectClass=posixGroup)
The map configuration allows you to override the attributes pushed
from LDAP. To override an attribute for a given map, specify the
attribute name and the new value. This is useful to ensure that the
shell is bash and the home directory is
map passwd homeDirectory "/home/cumulus" map passwd shell "/bin/bash"
In LDAP, the map refers to one of the supported maps specified in
the manpage for
nslcd.conf (such as passwd or group).
Here is an example configuration using Cumulus Linux ...
# /etc/nslcd.conf # nslcd configuration file. See nslcd.conf(5) # for details. # The user and group nslcd should run as. uid nslcd gid nslcd # The location at which the LDAP server(s) should be reachable. uri ldaps://myadserver.rtp.example.test # The search base that will be used for all queries. base ou=support,dc=rtp,dc=example,dc=test # The LDAP protocol version to use. #ldap_version 3 # The DN to bind with for normal lookups. # defconf-set-selections doesn't seem to set this. so have to manually set this. binddn CN=cumulus admin,CN=Users,DC=rtp,DC=example,DC=test bindpw 1Q2w3e4r! # The DN used for password modifications by root. #rootpwmoddn cn=admin,dc=example,dc=com # SSL options #ssl off (default) # Not good does not prevent man in the middle attacks #tls_reqcert demand(default) tls_cacertfile /etc/ssl/certs/rtp-example-ca.crt # The search scope. #scope sub # Add nested group support # Supported in nslcd 0.9 and higher. # default wheezy install of nslcd supports on 0.8. wheezy-backports has 0.9 nss_nested_groups yes # Mappings for Active Directory # (replace the SIDs in the objectSid mappings with the value for your domain) # "dsquery * -filter (samaccountname=testuser1) -attr ObjectSID" where cn == 'testuser1' pagesize 1000 referrals off idle_timelimit 1000 # Do not allow uids lower than 100 to login (aka Administrator) # not needed as pam already has this support # nss_min_uid 1000 # This filter says to get all users who are part of the cumuluslnxadm group. Supports nested groups. # Example, mary is part of the snrnetworkadm group which is part of cumuluslnxadm group # Ref: http://msdn.microsoft.com/en-us/library/aa746475%28VS.85%29.aspx (LDAP_MATCHING_RULE_IN_CHAIN) filter passwd (&(Objectclass=user)(!(objectClass=computer))(memberOf:1.2.840.1135126.96.36.1991:=cn=cumuluslnxadm,ou=groups,ou=support,dc=rtp,dc=example,dc=test)) map passwd uid sAMAccountName map passwd uidNumber objectSid:S-1-5-21-1391733952-3059161487-1245441232 map passwd gidNumber objectSid:S-1-5-21-1391733952-3059161487-1245441232 map passwd homeDirectory "/home/$sAMAccountName" map passwd gecos displayName map passwd loginShell "/bin/bash" # Filter for any AD group or user in the baseDN. the reason for filtering for the # user to make sure group listing for user files don't say '<user> <gid>'. instead will say '<user> <user>' # So for cosmetic reasons..nothing more. filter group (&(|(objectClass=group)(Objectclass=user))(!(objectClass=computer))) map group gidNumber objectSid:S-1-5-21-1391733952-3059161487-1245441232 map group cn sAMAccountName
This section provides troubleshooting tips.
nslcd Debug Mode
When setting up LDAP authentication for the first time, turn off the
nslcd service using the
systemctl stop nslcd.service command and run it in debug mode. Debug mode works whether you are using LDAP over SSL (port 636) or an unencrypted LDAP connection (port 389).
cumulus@switch:~$ sudo systemctl stop nslcd.service cumulus@switch:~$ sudo nslcd -d
After you enable debug mode, run the following command to test LDAP queries:
cumulus@switch:~$ sudo getent passwd
If LDAP is configured correctly, the following messages appear after you
nslcd: DEBUG: accept() failed (ignored): Resource temporarily unavailable nslcd: [8e1f29] DEBUG: connection from pid=11766 uid=0 gid=0 nslcd: [8e1f29] <passwd(all)> DEBUG: myldap_search(base="dc=example,dc=com", filter="(objectClass=posixAccount)") nslcd: [8e1f29] <passwd(all)> DEBUG: ldap_result(): uid=myuser,ou=people,dc=example,dc=com nslcd: [8e1f29] <passwd(all)> DEBUG: ldap_result(): ... 152 more results nslcd: [8e1f29] <passwd(all)> DEBUG: ldap_result(): end of results (162 total)
In the output above, <passwd(all)> indicates that the entire directory structure is queried.
You can query a specific user with the following command:
cumulus@switch:~$ sudo getent passwd myuser
You can replace myuser with any username on the switch. The following debug output indicates that user myuser exists:
nslcd: DEBUG: add_uri(ldap://10.50.21.101) nslcd: version 0.8.10 starting nslcd: DEBUG: unlink() of /var/run/nslcd/socket failed (ignored): No such file or directory nslcd: DEBUG: setgroups(0,NULL) done nslcd: DEBUG: setgid(110) done nslcd: DEBUG: setuid(107) done nslcd: accepting connections nslcd: DEBUG: accept() failed (ignored): Resource temporarily unavailable nslcd: [8b4567] DEBUG: connection from pid=11369 uid=0 gid=0 nslcd: [8b4567] <passwd="myuser"> DEBUG: myldap_search(base="dc=cumulusnetworks,dc=com", filter="(&(objectClass=posixAccount)(uid=myuser))") nslcd: [8b4567] <passwd="myuser"> DEBUG: ldap_initialize(ldap://<ip_address>) nslcd: [8b4567] <passwd="myuser"> DEBUG: ldap_set_rebind_proc() nslcd: [8b4567] <passwd="myuser"> DEBUG: ldap_set_option(LDAP_OPT_PROTOCOL_VERSION,3) nslcd: [8b4567] <passwd="myuser"> DEBUG: ldap_set_option(LDAP_OPT_DEREF,0) nslcd: [8b4567] <passwd="myuser"> DEBUG: ldap_set_option(LDAP_OPT_TIMELIMIT,0) nslcd: [8b4567] <passwd="myuser"> DEBUG: ldap_set_option(LDAP_OPT_TIMEOUT,0) nslcd: [8b4567] <passwd="myuser"> DEBUG: ldap_set_option(LDAP_OPT_NETWORK_TIMEOUT,0) nslcd: [8b4567] <passwd="myuser"> DEBUG: ldap_set_option(LDAP_OPT_REFERRALS,LDAP_OPT_ON) nslcd: [8b4567] <passwd="myuser"> DEBUG: ldap_set_option(LDAP_OPT_RESTART,LDAP_OPT_ON) nslcd: [8b4567] <passwd="myuser"> DEBUG: ldap_simple_bind_s(NULL,NULL) (uri="ldap://<ip_address>") nslcd: [8b4567] <passwd="myuser"> DEBUG: ldap_result(): end of results (0 total)
Notice how the
<passwd="myuser"> shows that the specific myuser user
This section discusses common problems.
- The FQDN of the LDAP server URI does not match the FQDN in the CA-signed server certificate exactly.
nslcdcannot read the SSL certificate and reports a Permission denied error in the debug during server connection negotiation. Check the permission on each directory in the path of the root SSL certificate. Ensure that it is readable by the
nscd cachedaemon is also enabled and you make some changes to the user from LDAP, you can clear the cache using the following commands:
nscd --invalidate = passwd nscd --invalidate = group
nscdpackage works with
nslcdto cache name entries returned from the LDAP server. This might cause authentication failures. To work around these issues:
cumulus@switch:~$ sudo nscd -K
cumulus@switch:~$ sudo systemctl restart nslcd.service
Try the authentication again.
The search filter returns wrong results. Check for typos in the search filter. Use
ldapsearchto test your filter.
Optionally, configure the basic LDAP connection and search parameters in
# ldapsearch -D 'cn=CLadmin' -w 'CuMuLuS' "(&(ObjectClass=inetOrgUser)(uid=myuser))"
When a local username also exists in the LDAP database, the order of the information sources in
/etc/nsswitchcan be updated to query LDAP before the local user database. This is generally not recommended. For example, the configuration below ensures that LDAP is queried before the local database.
# /etc/nsswitch.conf passwd: ldap compat
Configure LDAP Authorization
Linux uses the sudo command to allow non-administrator users (such as
the default cumulus user account) to perform privileged operations. To
control the users authorized to use sudo, the
/etc/sudoers file and
files located in the
/etc/sudoers.d/ directory have a series of rules
defined. Typically, the rules are based on groups, but can also be
defined for specific users. Therefore, sudo rules can be added using the
group names from LDAP. For example, if a group of users are associated
with the group netadmin, you can add a rule to give those users sudo
privileges. Refer to the sudoers manual (
man sudoers) for a complete
usage description. Here’s an illustration of this in
# The basic structure of a user specification is "who where = (as_whom) what ". %sudo ALL=(ALL:ALL) ALL %netadmin ALL=(ALL:ALL) ALL
Active Directory Configuration
Active Directory (AD) is a fully featured LDAP-based NIS server created by Microsoft. It offers unique features that classic OpenLDAP servers lack. Therefore, it can be more complicated to configure on the client and each version of AD is a little different in how it works with Linux-based LDAP clients. Some more advanced configuration examples, from testing LDAP clients on Cumulus Linux with Active Directory (AD/LDAP), are available in the Cumulus knowledge base.
LDAP Verification Tools
Typically, password and group information is retrieved from LDAP and cached by the LDAP client daemon. To test the LDAP interaction, you can use these command-line tools to trigger an LDAP query from the device. This helps to create the best filters and verify the information sent back from the LDAP server.
Identify a User with the id Command
id command performs a username lookup by following the lookup
information sources in NSS for the passwd service. This simply returns
the user ID, group ID and the group list retrieved from the information
source. In the following example, the user cumulus is locally defined
/etc/passwd, and myuser is on LDAP. The NSS configuration has the
passwd map configured with the sources
cumulus@switch:~$ id cumulus uid=1000(cumulus) gid=1000(cumulus) groups=1000(cumulus),24(cdrom),25(floppy),27(sudo),29(audio),30(dip),44(video),46(plugdev) cumulus@switch:~$ id myuser uid=1230(myuser) gid=3000(Development) groups=3000(Development),500(Employees),27(sudo)
getent command retrieves all records found with NSS for a given
map. It can also get a specific entry under that map. You can perform
tests with the passwd, group, shadow, or any other map configured in
/etc/nsswitch.conf. The output from this command is formatted
according to the map requested. Therefore, for the passwd service, the
structure of the output is the same as the entries in
group map outputs the same structure as
/etc/group. In this example,
looking up a specific user in the passwd map, the user cumulus is
locally defined in
/etc/passwd, and myuser is only in LDAP.
cumulus@switch:~$ getent passwd cumulus cumulus:x:1000:1000::/home/cumulus:/bin/bash cumulus@switch:~$ getent passwd myuser myuser:x:1230:3000:My Test User:/home/myuser:/bin/bash
In the next example, looking up a specific group in the group service,
the group cumulus is locally defined in
/etc/groups, and netadmin
is on LDAP.
cumulus@switch:~$ getent group cumulus cumulus:x:1000: cumulus@switch:~$ getent group netadmin netadmin:*:502:larry,moe,curly,shemp
Running the command
getent passwd or
getent group without a specific
request returns all local and LDAP entries for the passwd and group maps.
ldapsearch command performs LDAP operations directly on the LDAP
server. This does not interact with NSS. This command helps display what
the LDAP daemon process is receiving back from the server. The command
has many options. The simplest uses anonymous bind to the host and
specifies the search DN and the attribute to look up.
cumulus@switch:~$ ldapsearch -H ldap://ldap.example.com -b dc=example,dc=com -x uid=myuser
Click to expand the command output ...
# extended LDIF # # LDAPv3 # base <dc=example,dc=com> with scope subtree # filter: uid=myuser # requesting: ALL # # myuser, people, example.com dn: uid=myuser,ou=people,dc=example,dc=com cn: My User displayName: My User gecos: myuser gidNumber: 3000 givenName: My homeDirectory: /home/myuser initials: MU loginShell: /bin/bash mail: firstname.lastname@example.org objectClass: inetOrgPerson objectClass: posixAccount objectClass: shadowAccount objectClass: top shadowExpire: -1 shadowFlag: 0 shadowMax: 999999 shadowMin: 8 shadowWarning: 7 sn: User uid: myuser uidNumber: 1234 # search result search: 2 result: 0 Success # numResponses: 2 # numEntries: 1
There are several GUI LDAP clients available that help to work with LDAP servers. These are free tools to help show the structure of the LDAP database graphically.