The "No Apache MPM package installed" error typically occurs when Apache's Multi-Processing Module (MPM) isn't properly configured or recognized during virtual host setup. This is particularly common when working with Apache 2.2 on Debian-based systems.

First, verify which MPM is actually installed:

apache2 -l | grep mpm

Or check the installed packages:

dpkg -l | grep apache2-mpm

Even when prefork is installed, you might encounter this error after running:

a2ensite example.com
a2dissite default
/etc/init.d/apache2 reload

The system fails to properly detect the MPM module during configuration changes.

Try these steps to resolve the issue:

# Ensure the MPM module is enabled
a2enmod mpm_prefork

# Check the symlinks in mods-enabled
ls -l /etc/apache2/mods-enabled/mpm_*

# Restart Apache completely instead of reload
/etc/init.d/apache2 restart

Examine your Apache configuration files:

# Check the main configuration
grep -r "MPM" /etc/apache2/

# Verify the ports.conf file
cat /etc/apache2/ports.conf

If the issue persists, consider reinstalling the MPM package:

apt-get --reinstall install apache2-mpm-prefork

Run Apache's configuration test to identify potential issues:

apache2ctl -t
apache2ctl -S

Here's a proper virtual host configuration that works with MPM:

<VirtualHost *:80>
    ServerName example.com
    DocumentRoot /var/www/example.com/public_html
    ErrorLog /var/www/example.com/logs/error.log
    CustomLog /var/www/example.com/logs/access.log combined
</VirtualHost>

After making changes, verify everything works:

# Check running processes
ps aux | grep apache

# Verify listening ports
netstat -tulpn | grep apache

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When configuring virtual hosts on Apache 2.2, you might encounter the frustrating "No apache MPM package installed" error despite having prefork installed. This typically occurs during operations like a2ensite, a2dissite, or server reloads.

The error often surfaces when:

• The MPM module isn't properly enabled in Apache's configuration
• There's a mismatch between installed packages and enabled modules
• Configuration files reference non-existent MPM modules

First, verify your current MPM status:

apache2ctl -M | grep mpm
apache2 -l | grep mpm

Check installed packages on Debian/Ubuntu:

dpkg -l | grep apache2-mpm

If prefork is installed but not recognized:

# Enable the MPM module
sudo a2enmod mpm_prefork

# Verify module loading
sudo apache2ctl -t -D DUMP_MODULES

# Restart Apache properly
sudo systemctl restart apache2

Inspect these critical files:

/etc/apache2/mods-available/mpm_prefork.conf
/etc/apache2/mods-enabled/mpm_prefork.load
/etc/apache2/apache2.conf

For different server needs, consider:

# For worker MPM
sudo apt-get install apache2-mpm-worker
sudo a2dismod mpm_prefork
sudo a2enmod mpm_worker

After fixes, test with:

sudo apache2ctl configtest
sudo tail -f /var/log/apache2/error.log