What are HTTP headers and why are they important?

HTTP headers are key-value pairs sent with every HTTP request and response. They carry metadata about the communication: authentication credentials, content type, caching instructions, CORS policies, cookies, and more. Headers allow servers and clients to negotiate how data is exchanged without encoding everything in the URL or body.

What is the difference between request headers and response headers?

Request headers are sent by the client (browser/app) to the server — examples: Accept, Authorization, Content-Type, Cookie. Response headers are sent by the server back to the client — examples: Set-Cookie, Cache-Control, Content-Encoding, Location. Some headers (like Content-Type) are valid in both directions.

What is the Content-Type header used for?

Content-Type declares the media type of the request or response body. For example, `application/json` for JSON data, `text/html` for HTML pages, `multipart/form-data` for file uploads. Servers use it to parse the body correctly; browsers use it to render content appropriately. Mismatching Content-Type is a common source of API errors.

What are CORS headers and why do they matter?

CORS (Cross-Origin Resource Sharing) headers control whether web pages can make requests to a different domain. Key headers: `Access-Control-Allow-Origin` (which origins can access), `Access-Control-Allow-Methods` (allowed HTTP methods), `Access-Control-Allow-Headers` (custom headers allowed). Without proper CORS headers, browsers block cross-origin requests for security reasons.

What is the Authorization header and how is it structured?

The Authorization header carries credentials for HTTP authentication. Common schemes: `Basic base64(user:password)` for basic auth, `Bearer ` for OAuth 2.0 / JWT tokens, `Digest` for digest authentication. The format is always `Authorization: `. This header is typically set on every API request requiring authentication.

HTTP Headers Reference

Searchable reference table of HTTP request and response headers.

Header	Type	Category	Description
Accept	Request	Content Negotiation	Media types acceptable for the response (e.g. text/html, application/json).
Accept-Charset	Request	Content Negotiation	Character sets acceptable for the response.
Accept-Encoding	Request	Content Negotiation	Encoding algorithms the client supports (e.g. gzip, deflate, br).
Accept-Language	Request	Content Negotiation	Natural languages preferred for the response (e.g. en-US, fr).
Authorization	Request	Authentication	Credentials for authenticating the client with the server (e.g. Bearer token).
Cookie	Request	State	HTTP cookies previously set by the server, sent back with each request.
Expect	Request	Request Control	Indicates that particular server behaviors are required by the client (e.g. 100-continue).
Host	Request	Request Target	Domain name and port number of the server being addressed. Required in HTTP/1.1.
If-Match	Request	Conditional	Makes the request conditional based on matching ETag values.
If-Modified-Since	Request	Conditional	Makes the request conditional based on modification date.
If-None-Match	Request	Conditional	Makes the request conditional based on non-matching ETag values.
If-Unmodified-Since	Request	Conditional	Makes the request conditional on the resource not being modified since the given date.
Origin	Request	CORS	Indicates the origin from which the cross-site request is initiated.
Proxy-Authorization	Request	Authentication	Credentials for authenticating with a proxy server.
Range	Request	Partial Requests	Requests only part of a resource (e.g. bytes=0-1023 for the first 1 KB).
Referer	Request	Request Context	URL of the page from which the request originated.
TE	Request	Transfer Encoding	Specifies transfer encodings the client is willing to accept.
User-Agent	Request	Request Context	Contains information about the client software making the request.
X-Forwarded-For	Request	Proxies	Identifies the originating IP address when the request passes through proxies.
X-Requested-With	Request	Custom	Commonly used to identify Ajax requests (value: XMLHttpRequest).
Accept-Ranges	Response	Partial Requests	Indicates if the server supports range requests (bytes or none).
Age	Response	Caching	Time in seconds the object has been stored in a proxy cache.
Allow	Response	Request Methods	HTTP methods allowed for the requested resource (used in 405 responses).
Content-Disposition	Response	Response Body	Indicates whether content should be displayed inline or as a file download.
Content-Encoding	Response	Response Body	Encoding applied to the response body (e.g. gzip, br).
Content-Language	Response	Content Negotiation	Natural language(s) of the response body.
Content-Length	Response	Response Body	Size of the response body in bytes.
Content-Range	Response	Partial Requests	Indicates the range of bytes sent in a partial content response (206).
ETag	Response	Caching	Unique identifier for the specific version of a resource; used for cache validation.
Expires	Response	Caching	Date/time after which the response is considered stale.
Last-Modified	Response	Caching	Date and time the resource was last modified on the server.
Location	Response	Redirects	URL to redirect the client to in 3xx or 201 responses.
Proxy-Authenticate	Response	Authentication	Defines the authentication method to use for a proxy (407 response).
Retry-After	Response	Request Control	How long to wait before making a new request after a 429 or 503 response.
Server	Response	Server Info	Software information about the origin server.
Set-Cookie	Response	State	Sends a cookie from the server to the client for storage.
Strict-Transport-Security	Response	Security	HSTS: forces browsers to use HTTPS for subsequent requests.
Vary	Response	Caching	Tells caches which request headers to use as cache keys.
WWW-Authenticate	Response	Authentication	Defines the authentication method for accessing the resource (401 response).
X-Content-Type-Options	Response	Security	Prevents MIME sniffing. Use value: nosniff.
X-Frame-Options	Response	Security	Controls whether the page can be embedded in an iframe (DENY, SAMEORIGIN).
X-XSS-Protection	Response	Security	Enables XSS filtering in older browsers (legacy, mostly deprecated).
Cache-Control	Both	Caching	Directives for caching in requests and responses (e.g. no-cache, max-age=3600).
Connection	Both	Connection Management	Controls whether the network connection stays open (keep-alive or close).
Content-Type	Both	Response Body	Media type of the request or response body (e.g. application/json).
Date	Both	Timestamps	Date and time the message was sent.
Pragma	Both	Caching	Legacy cache control directive (no-cache). Superseded by Cache-Control.
Trailer	Both	Transfer Encoding	Indicates which headers will be present in the trailer of a chunked response.
Transfer-Encoding	Both	Transfer Encoding	Encoding applied to the message body for transfer (e.g. chunked).
Via	Both	Proxies	Added by proxies to indicate intermediate protocols and recipients.
Warning	Both	Request Control	General warnings about possible problems with the message (deprecated in RFC 9110).
Access-Control-Allow-Origin	Response	CORS	Specifies which origins are allowed for CORS requests.
Access-Control-Allow-Methods	Response	CORS	HTTP methods allowed for CORS preflight responses.
Access-Control-Allow-Headers	Response	CORS	HTTP headers allowed in CORS preflight responses.
Access-Control-Allow-Credentials	Response	CORS	Indicates if the response can be shared when credentials are included.
Access-Control-Max-Age	Response	CORS	How long the preflight response can be cached (in seconds).
Cross-Origin-Opener-Policy	Response	Security	Controls the browsing context group for cross-origin documents.
Cross-Origin-Resource-Policy	Response	Security	Prevents cross-origin reads of the response in certain contexts.
Content-Security-Policy	Response	Security	Specifies allowed sources for content types to prevent XSS and injection attacks.
Referrer-Policy	Response	Security	Controls how much referrer information is sent with requests.
Permissions-Policy	Response	Security	Controls browser features and APIs that are allowed to be used (formerly Feature-Policy).

About this tool

HTTP headers are metadata fields that travel with every web request and response, controlling everything from caching behavior to authentication and content negotiation. The HTTP Headers Reference is a searchable guide to the most common and important headers used in modern web development, helping developers quickly understand what each header does, what values it accepts, and when to use it.

Simply search for a header name or browse the categorized list to find detailed information about its purpose, syntax, and typical use cases. Whether you're debugging a CORS error, configuring cache control, setting security policies, or understanding redirect behavior, this reference cuts through the noise and gives you the exact information you need without leaving your page.

This tool is essential for backend developers, frontend engineers, DevOps professionals, and anyone managing HTTP infrastructure. It works entirely in your browser with no server calls, making it fast and private for offline reference while building APIs, configuring web servers, or troubleshooting request headers in production.

Frequently Asked Questions

Comments & Feedback

Comments are powered by Giscus. Sign in with GitHub to leave a comment.