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 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.

Install libnss-ldapd

The libldap-2.4-2 and libldap-common LDAP packages are already installed on the Cumulus Linux image; however you need to install these additional packages to use LDAP authentication:

• libnss-ldapd
• libpam-ldapd
• ldap-utils

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 does not connect to the internet, as they are in the cumulus-local-apt-archive repository that is embedded in the Cumulus Linux 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 passwd, group, and shadow services (press space to select a service). When done, select OK. This creates a 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 that returns from the LDAP server. nslcd appends ldap to the /etc/nsswitch.conf file, as well as the secondary information source for passwd, group, and shadow. nslcd references the local files (/etc/passwd, /etc/groups and /etc/shadow) first, as specified by the compat source.

passwd: compat ldap
group: compat ldap
shadow: compat ldap

Entering incorrect information during the installation process produces configuration errors. You can correct the information after installation by editing certain configuration files.

• Edit the /etc/nslcd.conf file to update the LDAP URI and search base DN (see Update the nslcd.conf File, below).
• Edit the /etc/nssswitch.conf file to update the service selections.

Be sure to restart nvued.service after editing the files.

cumulus@switch:~$ sudo systemctl restart nvued.service

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 queries. For details on all the available configuration options, read the nslcd.conf man page.

Connection

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 establishes without authentication (anonymous bind); otherwise, the client must provide a bind user and password. The variables you use 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, use the LDAPS protocol 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; if you do not specify the credentials, the switch assumes an anonymous bind. 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 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

Search Function

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 to the LDAP server use the configured search base, filter, and the desired entry (uid=myuser). If the LDAP directory is large, this search takes a long time. 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

Search Filters

To limit the search scope when authenticating users, use search filters to specify criteria when searching for objects within the directory. The default filters applied are:

filter passwd (objectClass=posixAccount)
filter group (objectClass=posixGroup)

Attribute Mapping

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 /home/cumulus:

map    passwd homeDirectory "/home/cumulus"
map    passwd shell "/bin/bash"

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) populates with the standard dotfiles from /etc/skel.

nslcd[5816]: 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

Example Configuration

The following 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:1.2.840.113556.1.4.1941:=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

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 that can use sudo, define a series of rules in the /etc/sudoers file and files in the /etc/sudoers.d/ directory. The rules apply to groups but you can also define specific users. You can add sudo rules using the group names from LDAP. For example, if a group of users are in 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 /etc/sudoers file:

# 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 the knowledge base.

LDAP Verification Tools

The LDAP client daemon retrieves and caches password and group information from LDAP. To verify the LDAP interaction, use these command-line tools to trigger an LDAP query from the device.

Identify a User with the id Command

The id command performs a username lookup by following the lookup information sources in NSS for the passwd service. This 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 compat ldap:

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

The 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 passwd, group, shadow, or any other map in the /etc/nsswitch.conf file. The output from this command formats 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 /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.

LDAP search

The ldapsearch command performs LDAP operations directly on the LDAP server. This does not interact with NSS. This command displays the information that the LDAP daemon process receives back from the server. The command has several 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

# 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: myuser@example.com
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

LDAP Browsers

The GUI LDAP clients are free tools that show the structure of the LDAP database graphically.

• Apache Directory Studio
• LDAPManager

Troubleshooting

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 nslcd@mgmt.service 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 you configure LDAP correctly, the following messages appear after you run the getent command:

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 example output above, <passwd(all)> shows a query of the entire directory structure.

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 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)

Common Problems

SSL/TLS

• The FQDN of the LDAP server URI does not match the FQDN in the CA-signed server certificate.
• nslcd cannot 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 nslcd user.

NSCD

• If the nscd cache daemon 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
  

• The nscd package works with nslcd to cache name entries returned from the LDAP server. This sometimes causes authentication failures. To work around these issues, disable nscd, restart the nslcd service, then retry authentication:

  cumulus@switch:~$ sudo nscd -K
  cumulus@switch:~$ sudo systemctl restart nslcd.service
  

cumulus@switch:~$ sudo nscd -K
cumulus@switch:~$ sudo systemctl restart nslcd@mgmt.service

LDAP

• The search filter returns incorrect results. Check for typos in the search filter. Use ldapsearch to test your filter.

• Optionally, configure the basic LDAP connection and search parameters in /etc/ldap/ldap.conf.

  # ldapsearch -D 'cn=CLadmin' -w 'CuMuLuS' "(&(ObjectClass=inetOrgUser)(uid=myuser))"
  

Related Information

• Debian - configuring LDAP authentication
• Debian - configuring PAM to use LDAP
• GitHub - Arthur de Jong nslcd.conf file
• Debian backports