LDAP Authentication and Authorization
Cumulus Linux uses Pluggable Authentication Modules (PAM) and Name Service Switch (NSS) for user authentication. NSS enables PAM to use LDAP to provide user authentication, group mapping, and information for other services on the system.
- NSS specifies the order of the information sources that are used to resolve names for each service. Using NSS with authentication and authorization provides the order and location 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.
There are three common ways to configure LDAP authentication on Linux: you can use
libnss-sss. This chapter describes
libnss-ldapd only. From internal testing, this library worked best with Cumulus Linux and is the easiest to configure, automate, and troubleshoot.
libldap-common LDAP packages are already installed on the Cumulus Linux image; however you need to install these additional packages to use LDAP authentication:
To install the additional packages, run the following command:
cumulus@switch:~$ sudo apt-get install libnss-ldapd libpam-ldapd ldap-utils nslcd
You can also install these packages even if the switch is not connected to the internet, as they are contained in the
cumulus-local-apt-archive repository that is embedded in the Cumulus Linux disk image.
Follow the interactive prompts to specify the LDAP URI, search base distinguished name (DN), and services that must have LDAP lookups enabled. You need to select at least the
shadow services (press space to select a service). When done, click OK. This creates a very basic LDAP configuration using anonymous bind and initiates user search under the base DN specified.
After the dialog closes, the install process prints information similar to the following:
/etc/nsswitch.conf: enable LDAP lookups for group /etc/nsswitch.conf: enable LDAP lookups for passwd /etc/nsswitch.conf: enable LDAP lookups for shadow
After the installation is complete, the name service caching daemon (
nslcd) runs. This service handles all the LDAP protocol interactions and caches information returned from the LDAP server.
ldap is appended in the
/etc/nsswitch.conf file, as 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.
Entering incorrect information during the installation process might produce configuration errors. You can correct the information after installation by editing certain configuration files.
- Edit the
/etc/nslcd.conffile to update the LDAP URI and search base DN (see Update the nslcd.conf File, below).
- Edit the
/etc/nssswitch.conffile to update the service selections.
Be sure to restart
netd after editing the files.
Update the nslcd.conf File
After installation, update the main configuration file (
/etc/nslcd.conf) to accommodate the expected LDAP server settings.
This section documents some of the more important options that relate to security and how queries are handled. For details on all the available configuration options, read the nslcd.conf man page.
After first editing the
/etc/nslcd.conf file and/or enabling LDAP in the
/etc/nsswitch.conf file, you must restart
netd with the
sudo systemctl restart netd command. If you disable LDAP, you need to restart the
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
ldap:// for clear text transport, or
ldaps:// for SSL/TLS encrypted transport. You can also specify an alternate port in the URI. In production environments, the LDAPS protocol is recommended so that all communications are 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
nslcd.conf (such as passwd or group).
Create Home Directory on Login
If you want to use unique home directories, run the
sudo pam-auth-update command and select
Create home directory on login in the PAM configuration dialog (press the space bar to select the option). Select OK, then press Enter to save the update and close the dialog.
cumulus@switch:~$ sudo pam-auth-update
The home directory for any user that logs in (using LDAP or not) is created and populated with the standard dotfiles from
/etc/skel if it does not already exist.
nslcd starts, you might see an error message similar to the following (where 5816 is the
nslcd: unable to dlopen /usr/lib/x86_64-linux-gnu/sasl2/libsasldb.so: libdb-5.3.so: cannot open shared object file: No such file or directory
You can safely ignore this message. The
libdb package and resulting log messages from
nslcd do not cause any issues when you use LDAP as a client for login and authentication.
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:1.2.840.113518.104.22.1681:=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/$sAMAccountName" map passwd gecos displayName map passwd loginShell "/bin/bash" # Filter for any AD group or user in the baseDN. the reason for filtering for the # user to make sure group listing for user files don't say '<user> <gid>'. instead will say '<user> <user>' # So for cosmetic reasons..nothing more. filter group (&(|(objectClass=group)(Objectclass=user))(!(objectClass=computer))) map group gidNumber objectSid:S-1-5-21-1391733952-3059161487-1245441232 map group cn sAMAccountName
nslcd Debug Mode
When setting up LDAP authentication for the first time, turn off the
nslcd service using the
systemctl stop nslcd.service command (or the
systemctl stop firstname.lastname@example.org if you are running the service in a management VRF) and run it in debug mode. Debug mode works whether you are using LDAP over SSL (port 636) or an unencrypted LDAP connection (port 389).
cumulus@switch:~$ 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:~$ getent passwd
If LDAP is configured correctly, the following messages appear after you run the
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:~$ getent passwd myuser
You can replace
myuser with any username on the switch. The following debug output indicates that user
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)
- 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
- If 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, disable
nscd, restart the
nslcdservice, then retry authentication:
cumulus@switch:~$ sudo nscd -K cumulus@switch:~$ sudo systemctl restart nslcd.service
If you are running the
nslcd service in a management VRF, you need to run the
systemctl restart email@example.com command instead of the
systemctl restart nslcd.service command. For example:
cumulus@switch:~$ sudo nscd -K cumulus@switch:~$ sudo systemctl restart firstname.lastname@example.org
The search filter returns incorrect 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 define a series of rules. Typically, the rules are based on groups, but can also be defined for specific users. You can add sudo rules 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. The following shows an example in the
# 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 create by Microsoft. It offers unique features that classic OpenLDAP servers do not have. AD can be more complicated to configure on the client and each version works a little differently 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 our 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 in
/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 retrieve a specific entry under that map. You can perform tests with the
shadow, or any other map configured in the
/etc/nsswitch.conf file. The output from this command is formatted according to the map requested. For the
passwd service, the structure of the output is the same as the entries in
/etc/passwd. The group map outputs the same structure as
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 option 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
To use NCLU, a user must be in either the
netedit NCLU group in the LDAP database. You can either:
- Add a user or one of their groups to the
- Add a user to the local
/etc/groupfile as a member of the
In the following example, a user that is not in the
netedit NCLU group in the LDAP database runs the NCLU
net show version command, which produces an error:
hsolo@switch:~$ net show version ERROR: 'getpwuid(): uid not found: 0922' See /var/log/netd.log for more details
To add user to the
netedit NCLU group in the LDAP database, either edit the
/etc/group file manually or use the
sudo adduser USERNAME netshow command, then restart
netd. For example, to add the user bill to the
cumulus@switch:~$ sudo adduser hsolo netshow Adding user `hsolo' to group `netshow' ... Adding user hsolo to group netshow Done. cumulus@switch:~$ sudo systemctl restart netd
Now, the user can run the NCLU
net show commands successfully:
hsolo@switch:~$ net show version NCLU_VERSION=1.0-cl4u1~1555625956.7cfe305 DISTRIB_ID="Cumulus Linux" DISTRIB_RELEASE=4.0.0~1555370771.772c26b6 DISTRIB_DESCRIPTION="Cumulus Linux 4.0.0~1555370771.772c26b6"
There are several GUI LDAP clients available that help you work with LDAP servers. These are free tools that show the structure of the LDAP database graphically.