apache httpd reference

key paths (debian/ubuntu)

Config root: /etc/apache2/
Virtual hosts: /etc/apache2/sites-available/ & sites-enabled/
Modules: /etc/apache2/mods-available/ & mods-enabled/
Logs: /var/log/apache2/access.log, error.log

service control

Action	Command
Start/Stop/Restart	`sudo systemctl start\|stop\|restart apache2`
Reload config	`sudo systemctl reload apache2`
Enable at boot	`sudo systemctl enable apache2`
Check status	`systemctl status apache2`

config checks

Task	Command
Syntax test	`sudo apachectl configtest`
List loaded modules	`apachectl -M`
Virtual host map	`apachectl -S`

enable/disable vhosts & modules

Action	Command
Enable site	`sudo a2ensite example.conf`
Disable site	`sudo a2dissite example.conf`
Enable module	`sudo a2enmod rewrite`
Disable module	`sudo a2dismod php8.2`
Reload after changes	`sudo systemctl reload apache2`

minimal virtual host (http)

<VirtualHost *:80>
    ServerName example.com
    ServerAlias www.example.com
    DocumentRoot /var/www/example
    ErrorLog ${APACHE_LOG_DIR}/example-error.log
    CustomLog ${APACHE_LOG_DIR}/example-access.log combined
</VirtualHost>

redirect http→https

<VirtualHost *:80>
    ServerName example.com
    RewriteEngine On
    RewriteRule ^/(.*)$ https://example.com/$1 [R=301,L]
</VirtualHost>

basic auth snippet

<Directory /var/www/secure>
    AuthType Basic
    AuthName \"Restricted\"
    AuthUserFile /etc/apache2/.htpasswd
    Require valid-user
</Directory>
# create user: htpasswd -c /etc/apache2/.htpasswd alice

ssl (with let's encrypt)

sudo apt install certbot python3-certbot-apache
sudo certbot --apache -d example.com -d www.example.com
sudo systemctl reload apache2

performance knobs

MPM: switch between mpm_prefork and mpm_event (a2dismod/a2enmod).
KeepAlive On; tune KeepAliveTimeout (1–5s typical).
Enable compression: a2enmod deflate and set AddOutputFilterByType DEFLATE text/html text/css application/javascript.
Cache static: ExpiresActive On, ExpiresByType text/css \"access plus 7 days\", etc.

logs and debugging

Tail combined logs: sudo tail -f /var/log/apache2/access.log /var/log/apache2/error.log
Per-vhost logs defined via ErrorLog and CustomLog.
Increase verbosity: set LogLevel debug inside a vhost/location for temporary troubleshooting.

common errors & fixes

403 forbidden: check DocumentRoot path permissions; ensure Require all granted present or proper AllowOverride with .htaccess.
ah00558 fqdn warning: set ServerName in /etc/apache2/apache2.conf (e.g., ServerName localhost).
redirect loop after https rewrite: guard with RewriteCond %{HTTPS} off before the RewriteRule.
permission denied on logs: check ownership/SELinux; on RHEL run restorecon -R /var/log/httpd.
module not loaded: enable needed module via a2enmod rewrite ssl headers then reload.

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