RADIUS AAA

Various add-on packages enable RADIUS users to log in to Cumulus Linux switches in a transparent way with minimal configuration. There is no need to create accounts or directories on the switch. Authentication is handled with PAM and includes login, ssh, sudo and su.

Install the RADIUS Packages

You can install the RADIUS 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 the RADIUS packages:

cumulus@switch:~$ sudo apt-get update
cumulus@switch:~$ sudo apt-get install libnss-mapuser libpam-radius-auth

After installation is complete, either reboot the switch or run the sudo systemctl restart netd command.

The libpam-radius-auth package supplied with the Cumulus Linux RADIUS client is a newer version than the one in Debian Buster. This package contains support for IPv6, the src_ip option described below, as well as a number of bug fixes and minor features. The package also includes VRF support, provides man pages describing the PAM and RADIUS configuration, and sets the SUDO_PROMPT environment variable to the login name for RADIUS mapping support.

The libnss-mapuser package is specific to Cumulus Linux and supports the getgrent, getgrnam and getgrgid library interfaces. These interfaces add logged in RADIUS users to the group member list for groups that contain the mapped_user (radius_user) if the RADIUS account is unprivileged, and add privileged RADIUS users to the group member list for groups that contain the mapped_priv_user (radius_priv_user) during the group lookups.

During package installation:

• The PAM configuration is modified automatically using pam-auth-update (8), and the NSS configuration file /etc/nsswitch.conf is modified to add the mapuser and mapuid plugins. If you remove or purge the packages, these files are modified to remove the configuration for these plugins.
• The radius_shell package is added, which installs the /sbin/radius_shell and setcap cap_setuid program used as the login shell for RADIUS accounts. The package adjusts the UID when needed, then runs the bash shell with the same arguments. When installed, the package changes the shell of the RADIUS accounts to /sbin//radius_shell, and to /bin/shell if the package is removed. This package is required for privileged RADIUS users to be enabled. It is not required for regular RADIUS client use.
• The radius_user account is added to the netshow group and the radius_priv_user account to the netedit and sudo groups. This change enables all RADUS logins to run NCLU net show commands and all privileged RADIUS users to also run net add, net del, and net commit commands, and to use sudo.

Configure the RADIUS Client

To configure the RADIUS client, edit the /etc/pam_radius_auth.conf file:

1. Add the hostname or IP address of at least one RADIUS server (such as a freeradius server on Linux), and the shared secret used to authenticate and encrypt communication with each server.

   The hostname of the switch must be resolvable to an IP address, which, in general, is fixed in DNS. If for some reason you cannot find the hostname in DNS, you can add the hostname to the /etc/hosts file manually. However, this can cause problems since the IP address is usually assigned by DHCP, which can change at any time.

   Multiple server configuration lines are verified in the order listed. Other than memory, there is no limit to the number of RADIUS servers you can use.

   The server port number or name is optional. The system looks up the port in the /etc/services file. However, you can override the ports in the /etc/pam_radius_auth.conf file.

2. If the server is slow or latencies are high, change the timeout setting. The setting defaults to 3 seconds.

3. If you want to use a specific interface to reach the RADIUS server, specify the src_ip option. You can specify the hostname of the interface, an IPv4, or an IPv6 address. If you specify the src_ip option, you must also specify the timeout option.

4. Set the vrf-name field. This is typically set to mgmt if you are using a management VRF. You cannot specify more than one VRF.

The configuration file includes the mapped_priv_user field that sets the account used for privileged RADIUS users and the priv-lvl field that sets the minimum value for the privilege level to be considered a privileged login (the default value is 15). If you edit these fields, make sure the values match those set in the /etc/nss_mapuser.conf file.

The following example provides a sample /etc/pam_radius_auth.conf file configuration:

mapped_priv_user   radius_priv_user
# server[:port]    shared_secret   timeout (secs)  src_ip
192.168.0.254      secretkey
other-server       othersecret     3               192.168.1.10
# when mgmt vrf is in use
vrf-name mgmt

As an optional step, you can set PAM configuration keywords by editing the /usr/share/pam-configs/radius file. After you edit the file, you must run the pam-auth-update --package command. PAM configuration keywords are described in the pam_radius_auth (8) man page.

Service-Type = Administrative-User,
Cisco-AVPair = "shell:roles=network-administrator",
Cisco-AVPair += "shell:priv-lvl=15"

Enable Login without Local Accounts

Because LDAP is not commonly used with switches and adding accounts locally is cumbersome, Cumulus Linux includes a mapping capability with the libnss-mapuser package.

Mapping is done using two NSS (Name Service Switch) plugins, one for account name, and one for UID lookup. These accounts are configured automatically in /etc/nsswitch.conf during installation and are removed when the package is removed. See the nss_mapuser (8) man page for the full description of this plugin.

A username is mapped at login to a fixed account specified in the configuration file, with the fields of the fixed account used as a template for the user that is logging in.

For example, if the name being looked up is dave and the fixed account in the configuration file is radius_user, and that entry in /etc/passwd is:

radius_user:x:1017:1002:radius user:/home/radius_user:/bin/bash

then the matching line returned by running getent passwd dave is:

cumulus@switch:~$ getent passwd dave
dave:x:1017:1002:dave mapped user:/home/dave:/bin/bash

The home directory /home/dave is created during the login process if it does not already exist and is populated with the standard skeleton files by the mkhomedir_helper command.

The configuration file /etc/nss_mapuser.conf is used to configure the plugins. The file includes the mapped account name, which is radius_user by default. You can change the mapped account name by editing the file. The nss_mapuser (5) man page describes the configuration file.

A flat file mapping is done based on the session number assigned during login, which persists across su and sudo. The mapping is removed at logout.

Local Fallback Authentication

If a site wants to allow local fallback authentication for a user when none of the RADIUS servers can be reached you can add a privileged user account as a local account on the switch. The local account must have the same unique identifier as the privileged user and the shell must be the same.

To configure local fallback authentication:

1. dd a local privileged user account. For example, if the radius_priv_user account in the /etc/passwd file is radius_priv_user:x:1002:1001::/home/radius_priv_user:/sbin/radius_shell, run the following command to add a local privileged user account named johnadmin:

   cumulus@switch:~$ sudo useradd -u 1002 -g 1001 -o -s /sbin/radius_shell johnadmin
   

2. To enable the local privileged user to run sudo and NCLU commands, run the following commands:

   cumulus@switch:~$ sudo adduser johnadmin netedit
   cumulus@switch:~$ sudo adduser johnadmin sudo
   cumulus@switch:~$ sudo systemctl restart netd
   

3. Edit the /etc/passwd file to move the local user line before to the radius_priv_user line:

   cumulus@switch:~$ sudo vi /etc/passwd
   ...
   johnadmin:x:1002:1001::/home/johnadmin:/sbin/radius_shell
   radius_priv_user:x:1002:1001::/home/radius_priv_user:/sbin/radius_shell
   

4. To set the local password for the local user, run the following command:

   cumulus@switch:~$ sudo passwd johnadmin
   

Verify RADIUS Client Configuration

To verify that the RADIUS client is configured correctly, log in as a non-privileged user and run a net add interface command.

In this example, the ops user is not a privileged RADIUS user so they cannot add an interface.

ops@leaf01:~$ net add interface swp1
ERROR: User ops does not have permission to make networking changes.

In this example, the admin user is a privileged RADIUS user (with privilege level 15) so is able to add interface swp1.

admin@leaf01:~$ net add interface swp1
admin@leaf01:~$ net pending
--- /etc/network/interfaces    2018-04-06 14:49:33.099331830 +0000
+++ /var/run/nclu/iface/interfaces.tmp    2018-04-06 16:01:16.057639999 +0000
@@ -3,10 +3,13 @@

  source /etc/network/interfaces.d/*.intf

  # The loopback network interface
  auto lo
  iface lo inet loopback

  # The primary network interface
  auto eth0
  iface eth0 inet dhcp
+
+auto swp1
iface swp1
...

Remove RADIUS Client Packages

Remove the RADIUS packages with the following command:

cumulus@switch:~$ sudo apt-get remove libnss-mapuser libpam-radius-auth

When you remove the packages, the plugins are removed from the /etc/nsswitch.conf file and from the PAM files.

To remove all configuration files for these packages, run:

cumulus@switch:~$ sudo apt-get purge libnss-mapuser libpam-radius-auth

To remove the home directories of the RADIUS users, first get the list by running:

cumulus@switch:~$ sudo ls -l /home | grep radius

For all users listed, except the radius_user, run this command to remove the home directories:

cumulus@switch:~$ sudo deluser --remove-home USERNAME

where USERNAME is the account name (the home directory relative portion). This command gives the following warning because the user is not listed in the /etc/passwd file.

userdel: cannot remove entry 'USERNAME' from /etc/passwd
/usr/sbin/deluser: `/usr/sbin/userdel USERNAME' returned error code 1. Exiting.

After removing all the RADIUS users, run the command to remove the fixed account. If the account has been changed in the /etc/nss_mapuser.conf file, use that account name instead of radius_user.

cumulus@switch:~$ sudo deluser --remove-home radius_user
cumulus@switch:~$ sudo deluser --remove-home radius_priv_user
cumulus@switch:~$ sudo delgroup radius_users

Limitations

• If two or more RADIUS users are logged in simultaneously, a UID lookup only returns the user that logged in first. Any processes run by either user get attributed to both, and all files created by either user get 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 fixed user from the password file. The current algorithm returns the first name matching the UID from the mapping file; this might be the first or second user that logged in.
• 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.