HTTP 431 Request Header Fields Too Large

Overview

The HTTP 431 Request Header Fields Too Large status code is returned by the server when the size of the client’s request headers exceeds the server’s limit. This response indicates that the server is unwilling to process the request because the request headers are too large.

Purpose

The HTTP 431 response is used to prevent clients from overwhelming servers with an excessive amount of request header data, which can impact server performance.

Usage

Client Behavior:

1. Send Request: The client sends an HTTP request with headers exceeding the server’s limit.
2. Receive 431 Response: The client receives an HTTP 431 status code, indicating that the request headers are too large.
3. Adjust Request: The client adjusts the request headers and resends the request.

Server Behavior:

1. Detect Header Size Exceedance: The server detects that the size of the client’s request headers exceeds the configured limit.
2. Send 431 Response: The server responds with an HTTP/1.1 431 Request Header Fields Too Large status code.

Scenarios

• Large Request Headers: Used when the client sends request headers that exceed the server’s defined limit.

Sequence Diagram

Illustrating the process for an HTTP 431 response:

sequenceDiagram
    participant Client
    participant Server as Web Server

    Note over Client: Step 1: Client sends a request with large headers
    Client->>Server: HTTP Request with Large Headers (Step 1)

    Note over Server: Step 2: Server detects header size exceedance
    Server->>Client: HTTP/1.1 431 Request Header Fields Too Large (Step 2)

Curl Request and Response Example

Sending a request with large headers using Curl:

curl -i --header "Host: example.com" --header "Large-Header: <large_data>" http://example.com/resource
# Expected response: HTTP/1.1 431 Request Header Fields Too Large

PHP cURL Request and Response Example

PHP script using cURL to handle a 431 Request Header Fields Too Large response:

<?php
$ch = curl_init('http://example.com/resource');
curl_setopt($ch, CURLOPT_HTTPHEADER, [
    'Host: example.com',
    'Large-Header: <large_data>',
]);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$response = curl_exec($ch);
if (curl_getinfo($ch, CURLINFO_HTTP_CODE) == 431) {
    echo "Request Header Fields Too Large";
}
curl_close($ch);
?>

Python Request and Response Example

Python script to handle a 431 Request Header Fields Too Large response:

import requests
headers = {
    'Host': 'example.com',
    'Large-Header': '<large_data>',
}
response = requests.get('http://example.com/resource', headers=headers)
if response.status_code == 431:
    print("Request Header Fields Too Large")

Apache Configuration for HTTP 431 Request Header Fields Too Large

Configuring Apache to set a limit on request header size:

<VirtualHost *:80>
    ServerName example.com
    LimitRequestFieldSize 8190
</VirtualHost>

NGINX Configuration for HTTP 431 Request Header Fields Too Large

Setting up NGINX to limit request header size:

server {
    listen 80;
    server_name example.com;
    http {
        client_header_buffer_size 8k;
    }
}

HTTP 429 Too Many Requests HTTP 444 No Response (Nginx)