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ACL Primer

  3 minute read.

Last Modified 2021-08-06 10:19 EDT

TrueNAS SCALE brings full ACL compatibility between Windows and Linux with NFSv4 ACLs on ZFS and eases the challenges of integrating Unix servers in Windows environments.

ACL Overview

In TrueNAS, ACLs specify which users or system processes (trustees) have access to datasets or shares. ACLs also determine what operations trustees may perform.

Each entry in an ACL specifies a trustee and an operation. For example, if an SMB share has an ACL that contains:

Who: User  
User: user1  
Permissions: Full Control  
Who: User  
User: user2  
Permissions: Read  

The ACL would give user1 permission to view, edit, and create files and directories within the share, but only give user2 viewing permissions.

NFSv4 in TrueNAS

While the POSIX ACL type has basic read, write, and execute permissions, the NFSv4 ACL type lets administrators fine-tune advanced read, modify (write), and traverse (execute) permissions.

For example, NFSv4 advanced permissions allow an administrator to set up a trustee that can read and write data, but not delete anything.

The NFSv4 ACL type also lets administrators apply basic or advanced inheritance flags. Basic flags enable or disable ACE (Access Control Entry) inheritance. Advanced flags give further control of how an ACE applies to a dataset’s files and directories.

For example, advanced flags allow an administrator to apply the ACL to new directories within a dataset, but not new files.

Preferred Configurations for SMB Shares

To properly configure ACLs on SMB shares, users should consider how they intend to access the dataset/share with other devices and services on the network.

Even though TrueNAS SCALE NFSv4 ACL support provides the best possible compatibility with a Windows file system security model, it is not the best choice for every situation.

When to use NFSv4 ACLs

TrueNAS administrators should use NFSv4 ACLs to losslessly migrate Windows-style ACLs across Active Directory domains (or stand-alone servers) that use ACL models richer than POSIX.

Since POSIX ACLs are a Linux-specific ZFS feature, administrators should use NFSv4 to maintain compatibility with TrueNAS Core, FreeBSD, or other non-Linux ZFS implementations

Administrators must use NFSv4 if they intend to replicate data from TrueNAS SCALE to a TrueNAS CORE disaster recovery target.

TrueNAS administrators should also use NFSv4 ACLs if their organization requires advanced NFSv4 ACL features.

  • If an organization requires managers to review all data before deletion, administrators can use advanced NFSv4 permissions to let employees access and create files, but not edit or delete existing files.
  • NFSv4 can operate alongside CIFS, allowing organizations that use UNIX-based processing systems features to use Windows-based clients.
  • NFSv4 can also cooperate with CIFS to bypass NFS’s 16 group limitation by generating NFS credentials based on Unix and Windows groups.

When to use POSIX ACLs

TrueNAS administrators should use POSIX ACLs when their organization’s data backup target does not support native NFSv4 ACLs. Since the Linux platform used POSIX for a long time, many backup products that access the server outside the SMB protocol can’t understand or preserve native NFSv4 ACLs.

Note: When deciding how to configuring ACLs, administrators should verify that they can correctly restore permissions from backups.

Inversely, TrueNAS administrators should use POSIX ACLs when TrueNAS is the backup target for clients that use POSIX ACLs. Since ZFS provides superb safety and data protection, many administrators use their TrueNAS system as a target for file-based backups from computers on their networks. Using POSIX ACLs preserves POSIX.1e ACLs from client systems.

TrueNAS administrators should also use POSIX ACLs if they wish to replicate SMB datasets to other non-TrueNAS Linux servers with ZFS, especially when the Linux server should seamlessly take over serving files during disaster recovery.