How to Use the Command 'runcon' (with examples)

The runcon command is a powerful tool used in systems employing Security-Enhanced Linux (SELinux). SELinux provides a mechanism for supporting access control, allowing administrators to define user permissions far more granularly. The runcon command lets users run a program in a different SELinux security context, which can be critical for maintaining strict system security protocols.

Use Case 1: Print the Security Context of the Current Execution Context

Code:

runcon

Motivation:

Understanding the security context of the current execution environment is essential when working in an SELinux-enabled system. This information can help diagnose permission issues, ensure correct context assignments, and refine security policy configurations. It’s an invaluable tool for system administrators who need to audit the current execution context to understand better how processes are being confined.

Explanation:

The command is executed without additional arguments, relying on SELinux to provide the context of the currently executing shell or process. Therefore, no specific manipulations are conducted on the security context itself.

Example Output:

user_u:role_r:domain_t:s0

This output indicates the current security context, broken down into user, role, domain, and level, reflecting the SELinux configuration in the environment.

Use Case 2: Specify the Domain to Run a Command In

Code:

runcon -t domain_t ls

Motivation:

Running a command under a specified domain can be vital for testing security policies, ensuring that particular operations perform as expected when confined under tightly controlled settings. For instance, listing directory contents under different domains can help verify access control lists and SELinux policy configurations.

Explanation:

• -t domain_t: This option specifies the target domain context the command should be executed in. Domains in SELinux are equivalent to types and control the process’s access rights based on the policy configuration.
• ls: This represents the command to be executed under the specified domain context. In this case, ls is used to list directory contents within the selected domain.

Example Output:

file1  file2  file3

This assumes the domain context allows access to the directory contents and operations carried out successfully.

Use Case 3: Specify the Context Role to Run a Command With

Code:

runcon -r role_r whoami

Motivation:

When a user needs to simulate or troubleshoot issues specific to a certain role within SELinux, utilizing the runcon command with a specific role can ensure that commands are executed with the required privileges or restrictions associated with that role. This technique, therefore, allows for extensive role-based access testing and validation in secure environments.

Explanation:

• -r role_r: This option allows you to specify the SELinux role in which the command should be executed. Roles in SELinux define a set of access permissions that can be applied to users or processes.
• whoami: This command is used to display the effective username of the current user operating the command line. When run under a specified role, it helps verify what SELinux role is effectively applied.

Example Output:

testuser

This indicates the user executing the command, assuming that role context switches do not change the user identity directly visible via whoami.

Use Case 4: Specify the Full Context to Run a Command With

Code:

runcon user_u:role_r:domain_t /bin/bash

Motivation:

There are times when a user needs to execute a command or script under a completely different security context to evaluate access controls, diagnose SELinux denials, or temporarily bypass certain policy restrictions. Using runcon to specify a full context comprising user, role, and domain enables security and policy management tasks such as validating comprehensive access rights.

Explanation:

• user_u:role_r:domain_t: This represents the fully specified security context. Here, user_u is the user identity, role_r is the role associated with this execution, and domain_t represents the domain or type context.
• /bin/bash: The command or shell being invoked under this specified context. In this example, a new bash session is started with the defined security context.

Example Output:

bash-4.2$

This output indicates that a new bash shell session is initiated under the specified security context.

Conclusion:

The runcon command is an essential component for tasks related to security and permissions management on SELinux-enabled systems. By allowing users to switch between contexts, it facilitates rigorous access controls, efficient policy troubleshooting, and comprehensive security strategy implementations. Proficiency with such tools ensures system administrators can maintain robust security postures sustainably.