Set Up Apache as a Reverse Proxy with mod_proxy

What is a reverse proxy?

A reverse proxy is like a receptionist. Visitors talk to Apache on the normal web ports. Apache then passes each request to another program running behind it and brings back the answer. Visitors never see the other program. This is useful for apps that listen on odd ports, like 3000.

mod_proxy is the Apache module that does this job. A module is a small add-on for Apache. Root is the all-powerful admin account. Sudo means "run this command as the admin".

Warning: A wrong setting can stop your website. Back up your Apache config files first.

Steps (Ubuntu or Debian)

Log in to your server with SSH.
Turn on the needed modules.
```
sudo a2enmod proxy proxy_http headers
```

Create a site file.

sudo nano /etc/apache2/sites-available/myapp.conf

Add this content. Change the domain and the port to match your app.
```
<VirtualHost *:80>
    ServerName app.example.com
    ProxyPreserveHost On
    ProxyPass / http://127.0.0.1:3000/
    ProxyPassReverse / http://127.0.0.1:3000/
</VirtualHost>
```
ProxyPass sends requests to your app. ProxyPassReverse fixes the addresses in the replies. 127.0.0.1 means "this same server".
Save the file. In nano, press Ctrl+O, Enter, then Ctrl+X.
Turn the site on.
```
sudo a2ensite myapp.conf
```
Test the config for typos.
```
sudo apachectl configtest
```
You want to see Syntax OK.
Reload Apache.
```
sudo systemctl reload apache2
```
Open your domain in a browser. You should see your app.

On CentOS-style systems

The modules are usually on already. Put the same VirtualHost block in a file inside /etc/httpd/conf.d/, for example myapp.conf. Test with httpd -t and reload with sudo systemctl reload httpd. If you see an error with SELinux on, run sudo setsebool -P httpd_can_network_connect 1.

Check that your app is running

Run curl http://127.0.0.1:3000 on the server. If it answers, the app is up. If not, start the app first.

Quick recap

A reverse proxy passes visitors to a hidden app.
Enable proxy and proxy_http.
Use ProxyPass and ProxyPassReverse in a VirtualHost.
Run a config test before you reload.