When dealing with Nginx configuration discrepancies between environments, the 500 error often points to subtle differences that aren't immediately apparent in standard log files. Here's how I approached troubleshooting this particular case:

First, ensure your logging is properly configured in both environments. The production server might need more verbose logging:

error_log /var/log/nginx/error.log debug;
http {
    log_format debug_format '$remote_addr - $remote_user [$time_local] '
                           '"$request" $status $body_bytes_sent '
                           '"$http_referer" "$http_user_agent" '
                           'rt=$request_time uct="$upstream_connect_time" '
                           'uht="$upstream_header_time" urt="$upstream_response_time"';
}

The 500 error might stem from environment-specific factors. Check these potential differences:

• Operating system and kernel versions
• File permission structures
• Available modules (run nginx -V on both machines)
• System limits (ulimit -a comparison)

If you're using proxy_pass or fastcgi, add these debugging directives:

location / {
    proxy_pass http://backend;
    proxy_intercept_errors on;
    proxy_next_upstream error timeout invalid_header http_500 http_502 http_503 http_504;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header Host $http_host;
    proxy_redirect off;
}

When standard logs don't reveal the issue:

1. Strace the Nginx worker process:

strace -p $(cat /var/run/nginx.pid) -f -s 200 -o /tmp/nginx.strace

2. Check for configuration differences with:

diff <(nginx -T on production) <(nginx -T on test)

Version 1.2.1 had known memory management quirks. Try adjusting these parameters:

worker_rlimit_nofile 100000;
worker_connections 4000;
multi_accept on;

List all compiled modules and compare between environments:

nginx -V 2>&1 | tr ' ' '\n' | grep -- '--with-'

Before concluding, perform these checks:

• Verify SELinux/AppArmor policies aren't interfering
• Check for hardware differences (particularly RAM)
• Test with a minimal configuration to isolate the issue
• Consider upgrading production to match test environment

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Recently, I encountered a puzzling scenario where Nginx 1.2.1 in production consistently returned 500 Internal Server Error, while an identical configuration on Nginx 1.4.1 (test environment) worked flawlessly. The access.log and error.log showed nothing unusual - which makes this a particularly sneaky issue.

When standard logs don't reveal the problem, we need deeper investigation techniques:

# Enable debug logging in nginx.conf
error_log /var/log/nginx/error.log debug;

# For more granular tracking in location blocks:
location /problematic-route {
    access_log /var/log/nginx/special-access.log debug;
    error_log /var/log/nginx/special-error.log debug;
    # Your proxy_pass or other config here
}

Between 1.2.1 and 1.4.1, several changes occurred that might affect behavior:

• Proxy header handling improvements
• Memory allocation changes
• Timeout handling modifications

Here's exactly what I did to identify the root cause:

# 1. Compare full configurations
diff -u /etc/nginx/nginx.conf.prod /etc/nginx/nginx.conf.test

# 2. Check for environment variables
env | grep -i nginx

# 3. Test with minimal configuration
# Create a stripped-down nginx.conf with just:
events {}
http {
    server {
        listen 80;
        location / {
            return 200 "OK";
        }
    }
}

After extensive testing, the issue was related to how Nginx 1.2.1 handles large headers. The solution was to add:

proxy_buffer_size 16k;
proxy_buffers 4 16k;

This wasn't needed in 1.4.1 due to improved buffer management in later versions.

To avoid similar issues:

1. Maintain version parity between environments
2. Implement comprehensive header size testing
3. Use configuration validation tools like:

nginx -t -c /path/to/nginx.conf