Understanding HTTP Headers

What are HTTP Headers?

HTTP headers are key-value pairs sent between clients and servers in HTTP requests and responses. They provide essential metadata about the request or response, including content type, encoding, caching directives, authentication credentials, and much more. Headers are invisible to end users but critical for proper communication between web browsers, APIs, and servers.

Request Headers

Request headers are sent by the client to provide information about the request and the client itself:

• Accept: Tells the server what content types the client can handle (e.g., application/json, text/html).
• Authorization: Contains credentials for authenticating the client with the server (e.g., Bearer tokens, Basic auth).
• User-Agent: Identifies the client software (browser, mobile app) making the request.
• Cookie: Sends cookies previously set by the server back with the request.
• Referer: The URL of the page that linked to the current request (useful for analytics).
• Accept-Encoding: Compression methods the client supports (gzip, deflate, br).
• Accept-Language: Preferred languages for the response content.
• If-None-Match: Used with ETags for conditional requests to check if content has changed.

Response Headers

Response headers are sent by the server to provide information about the response:

• Content-Type: Specifies the media type of the response body (e.g., text/html, application/json).
• Content-Length: The size of the response body in bytes.
• Set-Cookie: Instructs the client to store a cookie with specified attributes.
• Cache-Control: Directives for caching the response (e.g., max-age, no-cache, public).
• Location: Used in redirects to specify the new URL.
• Server: Information about the web server software handling the request.
• ETag: A unique identifier for a specific version of a resource, used for caching.
• Last-Modified: The date and time the resource was last modified.

Security Headers

Security headers protect against common web vulnerabilities:

• Content-Security-Policy (CSP): Controls which resources can be loaded, preventing XSS attacks.
• Strict-Transport-Security (HSTS): Forces browsers to use HTTPS, preventing downgrade attacks.
• X-Frame-Options: Prevents clickjacking by controlling whether the page can be embedded in frames.
• X-Content-Type-Options: Prevents MIME type sniffing, forcing browsers to respect declared content types.
• X-XSS-Protection: Enables browser XSS filters (legacy, mostly replaced by CSP).
• Referrer-Policy: Controls how much referrer information is sent with requests.

CORS Headers

Cross-Origin Resource Sharing (CORS) headers control cross-origin requests:

• Access-Control-Allow-Origin: Specifies which origins can access the resource.
• Access-Control-Allow-Methods: Lists allowed HTTP methods for cross-origin requests.
• Access-Control-Allow-Headers: Lists allowed headers in cross-origin requests.
• Access-Control-Allow-Credentials: Indicates if credentials can be included in cross-origin requests.

Common Use Cases

• API Debugging: Inspect request and response headers to troubleshoot API integration issues.
• Authentication Testing: Verify Authorization headers are correctly formatted and transmitted.
• Performance Optimization: Check caching headers to ensure proper browser and CDN caching.
• Security Audits: Verify security headers are present and correctly configured.
• CORS Troubleshooting: Debug cross-origin request issues by examining CORS headers.
• Content Negotiation: Ensure Accept and Content-Type headers align for proper data exchange.

Best Practices

• Always Use Security Headers: Implement CSP, HSTS, X-Frame-Options, and other security headers.
• Optimize Caching: Set appropriate Cache-Control headers to reduce server load and improve performance.
• Compress Responses: Use Content-Encoding: gzip to reduce bandwidth usage.
• Version APIs: Use custom headers or URL versioning for API version management.
• Rate Limiting Headers: Include headers like X-RateLimit-Remaining to inform clients of rate limits.
• CORS Configuration: Be specific with Access-Control-Allow-Origin rather than using wildcards in production.
• Remove Information Leakage: Hide or minimize Server headers to avoid revealing technology stack details.

Header Examples

Content-Type: application/json
Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
Accept: application/json
User-Agent: MyApp/1.0

Content-Type: text/html; charset=utf-8
Strict-Transport-Security: max-age=31536000; includeSubDomains
Content-Security-Policy: default-src 'self'
X-Frame-Options: DENY
X-Content-Type-Options: nosniff

Using This Parser

Paste HTTP headers into the input field (one per line, format: Header-Name: value) and click "Parse Headers." The tool will extract each header, display its value, and provide explanations for common headers. This is useful when debugging API requests, analyzing server responses, or learning about HTTP header functionality.