When working with HTTP protocols, certain header fields are absolutely mandatory for a properly formatted request. These headers form the foundation of web communication between clients and servers.

Every HTTP/1.1 request must include these essential headers:

GET /index.html HTTP/1.1
Host: www.example.com
Connection: close

1. The Request Line
This isn't technically a header but must appear first:

GET /path HTTP/1.1

2. Host Header
Required in HTTP/1.1 to support virtual hosting:

Host: api.example.com

While technically not mandatory, these headers are considered essential in modern web development:

User-Agent: Mozilla/5.0
Accept: text/html,application/xhtml+xml
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate

Here's a complete Python example using requests:

import requests

headers = {
    'Host': 'api.example.com',
    'User-Agent': 'MyApp/1.0',
    'Accept': 'application/json'
}

response = requests.get('http://example.com/api', headers=headers)

And a Node.js example:

const http = require('http');

const options = {
  hostname: 'example.com',
  path: '/api',
  method: 'GET',
  headers: {
    'Host': 'example.com',
    'User-Agent': 'MyApp/1.0',
    'Accept': 'application/json'
  }
};

const req = http.request(options, (res) => {
  // Handle response
});

Developers often forget that:

• HTTP/1.1 absolutely requires the Host header
• Some proxies and CDNs may reject requests missing certain headers
• Mobile clients sometimes omit important headers

For REST APIs, these additional headers are often necessary:

Content-Type: application/json
Authorization: Bearer token_value

Remember that while certain headers are technically optional, modern web infrastructure often expects them to be present for proper functioning.

---

When making HTTP requests, certain header fields are mandatory for the request to be properly processed by servers. While HTTP/1.1 is more lenient than HTTP/2 in terms of required headers, there are still fundamental fields that should always be included.

For a basic HTTP/1.1 request, these headers are essential:

GET /index.html HTTP/1.1
Host: www.example.com

Host Header (Required in HTTP/1.1):

Host: api.domain.com

This specifies the domain name of the server and is crucial for virtual hosting scenarios.

While not strictly mandatory, these headers should be included in production requests:

User-Agent: MyApp/1.0
Accept: */*
Accept-Language: en-US
Connection: keep-alive

HTTP/2 makes the Host header optional by using the :authority pseudo-header instead:

:method: GET
:path: /
:scheme: https
:authority: example.com

Missing Host headers often result in 400 Bad Request errors. Here's how to debug:

curl -v http://example.com/
* Rebuilt URL to: http://example.com/
*   Trying 93.184.216.34...
* TCP_NODELAY set
* Connected to example.com (93.184.216.34) port 80 (#0)
> GET / HTTP/1.1
> Host: example.com
> User-Agent: curl/7.64.1
> Accept: */*

For API requests, additional headers are often required:

POST /api/v1/users HTTP/1.1
Host: api.service.com
Content-Type: application/json
Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9
Content-Length: 78
Accept: application/json

{"username":"newuser","email":"user@example.com","password":"secure123"}

Browsers automatically include many headers that you need to specify manually in code:

// JavaScript fetch example
fetch('https://api.example.com/data', {
  method: 'GET',
  headers: {
    'Host': 'api.example.com',
    'Accept': 'application/json',
    'Authorization': 'Bearer token123'
  }
});

Use these tools to examine your request headers:

• Chrome Developer Tools (Network tab)
• Postman
• curl with -v flag
• Wireshark for low-level inspection