# HTTP header

Category: [Technical SEO](https://seo-geo-glossary.com/en/glossary/category/technical-seo/) › [Servers, HTTP & caching](https://seo-geo-glossary.com/en/glossary/category/technical-seo/servers-http-caching/) · Also known as: HTTP headers

An **HTTP header** is a name-value pair that a client or server sends along with an HTTP request or response to pass extra information, such as the content type, caching rules or indexing directives. Header names are not case-sensitive. With HTTP/2 and later they are written in lowercase.

## Which kinds of headers exist?

MDN groups headers by context:

- **Request headers** describe the client or what it wants, such as `User-Agent` or `Accept`.
- **Response headers** describe the answer or the server, such as `Location` or `Set-Cookie`.
- **Representation headers** describe the body, such as `Content-Type` and `Content-Encoding`.
- **Payload headers** carry information independent of the representation, such as `Content-Length`.

The status code, for example 200 or 404, is not a header; it belongs to the status line of the response.

## Headers that matter for SEO

- `X-Robots-Tag` carries robots directives such as `noindex` in the HTTP response. According to Google, any rule usable in a robots meta tag can also be given this way. It works for files without HTML, such as PDFs and images.
- `Link` can declare a canonical URL for non-HTML files.
- `Cache-Control` and `ETag` control caching and conditional requests.
- `Location` sends the target of a redirect.
- `Content-Type` tells crawlers what kind of file they received.
- `Strict-Transport-Security` enforces HTTPS in browsers.

Custom headers used to start with `X-`. That convention was deprecated in 2012 by RFC 6648, but `X-Robots-Tag` kept its name.

## Example

```http
HTTP/1.1 200 OK
Content-Type: application/pdf
X-Robots-Tag: noindex
Cache-Control: max-age=3600
```

## How can I view HTTP headers?

Run `curl -I https://example.com/` in a terminal, or open the Network tab of the browser's developer tools and select a request.

## HTTP versions compared

| Version | Transport | Concurrent requests | Header fields | Use by Google's crawlers |
|---|---|---|---|---|
| HTTP/1.1 (RFC 9112) | TCP | no multiplexing layer; pipelining possible, responses come back in request order; often several TCP connections for parallel requests | whitespace-delimited text fields; often repetitive and verbose according to RFC 9113 | supported; Google's default version |
| HTTP/2 (RFC 9113) | TCP | yes, many requests interleaved on one connection; TCP-level packet loss still stalls all streams | binary frames, HPACK compression | supported since November 2020; no ranking boost; opt out with status code 421 |
| HTTP/3 (RFC 9114) | QUIC over UDP | yes, over independent QUIC streams; one blocked stream does not hold up the others | QPACK compression | not stated by Google; the crawler documentation lists only HTTP/1.1 and HTTP/2 |

Sources: [RFC 9112: HTTP/1.1](https://www.rfc-editor.org/rfc/rfc9112.html); [RFC 9113: HTTP/2](https://www.rfc-editor.org/rfc/rfc9113.html); [RFC 9114: HTTP/3](https://www.rfc-editor.org/rfc/rfc9114.html); [RFC 9000: QUIC](https://www.rfc-editor.org/rfc/rfc9000.html); [Google Crawling Infrastructure: Overview of Google crawlers and fetchers (last updated 2026-06-12)](https://developers.google.com/crawling/docs/crawlers-fetchers/overview-google-crawlers); [Google Search Central Blog: Googlebot will soon speak HTTP/2 (2020-09-17)](https://developers.google.com/search/blog/2020/09/googlebot-will-soon-speak-http2). Last checked: October 8, 2026

## Related terms

- [X-Robots-Tag](https://seo-geo-glossary.com/en/glossary/x-robots-tag/index.md)
- [HTTP Link header](https://seo-geo-glossary.com/en/glossary/http-link-header/index.md)
- [HTTP status code](https://seo-geo-glossary.com/en/glossary/http-status-code/index.md)
- [Cache-Control](https://seo-geo-glossary.com/en/glossary/cache-control/index.md)
- [HTTP/2](https://seo-geo-glossary.com/en/glossary/http-2/index.md)

## Sources

- [HTTP headers – MDN Web Docs](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers)
- [Robots meta tag and X-Robots-Tag – Google Search Central](https://developers.google.com/search/docs/crawling-indexing/robots-meta-tag)
- [Consolidate duplicate URLs (Link header for canonicals) – Google Search Central](https://developers.google.com/search/docs/crawling-indexing/consolidate-duplicate-urls)
- [RFC 6648: Deprecating the X- Prefix – IETF](https://www.rfc-editor.org/rfc/rfc6648)

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Last updated: October 8, 2026
URL: https://seo-geo-glossary.com/en/glossary/http-header/