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Cumulus Linux

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TACACS

Cumulus Linux implements TACACS+ client AAA 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.

TACACS+ in Cumulus Linux:

  • Uses PAM authentication and includes login, ssh, sudo and su.
  • Allows users with privilege level 15 to run any command with sudo.
  • Allows users with privilege level 15 to run NVUE nv set, nv unset, and nv apply commands in addition to nv show commands. TACACS+ users with a lower privilege level can only execute nv show commands.
  • Supports up to seven TACACS+ servers. Be sure to configure your TACACS+ servers in addition to the TACACS+ client. Refer to your TACACS+ server documentation.

TACACS+ Client Packages

NVUE automatically installs the TACACS+ packages; you do not have to install the packages if you use NVUE commands to configure TACACS+.

If you use Linux commands to configure TACACS+, you must install the TACACS+ 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

Required TACACS+ Client Configuration

Configure the following required settings on the switch (the TACACS+ client).

  • Set the IP address or hostname of at least one TACACS+ server.
  • Set the secret (key) shared between the TACACS+ server and client.
  • Set the VRF you want to use to communicate with the TACACS+ server. This is typically the management VRF (mgmt), which is the default VRF on the switch.

If you use NVUE commands to configure TACACS+, you must also set the priority for the authentication order for local and TACACS+ users, and enable TACACS+.

After you configure any TACACS+ settings with NVUE and you run nv config apply, you must restart the NVUE service with the sudo systemctl restart nvued.service command.

NVUE commands require you to specify the priority for each TACACS+ server. You must set a priority even if you only specify one server.

The following example commands set:

  • The TACACS+ server priority to 5.
  • The IP address of the server to 192.168.0.30.
  • The secret to mytacac$key.

If you include special characters in the password (such as $), you must enclose the password in single quotes (').

  • The VRF to mgmt.
  • The authentication order so that TACACS+ authentication has priority over local (the lower number has priority).
  • TACACS+ to enabled.
cumulus@switch:~$ nv set system aaa tacacs server 5 host 192.168.0.30
cumulus@switch:~$ nv set system aaa tacacs server 5 secret 'mytacac$key'
cumulus@switch:~$ nv set system aaa tacacs vrf mgmt 
cumulus@switch:~$ nv set system aaa authentication-order 5 tacacs
cumulus@switch:~$ nv set system aaa authentication-order 10 local
cumulus@switch:~$ nv set system aaa tacacs enable on
cumulus@switch:~$ nv config apply

If you want the server to use IPv6, you must add the nv set system aaa tacacs server <priority> prefer-ip-version 6 command:

cumulus@switch:~$ nv set system aaa tacacs server 5 host server5
cumulus@switch:~$ nv set system aaa tacacs server 5 prefer-ip-version 6
...

If you configure more than one TACACS+ server, you need to set the priority for each server. If the switch cannot establish a connection with the server that has the highest priority, it tries to establish a connection with the next highest priority server. The server with the lower number has the higher prioritity. In the example below, server 192.168.0.30 with a priority value of 5 has a higher priority than server 192.168.1.30, which has a priority value of 10.

cumulus@switch:~$ nv set system aaa tacacs server 5 host 192.168.0.30
cumulus@switch:~$ nv set system aaa tacacs server 5 secret 'mytacac$key' 
cumulus@switch:~$ nv set system aaa tacacs server 10 host 192.168.1.30
cumulus@switch:~$ nv set system aaa tacacs server 10 secret 'mytacac$key2'
cumulus@switch:~$ nv config apply

  1. 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:

    cumulus@switch:~$ sudo nano /etc/tacplus_servers
secret=mytacac$key
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 in the file.

    cumulus@switch:~$ sudo nano /etc/tacplus_servers
secret=mytacac$key
server=192.168.0.30
secret=mytacac$key2
server=192.168.1.30

    If you want the server to use IPv6, you must add the prefer_ip_version=6 parameter in the /etc/tacplus_servers file:

    cumulus@switch:~$ sudo nano /etc/tacplus_servers
secret=mytacac$key
server=server5
prefer_ip_version=ipv6 
secret=mytacac$key2
server=server6
prefer_ip_version=ipv6

  2. Uncomment the vrf=mgmt line:

    # 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
# will be made through the named vrf.
vrf=mgmt

  3. Restart auditd:

    cumulus@switch:~$ sudo systemctl restart auditd

Optional TACACS+ Configuration

You can configure the following optional TACACS+ settings:

  • The port to use for communication between the TACACS+ server and client. By default, Cumulus Linux uses IP port 49.
  • The TACACS timeout value, which is the number of seconds to wait for a response from the TACACS+ server before trying the next TACACS+ server. You can specify a value between 0 and 60. The default is 5 seconds.
  • The source IP address to use when communicating with the TACACS+ server so that the server can identify the client switch. You must specify an IPv4 address, which must be valid for the interface you use. This source IP address is typically the loopback address on the switch.
  • The TACACS+ authentication type. You can specify PAP to send clear text between the user and the server, CHAP to establish a PPP connection between the user and the server, or login. The default is PAP.
  • The users you do not want to send to the TACACS+ server for authentication; for example, local user accounts that exist on the switch, such as the cumulus user.
  • 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 /etc/passwd. If the home directory does not exist, the mkhomedir_helper program creates it. This option does not apply to accounts with restricted shells (users mapped to a TACACS privilege level that has enforced per-command authorization).

The following example commands set the timeout to 10 seconds and the TACACS+ server port to 32:

cumulus@switch:~$ nv set system aaa tacacs timeout 10
cumulus@switch:~$ nv set system aaa tacacs server 5 port 32
cumulus@switch:~$ nv config apply

The following example commands set the source IP address to 10.10.10.1 and the authentication type to CHAP:

cumulus@switch:~$ nv set system aaa tacacs source-ip 10.10.10.1
cumulus@switch:~$ nv set system aaa tacacs authentication mode chap
cumulus@switch:~$ nv config apply

The following example commands exclude the user USER1 from going to the TACACS+ server for authentication and enables Cumulus Linux to create a separate home directory for each TACACS+ user when the TACACS+ user first logs in:

cumulus@switch:~$ nv set system aaa tacacs exclude-user USER1
cumulus@switch:~$ nv set system aaa tacacs authentication per-user-homedir on
cumulus@switch:~$ nv config apply
  • To set the server port (use the format server:port), source IP address, authentication type, and enable Cumulus Linux to create a separate home directory for each TACACS+ user, edit the /etc/tacplus_servers file, then restart auditd.
  • To set the timeout and the usernames to exclude from TACACS+ authentication, edit the /etc/tacplus_nss.conf file (you do not need to restart auditd).

The following example sets the server port to 32, the authentication type to CHAP, the source IP address to 10.10.10.1, and enables Cumulus Linux to create a separate home directory for each TACACS+ user when the TACACS+ user first logs in:

cumulus@switch:~$ sudo nano /etc/tacplus_servers
...
secret=mytacac$key
server=192.168.0.30:32
...
# Sets the IPv4 address used as the source IP address when communicating with
# the TACACS+ server.  IPv6 addresses are not supported, nor are hostnames.
# The address must work when passsed to the bind() system call, that is, it must
# be valid for the interface being used.
source_ip=10.10.10.1
...
# If user_homedir=1, then tacacs users will be set to have a home directory
# based on their login name, rather than the mapped tacacsN home directory.
# mkhomedir_helper is used to create the directory if it does not exist (similar
# to use of pam_mkhomedir.so). This flag is ignored for users with restricted
# shells, e.g., users mapped to a tacacs privilege level that has enforced
# per-command authorization (see the tacplus-restrict man page).
user_homedir=1
...
login=chap

cumulus@switch:~$ sudo systemctl restart auditd

The following example sets the timeout to 10 seconds and excludes the user USER1 from going to the TACACS+ server for authentication:

cumulus@switch:~$ sudo nano /etc/tacplus_nss.conf
...
# The connection timeout for an NSS library should be short, since it is
# invoked for many programs and daemons, and a failure is usually not
# catastrophic.  Not set or set to a negative value disables use of poll().
# This follows the include of tacplus_servers, so it can override any
# timeout value set in that file.
# It's important to have this set in this file, even if the same value
# as in tacplus_servers, since tacplus_servers should not be readable
# by users other than root.
timeout=10
...
# This is a comma separated list of usernames that are never sent to
# a tacacs server, they cause an early not found return.
#
# "*" is not a wild card.  While it's not a legal username, it turns out
# that during pathname completion, bash can do an NSS lookup on "*"
# To avoid server round trip delays, or worse, unreachable server delays
# on filename completion, we include "*" in the exclusion list.
exclude_users=root,daemon,nobody,cron,radius_user,radius_priv_user,sshd,cumulus,quagga,frr,snmp,www-data,ntp,man,_lldpd,USER1,*

Cumulus Linux supports the following additional Linux parameters in the etc/tacplus_nss.conf file. Currently, there are no equivalent NUVE commands.

Linux Parameter Description
include Configures a supplemental configuration file to avoid duplicating configuration information. You can include up to eight additional configuration files. For example: include=/myfile/myname.
min_uid Configures the minimum user ID that the NSS plugin can look 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).

TACACS+ Accounting

When you install the TACACS+ packages and configure the basic TACACS+ settings (set the server and shared secret), accounting is on and there is no additional configuration required.

TACACS+ accounting uses the audisp module, with an additional plugin for auditd and 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.

  • All sudo commands run by TACACS+ users generate accounting records against the original TACACS+ login name.
  • All Linux and NVUE commands result in an accounting record, including login commands and sub-processes of other commands. This can generate a lot of accounting records.

By default, Cumulus Linux sends accounting records to all servers. You can change this setting to send accounting records to the server that is first to respond:

cumulus@switch:~$ nv set system aaa tacacs accounting send-records first-response
cumulus@switch:~$ nv config apply

To reset to the default configuration (send accounting records to all servers), run the nv set system aaa tacacs accounting send-records all command.

  1. Edit the /etc/audisp/audisp-tac_plus.conf file and change the acct_all parameter to 0:

    cumulus@switch:~$ sudo nano /etc/audisp/audisp-tac_plus.conf
...
acct_all=0

  2. Restart auditd:

    cumulus@switch:~$ sudo systemctl restart auditd

To reset to the default configuration (send accounting records to all servers), change the value of acct_all to 1 (acct_all=1).

To disable TACACS+ accounting:

cumulus@switch:~$ nv set system aaa tacacs accounting enable off
cumulus@switch:~$ nv config apply

  1. Edit the /etc/audit/plugins.d/audisp-tacplus.conf file and change the active parameter to no:

    cumulus@switch:~$ sudo nano /etc/audit/plugins.d/audisp-tacplus.conf
...
# default to enabling tacacs accounting; change to no to disable
active = no

  2. Restart auditd:

    cumulus@switch:~$ sudo systemctl restart auditd

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.

NVUE does not provide commands to configure local fallback authentication.

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 with the following command:

    cumulus@switch:~$ sudo systemctl restart nvued

TACACS+ Per-command Authorization

TACACS+ per-command authorization lets you configure the commands that TACACS+ users at different privilege levels can run.

To reach the TACACS+ server through the default VRF, you must specify the egress interface you use in the default VRF. Either run the NVUE nv set system aaa tacacs vrf <interface> command (for example, nv set system aaa tacacs vrf swp51) or set the vrf=<interface> option in the /etc/tacplus_servers file (for example, vrf=swp51).

The following command allows TACACS+ users at privilege level 0 to run the nv and ip commands (if authorized by the TACACS+ server):

cumulus@switch:~$ nv set system aaa tacacs authorization 0 command ip 
cumulus@switch:~$ nv set system aaa tacacs authorization 0 command nv
cumulus@switch:~$ nv config apply

To show the per-command authorization settings, run the nv show system aaa tacacs authorization command:

cumulus@switch:~$ nv show system aaa tacacs authorization
Privilege Level  role          command
---------------  ------------  -------
0                nvue-monitor  ip     
                               nv  

tacuser0@switch:~$ sudo tacplus-restrict -i -u tacacs0 -a ip nv

The tacplus-auth command handles authorization for each command. To make this an enforced authorization, change the TACACS+ log in 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 following table provides the tacplus-restrict command options:

Option Description
-i Initializes the environment. You only need to issue this option one time per username.
-a You can invoke the utility with the -a option as often as you like. For each command in the -a list, the utility creates a symbolic link 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 your TACACS+ server documentation). It is common for the server to allow some options to a command, but not others.
-f Re-initializes the environment. If you need to restart, run the -f option with -i to force re-initialization; otherwise, the utility ignores repeated use of -i.
During initialization:
- The user shell changes to /bin/rbash.
- The utility saves any existing dot files.

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

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

If you 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

To remove all commands:

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

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

Show TACACS+ Configuration

Run the following commands to show TACACS+ configuration:

  • To show all TACACS+ configuration (NVUE hides server secret keys), run the nv show aaa tacacs command.
  • To show TACACS+ authentication configuration , run the nv show system aaa tacacs authentication command.
  • To show TACACS+ accounting configuration , run the nv show system aaa tacacs accounting command.
  • To show TACACS+ server configuration, run the nv show system aaa tacacs server command.
  • To show TACACS+ server priority configuration, run the nv show system aaa tacacs server <priority-id> command.
  • To show the list of users excluded from TACACS+ server authentication, run the nv show system aaa tacacs exclude-user command.

The following example command shows all TACACS+ configuration:

cumulus@switch:~$ nv show system aaa tacacs
                    applied
------------------  -------
enable              off    
debug-level         0      
timeout             5      
vrf                 mgmt   
accounting                 
  enable            off    
authentication             
  mode              pap    
  per-user-homedir  off    
[server]            5      
[server]            10

The following command shows the list of users excluded from TACACS+ server authentication:

cumulus@switch:~$ nv show system aaa tacacs exclude-user
          applied
--------  -------
username  USER1

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, you can use debugging. Add the debug=1 parameter to the /etc/tacplus_servers and /etc/tacplus_nss.conf files; see the Linux Commands under Optional TACACS+ Configuration above. You can also add debug=1 to individual pam_tacplus lines in /etc/pam.d/common*.

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

Debug 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+ Package Descriptions

Cumulus Linux uses the following packages for TACACS.

Package
 Description
audisp-tacplus Uses auditing data from auditd to send accounting records to the TACACS+ server and starts as part of auditd.
libtac2 Provides basic TACACS+ server utility and communication routines.
libnss-tacplus Provides an interface between libc username lookups, the mapping functions, and the TACACS+ server.
tacplus-auth Includes the tacplus-restrict setup utility, which enables you to perform per-command TACACS+ authorization. Per-command authorization is not the default.
libpam-tacplus Provides a modified version of the standard Debian package.
libtacplus-map1 Provides mapping between local and TACACS+ users on the server. The package:
- Sets the immutable sessionid and auditing UID to ensure that you can track the original user through multiple processes and privilege changes.
- Sets the auditing loginuid as immutable.
- Creates and maintains a status database in /run/tacacs_client_map to manage and lookup mappings.
libsimple-tacacct1 Provides an interface for programs to send accounting records to the TACACS+ server. audisp-tacplus uses this package.
libtac2-bin Provides the tacc testing program and TACACS+ man page.

TACACS+ Client Configuration Files

The following table describes the TACACS+ client configuration files that Cumulus Linux uses.

Filename
 Description
/etc/tacplus_servers The primary file that requires configuration after installation. All packages with include=/etc/tacplus_servers parameters use this file. Typically, this file contains the shared secrets; make sure that the Linux file mode is 600.
/etc/nsswitch.conf When the libnss_tacplus package installs, this file configures tacplus lookups through libnss_tacplus. If you replace this file by automation, you need to add tacplus as the first lookup method for the passwd database line.
/etc/tacplus_nss.conf Sets the basic parameters for libnss_tacplus. The file includes a debug variable for debugging NSS lookups separately from other client packages.
/usr/share/pam-configs/tacplus The configuration file for pam-auth-update to generate the files in the next row. The file uses these configurations at login, by su, and by ssh.
/etc/pam.d/common-* The /etc/pam.d/common-* files update for tacplus authentication. The files update with pam-auth-update when you install or remove libpam-tacplus.
/etc/sudoers.d/tacplus 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 a password and provides an example of how to enable all TACACS users to run specific commands with sudo. Only edit this file with the visudo -f /etc/sudoers.d/tacplus command.
/etc/audisp/plugins.d/audisp-tacplus.conf The audisp plugin configuration file. You do not need to modify this file.
/etc/audisp/audisp-tac_plus.conf 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.
/etc/audit/rules.d/audisp-tacplus.rules The auditd rules for TACACS+ accounting. The augenrules command uses all rule files to generate the rules file.
/etc/audit/audit.rules The audit rules file that generate when you install auditd.

Considerations

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.

As a result, 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 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 the 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.

TACACS+ and PAM

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.

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 pam-auth-update.

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+ Client Sequencing

Cumulus Linux requires the following information 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.

Multiple Servers with Different User Accounts

If you configure multiple TACACS+ servers that have different user accounts:

  • TACACS+ authentication allows for fall through; if the first reachable server does not authenticate the user, the client tries the second server, and so on.
  • TACACS authorization does not fall through. If the first reachable server returns an unauthorized result, the client does not try the next server.

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