13.14. Shared Administration with Sudo

Contributed by Tom Rhodes.

System administrators often need the ability to grant enhanced permissions to users so they may perform privileged tasks. The idea that team members are provided access to a FreeBSD system to perform their specific tasks opens up unique challenges to every administrator. These team members only need a subset of access beyond normal end user levels; however, they almost always tell management they are unable to perform their tasks without superuser access. Thankfully, there is no reason to provide such access to end users because tools exist to manage this exact requirement.

Up to this point, the security chapter has covered permitting access to authorized users and attempting to prevent unauthorized access. Another problem arises once authorized users have access to the system resources. In many cases, some users may need access to application startup scripts, or a team of administrators need to maintain the system. Traditionally, the standard users and groups, file permissions, and even the su(1) command would manage this access. And as applications required more access, as more users needed to use system resources, a better solution was required. The most used application is currently Sudo.

Sudo allows administrators to configure more rigid access to system commands and provide for some advanced logging features. As a tool, it is available from the Ports Collection as security/sudo or by use of the pkg(8) utility. To use the pkg(8) tool:

# pkg install sudo

After the installation is complete, the installed visudo will open the configuration file with a text editor. Using visudo is highly recommended as it comes with a built in syntax checker to verify there are no errors before the file is saved.

The configuration file is made up of several small sections which allow for extensive configuration. In the following example, web application maintainer, user1, needs to start, stop, and restart the web application known as webservice. To grant this user permission to perform these tasks, add this line to the end of /usr/local/etc/sudoers:

user1   ALL=(ALL)       /usr/sbin/service webservice *

The user may now start webservice using this command:

% sudo /usr/sbin/service webservice start

While this configuration allows a single user access to the webservice service; however, in most organizations, there is an entire web team in charge of managing the service. A single line can also give access to an entire group. These steps will create a web group, add a user to this group, and allow all members of the group to manage the service:

# pw groupadd -g 6001 -n webteam

Using the same pw(8) command, the user is added to the webteam group:

# pw groupmod -m user1 -n webteam

Finally, this line in /usr/local/etc/sudoers allows any member of the webteam group to manage webservice:

%webteam   ALL=(ALL)       /usr/sbin/service webservice *

Unlike su(1), Sudo only requires the end user password. This adds an advantage where users will not need shared passwords, a finding in most security audits and just bad all the way around.

Users permitted to run applications with Sudo only enter their own passwords. This is more secure and gives better control than su(1), where the root password is entered and the user acquires all root permissions.

Tip:

Most organizations are moving or have moved toward a two factor authentication model. In these cases, the user may not have a password to enter. Sudo provides for these cases with the NOPASSWD variable. Adding it to the configuration above will allow all members of the webteam group to manage the service without the password requirement:

%webteam   ALL=(ALL)       NOPASSWD: /usr/sbin/service webservice *

13.14.1. Logging Output

An advantage to implementing Sudo is the ability to enable session logging. Using the built in log mechanisms and the included sudoreplay command, all commands initiated through Sudo are logged for later verification. To enable this feature, add a default log directory entry, this example uses a user variable. Several other log filename conventions exist, consult the manual page for sudoreplay for additional information.

Defaults iolog_dir=/var/log/sudo-io/%{user}

Tip:

This directory will be created automatically after the logging is configured. It is best to let the system create directory with default permissions just to be safe. In addition, this entry will also log administrators who use the sudoreplay command. To change this behavior, read and uncomment the logging options inside sudoers.

Once this directive has been added to the sudoers file, any user configuration can be updated with the request to log access. In the example shown, the updated webteam entry would have the following additional changes:

%webteam ALL=(ALL) NOPASSWD: LOG_INPUT: LOG_OUTPUT: /usr/sbin/service webservice *

From this point on, all webteam members altering the status of the webservice application will be logged. The list of previous and current sessions can be displayed with:

# sudoreplay -l

In the output, to replay a specific session, search for the TSID= entry, and pass that to sudoreplay with no other options to replay the session at normal speed. For example:

# sudoreplay user1/00/00/02

Warning:

While sessions are logged, any administrator is able to remove sessions and leave only a question of why they had done so. It is worthwhile to add a daily check through an intrusion detection system (IDS) or similar software so that other administrators are alerted to manual alterations.

The sudoreplay is extremely extendable. Consult the documentation for more information.

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