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:

• libnss-ldap
• libnss-ldapd
• libnss-sss

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.

Install libnss-ldapd

The libpam-ldapd package depends on nslcd. To install libnss-ldapd, libpam-ldapd, and 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.

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 the /etc/nsswitch.conf file, ldap is appended and is the secondary information source for passwd, group, and shadow. The local files (/etc/passwd, /etc/groups and /etc/shadow) are used first, as specified by the compat source.

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

Configure nslcd.conf

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.

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 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. 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

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 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

Search Filters

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)

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"

Example Configuration

# /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

Troubleshooting

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 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 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 was queried.

Common Problems

This section discusses common problems.

SSL/TLS

• The FQDN of the LDAP server URI does not match the FQDN in the CA-signed server certificate exactly.
• 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 might cause authentication failures. To work around these issues:

  1. Disable nscd by running:

     cumulus@switch:~$ sudo nscd -K
     

  2. Restart the nslcd service:

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

  3. Try the authentication again.

LDAP

• The search filter returns wrong 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))"
  

• When a local username also exists in the LDAP database, the order of the information sources in /etc/nsswitch can 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 /etc/sudoers:

# 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

The 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 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 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 /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 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

# 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

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.

• Apache Directory Studio
• LDAPManager

Related Information

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