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4xx Client & Proxy Request

HTTP 400 Bad Request — Reverse Proxy & Gateway Troubleshooting

HTTP 400 Bad Request indicates that the server or edge reverse proxy refused to process the request due to malformed HTTP syntax, oversized cookie headers, invalid TLS-on-plaintext port traffic, or strict Host header validation failure.

Status Code400 Bad Request
Specification / OriginRFC 9110 §15.5.1
Failure LayerReverse Proxy / Ingress Parser
Retry SafetyDo not blind-retry without fixing request syntax, auth, or origin certificate

Interactive HTTP 400 Live Probe & Log Command Builder

Enter your target endpoint below to generate copy-ready curl latency breakdown probes and origin bypass tests:


  

    Related diagnostics:
    Nginx & Caddy Proxy Generator → ·
    X.509 SSL Certificate Decoder → ·
    Offline JWT Expiry Inspector →
  






  
Production Root Causes & Remediation Guide

  
In production Nginx, Envoy, and AWS ALB deployments, 400 errors most often stem from oversized SSO/JWT cookies exceeding large_client_header_buffers, plain HTTP sent to an HTTPS listener port, or malformed percent-encoding in query strings.

  
Step-by-Step Engineering Fix

  
Inspect reverse-proxy error logs for 'Request Header Or Cookie Too Large', verify the Host header matches server_name or Kubernetes Ingress rules, and expand header buffer allocations carefully if enterprise OAuth cookies exceed 8 KiB.

  
Recommended Nginx / Reverse-Proxy Configuration Snippet

  
large_client_header_buffers 4 32k;
client_header_buffer_size 8k;







  
Frequently Asked Questions (HTTP 400)

  
  

    Why does Nginx return 400 Bad Request only after users log in?
    
Authentication providers often set large session or JWT cookies that push the Cookie header past Nginx's default 8 KiB buffer; raising large_client_header_buffers to 4 32k resolves it.

  

  
  

    Which infrastructure layer generates HTTP 400 Bad Request?
    
HTTP 400 (RFC 9110 §15.5.1) typically originates at the Reverse Proxy / Ingress Parser layer. HTTP 400 Bad Request indicates that the server or edge reverse proxy refused to process the request due to malformed HTTP syntax, oversized cookie headers, invalid TLS-on-plaintext port traffic, or strict Host header validation failure.

  

  
  

    Should API clients automatically retry HTTP 400 responses?
    
Do not blind-retry HTTP 400 without fixing the underlying request payload, authentication token, or origin TLS certificate first.

  

  






  
Browse All 25 Gateway, Proxy & Cloud HTTP Error Guides

  

    
      HTTP 400 (Bad Request)
    
      HTTP 401 (Unauthorized)
    
      HTTP 403 (Forbidden)
    
      HTTP 404 (Not Found)
    
      HTTP 405 (Method Not Allowed)
    
      HTTP 408 (Request Timeout)
    
      HTTP 413 (Content Too Large)
    
      HTTP 422 (Unprocessable Content)
    
      HTTP 429 (Too Many Requests)
    
      HTTP 431 (Request Header Fields Too Large)
    
      HTTP 499 (Client Closed Request)
    
      HTTP 500 (Internal Server Error)
    
      HTTP 501 (Not Implemented)
    
      HTTP 502 (Bad Gateway)
    
      HTTP 503 (Service Unavailable)
    
      HTTP 504 (Gateway Timeout)
    
      HTTP 507 (Insufficient Storage)
    
      HTTP 511 (Network Authentication Required)
    
      HTTP 520 (Web Server Returned an Unknown Error)
    
      HTTP 521 (Web Server Is Down)
    
      HTTP 522 (Connection Timed Out)
    
      HTTP 523 (Origin Is Unreachable)
    
      HTTP 524 (A Timeout Occurred)
    
      HTTP 525 (SSL Handshake Failed)
    
      HTTP 526 (Invalid SSL Certificate)
    
  






  


  
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