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 configured TACACS+ servers by default. Use of 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 is installed by 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, as they are contained in the
cumulus-local-apt-archive repository that is embedded in the Cumulus Linux disk 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 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. For example, the timeout value for NSS lookups (see description below) is set to 5 seconds by default in the
/etc/tacplus_nss.conf file, whereas the timeout value for other packages is 10 seconds and is set in the
/etc/tacplus_servers file. The timeout value is per connection to the TACACS+ servers. (If authorization is configured per command, the timeout occurs for each command.) There are several (typically four) 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 per 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.
|This is the primary file that requires configuration after installation. The file is used by all packages with |
|When the |
|This file sets the basic parameters for |
|This is the configuration file for |
|This file allows TACACS+ privilege level 15 users to run commands with |
|This is the |
|This 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.|
|This is the audit rules file generated when |
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
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+ Client Sequencing
Due to SSH and login processing mechanisms, Cumulus Linux needs to know the following very early in the AAA sequence:
- Whether the user is a valid TACACS+ user
- The user’s privilege level
The only way to do this for non-local users — that is, users not present in the local password file — is to send a TACACS+ authorization request as the first communication with the TACACS+ server, prior to the authentication and before a password is requested from the user logging in.
Some TACACS+ servers need special configuration to allow authorization requests prior to authentication. Contact your TACACS+ server vendor for the proper configuration if your TACACS+ server does not allow the initial authorization request.
Local Fallback Authentication
If a site wants to allow local fallback authentication for a user when none of the TACACS servers can be reached 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.)
An example of the
/etc/nsswitch.conffile with the keyword
tacplusremoved from the line starting with
passwdis shown below.
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 NCLU 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 netedit 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).
netdservice with the following command:
cumulus@switch:~$ sudo systemctl restart netd
TACACS+ accounting is implemented with the
audisp module, with an additional plugin for
audisp. The plugin maps the auid in the accounting record to a TACACS login, based on 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 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
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
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.
Configure 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.
For any TACACS+ users to execute
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. 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
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
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
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
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
The following table provides the command options:
|Initializes the environment. You only need to issue this option once per username.|
|You 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.|
|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
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
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
If you mistakenly add potential commands with the
-a option, you can remove them. The example below shows how to remove the
cumulus@switch:~$ sudo rm ~tacacs0/bin/net
You can remove all commands as follows:
cumulus@switch:~$ sudo rm ~tacacs0/bin/*
man command on the switch for more information on
cumulus@switch:~$ man tacplus-auth tacplus-restrict
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
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
If the user is not found, a mapped lookup is performed using the
libtacplus.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
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
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 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.
|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.|
Note: When sending accounting records, the record is sent to all servers in the list if acct_all=1, which is the default.
|source_ip=ipv4-address||Sets the IP address used as the source IP address when communicating with the TACACS+ server. You must specify an IPv4 address. IPv6 addresses and hostnames are not supported. The address must must be valid for the interface being used.|
|timeout=seconds||TACACS+ server(s) 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. Setting it to 0 means uid 0 (root) is never looked up, which is desirable for performance reasons. The value should not be greater than the local TACACS+ user IDs (0 through 15), to ensure they can be looked up.|
|exclude_users=user1,user2,…||A comma-separated list of usernames that are never looked up by the NSS plugin, set in the tacplus_nss.conf file. You cannot use * (asterisk) as a wild card in the list. While it’s not a legal username, bash may lookup this as a user name during pathname completion, so it is included in this list as a username string.|
Note: Do not remove the cumulus user from the exclude_users list; doing so can make it impossible to log in as the cumulus user, which is the primary administrative account in Cumulus Linux. If you do remove the cumulus user, add some other local fallback user that does not rely on TACACS but is a member of sudo and netedit groups, so that these accounts can run sudo and NCLU commands.
|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 way 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=seconds||Sets 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=vrf-name||If the management network is in a VRF, set this variable to the VRF name. This is typically 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.|
Remove 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
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:
You can also add
debug=1 to individual pam_tacplus lines in
All log messages are stored in
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
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 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=4.1.0 DISTRIB_DESCRIPTION="Cumulus Linux 4.1.0"
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.
|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||This package 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. Used by |
|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 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
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
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 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
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.
When Both TACACS+ and RADIUS AAA Clients are Installed
When you have both the TACACS+ and the RADIUS AAA client installed, RADIUS login is not attempted. As a workaround, do not install both the TACACS+ and the RADIUS AAA client on the same switch.