421 - Misdirected Request

1// Client-side: Handle 421 by retrying with correct endpoint
2async function fetchWithRetry(url) {
3  let response = await fetch(url);
4  
5  if (response.status === 421) {
6    console.warn('Request misdirected - server cannot handle this authority');
7    
8    // Extract alternative URL from response headers
9    const altUrl = response.headers.get('Location') || 
10                   response.headers.get('X-Alternative-Url') ||
11                   response.headers.get('X-Correct-Server');
12    
13    if (altUrl) {
14      console.log(`Retrying with correct server: ${altUrl}`);
15      response = await fetch(altUrl);
16    } else {
17      // Try with different scheme or reconstruct URL
18      const urlObj = new URL(url);
19      
20      // Try HTTPS if HTTP was used
21      if (urlObj.protocol === 'http:') {
22        urlObj.protocol = 'https:';
23        response = await fetch(urlObj.toString());
24      } else {
25        // Try different hostname (if known)
26        throw new Error('Request misdirected and no alternative server provided');
27      }
28    }
29  }
30  
31  return response.json();
32}
33
34// For HTTP/2, close connection and retry
35async function fetchWithHTTP2Handling(url) {
36  try {
37    const response = await fetch(url);
38    
39    if (response.status === 421) {
40      // HTTP/2 connection issue - may need to close connection
41      console.warn('HTTP/2 connection misdirected');
42      // Browser will handle connection reuse, but log for debugging
43    }
44    
45    return response.json();
46  } catch (error) {
47    console.error('Request failed:', error);
48    throw error;
49  }
50}

1// Server-side: Return 421 for misdirected requests
2const express = require('express');
3const app = express();
4
5// Virtual host configuration
6const virtualHosts = {
7  'api.example.com': app,
8  'api-staging.example.com': require('./staging-app'),
9};
10
11// Middleware to check virtual host
12app.use((req, res, next) => {
13  const host = req.get('host');
14  const expectedHost = process.env.API_HOST || 'api.example.com';
15  
16  // Check if request is for this server
17  if (host !== expectedHost && !host.includes('localhost')) {
18    return res.status(421)
19      .set('X-Correct-Server', `https://${expectedHost}${req.originalUrl}`)
20      .json({
21        error: 'Misdirected Request',
22        message: `This server cannot handle requests for ${host}`,
23        correctServer: expectedHost,
24        requestedHost: host,
25      });
26  }
27  
28  next();
29});
30
31// HTTP/2 authority check (if using HTTP/2)
32app.use((req, res, next) => {
33  const authority = req.headers[':authority'] || req.get('host');
34  const scheme = req.protocol;
35  
36  // Check if server is configured for this authority/scheme combination
37  if (!isConfiguredForAuthority(authority, scheme)) {
38    return res.status(421).json({
39      error: 'Misdirected Request',
40      message: 'Server not configured for this authority',
41      authority: authority,
42      scheme: scheme,
43    });
44  }
45  
46  next();
47});
48
49function isConfiguredForAuthority(authority, scheme) {
50  // Check server configuration
51  const configuredAuthorities = ['api.example.com', 'api-staging.example.com'];
52  return configuredAuthorities.includes(authority);
53}

1# Nginx: Configure virtual hosts to prevent 421
2server {
3    listen 443 ssl http2;
4    server_name api.example.com;
5    
6    # SSL configuration
7    ssl_certificate /etc/ssl/certs/api.example.com.crt;
8    ssl_certificate_key /etc/ssl/private/api.example.com.key;
9    
10    # Handle requests for this virtual host
11    location / {
12        proxy_pass http://backend;
13        proxy_set_header Host $host;
14        proxy_set_header X-Real-IP $remote_addr;
15    }
16}
17
18# Catch-all server for misdirected requests
19server {
20    listen 443 ssl http2 default_server;
21    server_name _;
22    
23    # Return 421 for requests to wrong virtual host
24    return 421 '{"error":"Misdirected Request","message":"This server cannot handle requests for this hostname"}';
25    default_type application/json;
26}
27
28# Or redirect to correct server
29# server {
30#     listen 443 ssl http2;
31#     server_name wrong-host.example.com;
32#     return 301 https://api.example.com$request_uri;
33# }