When Nginx and Apache run on the same server, using Unix domain sockets instead of TCP connections can significantly reduce overhead. This eliminates TCP/IP stack processing, connection state maintenance, and port allocation - three major performance bottlenecks in local inter-process communication.

First, modify your Apache virtual host to listen on a Unix socket instead of a TCP port. Edit your Apache configuration file (typically in /etc/apache2/sites-available/):

<VirtualHost *:82>
    # Change this to use Unix socket
    Listen /var/run/apache2/apache.sock
    
    ServerName site.net
    DocumentRoot /var/www/html
    
    <Directory /var/www/html>
        Options Indexes FollowSymLinks
        AllowOverride All
        Require all granted
    </Directory>
</VirtualHost>

Then update Apache's main configuration to enable the Unix socket module (if not already enabled):

LoadModule unixd_module modules/mod_unixd.so

Now modify your Nginx configuration to use the Unix socket instead of localhost:82:

server {
    server_name site.net;
    
    location / {
        proxy_pass http://unix:/var/run/apache2/apache.sock:;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        
        # Important for Unix socket performance
        proxy_http_version 1.1;
        proxy_set_header Connection "";
    }
}

Unix sockets require proper permissions. After restarting Apache, verify the socket file exists and has correct permissions:

sudo chown www-data:www-data /var/run/apache2/apache.sock
sudo chmod 660 /var/run/apache2/apache.sock

You may need to adjust these based on your specific user/group configuration.

To verify the performance improvement, you can use ab (Apache Benchmark):

ab -n 10000 -c 100 http://site.net/test.html

Compare the results with your previous TCP-based configuration. Typically, you'll observe:

• 20-30% reduction in connection time
• Lower CPU usage during high concurrency
• More stable performance under heavy load

If you encounter 502 Bad Gateway errors:

1. Verify the socket file exists after Apache starts
2. Check Nginx error logs: tail -f /var/log/nginx/error.log
3. Confirm SELinux/apparmor isn't blocking socket access
4. Ensure the socket path in Nginx exactly matches Apache's configuration

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Many developers use Nginx as a reverse proxy for Apache with TCP connections like this:

server {
    server_name site.net;
    location / {
        proxy_pass http://localhost:82;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
    }
}

Unix domain sockets offer several advantages over TCP for local communication:

• Lower latency (no TCP stack overhead)
• Higher throughput
• Better security (filesystem permissions instead of network ports)
• No port conflicts

First, modify your Apache configuration to listen on a Unix socket instead of TCP:

# In httpd.conf or apache2.conf
Listen /var/run/apache2/apache.sock

<VirtualHost unix:/var/run/apache2/apache.sock>
    # Your virtual host configuration
</VirtualHost>

Make sure the socket directory exists and has proper permissions:

sudo mkdir -p /var/run/apache2
sudo chown www-data:www-data /var/run/apache2

Modify your Nginx configuration to use the Unix socket:

server {
    server_name site.net;
    location / {
        proxy_pass http://unix:/var/run/apache2/apache.sock;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }
}

For optimal performance with Unix sockets, consider these additional settings:

location / {
    proxy_pass http://unix:/var/run/apache2/apache.sock;
    proxy_http_version 1.1;
    proxy_set_header Connection "";
    proxy_buffering off;
    proxy_request_buffering off;
}

If you encounter problems:

1. Check socket permissions: ls -la /var/run/apache2/
2. Verify Apache is listening: ss -a | grep apache.sock
3. Check error logs for both Nginx and Apache

Use tools like ab or wrk to compare performance:

ab -n 10000 -c 100 http://site.net/test.html

Expect to see significant improvements in requests per second and reduced latency.