On Linux systems, standard group permissions don't automatically grant members the ability to modify file permissions - even when they have write access. This creates a specific challenge when you need a group (like ftpusers) to manage permissions within a directory tree.

Unlike BSD's chmod +a command, Linux uses a different permission model. The key is combining these elements:

1. Directory ownership (chown)
2. Special permission bits (chmod g+s)
3. Umask settings

Here's the complete solution for Debian/GNU Linux:

# 1. Set directory ownership
sudo chown -R :ftpusers /path/to/directory

# 2. Set SGID bit to maintain group ownership
sudo chmod -R g+s /path/to/directory

# 3. Set ACL for permission modification
sudo setfacl -R -m g:ftpusers:rwx /path/to/directory
sudo setfacl -dR -m g:ftpusers:rwx /path/to/directory

# 4. Verify the setup
getfacl /path/to/directory

For more granular control (kernel 2.6.24+):

# Grant CAP_FOWNER capability to specific binaries
sudo setcap cap_fowner+ep /usr/bin/chmod

Always:

• Restrict access to necessary directories only
• Monitor permission changes with auditd
• Consider using filesystem quotas

# Check effective permissions
namei -l /path/to/file

# Verify group membership
groups ftpuser1

# Check SELinux context
ls -Z /path/to/directory

When administering Linux servers, particularly in web hosting environments, we often need to delegate permission management capabilities to specific user groups. The scenario where members of the 'ftpusers' group need to modify permissions within a designated directory is common yet requires careful implementation.

Unlike BSD's ACL system (which uses +a flags), Linux provides several approaches for granular permission control:

1. Traditional UNIX permissions (owner/group/others)
2. POSIX ACLs (Access Control Lists)
3. Special permission bits (setuid/setgid/sticky bit)

For Debian/GNU systems, we'll combine two methods to achieve the desired functionality:

1. Setting the Directory Structure

First, ensure proper ownership and permissions on the parent directory:

sudo chown -R :ftpusers /path/to/ftp_root
sudo chmod -R 2770 /path/to/ftp_root  # Note the setgid bit (2)

The 2770 permission breaks down as:

• 2: setgid bit ensures new files inherit group ownership
• 7: owner gets rwx
• 7: group gets rwx
• 0: others get no permissions

2. Enabling Permission Modification Capability

To allow group members to change permissions, we'll use POSIX ACLs:

sudo apt install acl  # Install ACL utilities if not present
sudo setfacl -Rm g:ftpusers:rwx /path/to/ftp_root
sudo setfacl -Rdm g:ftpusers:rwx /path/to/ftp_root

The -d flag sets default ACLs for new files/directories.

Create a test environment to verify the configuration:

# As admin:
sudo mkdir -p /path/to/ftp_root/testdir
sudo touch /path/to/ftp_root/testfile

# As ftpuser:
su ftpuser
cd /path/to/ftp_root
chmod 755 testfile  # Should succeed
ls -la testfile     # Verify permissions changed

For production environments, consider these additional measures:

• Implement filesystem quotas
• Set up audit logging with auditd
• Consider using chattr +i for critical files
• Review SELinux/AppArmor policies if enabled

If permission changes still fail, check:

# Verify group membership
groups ftpuser

# Check effective permissions
getfacl /path/to/ftp_root

# Ensure no conflicting SELinux contexts
ls -Z /path/to/ftp_root