Apache Guide: The Versatile HTTP Server

Apache HTTP Server (httpd) has powered the web for decades. Its module system, .htaccess support, and broad OS compatibility make it a common choice for shared hosting, legacy applications, and environments that need runtime configuration flexibility.

Installation

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

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

# Verify
curl -I http://localhost

Configuration Files

On Debian-based systems, the main config is /etc/apache2/apache2.conf. On RHEL-based systems, it is /etc/httpd/conf/httpd.conf.

Debian uses a modular layout with helper commands:

# Enable a module
sudo a2enmod rewrite
sudo a2enmod ssl
sudo a2enmod headers

# Enable a site
sudo a2ensite example.com.conf

# Disable a site
sudo a2dissite 000-default.conf

# Test configuration
sudo apachectl configtest

# Reload
sudo systemctl reload apache2

VirtualHost Configuration

# /etc/apache2/sites-available/example.com.conf
<VirtualHost *:80>
    ServerName example.com
    ServerAlias www.example.com
    DocumentRoot /var/www/example.com/html

    <Directory /var/www/example.com/html>
        Options -Indexes +FollowSymLinks
        AllowOverride All
        Require all granted
    </Directory>

    ErrorLog  ${APACHE_LOG_DIR}/example.com-error.log
    CustomLog ${APACHE_LOG_DIR}/example.com-access.log combined
</VirtualHost>

AllowOverride All permits .htaccess files in that directory tree. Set it to None for better performance if .htaccess is not needed.

.htaccess and mod_rewrite

.htaccess files provide per-directory configuration without editing the main server config. The most common use is URL rewriting:

# /var/www/example.com/html/.htaccess
RewriteEngine On

# Redirect HTTP to HTTPS
RewriteCond %{HTTPS} off
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

# Pretty URLs -- route everything to index.php
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^ index.php [L]

Performance note: Apache checks for .htaccess in every directory along the path for every request. Disable AllowOverride in production and move rules into the VirtualHost block when possible.

mod_ssl -- HTTPS with Apache

<VirtualHost *:443>
    ServerName example.com
    DocumentRoot /var/www/example.com/html

    SSLEngine on
    SSLCertificateFile    /etc/letsencrypt/live/example.com/fullchain.pem
    SSLCertificateKeyFile /etc/letsencrypt/live/example.com/privkey.pem

    <Directory /var/www/example.com/html>
        AllowOverride All
        Require all granted
    </Directory>
</VirtualHost>

Multi-Processing Modules (MPMs)

Apache offers three MPMs that control how it handles connections:

MPM	Model	Best For
prefork	One process per connection	Legacy PHP (mod_php)
worker	Threads within processes	Thread-safe apps, moderate traffic
event	Async with dedicated listener	High concurrency, keep-alive heavy

Switch MPMs on Debian:

sudo a2dismod mpm_prefork
sudo a2enmod mpm_event
sudo systemctl restart apache2

Tune the 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>

Useful Modules

Module	Purpose
mod_rewrite	URL rewriting and redirects
mod_ssl	TLS termination
mod_headers	Set/modify HTTP headers
mod_proxy	Reverse proxy
mod_deflate	Response compression
mod_security	Web application firewall
mod_expires	Cache control headers

Common Troubleshooting

# Check syntax
sudo apachectl configtest

# View error log
sudo tail -f /var/log/apache2/error.log

# List loaded modules
apache2ctl -M

# List enabled sites
ls /etc/apache2/sites-enabled/

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