Understanding .cagefs/tmp in CloudLinux CageFS

Table of Contents: Introduction Purpose of .cagefs/tmp Permission Denied Error No Space Left Error PHP Sessions Not Working File Uploads Failing Summary of Fixes

The .cagefs/tmp directory is part of the CloudLinux CageFS system, which isolates users in a secure environment. This directory is used for temporary files created by web applications running under CageFS.

1. Purpose of .cagefs/tmp

The .cagefs/tmp directory serves several critical functions in the CloudLinux security architecture:

• Secure Temporary Storage: Each user has their own virtual /tmp directory inside .cagefs, preventing access to other users' temporary files.
• Prevents Privilege Escalation: Users cannot escape their CageFS jail to access the system-level /tmp directory or other users' files.
• Ensures Compatibility: Applications requiring temporary file storage (like PHP sessions, file uploads, caching) use .cagefs/tmp instead of the global /tmp.

2. Fix "Permission Denied" Error in .cagefs/tmp

If a web application cannot write to .cagefs/tmp, you'll see permission errors in logs. This typically happens after incorrect manual file operations or service changes.

# Set correct ownership (replace 'username' with actual username)
sudo chown -R username:username /home/username/.cagefs/tmp

# Set secure permissions with sticky bit
sudo chmod -R 1777 /home/username/.cagefs/tmp

Restart CageFS and Related Services

sudo systemctl restart cagefs
sudo systemctl restart apache2  # or httpd on CentOS
sudo systemctl restart php7.4-fpm  # adjust PHP version as needed

3. Fix "No Space Left on Device" Error

When .cagefs/tmp fills up, PHP sessions, file uploads, and other temporary operations will fail with disk space errors.

Check Disk Usage

# Check specific user's tmp usage
du -sh /home/username/.cagefs/tmp

# Check inode usage (sometimes files are small but numerous)
find /home/username/.cagefs/tmp -type f | wc -l

Clear Temporary Files

# Remove all files (preserves directory structure)
sudo rm -rf /home/username/.cagefs/tmp/*

# Or remove only old files (safer, preserves recent files)
sudo find /home/username/.cagefs/tmp -type f -mtime +3 -delete

Monitor and Prevent Recurrence

# Set up monitoring
df -h /home/username/.cagefs/tmp

# Consider automated cleanup via cron
# Add to crontab: 0 3 * * * find /home/username/.cagefs/tmp -type f -mtime +7 -delete

4. Fix PHP Sessions Not Working (session_start() Fails)

If PHP cannot store session files, session_start() will fail, breaking user logins and sessions in web applications.

Identify Current Session Path

# Check current PHP session configuration
php -i | grep session.save_path

# For web server PHP, create a test file:
echo "<?php phpinfo(); ?>" > /home/username/public_html/session_test.php
# Then visit http://domain.com/session_test.php and search for session.save_path
# REMOVE this file after testing!

Set Correct session.save_path

# Edit php.ini (adjust path for your system)
# For CentOS/RHEL:
sudo nano /etc/php.ini

# For Ubuntu/Debian Apache:
sudo nano /etc/php/7.4/apache2/php.ini

# For Ubuntu/Debian PHP-FPM:
sudo nano /etc/php/7.4/fpm/php.ini

session.save_path = "/home/username/.cagefs/tmp"

Restart Web Services

sudo systemctl restart apache2  # or httpd
sudo systemctl restart php7.4-fpm  # adjust version

5. Fix PHP File Uploads Failing (upload_tmp_dir Issues)

File uploads in PHP applications fail when the temporary upload directory is not writable or incorrectly configured.

Check Current Upload Configuration

php -i | grep upload_tmp_dir

Set Correct upload_tmp_dir

# Edit php.ini (adjust path for your system)
sudo nano /etc/php.ini  # or appropriate php.ini

# Find and update upload_tmp_dir:
upload_tmp_dir = "/home/username/.cagefs/tmp"

# Also ensure these related settings are appropriate:
upload_max_filesize = 64M
post_max_size = 64M
max_file_uploads = 20

Verify Directory is Writable

# Test if PHP can write to the directory
sudo -u username php -r "echo is_writable('/home/username/.cagefs/tmp') ? 'Writable' : 'NOT Writable';"

# If not writable, ensure permissions are correct (as in Section 2)

Restart Services

sudo systemctl restart apache2
sudo systemctl restart php7.4-fpm
sudo systemctl restart cagefs

6. Summary of Fixes

Understanding and properly configuring the .cagefs/tmp directory is essential for maintaining secure, functional web applications in CloudLinux CageFS environments!