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Extended Access Control Lists (ACLs) are a tool that allows for more flexible management of file and directory permissions in Linux operating systems. Unlike traditional methods where each file or directory can have permissions set only for the owner, group, and others, extended ACLs enable defining more detailed and specific access rules for different users and groups.

Tools for Working with Extended ACL

Key tools for working with extended ACL in Linux are setfacl and getfacl. These tools allow users to set and retrieve information about extended ACLs on files and directories.

Setfacl: Setting Extended ACL

The setfacl tool is used to add, modify, or remove ACLs on files and directories. It enables defining permissions for specific users and groups beyond basic permissions. The syntax of the command is as follows:

setfacl -m u:user:permissions file

Where -m indicates modification, u denotes a user (for groups, g is used), user is the username, permissions are the permissions (e.g., rwx), and file is the path to the file or directory.

Example usage:

setfacl -m u:john:rw document.txt

This command sets read and write permissions for the user john on the file document.txt.

Getfacl: Retrieving Information about Extended ACL

On the other hand, getfacl is a tool used to display a list of ACLs set on files and directories. With this command, users can easily find out what specific permissions have been assigned to a file or directory. The syntax of the command is:

getfacl file

Example usage:

getfacl document.txt

This command displays a list of all ACLs applied to document.txt, including permissions for specific users and groups.

Practical Examples and Tips

For effective management of extended ACLs, it is important to understand the capabilities offered by setfacl and getfacl. Here are some tips and examples for common tasks:

• Adding permissions for multiple users: Multiple -m options can be used in a single setfacl command to set permissions for multiple users at once.
• Removing specific ACL: Using the -x option with setfacl allows you to remove specific permissions for users or groups. For example, removing permissions for the user john from document.txt can be done with the command:

  setfacl -x u:john document.txt
  ​

• Setting default ACL for directories: Using the -d option, you can define default ACLs for all newly created files and subdirectories within a directory. This is useful for maintaining consistent permission settings within a directory structure.

  setfacl -d -m u:john:rw directory
  ​

• This command ensures that all new files created in directory will automatically be assigned read and write permissions for the user john.

• Copying ACL between files: The getfacl command can be combined with setfacl to copy ACL from one file to another. First, you retrieve the ACL from the source file using getfacl and then apply it to the target file using setfacl:

  getfacl source_file | setfacl --set-file=- target_file
  ​

• Using mask to limit permissions: The mask in ACL limits effective permissions for all users and groups. It is important to understand the impact of the mask on the set permissions, as it can lead to unintended access restrictions. Setting the mask can be done using:

  setfacl -m m:rwx file
  ​

  This command sets the mask to rwx (read, write, execute), allowing maximum permissions defined by other ACLs.

 

Managing extended ACLs in Linux offers significant flexibility in defining access permissions to files and directories. The tools setfacl and getfacl are powerful aids for customizing permissions according to users' and groups' needs. While their usage may seem complex at first glance, practice and understanding of basic principles enable efficient utilization of extended ACLs for system security and management. It is important to note that incorrect settings can lead to unintended security risks, so it is advisable to familiarize oneself with recommended practices and thoroughly test configurations in a secure environment before implementation.