Apache guide

What Apache does in production

Apache listens on ports (typically 80 and 443), matches each request to a virtual host, and either serves files from disk or forwards the request to a backend via mod_proxy. Access control, URL rewriting, TLS termination, and compression are handled by loadable modules. On SadServers scenarios you debug real configs: wrong Listen ports, broken vhosts, missing modules, and permission errors on files or document directories and analize logs.

How a request is handled

When a client connects, the typical workflow is:

1. TCP accept — Apache accepts the connection on a Listen port
2. TLS handshake — if HTTPS, mod_ssl negotiates the certificate (SNI determines which certificate to present); vhost selection happens in step 3
3. IP/Port match — Apache selects candidate <VirtualHost> blocks that match the local IP address and port the client connected to
4. Name based virtual host match — Apache then looks at the Host header and picks a <VirtualHost> using ServerName, ServerAlias
5. Request processing — Apache determines the handler via DocumentRoot, Alias, ProxyPass, or rewrite rules
6. Access check — Require directives allow or deny the request
7. Response — static file, CGI, or proxied backend response is served; then logged to access_log / error_log

If no vhost matches, the first vhost for that port is used as the default — a common source of "wrong site" bugs.

Configuration layout

Paths differ by distribution:

• Debian/Ubuntu (package: `apache2`) — /etc/apache2/apache2.conf, sites in sites-available/ / sites-enabled/, modules via a2enmod / a2dismod
• RHEL/CentOS/Rocky (packate: `httpd`) — /etc/httpd/conf/httpd.conf, vhosts in conf.d/, modules in modules/

Always run a syntax check before reload: apache2ctl configtest (Debian) or httpd -t (RHEL).

Key concepts to know

• Virtual hosts — IP / Port first, then ServerName / ServerAlias matching against the Host header
• Modules — LoadModule directive (or a2enmod on Debian), MPM choice (prefork/worker/event) affects performance and threading
• Logs — ErrorLog for failures; CustomLog for access; check these first
• Reverse proxy — ProxyPass / ProxyPassReverse to app servers (Gunicorn, Node, etc.) ProxyPreserveHost is a common pitfall
• systemd — service unit apache2 or httpd; reload gracefully restart workers without dropping existing connections
• .htaccess — per-directory overrides (if AllowOverride permits); slower than vhost config because Apache re-parses it per request; prefer vhost config for production

Learning resources

• Apache HTTP Server documentation — httpd.apache.org/docs/2.4 (official 2.4 reference)
• Virtual hosts — httpd.apache.org/docs/2.4/vhosts (name-based vhost guide)
• mod_proxy — httpd.apache.org/docs/2.4/mod/mod_proxy (reverse proxy directives)
• apache2ctl(8) — manpages.debian.org/apache2ctl (Debian control tool)