How to troubleshoot Nginx logs

Using Nginx logs to troubleshoot server failures is an efficient operation and maintenance method, which can help you understand the server's operating status, accurately locate problems and implement optimization. This article will introduce several common Nginx log types and troubleshooting methods.

1. Access log

The access log records all request information for accessing the Nginx server.

Key fields:

• $remote_addr : Client IP address
• $http_user_agent : Client User-Agent string
• $http_referer : Request source page
• $status : HTTP status code
• $body_bytes_sent : The number of response body bytes sent to the client
• $request_time : Request processing time

Troubleshooting steps:

1. Identify 4xx and 5xx errors: Find requests with status codes of 4xx (client error) and 5xx (server error), analyze error information and request paths.

    grep ' [45][0-9]{2} ' /var/log/nginx/access.log

2. Analyze access mode: Use awk or grep to analyze visits, visit frequency and popular pages.

    awk '{print $7}' /var/log/nginx/access.log | cut -d'/' -f3 | sort | uniq -c | sort -nr

3. Detect slow requests: Based on the $request_time field, find requests that have been processed for too long.

    awk '$4 > 1' /var/log/nginx/access.log

2. Error log (error log)

The error log records error information during Nginx running.

Key fields:

• $time_local : local time
• $remote_addr : Client IP address
• $request : request line
• $status : HTTP status code
• $body_bytes_sent : The number of response body bytes sent to the client
• $http_user_agent : Client User-Agent string

Troubleshooting steps:

1. View the latest error: View the most recent error information in the error log to understand the cause of the error.

    tail -n 100 /var/log/nginx/error.log

2. Analyze error types: Use grep to find specific types of errors (such as configuration errors, permission issues).

    grep 'permission denied' /var/log/nginx/error.log

3. Custom logs

Nginx supports custom log formats to enable more granular recording of specific information.

Troubleshooting steps:

1. Configure custom logs: Configure custom log formats in Nginx configuration files.

    http {
       log_format custom '$remote_addr - $remote_user [$time_local] "$request" '
                         '$status $body_bytes_sent "$http_referer" '
                         '"$http_user_agent" "$http_x_forwarded_for"';
       access_log /var/log/nginx/access_custom.log custom;
   }

2. View custom logs: View custom log files for more detailed information.

    tail -n 100 /var/log/nginx/access_custom.log

Summarize

By analyzing Nginx access logs and error logs, you can effectively troubleshoot, understand the server's health status, and take corresponding optimization measures. Remember to check and clean log files regularly to avoid taking up too much disk space.

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