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Nginx Proxy Pass Not Working

Fix Nginx proxy_pass connection failures and upstream communication errors

Published: Nov 19, 20258 min readBy FixWikiHub Editorial Team

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Introduction

Nginx is configured to proxy requests to an upstream server using proxy_pass, but the proxy connection is failing. Requests return errors like 502 Bad Gateway, 504 Gateway Timeout, or connection refused. The upstream server may be unreachable, misconfigured, or rejecting connections.

Symptoms

```bash $ curl http://localhost/api <html> <head><title>502 Bad Gateway</title></head> <body> <center><h1>502 Bad Gateway</h1></center> </body> </html>

$ tail -f /var/log/nginx/error.log 2024/01/01 12:00:00 [error] connect() failed (111: Connection refused) while connecting to upstream 2024/01/01 12:00:00 [error] connect() failed (113: No route to host) while connecting to upstream 2024/01/01 12:00:00 [error] no live upstreams while connecting to upstream

$ curl -I http://localhost/api HTTP/1.1 502 Bad Gateway

# Common error codes in logs: # (111) Connection refused - Service not running or wrong port # (113) No route to host - Network/firewall issue # (110) Connection timed out - Service slow or unreachable # (101) Network is unreachable - Network configuration issue ```

Common Causes

1.Backend not running: Upstream service is down
2.Wrong port: proxy_pass pointing to wrong port
3.Wrong host: Incorrect hostname or IP address
4.DNS resolution failure: Cannot resolve upstream hostname
5.Firewall blocking: Network rules preventing connection
6.Wrong protocol: HTTP vs HTTPS mismatch
7.SELinux blocking: Security policy preventing connection
8.Upstream server overload: Backend refusing new connections
9.Proxy headers missing: Upstream rejecting due to missing headers
10.Keepalive issues: Connection pooling problems

Step-by-Step Fix

1.Check logs for specific error messages
2.Verify configuration settings
3.Test network connectivity
4.Review recent changes
5.Apply corrective action
6.Verify the fix

Step 1: Verify Backend Server

Confirm the upstream is running:

```bash # Check if backend is running on expected port netstat -tulpn | grep :3000 ss -tulpn | grep :3000

# Check process ps aux | grep node ps aux | grep python systemctl status myapp

# Test direct connection to backend curl -I http://127.0.0.1:3000 curl -I http://localhost:3000 curl -I http://backend-server:8080

# Test with nc nc -zv 127.0.0.1 3000 telnet 127.0.0.1 3000

# For Docker/containerized backends docker ps docker logs backend-container ```

Step 2: Check Proxy Configuration

Verify proxy_pass settings:

bash

# View current configuration
grep -r "proxy_pass" /etc/nginx/
nginx -T 2>/dev/null | grep -A 20 "proxy_pass"

Correct proxy_pass syntax: ```nginx # Basic proxy location /api { proxy_pass http://127.0.0.1:3000; }

# With upstream block upstream backend { server 127.0.0.1:3000; }

location /api { proxy_pass http://backend; }

# With trailing slash (changes URI) location /api/ { proxy_pass http://127.0.0.1:3000/; # /api/users -> /users }

# Without trailing slash location /api/ { proxy_pass http://127.0.0.1:3000; # /api/users -> /api/users } ```

Step 3: Fix Wrong Port or Host

Correct upstream address:

```nginx # WRONG: Wrong port location /api { proxy_pass http://127.0.0.1:3000; # But app runs on 8080 }

# CORRECT: Match actual port location /api { proxy_pass http://127.0.0.1:8080; # Correct port }

# WRONG: Wrong hostname location /api { proxy_pass http://backend-server:3000; # Wrong hostname }

# CORRECT: Correct hostname location /api { proxy_pass http://app-server:3000; }

# For Docker services location /api { proxy_pass http://container-name:3000; # Or use Docker network name } ```

Step 4: Add Required Proxy Headers

Include necessary headers:

```nginx location /api { proxy_pass http://127.0.0.1:3000;

# Essential headers proxy_set_header Host $host; proxy_set_header X-Real-IP $remote_addr; proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for; proxy_set_header X-Forwarded-Proto $scheme; proxy_set_header X-Forwarded-Host $host; proxy_set_header X-Forwarded-Port $server_port; }

# For WebSocket location /ws { proxy_pass http://127.0.0.1:3000; proxy_http_version 1.1; proxy_set_header Upgrade $http_upgrade; proxy_set_header Connection "upgrade"; proxy_set_header Host $host; }

# For HTTPS upstream location /api { proxy_pass https://api.example.com; proxy_ssl_server_name on; proxy_set_header Host api.example.com; } ```

Step 5: Fix DNS Resolution Issues

Handle DNS problems:

```nginx # Using resolver for dynamic upstreams location /api { resolver 8.8.8.8 8.8.4.4 valid=300s; resolver_timeout 5s;

# Use variable to force DNS resolution set $upstream http://backend.example.com:3000; proxy_pass $upstream; }

# For Kubernetes services location /api { resolver kube-dns.kube-system.svc.cluster.local valid=5s; proxy_pass http://my-service.default.svc.cluster.local:3000; }

# For Docker Compose location /api { proxy_pass http://backend:3000; # Use service name } ```

Check DNS resolution: ```bash # Test DNS resolution nslookup backend.example.com dig backend.example.com

# Test from nginx server getent hosts backend.example.com

# Add to /etc/hosts if needed echo "192.168.1.100 backend.example.com" | sudo tee -a /etc/hosts ```

Step 6: Configure Timeouts

Set appropriate timeouts:

```nginx location /api { proxy_pass http://backend:3000;

# Connection timeout proxy_connect_timeout 60s;

# Read timeout (waiting for response) proxy_read_timeout 60s;

# Send timeout (sending request) proxy_send_timeout 60s;

# For long-running requests proxy_read_timeout 300s; }

# Upstream-level timeouts upstream backend { server 127.0.0.1:3000; keepalive 32; keepalive_timeout 60s; } ```

Step 7: Configure Connection Pooling

Enable keepalive connections:

```nginx upstream backend { server 127.0.0.1:3000; keepalive 32; # Keep 32 connections open keepalive_timeout 60s; keepalive_requests 1000; }

server { location /api { proxy_pass http://backend;

# Required for keepalive proxy_http_version 1.1; proxy_set_header Connection "";

# Don't close connections proxy_set_header Connection ""; } } ```

Step 8: Handle Firewall and SELinux

Allow network connections:

```bash # Check firewall rules sudo iptables -L -n sudo firewall-cmd --list-all

# Allow connections to backend sudo firewall-cmd --add-port=3000/tcp --permanent sudo firewall-cmd --reload

# For SELinux (CentOS/RHEL) getenforce # If Enforcing, allow HTTP network connections sudo setsebool -P httpd_can_network_connect 1 sudo setsebool -P httpd_can_network_relay 1

# Check SELinux denials sudo ausearch -m avc -ts recent | grep nginx

# Create SELinux policy if needed sudo grep nginx /var/log/audit/audit.log | audit2allow -M nginx-network sudo semodule -i nginx-network.pp ```

Step 9: Configure Buffer Settings

Handle large requests/responses:

```nginx location /api { proxy_pass http://backend:3000;

# Request body buffering proxy_request_buffering on; client_max_body_size 100m; client_body_buffer_size 1m;

# Response buffering proxy_buffering on; proxy_buffer_size 4k; proxy_buffers 8 16k; proxy_busy_buffers_size 32k;

# For large responses proxy_buffers 8 32k; proxy_buffer_size 64k;

# For streaming (disable buffering) proxy_buffering off; proxy_request_buffering off; } ```

Step 10: Debug Proxy Issues

Enable detailed logging:

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

# Log upstream address log_format proxy_log '$remote_addr - $remote_user [$time_local] ' '"$request" $status $body_bytes_sent ' 'upstream: $upstream_addr ' 'upstream_status: $upstream_status ' 'request_time: $request_time ' 'upstream_response_time: $upstream_response_time';

access_log /var/log/nginx/proxy.log proxy_log; ```

```bash # Watch proxy logs tail -f /var/log/nginx/error.log | grep -i upstream

# Test specific endpoint curl -v http://localhost/api/test

# Check upstream connectivity ss -tnp | grep nginx

# Monitor connections watch -n 1 'ss -tnp | grep nginx | wc -l' ```

Step 11: Test Upstream Directly

Bypass Nginx to isolate issue:

```bash # Test backend directly curl -v http://127.0.0.1:3000/api/test curl -v http://backend-server:8080/health

# Test with same headers Nginx sends curl -H "Host: localhost" \ -H "X-Real-IP: 127.0.0.1" \ http://127.0.0.1:3000/api/test

# Test specific endpoints for endpoint in /api/health /api/status /api/test; do echo "Testing $endpoint:" curl -s -o /dev/null -w "%{http_code}\n" http://127.0.0.1:3000$endpoint done

# Check if backend is listening on expected interface # May be listening on 127.0.0.1 but not 0.0.0.0 netstat -tulpn | grep 3000 ```

Step 12: Fix Common Upstream Issues

Address backend problems:

```bash # If backend not running, start it systemctl start myapp # Or for Node.js pm2 start app.js # Or for Python gunicorn app:app

# If wrong interface binding, reconfigure app # Change from 127.0.0.1 to 0.0.0.0 to accept all interfaces

# Check backend logs journalctl -u myapp -f tail -f /var/log/app/error.log

# For Docker docker logs -f backend-container docker inspect backend-container | grep IPAddress ```

Verification

Confirm proxy is working:

```bash # Test proxy endpoint curl -I http://localhost/api # Should return 200 OK

# Check error log for proxy errors tail -20 /var/log/nginx/error.log | grep -i "proxy|upstream" # Should not show connection errors

# Test multiple requests for i in {1..10}; do curl -s -o /dev/null -w "Request $i: %{http_code}\n" http://localhost/api done

# Verify proxy headers being sent curl -v http://localhost/api 2>&1 | grep -E "X-Forwarded|X-Real"

# Check upstream connectivity ss -tnp | grep :3000

# Monitor during load ab -n 100 -c 10 http://localhost/api/ ```

Proxy connections should now succeed and return proper responses from the upstream server.

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How do I know if this nginx-errors troubleshooting guide applies to my situation?

This guide is designed for nginx-errors issues. If you're experiencing similar symptoms described in the article, follow the step-by-step instructions. Start with the most common causes and work through the diagnostic process.

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Last updated: Nov 19, 2025

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Nginx Proxy Pass Not Working

Introduction

Symptoms

Common Causes

Step-by-Step Fix

Step 1: Verify Backend Server

Step 2: Check Proxy Configuration

Step 3: Fix Wrong Port or Host

Step 4: Add Required Proxy Headers

Step 5: Fix DNS Resolution Issues

Step 6: Configure Timeouts

Step 7: Configure Connection Pooling

Step 8: Handle Firewall and SELinux

Step 9: Configure Buffer Settings

Step 10: Debug Proxy Issues

Step 11: Test Upstream Directly

Step 12: Fix Common Upstream Issues

Verification

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FixWikiHub Editorial Team

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Official Documentation & Further Reading

Nginx Proxy Pass Not Working

Introduction

Symptoms

Common Causes

Step-by-Step Fix

Step 1: Verify Backend Server

Step 2: Check Proxy Configuration

Step 3: Fix Wrong Port or Host

Step 4: Add Required Proxy Headers

Step 5: Fix DNS Resolution Issues

Step 6: Configure Timeouts

Step 7: Configure Connection Pooling

Step 8: Handle Firewall and SELinux

Step 9: Configure Buffer Settings

Step 10: Debug Proxy Issues

Step 11: Test Upstream Directly

Step 12: Fix Common Upstream Issues

Verification

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People also search for

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Nginx Troubleshooting FAQs

FixWikiHub Editorial Team

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