TACACS

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.

Supported Features

• Authentication using PAM; includes login, ssh, sudo 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 the libtacplus-map1 package installs
• 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

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

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. To specify multiple servers, add one per line to the /etc/tacplus_servers file.

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.

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.

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

You can configure the switch to allow local fallback authentication for a user when the TACACS servers are unreachable, do not include the user for authentication, or have the user in the exclude user list.

To allow local fallback authentication for a user, add a local privileged user account on the switch with the same username as a TACACS user. A local user is always active even when the TACACS service is not running.

To configure local fallback authentication:

1. Edit the /etc/nsswitch.conf file to remove the keyword tacplus from the line starting with passwd. (You need to add the keyword back in step 3.)

   The following example shows the /etc/nsswitch.conf file with no tacplus keyword in the line starting with passwd.

   cumulus@switch:~$ sudo nano /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
   ...
   

2. To enable the local privileged user to run sudo and NVUE commands, run the adduser commands shown below. In the example commands, the TACACS account name is tacadmin.

   The first adduser command 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
   

3. Edit the /etc/nsswitch.conf file to add the keyword tacplus back to the line starting with passwd (the keyword you removed in the first step).

    cumulus@switch:~$ sudo nano /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:         tacplus files
    group:          tacplus files
    shadow:         files
    gshadow:        files
    ...
   

4. Restart the nvued service and the nginx service with the following commands:

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

TACACS+ Accounting

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

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.

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 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 /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.

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

TACACS+ Per-command Authorization

The 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 0.

tacuser0@switch:~$ sudo tacplus-restrict -i -u tacacs0 -a command1 command2 command3

The following table provides the command options:

For example, if you want to allow the user to be able to run the nv and 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 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 nv -> /usr/sbin/tacplus-auth

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

If add commands with the -a option by mistake, you can remove them. The example below removes the nv command:

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

You can remove all commands:

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

For more information on tacplus-auth and tacplus-restrict, run the man command.

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

NSS Plugin

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 in the local password file, provides the getpwnam() and 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 libnss_tacplus package, 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 tacacs0

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 tacacs0 and 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 libpam-tacplus package.

The table below describes the configuration options available:

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

Troubleshooting

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:

• /etc/tacplus_servers
• /etc/tacplus_nss.conf

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

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 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

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.

Considerations

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.

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 clagctl and 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 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 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.