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What Are HTTP Headers?

HTTP headers are the name-value lines sent before the body of every HTTP request and response, such as User-Agent, Accept-Language and Cookie. They tell the server who is asking and what it accepts, and tell the client how to handle the reply.

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how it works

How HTTP headers work

A request starts with a line like GET /page HTTP/1.1, followed by headers, one per line, and an empty line. HTTP/2 and HTTP/3 send the same headers in compressed binary form, with names in lowercase. Some headers are end to end and reach the website; hop-by-hop ones, such as Proxy-Authorization and Connection, are meant only for the next hop.

  • 1

    User-Agent

    Sent by
    Client
    What it tells the other side
    Browser, version and operating system
  • 2

    Accept-Language

    Sent by
    Client
    What it tells the other side
    Preferred languages, such as en-US,en;q=0.9
  • 3

    Cookie / Set-Cookie

    Sent by
    Client / server
    What it tells the other side
    Session and preference data
  • 4

    Proxy-Authorization

    Sent by
    Client, to the proxy
    What it tells the other side
    Proxy credentials; never forwarded to the site
  • 5

    X-Forwarded-For / Via

    Sent by
    Proxies
    What it tells the other side
    The original client IP / that a proxy was used
  • 6

    Retry-After

    Sent by
    Server
    What it tells the other side
    How long to wait after a 429 or 503
why it matters

Why headers matter for proxy users

Headers are where many setups give themselves away. The IP comes from the proxy, but nearly every header comes from your client.

CleanProxies rotating residential proxies and ISP proxies add no identifying headers. Check what a site receives with the User-Agent checker.

HTTP headers: what it means for you
  • Through a CONNECT tunnel the proxy cannot add headers to HTTPS requests, so no proxy headers reach the site. On plain http:// a poorly configured proxy may add X-Forwarded-For or Via; see anonymity levels.
  • Websites compare headers with each other: a Chrome User-Agent without the sec-ch-ua client hints Chrome sends, or in an unusual order, does not look like Chrome.
  • Match Accept-Language and the time zone of your client to the proxy's country when the site localises content.
  • Send Accept-Encoding: gzip, br so responses are compressed and per-GB plans go further.
See the headers a site receivesShell
curl -x http://USERNAME:PASSWORD@HOST:PORT https://httpbin.org/headers

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Setup guides and best practices from the CleanProxies blog.

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Related terms

Terms that come up alongside HTTP headers.

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HTTP headers FAQs

Straight answers about HTTP headers.

They carry metadata about a request or response: who the client is, what formats and languages it accepts, cookies, caching rules, authentication and more.

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