Understanding request and response

✅ Security Headers (Response)

These help protect against XSS, clickjacking, MIME sniffing, and enforce secure communication.

Header	Purpose
Strict-Transport-Security	Forces HTTPS for all requests (HSTS).
Content-Security-Policy	Controls resources the user agent is allowed to load.
X-Content-Type-Options: nosniff	Prevents MIME sniffing.
X-Frame-Options: DENY / SAMEORIGIN	Prevents clickjacking (iframe embedding).
X-XSS-Protection: 1; mode=block	Enables browser XSS filter (legacy).
Referrer-Policy	Controls how much referrer info is sent.
Permissions-Policy	Controls access to browser features like camera, mic, geolocation.
Cross-Origin-Embedder-Policy	Enforces stricter isolation for cross-origin resources.
Cross-Origin-Opener-Policy	Prevents window from being shared with cross-origin content.
Cross-Origin-Resource-Policy	Controls sharing of resources with other origins.
Set-Cookie: Secure; HttpOnly; SameSite=Strict	Protects cookies from XSS and CSRF.
X-Permitted-Cross-Domain-Policies	Restricts Flash/Adobe cross-domain policies.
X-Download-Options: noopen	Prevents file auto-open in IE.

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🌍 CORS Headers (Response)

These control cross-origin access to resources.

Header	Purpose
Access-Control-Allow-Origin	Specifies allowed origins.
Access-Control-Allow-Credentials	Allows cookies/auth headers in CORS requests.
Access-Control-Allow-Methods	Specifies allowed HTTP methods for CORS.
Access-Control-Allow-Headers	Specifies allowed custom headers.
Access-Control-Expose-Headers	Allows specific response headers to be accessed by client.
Vary: Origin	Ensures correct caching based on origin.

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🔒 Security-Relevant Request Headers

Header	Purpose
Authorization	Sends credentials (Basic, Bearer tokens).
Cookie	Sends cookies (can be secured with flags).
Origin	Indicates origin of the cross-site request.
Referer	Tells server which page made the request.
User-Agent	Identifies client software (can be fingerprinted).
Host	Specifies the domain being accessed.
X-Requested-With	Used in AJAX requests to detect JavaScript-based access.
Forwarded / X-Forwarded-For / X-Real-IP	Reveal client’s original IP in proxied setups.

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🚦 Rate Limiting & Abuse Protection (Response)

Header	Purpose
X-RateLimit-Limit	Max number of requests allowed.
X-RateLimit-Remaining	Requests remaining in window.
X-RateLimit-Reset	Time when the rate limit resets.
Retry-After	Suggests when to retry after rate limiting or 503.

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⚡ Performance & Caching Headers

Header	Purpose
Cache-Control	Controls caching behavior (e.g., no-store, public).
ETag	Identifier for a specific version of a resource.
Last-Modified	Timestamp of last resource update.
Expires	Date/time after which response is considered stale.
Vary	Instructs caches to vary based on request headers.
Transfer-Encoding: chunked	Allows streaming responses in chunks.

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📦 Content & File Management Headers

Header	Purpose
Content-Type	MIME type of the resource.
Content-Length	Size of the body in bytes.
Content-Disposition	Controls if content is inline or attachment (download).
Content-Encoding	Indicates compression method (e.g., gzip, br).
Accept	Media types the client accepts.
Accept-Encoding	Compression types client supports.
Accept-Language	Languages the client prefers.

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🔁 Redirection & Response Control

Header	Purpose
Location	Used with 3xx responses to redirect.
Retry-After	Used with 429/503 to indicate retry time.
Link	Preloading resources, pagination, etc.

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🛠️ Custom Headers (X-Headers)

Header	Purpose
X-Request-ID / X-Correlation-ID	Trace a request across systems.
X-Powered-By	Reveals backend tech (often removed for security).
X-App-Version	Indicates the API or app version.

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Headers from DHL:

1. HttpOnly - Mitigates XSS
2. no-cache: Forces revalidation with the server before use.
3. no-store: Don’t store any part of the response.
4. max-age=0: The response is immediately stale.
5. must-revalidate: Client must validate the response with the server before using cached data.

   CORS (Cross-Origin Resource Sharing) Headers

Vary: Access-Control-Request-Headers

• Instructs caches to consider request headers like Access-Control-Request-Headers when deciding if a cached response is valid. Access-Control-Expose-Headers: Authorization
• Allows the client (JavaScript) to access the Authorization header in the response. Access-Control-Allow-Credentials: true
• Indicates that the response can be shared with requests that include credentials (like cookies, HTTP auth). Access-Control-Allow-Origin: *
• Allows any domain to access the resource. Not recommended with Allow-Credentials: true

Rate Limiting Headers

X-RateLimit-Remaining: -1

• Indicates how many requests are remaining in the current rate limit window. -1 may mean unlimited or rate limit not enforced. X-RateLimit-Requested-Tokens: 1
• Custom header indicating how many tokens (or cost) this request used. X-RateLimit-Burst-Capacity: 50
• Max number of requests that can be made in a burst. X-RateLimit-Replenish-Rate: 25
• How quickly the token bucket is refilled (requests per second/minute etc.).

Security Headers

X-XSS-Protection: 1; mode=block

• Enables XSS filter in some browsers and blocks detected attacks. Strict-Transport-Security: max-age=631138519
• Enforces HTTPS connections for a specified period (in seconds). Helps prevent man-in-the-middle attacks. X-Frame-Options: DENY
• Prevents the page from being embedded in an iframe (clickjacking protection). X-Content-Type-Options: nosniff
• Prevents browsers from MIME-sniffing a response away from the declared content type. Referrer-Policy: no-referrer
• Specifies that the browser should not send any Referer header. Content-Security-Policy: default-src 'none'
• Restricts where resources (scripts, images, etc.) can be loaded from. 'none' blocks everything unless overridden. X-Download-Options: noopen
• Prevents file downloads from automatically opening in Internet Explorer. X-Permitted-Cross-Domain-Policies: none
• Restricts Flash/Adobe PDF cross-domain data loading policies.