Steps to solve the problem of 502 errors in Nginx reverse proxy

502 error in Nginx reverse proxy. The following are the specific steps:

1. The nature of 502 error

502 error full nameBad Gateway, the essence isNginx is a proxy server and cannot get a valid response from upstream servers (such as Tomcat/PHP/FastCGI). Common reasons include:

• Upstream server down or unresponsive
• • The proxy timeout is too short
• • Insufficient number of connections/processes
• • Firewall/SELinux Intercept
• • DNS resolution failed

2. Step-by-step troubleshooting and solutions

1. Step 1: Confirm whether the upstream server is alive

operate: Bypass Nginx directly and usecurlAccess backend services

Example：

# Assume that the backend service port is 8080curl http://127.0.0.1:8080

• If return to normal content: The problem is in Nginx configuration
• • If timeout/connection fails: Repair the backend service first

2. Step 2: Adjust the Nginx timeout parameters

Nginx has a short default timeout (such as 60 seconds), which can be extended by the following configuration:

Modify Nginx configuration(path:/etc/nginx/or/*.conf）：

location / {
    proxy_pass http://backend_server;
        # Timeout time for connecting to the upstream server (default 60s)            proxy_connect_timeout 120s;
                # Timeout time for reading response from upstream server (default 60s)                    proxy_read_timeout 120s;
                        # Timeout time for sending a request to the upstream server (default 60s)                            proxy_send_timeout 120s;
                            }

Take effect：

sudo nginx -s reload

3. Step 3: Solve the problem of insufficient connections

Phenomenon: A large number of 502 errors, accompanied by Nginx logsupstream prematurely closed connection

Solution：

• Increase the number of Nginx worker connections：

  worker_processes  4;  # Adjust according to the number of CPU cores    worker_connections  10240;  # Maximum number of connections per worker

• Configure the backend server Keepalive(Reduce frequent new connections):

  upstream backend_server {
        server 192.168.1.10:8080;
              keepalive 32;  # Keep 32 idle connections                }
                  location / {
                        proxy_pass http://backend_server;
                              proxy_http_version 1.1;
                                    proxy_set_header Connection "";  # Close the proxy layer's Connection: close                                      }

4. Step 4: Check the firewall and SELinux

Firewall(Take CentOS as an example):

# Open the port for Nginx to communicate with the backend serversudo firewall-cmd --add-port=8080/tcp --permanent
sudo firewall-cmd --reload

SELinux(Temporarily close the test):

sudo setenforce 0  # Temporary Close
# Or permanently closed（Revise/etc/selinux/config）

5. Step 5: Fix DNS resolution problem

Phenomenon: Use domain names in proxy configuration (such asproxy_pass 502 appears when )

Solution：

• Plan 1: Use the IP address directly
• Plan 2: Configure Nginx dedicated DNS resolver:

  upstream backend_server {
        server  resolve;  # Enable parsing              resolver 8.8.8.8 8.8.4.4;  # Specify the DNS server                    resolver_timeout 5s;  # parse timeout                      }

6. Step 6: Other FAQs

• FastCGI/PHP configuration error(Take PHP as an example):

  location ~ \.php$ {
        fastcgi_pass   127.0.0.1:9000;
              fastcgi_param  SCRIPT_FILENAME  $document_root$fastcgi_script_name;
                    include        fastcgi_params;
                      }

Need to be surephp-fpmThe service runs, andlistenThe port is correct.

• Improper load balancing configuration：
• If usedleast_connorip_hash, you need to check whether the backend server is all healthy.

3. Log positioning skills

View Nginx error log (path:/var/log/nginx/）：

tail -f /var/log/nginx/

Key Error Keywords：

• connect() failed: Connection failed (IP/port error)
• • upstream timed out: Timeout (adjust the timeout parameter)
• • no live upstreams: There are no surviving nodes on the upstream server (check load balancing configuration)
• Follow the above steps to check gradually, 90% of 502 errors can be solved. If the problem remains the same, it is recommended to check the logs of the backend server (such as Tomcat/PHP-FPM logs) for further location.

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