What are common Nginx problems? How to troubleshoot?

Nginx may encounter various problems during operation. Mastering troubleshooting methods is crucial for quickly resolving issues.

Common Problems and Solutions:

1. 502 Bad Gateway

Cause: Backend service unavailable or connection timeout

Troubleshooting Steps:

bash
# Check backend service status
systemctl status php-fpm
systemctl status nginx

# Check backend service port
netstat -tlnp | grep :9000

# Check Nginx error log
tail -f /var/log/nginx/error.log

# Check backend service log
tail -f /var/log/php-fpm/error.log

Solution:

nginx
# Increase timeout
proxy_connect_timeout 60s;
proxy_send_timeout 60s;
proxy_read_timeout 60s;

# Check backend service configuration
fastcgi_connect_timeout 60s;
fastcgi_send_timeout 60s;
fastcgi_read_timeout 60s;

2. 504 Gateway Timeout

Cause: Backend service processing time too long

Troubleshooting Steps:

bash
# Check backend service performance
top -u nginx
htop

# Check database connections
mysql -u root -p -e "SHOW PROCESSLIST;"

# Check slow query log
tail -f /var/log/mysql/slow.log

Solution:

nginx
# Increase timeout
proxy_read_timeout 300s;
fastcgi_read_timeout 300s;

# Optimize backend service performance
# Optimize database queries
# Add caching

3. 403 Forbidden

Cause: Insufficient permissions or access control restrictions

Troubleshooting Steps:

bash
# Check file permissions
ls -la /var/www/html

# Check Nginx user
ps aux | grep nginx

# Check SELinux status
getenforce

# Check firewall rules
iptables -L -n

Solution:

bash
# Modify file permissions
chown -R nginx:nginx /var/www/html
chmod -R 755 /var/www/html

# Temporarily disable SELinux
setenforce 0

# Add firewall rules
firewall-cmd --add-service=http --permanent
firewall-cmd --reload

4. 404 Not Found

Cause: File doesn't exist or path configuration error

Troubleshooting Steps:

bash
# Check if file exists
ls -la /var/www/html

# Check Nginx configuration
nginx -T | grep root

# Check symbolic links
readlink -f /var/www/html

Solution:

nginx
# Check root configuration
server {
    listen 80;
    server_name example.com;
    root /var/www/html;
    index index.html index.php;
    
    location / {
        try_files $uri $uri/ =404;
    }
}

5. 413 Request Entity Too Large

Cause: Uploaded file exceeds limit

Solution:

nginx
# Increase client_max_body_size
client_max_body_size 100m;

# PHP configuration
# /etc/php.ini
upload_max_filesize = 100M
post_max_size = 100M

6. Insufficient Connections

Cause: worker_connections set too small

Troubleshooting Steps:

bash
# Check current connections
netstat -an | grep :80 | wc -l

# Check Nginx status
curl http://localhost/nginx_status

# Check system limits
ulimit -n

Solution:

nginx
# Increase connections
events {
    worker_connections 10240;
}

# Increase file descriptor limit
worker_rlimit_nofile 65535;

Diagnostic Tools:

1. Configuration Testing

bash
# Test configuration file
nginx -t

# Display configuration
nginx -T

# Check configuration syntax
nginx -c /etc/nginx/nginx.conf -t

2. Status Monitoring

nginx
# Enable status page
location /nginx_status {
    stub_status on;
    access_log off;
    allow 127.0.0.1;
    deny all;
}

3. Log Analysis

bash
# Real-time error log viewing
tail -f /var/log/nginx/error.log

# View last 100 lines of errors
tail -n 100 /var/log/nginx/error.log

# Search for specific errors
grep "502" /var/log/nginx/error.log

# Count errors
awk '{print $9}' /var/log/nginx/access.log | sort | uniq -c | sort -rn

4. Performance Analysis

bash
# Use strace to trace system calls
strace -p $(pidof nginx)

# Use tcpdump to capture packets
tcpdump -i eth0 port 80 -w nginx.pcap

# Use netstat to view connections
netstat -an | grep :80 | awk '{print $6}' | sort | uniq -c

Performance Problem Troubleshooting:

1. High CPU Usage

bash
# Check CPU usage
top -p $(pidof nginx)

# Check worker process count
ps aux | grep nginx | wc -l

# Check CPU affinity
taskset -cp $(pidof nginx)

Solution:

nginx
# Adjust worker_processes
worker_processes auto;

# Bind CPU cores
worker_cpu_affinity auto;

# Enable efficient file transfer
sendfile on;
tcp_nopush on;

2. High Memory Usage

bash
# Check memory usage
free -m

# Check process memory
ps aux | grep nginx | awk '{print $6}' | awk '{sum+=$1} END {print sum}'

# Check memory leaks
valgrind --leak-check=full nginx

Solution:

nginx
# Reduce buffer sizes
client_body_buffer_size 128k;
client_header_buffer_size 1k;

# Optimize connections
worker_connections 4096;

# Enable file caching
open_file_cache max=100000 inactive=20s;

3. Slow Response

bash
# Check response time
curl -w "@curl-format.txt" -o /dev/null -s http://example.com

# Check network latency
ping example.com

# Check DNS resolution
nslookup example.com

Solution:

nginx
# Enable Gzip compression
gzip on;
gzip_min_length 1024;

# Enable caching
proxy_cache_path /var/cache/nginx levels=1:2 keys_zone=cache:10m;

# Optimize TCP parameters
tcp_nodelay on;
tcp_nopush on;

Security Problem Troubleshooting:

1. DDoS Attacks

bash
# Check abnormal connections
netstat -an | grep :80 | awk '{print $5}' | cut -d: -f1 | sort | uniq -c | sort -rn

# Check request frequency
awk '{print $1}' /var/log/nginx/access.log | sort | uniq -c | sort -rn | head -20

Solution:

nginx
# Enable rate limiting
limit_req_zone $binary_remote_addr zone=limit:10m rate=10r/s;
limit_req zone=limit burst=20 nodelay;

# Limit connections
limit_conn_zone $binary_remote_addr zone=conn:10m;
limit_conn conn 10;

2. Malicious Access

bash
# Check suspicious User-Agents
grep "bot" /var/log/nginx/access.log

# Check SQL injection attempts
grep "union.*select" /var/log/nginx/access.log

Solution:

nginx
# Block malicious User-Agents
if ($http_user_agent ~* (bot|crawl|spider)) {
    return 403;
}

# Prevent SQL injection
if ($args ~* "union.*select.*\(") {
    return 403;
}

Monitoring and Alerting:

1. System Monitoring

bash
# Use Prometheus + Grafana
# Use Zabbix
# Use Nagios

2. Log Monitoring

bash
# Use ELK Stack
# Use Graylog
# Use Fluentd

3. Automatic Alerting

bash
# Use Alertmanager
# Use PagerDuty
# Use Slack integration

Best Practices:

Regular configuration backups: Backup Nginx configuration files
Monitor logs: Real-time monitoring of error logs
Performance testing: Regular stress testing
Documentation: Record common problems and solutions
Automated deployment: Use configuration management tools
Version control: Use Git to manage configuration files
Regular updates: Keep Nginx version up to date
Security audits: Regular security checks

Troubleshooting Workflow:

shell
1. Confirm problem symptoms
   ↓
2. Check Nginx status
   ↓
3. View error logs
   ↓
4. Check configuration files
   ↓
5. Check backend services
   ↓
6. Check system resources
   ↓
7. Apply solution
   ↓
8. Verify fix
   ↓
9. Document problem and solution