Apache Configuration Basics

Apache HTTP Server is the most widely deployed web server in the world. Its configuration is file-driven — understanding the httpd.conf structure, MPM selection, and service management is the foundation of running Apache in production.

httpd.conf Structure & Core Directives

The main config file is typically at /etc/apache2/apache2.conf (Debian/Ubuntu) or /etc/httpd/conf/httpd.conf (RHEL/CentOS). It uses a directive-based syntax inside block containers.

# Core server identity directives
ServerName www.example.com
ServerAlias example.com
ServerAdmin webmaster@example.com

# Document root — where your site files live
DocumentRoot "/var/www/html"

# Directory block — controls access to a path
<Directory "/var/www/html">
    Options Indexes FollowSymLinks
    AllowOverride All          # Allow .htaccess to override directives
    Require all granted        # Apache 2.4 access control (replaces Order/Allow/Deny)
</Directory>

# Restrict access to a sensitive directory
<Directory "/var/www/html/admin">
    Require ip 192.168.1.0/24  # Only allow internal network
</Directory>

# Options values:
#   Indexes        — show directory listing if no index file exists
#   FollowSymLinks — follow symbolic links
#   ExecCGI        — allow CGI scripts
#   None           — disable all options
#   -Indexes       — disable Indexes specifically

# Listen on multiple ports
Listen 80
Listen 443

# Include additional config files
Include conf.d/*.conf
IncludeOptional sites-enabled/*.conf

MPM — Multi-Processing Modules

The MPM controls how Apache handles concurrent connections. Choosing the right one impacts performance, memory usage, and PHP compatibility.

# Check active MPM
apachectl -V | grep MPM
apache2ctl -V | grep "Server MPM"

# prefork  — one process per connection; required for non-thread-safe PHP (mod_php)
# worker   — threads + processes; better memory efficiency
# event    — like worker but handles keep-alive connections in a dedicated thread (best for modern workloads)

# Configure event MPM in /etc/apache2/mods-available/mpm_event.conf
<IfModule mpm_event_module>
    StartServers             2
    MinSpareThreads         25
    MaxSpareThreads         75
    ThreadLimit             64
    ThreadsPerChild         25
    MaxRequestWorkers      150
    MaxConnectionsPerChild   0
</IfModule>

# Switch MPM on Ubuntu/Debian
sudo a2dismod mpm_prefork
sudo a2enmod mpm_event
sudo systemctl restart apache2

# On RHEL/CentOS edit /etc/httpd/conf.modules.d/00-mpm.conf
# Comment out LoadModule mpm_prefork_module and uncomment mpm_event_module

Service Management

# systemctl (modern — use on all systemd distros)
sudo systemctl start apache2        # Start
sudo systemctl stop apache2         # Stop
sudo systemctl restart apache2      # Full restart (drops connections)
sudo systemctl reload apache2       # Graceful reload (no dropped connections)
sudo systemctl enable apache2       # Auto-start on boot
sudo systemctl status apache2       # Show status + recent logs

# apachectl / apache2ctl (cross-platform alternative)
sudo apachectl start
sudo apachectl graceful             # Graceful restart (waits for in-flight requests)
sudo apachectl configtest           # Validate config syntax BEFORE reloading
sudo apachectl -t                   # Same as configtest
sudo apachectl -S                   # Show parsed virtual host config

# RHEL/CentOS uses httpd instead of apache2
sudo systemctl restart httpd
sudo apachectl configtest

# Log files
tail -f /var/log/apache2/access.log
tail -f /var/log/apache2/error.log
tail -f /var/log/httpd/error_log    # RHEL/CentOS path

# Custom log format
LogFormat "%h %l %u %t "%r" %>s %b "%{Referer}i" "%{User-Agent}i"" combined
CustomLog /var/log/apache2/access.log combined
ErrorLog /var/log/apache2/error.log
LogLevel warn