Cumulus Linux implements TACACS+ client AAA (Accounting, Authentication, and Authorization) in a transparent way with minimal configuration. The client implements the TACACS+ protocol as described in this IETF document. There is no need to create accounts or directories on the switch. Accounting records are sent to all configuredTACACS+ servers by default. Use of per-command authorization requires additional setup on the switch.

Contents

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Supported Features

  • Authentication using PAM; includes loginsshsudo and su
  • Runs over the eth0 management interface

  • Ability to run in the management VRF
  • TACACS+ privilege 15 users can run any command with sudo using the /etc/sudoers.d/tacplus file that is installed by the libtacplus-map1 package
  • Up to seven TACACS+ servers

Installing the TACACS+ Client Packages

TACACS+ requires the following packages to be installed on Cumulus Linux. These packages are not part of the base Cumulus Linux image installation.

To install all required packages, run these commands:

cumulus@switch:~$ sudo -E apt-get update
cumulus@switch:~$ sudo -E apt-get install tacplus-client

Configuring the TACACS+ Client

After installing TACACS+, edit the /etc/tacplus_servers file to add at least one server and one shared secret (key). You can specify the server and secret parameters in any order anywhere in the file. Whitespace (spaces or tabs) are not allowed. For example, if your TACACS+ server IP address is 192.168.0.30 and your shared secret is tacacskey, add these parameters to the /etc/tacplus_servers file:

secret=tacacskey
server=192.168.0.30

Cumulus Linux supports a maximum of seven TACACS+ servers. Connections are made in the order in which they are listed in this file. In most cases, you do not need to change any other parameters. You can add  parameters used by any of the packages to this file, which affects all the TACACS+ client software. You can also configure some of the packages through individual configuration files. For example, the timeout value (see description below) is set to 5 seconds by default for NSS lookups in /etc/tacplus_nss.conf, while other packages use a value of 10 seconds in /etc/tacplus_servers.

When you add or remove TACACS+ servers, you must restart auditd  (with the  systemctl restart auditd command) or you must send a signal (with killall -HUP audisp-tacplus) before audisp-tacplus  rereads the configuration to see the changed server list.

This is the complete list of the TACACS+ client configuration files, and their use. The full list of TACACS+ parameters is shown in TACACS Parameters below.

FilenameDescription
/etc/tacplus_serversThis is the primary file that requires configuration after installation. The file is used by all packages with include=/etc/tacplus_servers parameters in the other configuration files that are installed. Typically, this file contains the shared secrets; make sure that the Linux file mode is 600.
/etc/nsswitch.confWhen the libnss_tacplus package is installed, this file is configured to enable tacplus lookups via libnss_tacplus.  If you replace this file by automation or other means, you need to add tacplus as the first lookup method for the passwd  database line.
/etc/tacplus_nss.confThis file sets the basic parameters for libnss_tacplus. It includes a debug variable for debugging NSS lookups separately from other client packages.
/usr/share/pam-configs/tacplusThis is the configuration file for pam-auth-update to generate the files in the next row. These configurations are used at login, by su, and by ssh.
/etc/pam.d/common-*The /etc/pam.d/common-* files are updated for tacplus authentication. The files are updated with pam-auth-update, when libpam-tacplus is installed or removed.
/etc/sudoers.d/tacplusThis file allows TACACS+ privilege level 15 users to run commands with sudo. The file includes an example (commented out) of how to enable privilege level 15 TACACS users to use sudo without having to enter a password and provides an example of how to enable all TACACS users to run specific commands with sudo. Only edit this wile with the command visudo -f /etc/sudoers.d/tacplus.
 audisp-tacplus.confThis is the audisp plugin configuration file. Typically, no modifications are required.
/etc/audisp/audisp-tac_plus.confThis is the TACACS+ server configuration file for accounting. Typically, no modifications are required. You can use this configuration file when you only want to debug TACACS+ accounting issues, not all TACACS+ users.
/etc/audit/rules.d/audisp-tacplus.rulesThe auditd rules for TACACS+ accounting.  The augenrules command uses all rule files to generate the rules file (described below).
/etc/audit/audit.rulesThis is the audit rules file generated when auditd is installed.

You can edit the /etc/pam.d/common-* files manually. However, if you run pam-auth-update again after making the changes, the update fails. Only perform configuration in /usr/share/pam-configs/tacplus, then run pam-auth-update.

TACACS+ Authentication (login)

The initial authentication configuration is done through the PAM modules and an updated version of the libpam-tacplus package. When the package is installed, the PAM configuration is updated in /etc/pam.d with the pam-auth-update command. If you have made changes to your PAM configuration, you need to integrate these changes yourself. If you are also using LDAP with the libpam-ldap package, you might need to edit the PAM configuration to ensure the LDAP and TACACS ordering that you prefer. The libpam-tacplus are configured to skip over rules and the values in the success=2 might require adjustments to skip over LDAP rules.

A user privilege level is determined by the TACACS+ privilege attribute priv_lvl for the user that is returned by the TACACS+ server during the user authorization exchange. The client accepts the attribute in either the mandatory or optional forms and also accepts priv-lvl as the attribute name. The attribute value must be a numeric string in the range 0 to 15, with 15 the most privileged level.

By default, TACACS+ users at privilege levels other than 15 are not allowed to run sudo commands and are limited to commands that can be run with standard Linux user permissions.

TACACS+ Accounting

TACACS+ accounting is implemented with the audisp module, with an additional plugin for auditd/audisp. The plugin maps the auid in the accounting record to a TACACS login, based on the auid and sessionid. The audisp module requires libnss_tacplus and uses the libtacplus_map.so library interfaces as part of the modified lipam_tacplus package.

Communication with the TACACS+ servers is done with the libsimple-tacact1 library, through dlopen(). A maximum of 240 bytes of command name and arguments are sent in the accounting record, due to the TACACS+ field length limitation of 255 bytes.

All Linux commands result in an accounting record, including commands run as part of the login process or as a sub-processes of other commands. This can sometimes generate a large number of accounting records.

Configure the IP address and encryption key of the server  in the /etc/tacplus_servers file. Minimal configuration to auditd and audisp is necessary to enable the audit records necessary for accounting. These records are installed as part of the package.

audisp-tacplus installs the audit rules for command accounting. Modifying the configuration files is not usually necessary. However, when a management VRF is configured, the accounting configuration does need special modification because the auditd service starts prior to networking. It is necessary to add the vrf parameter and to signal the audisp-tacplus process to reread the configuration. The example below shows that the management VRF is named mgmt. You can place the vrf parameter in either the /etc/tacplus_servers file or in the /etc/audisp/audisp-tac_plus.conf file.

vrf=mgmt

After editing the configuration file, send the HUP signal killall -HUP audisp-tacplus to notify the accounting process to reread the file.

All sudo commands run by TACACS+ users generate accounting records against the original TACACS+ login name.

For more information, refer to the audisp.8 and auditd.8 man pages.

Configuring NCLU for TACACS+ Users

When you install or upgrade TACACS+ packages, mapped user accounts are created automatically. All tacacs0 through tacacs15 users are added to the netshow group.

In order for any TACACS+ users to execute net add, net del, and net commit commands and to restart services with NCLU, you need to add those users to the users_with_edit variable in the /etc/netd.conf file. Cumulus Networks recommends you add the tacacs15 user and, depending upon your policies, other users (tacacs1 through tacacs14) to this variable. 

To give a TACACS+ user access to the show commands, add the tacacs group to the groups_with_show variable.

Do not add the tacacs group to the groups_with_edit variable; this is dangerous and can potentially enable any user to log into the switch as the root user.

To add the users, edit the /etc/netd.conf file:

cumulus@switch:~$ sudo nano /etc/netd.conf
  
...
  
# Control which users/groups are allowed to run "add", "del",
# "clear", "abort", and "commit" commands.
users_with_edit = root, cumulus, tacacs15
groups_with_edit = netedit
  
# Control which users/groups are allowed to run "show" commands
users_with_show = root, cumulus
groups_with_show = netshow, netedit, tacacs
  
...

After you save and exit the netd.conf file, restart the netd service. Run:

cumulus@switch:~$ sudo systemctl restart netd

TACACS+ Per-command Authorization

The tacplus-auth command handles the per-command authorization. To make this an enforced authorization, you must change the TACACS+ login to use a restricted shell, with a very limited executable search path. Otherwise, the user can bypass the authorization. The tacplus-restrict utility simplifies the setup of the restricted environment. The example below initializes the environment for the tacacs0 user account. This is the account used for TACACS+ users at privilege level 0

tacuser0@switch:~$ sudo tacplus-restrict -i -u tacacs0 -a command1 command2 ... commandN

If the user/command combination is not authorized by the TACACS+ server, a message similar to the following displays:

tacuser0@switch:~$ net show version
net not authorized by TACACS+ with given arguments, not executing

Command Options

OptionDescription
-iInitializes the environment. You only need to issue this option once per username.
-aYou can invoke the utility with the -a option as many times as desired. For each command in the -a list, a symbolic link is created from tacplus-auth to the relative portion of the command name in the local bin subdirectory. You also need to enable these commands on the TACACS+ server (refer to the TACACS+ server documentation). It is common to have the server allow some options to a command, but not others.
-f

Re-initializes the environment. If you need to restart, issue the -f option with -i to force the re-initialization; otherwise, repeated use of -i is ignored.

As part of the initialization:

  • The user's shell is changed to /bin/rbash.
  • Any existing dot files are saved.
  • A limited environment is set up that does not allow general command execution, but instead allows only commands from the user's local bin subdirectory.

For example, if you want to allow the user to be able to run the net and ip commands (if authorized by the TACACS+ server), use the command:

cumulus@switch:~$ sudo tacplus-restrict -i -u tacacs0 -a ip net

After running this command, examine the tacacs0 directory::

cumulus@switch:~$ sudo ls -lR ~tacacs0
total 12
lrwxrwxrwx 1 root root 22 Nov 21 22:07 ip -> /usr/sbin/tacplus-auth
lrwxrwxrwx 1 root root 22 Nov 21 22:07 net -> /usr/sbin/tacplus-auth

Other than shell built-ins, the only two commands the privilege level 0 TACACS users can run are the ip and net commands.

If you mistakenly add potential commands with the -a option, you can remove them. The example below shows how to remove the net command:

cumulus@switch:~$ sudo rm ~tacacs0/bin/net

You can remove all commands as follows:

cumulus@switch:~$ sudo rm ~tacacs0/bin/*

Use the man command on the switch for more information on tacplus-auth and tacplus-restrict.

cumulus@switch:~$ man tacplus-auth tacplus-restrict

NSS Plugin

When used with pam_tacplus, TACACS+ authenticated users can log in without a local account on the system using the NSS plugin that comes with the tacplus_nss package. The plugin uses the mapped tacplus information if the user is not found in the local password file, provides the getpwnam() and getpwuid()entry point,s and uses the TACACS+ authentication functions.

The plugin asks the TACACS+ server if the user is known, and then for relevant attributes to determine the privilege level of the user. When the libnss_tacplus package is installed, nsswitch.conf is modified to set tacplus as the first lookup method for passwd. If the order is changed, lookups return the local accounts, such as tacacs0

If the user is not found, a mapped lookup is performed using the libtacplus_map.so exported functions. The privilege level is appended to tacacs and the lookup searches for the name in the local password file. For example, privilege level 15 searches for the tacacs15 user. If the user is found, the password structure is filled in with information for the user.

If the user is not found, the privilege level is decremented and checked again until privilege level 0 (user tacacs0) is reached. This allows use of only the two local users tacacs0 and tacacs15, if minimal configuration is desired.

TACACS Configuration Parameters

The recognized configuration options are the same as the libpam_tacplus command line arguments; however, not all pam_tacplus options are supported. These configuration parameters are documented in the tacplus_servers.5 man page, which is part of the libpam-tacplus package.

The table below describes the configuration options available:

Configuration OptionDescription
debug

The output debugging information through syslog(3).

Debugging is heavy, including passwords. Do not leave debugging enabled on a production switch after you have completed troubleshooting.

secret=STRINGThe secret key used to encrypt and decrypt packets sent to and received from the server. You can specify the secret key more than once in any order with respect to the server= parameter.  When fewer secret= parameters are specified, the last secret given is used for the remaining servers. Only use this parameter in files such as /etc/tacplus_servers that are not world readable.

server=HOSTNAME

server=IP_ADDR

Adds a TACACS+ server to the servers list. Servers are queried in turn until a match is found, or no servers remain in the list. Can be specified up to 7 times. An IP address can be optionally followed by a port number, preceded by a ":".  The default port is 49.

When sending accounting records, the record is sent to all servers in the list if acct_all=1, which is the default.

timeout=SECONDSTACACS+ server(s) communication timeout. The default value is 5 seconds.
login=STRING

TACACS+ authentication service (pap, chap, or login). The default value is pap.

user_homedir=1

This is not enabled by default. When enabled, a separate home directory for each TACACS+ user is created when the TACACS+ user first logs in. By default, the home directory in the mapping accounts in /etc/passwd (/home/tacacs0 ... /home/tacacs15) is used. If the home directory does not exist, it is created with the mkhomedir_helper program, in the same manner as pam_mkhomedir.

This option is not honored for accounts with restricted shells when per-command authorization is enabled.

acct_all=1

Configuration option for audisp_tacplus and pam_tacplus sending accounting records to all supplied servers (1), or the first server to respond (0). The default value is 1.

timeout=SECSSets the timeout in seconds for connections to each TACACS+ server.   The default is 10 seconds for all lookups except that NSS lookups use a 5 second timeout.
vrf=VRFNAME

If the management network is in a VRF, set this variable to the VRF name. This  would  usually  be  "mgmt".  When this variable is set, the connection to the TACACS+ accounting servers is made through the named VRF.

service

TACACS+ accounting and authorization service. Examples include shell, pap, raccess, ppp, and slip.

The default value is shell.

protocol

TACACS+ protocol field. This option is use dependent.

PAM uses the SSH protocol.

Removing the TACACS+ Client Packages

To remove all of the TACACS+ client packages, use the following commands:

cumulus@switch:~$ sudo -E apt-get remove tacplus-client
cumulus@switch:~$ sudo -E apt-get autoremove

To remove the TACACS+ client configuration files as well as the packages (recommended), use this command:

cumulus@switch:~$ sudo -E apt-get autoremove --purge

Troubleshooting TACACS+

Debugging Basic Server Connectivity or NSS Issues

You can use the getent command to determine if TACACS+ is configured correctly and if the local password is stored in the configuration files. In the example commands below, the cumulus user represents the local user, while cumulusTAC represents the TACACS user.

To look up the username within all NSS methods:

cumulus@switch:~$ sudo getent passwd cumulusTAC
cumulusTAC:x:1016:1001:TACACS+ mapped user at privilege level 15,,,:/home/tacacs15:/bin/bash

To look up the user within the local database only:

cumulus@switch:~$ sudo getent -s compat passwd cumulus
cumulus:x:1000:1000:cumulus,,,:/home/cumulus:/bin/bash

To look up the user within the TACACS+ database only:

cumulus@switch:~$ sudo getent -s tacplus passwd cumulusTAC
cumulusTAC:x:1016:1001:TACACS+ mapped user at privilege level 15,,,:/home/tacacs15:/bin/bash

If TACACS does not appear to be working correctly, debug the following configuration files by adding the debug=1 parameter to one or more of these files:

  • /etc/tacplus_servers

  • /etc/tacplus_nss.conf

You can also add debug=1 to individual pam_tacplus lines in /etc/pam.d/common*.

All log messages are stored in /var/log/syslog.

Incorrect Shared Key

The TACACS client on the switch and the TACACS server should have the same shared secret key. If this key is incorrect, the following message is printed to syslog:

2017-09-05T19:57:00.356520+00:00 leaf01 sshd[3176]: nss_tacplus: TACACS+ server 192.168.0.254:49 read failed with protocol error (incorrect shared secret?) user cumulus 

Debugging Issues with Per-command Authorization

To debug TACACS user command authorization, have the TACACS+ user enter the following command at a shell prompt, then try the command again:

tacuser0@switch:~$ export TACACSAUTHDEBUG=1 

When this debugging is enabled, additional information is shown for the command authorization conversation with the TACACS+ server:

tacuser0@switch:~$ net pending
tacplus-auth: found matching command (/usr/bin/net) request authorization
tacplus-auth: error connecting to 10.0.3.195:49 to request authorization for net: Transport endpoint is not connected
tacplus-auth: cmd not authorized (16)
tacplus-auth: net not authorized from 192.168.3.189:49
net not authorized by TACACS+ with given arguments, not executing

tacuser0@switch:~$ net show version
tacplus-auth: found matching command (/usr/bin/net) request authorization
tacplus-auth: error connecting to 10.0.3.195:49 to request authorization for net: Transport endpoint is not connected
tacplus-auth: 192.168.3.189:49 authorized command net
tacplus-auth: net authorized, executing
DISTRIB_ID="Cumulus Linux"
DISTRIB_RELEASE=3.4.0
DISTRIB_DESCRIPTION="Cumulus Linux 3.4.0"

To disable debugging:

tacuser0@switch:~$ export -n TACACSAUTHDEBUG

Debug Issues with Accounting Records

If you have added or deleted TACACS+ servers from the configuration files, make sure you notify the audisp plugin with this command:

cumulus@switch:~$ sudo killall -HUP audisp-tacplus

If accounting records are still not being sent, add debug=1 to the /etc/audisp/audisp-tac_plus.conf file, then issue the command above to notify the plugin. Ask the TACACS+ user to run a command and examine the end of /var/log/syslog for messages from the plugin. You can also check the auditing log file /var/log/audit/audit.log to be sure the auditing records are being written.  If they are not, restart the audit daemon with:

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

TACACS Component Software Descriptions

The following table describes the different pieces of software involved with delivering TACACS.

Package NameDescription
audisp-tacplus_1.0.0-1-cl3u3This package uses auditing data from auditd to send accounting records to the TACACS+ server and is started as part of auditd.
libtac2_1.4.0-cl3u2Basic TACACS+ server utility and communications routines.
libnss-tacplus_1.0.1-cl3u3Provides an interface between libc username lookups, the mapping functions, and the TACACS+ server.
tacplus-auth-1.0.0-cl3u1This package includes the tacplus-restrict setup utility, which enables you to perform per-command TACACS+ authorization. Per-command authorization is not done by default.
libpam-tacplus_1.4.0-1-cl3u2A modified version of the standard Debian package.
libtacplus-map1_1.0.0-cl3u2The mapping functionality between local and TACACS+ users on the server. Sets the immutable sessionid and auditing UID to ensure the original user can be tracked through multiple processes and privilege changes. Sets the auditing loginuid as immutable if supported. Creates and maintains a status database in /run/tacacs_client_map to manage and lookup mappings.
libsimple-tacacct1_1.0.0-cl3u2Provides an interface for programs to send accounting records to the TACACS+ server. Used by audisp-tacplus.
libtac2-bin_1.4.0-cl3u2

 

Provides the tacc testing program and TACACS+ man page.

Limitations

TACACS+ Client Is only Supported through the Management Interface

The TACACS+ client is only supported through the management interface on the switch: eth0, eth1, or the VRF management interface. The TACACS+ client is not supported through bonds, switch virtual interfaces (SVIs), or switch port interfaces (swp).

Multiple TACACS+ Users

If two or more TACACS+ users are logged in simultaneously with the same privilege level, while the accounting records are maintained correctly, a lookup on either name will match both users, while a UID lookup will only return the user that logged in first.

This means that any processes run by either user will be attributed to both, and all files created by either user will be attributed to the first name matched. This is similar to adding two local users to the password file with the same UID and GID, and is an inherent limitation of using the UID for the base user from the password file.

The current algorithm returns the first name matching the UID from the mapping file; this can be the first or the second user that logged in.

To work around this issue, you can use the switch audit log or the TACACS server accounting logs to determine which processes and files are created by each user.

  • For commands that do not execute other commands (for example, changes to configurations in an editor, or actions with tools like clagctl and vtysh), no additional accounting is done.

  • Per-command authorization is implemented at the most basic level (commands are permitted or denied based on the standard Linux user permissions for the local TACACS users and only privilege level 15 users can run sudo commands by default).

The Linux auditd system does not always generate audit events for processes when terminated with a signal (with the kill system call or internal errors such as SIGSEGV). As a result, processes that exit on a signal that is not caught and handled, might not generate a STOP accounting record.

Issues with deluser Command

TACACS+ and other non-local users that run the deluser command with the --remove-home option will see an error about not finding the user in /etc/passwd:

tacuser0@switch: deluser --remove-home USERNAME
userdel: cannot remove entry ‘USERNAME’ from /etc/passwd
/usr/sbin/deluser: `/usr/sbin/userdel USERNAME' returned error code 1. Exiting

However, the command does remove the home directory. The user can still log in on that account, but will not have a valid home directory. This is a known upstream issue with the deluser command for all non-local users.

Only use the --remove-home option when the user_homedir=1 configuration command is in use.