The error message connect() failed (111: Connection refused) while connecting to upstream typically occurs when Nginx cannot establish a connection to the PHP-FPM service. This results in a 502 Bad Gateway error, indicating the web server cannot communicate with the upstream (PHP processor).

Several factors could trigger this issue:

• PHP-FPM service not running
• Incorrect socket or port configuration
• Firewall blocking the connection
• Permission issues
• Resource exhaustion

First, verify if PHP-FPM is running:

sudo systemctl status php-fpm

If it's not running, start it with:

sudo systemctl start php-fpm

Ensure your Nginx server block has the correct FastCGI configuration. Here's a proper example:

location ~ \.php$ {
    include snippets/fastcgi-php.conf;
    fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
    # or use TCP connection:
    # fastcgi_pass 127.0.0.1:9000;
}

You can configure PHP-FPM to use either Unix sockets or TCP ports. Check your /etc/php/7.4/fpm/pool.d/www.conf (adjust version as needed):

; For Unix socket:
listen = /var/run/php/php7.4-fpm.sock

; For TCP connection:
; listen = 127.0.0.1:9000

If using Unix sockets, ensure proper permissions:

sudo chown www-data:www-data /var/run/php/php7.4-fpm.sock
sudo chmod 660 /var/run/php/php7.4-fpm.sock

For TCP connections, verify the port isn't blocked:

sudo ufw allow 9000/tcp
sudo netstat -tulnp | grep 9000

If the issue persists, check PHP-FPM logs:

sudo tail -f /var/log/php7.4-fpm.log

And increase PHP-FPM's pm.max_children if you're hitting resource limits:

pm.max_children = 50
pm.start_servers = 20
pm.min_spare_servers = 10
pm.max_spare_servers = 30

Here's a complete Nginx server block configuration that works with PHP-FPM:

server {
    listen 80;
    server_name example.com;
    root /var/www/html;
    index index.php index.html index.htm;

    location / {
        try_files $uri $uri/ /index.php?$query_string;
    }

    location ~ \.php$ {
        include snippets/fastcgi-php.conf;
        fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        include fastcgi_params;
    }

    location ~ /\.ht {
        deny all;
    }
}

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The error message connect() failed (111: Connection refused) while connecting to upstream typically occurs when Nginx cannot establish a connection to the PHP-FPM service. This results in a 502 Bad Gateway error being returned to the client.

Several factors could lead to this issue:

• PHP-FPM service is not running
• Incorrect socket or port configuration
• Firewall blocking the connection
• Permission issues on the socket file
• Resource exhaustion (too many connections)

First, check if PHP-FPM is running:

sudo systemctl status php-fpm
# or
sudo service php-fpm status

If it's not running, start it:

sudo systemctl start php-fpm
# or
sudo service php-fpm start

Verify your Nginx and PHP-FPM configuration. Here's a typical setup:

Nginx configuration (usually in /etc/nginx/sites-available/your-site):

location ~ \.php$ {
    include snippets/fastcgi-php.conf;
    fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
    # or for TCP connection:
    # fastcgi_pass 127.0.0.1:9000;
}

PHP-FPM pool configuration (usually in /etc/php/7.4/fpm/pool.d/www.conf):

[www]
listen = /var/run/php/php7.4-fpm.sock
; or for TCP connection:
; listen = 127.0.0.1:9000

listen.owner = www-data
listen.group = www-data
listen.mode = 0660

If using Unix sockets (recommended for better performance):

1. Ensure the socket file exists
2. Verify permissions (www-data should have access)
3. Check the path matches in both Nginx and PHP-FPM config

If using TCP connection:

1. Verify PHP-FPM is listening on the correct port
2. Check firewall rules (e.g., ufw or iptables)
3. Test connectivity with telnet 127.0.0.1 9000

If the issue persists, try these steps:

# Check for port conflicts
sudo netstat -tulnp | grep 9000

# Check socket file permissions
ls -la /var/run/php/

# Increase PHP-FPM connection limits
pm.max_children = 50
pm.start_servers = 5
pm.min_spare_servers = 2
pm.max_spare_servers = 8

After making changes:

sudo systemctl restart php-fpm
sudo systemctl restart nginx
sudo tail -f /var/log/nginx/error.log

Remember to check both Nginx and PHP-FPM error logs for more detailed information about the issue.