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 go to all configured TACACS+ servers by default. Using per-command authorization requires additional setup on the switch.
- Authentication using PAM; includes
- 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/tacplusfile that the
- Up to seven TACACS+ servers
Install the TACACS+ Client Packages
You can install the TACACS+ packages even if the switch is not connected to the internet; the packages are in the
cumulus-local-apt-archive repository in the Cumulus Linux image.
To install all required packages, run these commands:
cumulus@switch:~$ sudo -E apt-get update cumulus@switch:~$ sudo -E apt-get install tacplus-client
Configure 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
Cumulus Linux supports a maximum of seven TACACS+ servers. To specify multiple servers, add one per line to the
Connections establish in the order that this file lists. In most cases, you do not need to change any other parameters. You can add parameters the packages use to this file, which affects all the TACACS+ client software. For example, the timeout value for NSS lookups is 5 seconds by default in the
/etc/tacplus_nss.conf file, whereas the timeout value for other packages is 10 seconds in the
/etc/tacplus_servers file. The timeout value applies to each connection to the TACACS+ servers. (If you configure authorization per command, the timeout occurs for each command.) There are several connections to the server per login attempt from PAM, as well as two or more through NSS. Therefore, with the default timeout values, a TACACS+ server that is not reachable can delay logins by a minute or more for each unreachable server. If you must list unreachable TACACS+ servers, place them at the end of the server list and consider reducing the timeout values.
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.
You can also configure the IP address used as the source IP address when communicating with the TACACS+ server. See TACACS Configuration Parameters below for the full list of TACACS+ parameters.
Following is the complete list of the TACACS+ client configuration files, and their use.
|The primary file that requires configuration after installation. All packages with |
|When the |
|Sets the basic parameters for |
|The configuration file for |
|Allows TACACS+ privilege level 15 users to run commands with |
|The TACACS+ server configuration file for accounting. You do not need to modify this file. You can use this configuration file when you only want to debug TACACS+ accounting issues, not all TACACS+ users.|
|The audit rules file that generate when you install |
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 configure
/usr/share/pam-configs/tacplus, then run
TACACS+ Authentication (login)
PAM modules and an updated version of the
libpam-tacplus package configure authentication initially. When you install the package, the
pam-auth-update command updates the PAM configuration in
/etc/pam.d. If you make changes to your PAM configuration, you need to integrate these changes. If you also use LDAP with the
libpam-ldap package, you need to edit the PAM configuration with the LDAP and TACACS ordering you prefer. The
libpam-tacplus package ignore rules and the values in
success=2 require adjustments to ignore LDAP rules.
The TACACS+ privilege attribute
priv_lvl determines the privilege level for the user that the TACACS+ server returns 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 cannot run
sudo commands and can only run commands with standard Linux user permissions.
TACACS+ Client Sequencing
Due to SSH and login processing mechanisms, Cumulus Linux needs to know the following at the beginning of the AAA sequence:
- Whether the user is a valid TACACS+ user
- The user privilege level
For non-local users (users not in the local password file) you need to send a TACACS+ authorization request as the first communication with the TACACS+ server, before authentication and before the user logging in requests a password.
You need to configure certain TACACS+ servers to allow authorization requests before authentication. Contact your TACACS+ server vendor for information.
Local Fallback Authentication
If a site wants to allow local fallback authentication for a user when none of the TACACS servers are reachable, you can add a privileged user account as a local account on the switch.
To configure local fallback authentication:
/etc/nsswitch.conffile to remove the keyword
tacplusfrom the line starting with
passwd. (You need to add the keyword back in step 3.)
The following example shows the
/etc/nsswitch.conffile with no
tacpluskeyword in the line starting with
cumulus@switch:~$ sudo vi /etc/nsswitch.conf # # Example configuration of GNU Name Service Switch functionality. # If you have the `glibc-doc-reference' and `info' packages installed, try: # `info libc "Name Service Switch"' for information about this file. passwd: files group: tacplus files shadow: files gshadow: files ...
To enable the local privileged user to run
sudoand NVUE commands, run the
addusercommands shown below. In the example commands, the TACACS account name is tacadmin.
addusercommand prompts for information and a password. You can skip most of the requested information by pressing ENTER.
cumulus@switch:~$ sudo adduser --ingroup tacacs tacadmin cumulus@switch:~$ sudo adduser tacadmin nvset cumulus@switch:~$ sudo adduser tacadmin nvapply cumulus@switch:~$ sudo adduser tacadmin sudo
/etc/nsswitch.conffile to add the keyword
tacplusback to the line starting with
passwd(the keyword you removed in the first step).
nvuedservice with the following command:
cumulus@switch:~$ sudo systemctl restart nvued
TACACS+ accounting uses the
audisp module, with an additional plugin for
audisp. The plugin maps the auid in the accounting record to a TACACS login, which it bases on the
audisp module requires
libnss_tacplus and uses the
libtacplus_map.so library interfaces as part of the modified
Communication with the TACACS+ servers occurs with the
libsimple-tacact1 library, through
dlopen(). A maximum of 240 bytes of command name and arguments send in the accounting record, due to the TACACS+ field length limitation of 255 bytes.
All Linux commands result in an accounting record, including login commands and sub-processes of other commands. This can generate a lot of accounting records.
Configure the IP address and encryption key of the server in the
/etc/tacplus_servers file. Minimal configuration to
audisp is necessary to enable the audit records needed for accounting. These records install as part of the package.
audisp-tacplus installs the audit rules for command accounting. When you configure a management VRF, you must add the vrf parameter and signal the
audisp-tacplus process to reread the configuration. The example below shows that the management VRF name is mgmt. You can add the vrf parameter to either the
/etc/tacplus_servers file or the
After editing the configuration file, send the HUP signal
killall -HUP audisp-tacplus to notify the accounting process to reread the file.
sudo commands run by TACACS+ users generate accounting records against the original TACACS+ login name.
For more information, refer to the
auditd.8 man pages.
TACACS+ Per-command Authorization
tacplus-auth command handles authorization for each command. To make this an enforced authorization, 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 setting up the restricted environment. The example below initializes the environment for the tacacs0 user account. This is the account for TACACS+ users at privilege level
tacuser0@switch:~$ sudo tacplus-restrict -i -u tacacs0 -a command1 command2 command3
The following table provides the command options:
|Initializes the environment. You only need to issue this option one time per username.|
|You can invoke the utility with the |
|Re-initializes the environment. If you need to restart, issue the |
As part of the initialization:
- The user shell changes to
- The utility saves any existing dot files.
For example, if you want to allow the user to be able to run the
ip commands (if authorized by the TACACS+ server):
cumulus@switch:~$ sudo tacplus-restrict -i -u tacacs0 -a ip nv
After running this command, examine the
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 nv -> /usr/sbin/tacplus-auth
Other than shell built-ins, privilege level 0 TACACS users can only run the
If add commands with the
-a option by mistake, you can remove them. The example below removes the
cumulus@switch:~$ sudo rm ~tacacs0/bin/nv
You can remove all commands:
cumulus@switch:~$ sudo rm ~tacacs0/bin/*
For more information on
tacplus-restrict, run the
cumulus@switch:~$ man tacplus-auth tacplus-restrict
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 in the local password file, provides the
getpwuid()entry points, and uses the TACACS+ authentication functions.
The plugin asks the TACACS+ server if it knows the user, and then for relevant attributes to determine the privilege level of the user. When you install the
nsswitch.conf changes to set
tacplus as the first lookup method for
passwd. If you change the order, lookups return the local accounts, such as
If TACACS+ server does not find the user, it uses the
libtacplus.so exported functions to do a mapped lookup. The privilege level appends 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 TACACS+ server finds the user, it adds information for the user in the password structure.
If the TACACS+ server does not find the user, it decrements the privilege level and checks again until it reaches privilege level 0 (user
tacacs0). This allows you to use only the two local users
tacacs15, for minimal configuration.
TACACS Configuration Parameters
The recognized configuration options are the same as the
libpam_tacplus command line arguments; however, Cumulus Linux does not support all
pam_tacplus options. For a description of the configuration parameters, refer to the
tacplus_servers.5 man page, which is part of the
The table below describes the configuration options available:
|debug||The output debugging information through syslog(3).|
Note: Debugging is heavy, including passwords. Do not leave debugging enabled on a production switch after you have completed troubleshooting.
|secret=STRING||The secret key to encrypt and decrypt packets sent to and received from the server.|
You can specify the secret key more than one time in any order.
Only use this parameter in files such as /etc/tacplus_servers that are not world readable.
|Adds a TACACS+ server to the servers list. Cumulus Linux queries servers in turn until it finds a match or no servers remain in the list. You can provide an IP address with a port number, preceded by a colon (:). The default port is 49.|
Note: When sending accounting records, the record sends to all servers in the list if
|source_ip=ipv4-address||Sets the IP address as the source IP address when communicating with the TACACS+ server. You must specify an IPv4 address. You cannot use IPv6 addresses and hostnames. The address must be valid for the interface you use.|
|timeout=seconds||TACACS+ servers communication timeout.|
This parameter defaults to 10 seconds in the /etc/tacplus_servers file, but defaults to 5 seconds in the /etc/tacplus_nss.conf file.
|include=/file/name||A supplemental configuration file to avoid duplicating configuration information. You can include up to 8 more configuration files.|
|min_uid=value||The minimum user ID that the NSS plugin looks up. 0 specifies that the plugin never looks up uid 0 (root). Do not specify a value greater than the local TACACS+ user IDs (0 through 15).|
|exclude_users=user1,user2,||A comma-separated list of usernames in the |
Note: Do not remove the cumulus user from the
|login=string||TACACS+ authentication service (pap, chap, or login).|
The default value is pap.
|user_homedir=1||This option is off by default. When you enable this option, Cumulus Linux creates a separate home directory for each TACACS+ user when the TACACS+ user first logs in. By default, the switch uses the home directory in the mapping accounts in |
This option does not apply for accounts with restricted shells when you enable per-command authorization.
|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=seconds||Sets the timeout in seconds for connections to each TACACS+ server.|
The default is 10 seconds for all lookups. NSS lookups use a 5 second timeout.
|vrf=vrf-name||If the management network is in a VRF, set this variable to the VRF name. This is typically |
|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 user dependent. PAM uses the SSH protocol.|
Remove the TACACS+ Client Packages
To remove all 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
Basic Server Connectivity or NSS Issues
You can use the
getent command to determine if you configured TACACS+ correctly and if the local password is 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 is not working correctly, debug the following configuration files by adding the debug=1 parameter to one or more of these files:
You can also add
debug=1 to individual pam_tacplus lines in
All log messages are in
Incorrect Shared Key
The TACACS client on the switch and the TACACS server must have the same shared secret key. If this key is incorrect, the following message prints to
2017-09-05T19:57:00.356520+00:00 leaf01 sshd: nss_tacplus: TACACS+ server 192.168.0.254:49 read failed with protocol error (incorrect shared secret?) user cumulus
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 you enable debugging, the command authorization conversation with the TACACS+ server shows additional information.
To disable debugging:
tacuser0@switch:~$ export -n TACACSAUTHDEBUG
Debug Issues with Accounting Records
If you add or delete 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 do not send, add debug=1 to the
/etc/audisp/audisp-tac_plus.conf file, then run 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 exist. If the auditing records do not exist, restart the audit daemon with:
cumulus@switch:~$ sudo systemctl restart auditd.service
TACACS Component Software Descriptions
Cumulus Linux uses the following packages for TACACS.
|audisp-tacplus_1.0.0-1-cl3u3||This package uses auditing data from |
|libtac2_1.4.0-cl3u2||Basic TACACS+ server utility and communications routines.|
|libnss-tacplus_1.0.1-cl3u3||Provides an interface between |
|tacplus-auth-1.0.0-cl3u1||Includes the |
|libpam-tacplus_1.4.0-1-cl3u2||A modified version of the standard Debian package.|
|libtacplus-map1_1.0.0-cl3u2||The mapping functionality between local and TACACS+ users on the server. Sets the immutable |
|libsimple-tacacct1_1.0.0-cl3u2||Provides an interface for programs to send accounting records to the TACACS+ server. |
|libtac2-bin_1.4.0-cl3u2||Provides the |
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 log in simultaneously with the same privilege level, while the accounting records are correct, a lookup on either name matches both users, while a UID lookup only returns the user that logs in first.
This means that any processes that either user runs apply to both, and all files either user creates apply 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; either the first or the second user that logs 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 each user creates.
- For commands that do not execute other commands (for example, changes to configurations in an editor or actions with tools like
clagctland vtysh), there is no additional accounting.
- Per-command authorization is at the most basic level (Cumulus Linux uses standard Linux user permissions for the local TACACS users and only privilege level 15 users can run
sudocommands by default).
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 you do not handle, 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 see the following error:
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
The command does remove the home directory. The user can still log in on that account but does 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 with the
user_homedir=1 configuration command.
Both TACACS+ and RADIUS AAA Clients
When you install both the TACACS+ and the RADIUS AAA client, Cumulus Linux does not attempt RADIUS login. As a workaround, do not install both the TACACS+ and the RADIUS AAA client on the same switch.