Apache HTTP Server: Fundamentals & Core Configuration

Apache HTTP Server is the world's most widely deployed web server. It's a module-based server that handles everything from static file serving to proxying, SSL termination, and URL rewriting.

Apache vs Nginx

• Apache: process/thread per connection (event MPM is more efficient), .htaccess per-directory config, rich module ecosystem, battle-tested — good for shared hosting, dynamic content via mod_php

• Nginx: event-driven async architecture, no .htaccess (config reload needed), excellent static file serving, better at high concurrency — preferred for modern deployments

• Practical choice: Nginx as reverse proxy → Apache as backend is a common combo

Installation

# Debian/Ubuntu
sudo apt update && sudo apt install apache2
sudo systemctl enable --now apache2

# RHEL/CentOS/Amazon Linux
sudo dnf install httpd
sudo systemctl enable --now httpd

# macOS (via Homebrew)
brew install httpd
brew services start httpd

MPM (Multi-Processing Module)

• prefork: one process per request, no threading, compatible with non-thread-safe libs (older mod_php). High memory use.

• worker: multi-process + multi-threaded. More concurrent connections with less memory.

• event: like worker but keeps connections alive without tying up threads. Best for modern workloads. Default on Ubuntu.

# Check active MPM
apache2ctl -V | grep MPM
httpd -V | grep MPM

# Switch MPM (Ubuntu)
sudo a2dismod mpm_prefork
sudo a2enmod mpm_event
sudo systemctl restart apache2

Key Configuration Files

# Ubuntu/Debian paths
/etc/apache2/apache2.conf       # Main config
/etc/apache2/ports.conf         # Listen directives
/etc/apache2/sites-available/   # Virtual host configs (enable with a2ensite)
/etc/apache2/sites-enabled/     # Symlinks to active sites
/etc/apache2/mods-available/    # Available modules
/etc/apache2/mods-enabled/      # Enabled modules (symlinks)
/var/log/apache2/access.log     # Access log
/var/log/apache2/error.log      # Error log

# RHEL/CentOS paths
/etc/httpd/conf/httpd.conf       # Main config
/etc/httpd/conf.d/               # Additional configs
/var/log/httpd/access_log
/var/log/httpd/error_log

Core Directives

# apache.conf / httpd.conf key settings

ServerName www.example.com
ServerAdmin webmaster@example.com

# Document root for default site
DocumentRoot /var/www/html

# Directory permissions
<Directory /var/www/html>
    Options -Indexes +FollowSymLinks    # Disable directory listing
    AllowOverride All                   # Allow .htaccess (use None for performance)
    Require all granted
</Directory>

# Logging
LogLevel warn
ErrorLog ${APACHE_LOG_DIR}/error.log
CustomLog ${APACHE_LOG_DIR}/access.log combined

# Log format definition
LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" combined

# Keep-Alive
KeepAlive On
MaxKeepAliveRequests 100
KeepAliveTimeout 5

# Listen on ports
Listen 80
Listen 443

apachectl Commands

# Start / stop / restart
sudo apachectl start
sudo apachectl stop
sudo apachectl restart       # Full restart (brief downtime)
sudo apachectl graceful      # Reload config, finish active connections first
sudo apachectl graceful-stop

# Validate config before restarting
sudo apachectl configtest
sudo apachectl -t            # Same thing

# Show compiled-in settings
apache2ctl -V

# List loaded modules
apache2ctl -M

mod_status — Server Status

# Enable mod_status
LoadModule status_module modules/mod_status.so

<Location /server-status>
    SetHandler server-status
    Require ip 127.0.0.1 10.0.0.0/8    # Restrict access
</Location>

ExtendedStatus On    # Include per-request details

# Enable on Ubuntu
sudo a2enmod status
curl http://localhost/server-status?auto   # Machine-readable output