The HTTP header - an overview for Internet users

When you visit a website, your browser sends a request to the web server to obtain data or information from it, e.g. an HTML file (i.e. a web page). Both in the request - the HTTP-Request – and in the server's response, some meta-information is exchanged in addition to the actual data. This is summarized in the HTTP header. We explain the function of the HTTP header and its most important fields.

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Function of a Header, explained with an example

When the website is opened, the web server not only opens the website itself, but also sends out – invisible to users – the following header:

The individual lines are called “header fields”. Each (except the first) consists of a name/value pair separated by a colon.

Key to the individual symbols:

  • HTTP/1.1 is the valid HTTP protocol version.
  • 200 OK is the Status-Code. It says that the server has received, understood and accepted the request.
  • Content-Encoding and Content-Type tell us about the type of file.
  • Age, Cache-Control, Expires, Vary and X-Cache refer to the caching of the file.
  • Etag and Last-Modified are used for version control of the delivered file.
  • Server refers to the web server software.
  • Content-Length is the file size in bytes.

As you can see, this header information is mainly used for coordination between the client (browser) and the server. It is ensured that the client can understand the form of the file, that the file is sufficiently up-to-date and that the file size meets the browser's expectations.

The header lines shown in the example are only a small part of the available header fields. In total, there are almost 100 HTTP header fields, of which about 30 are for specifying HTTP requests, about 30 for the server response, and a whole range of other header fields that serve different purposes and are partly (still) not standardized.

The following overview explains the most important header fields.

The most important HTTP request and response headers: An Overview

HTTP Request (Client Request)

Header Field Meaning Example
Accept Which content types the client can process; if the field is empty, these are all content types. Accept: text/html, application/xml
Accept-Charset Which character sets the client can display. Accept-Charset: utf-8
Accept-Encoding Which compressed formats the client supports. Accept-Encoding: gzip
Accept-Language Requested language version Accept-Language: en-US
Authorization Authentication data (e.g. for a login) Basic WjbU7D25zTAlV2tZ7==
Cache-Control Options of the caching mechanism Cache-Control: no-cache
Cookie Cookie stored for this server Cookie: $Version=1; Content=23
Content-Length Length of the request body Content-Length: 212
Content-Type MIME type of the body; relevant for POST and PUT requests Content-Type: application/x_222-form-urlencoded
Date Date and time of the request Date: Mon, 9 March 2020 09:02:22 GMT
Expect Sends an expectation to the server, usually the receipt of a large request. Expect: 100-continue (the server should send code 100 when it is ready to receive the request)
Host Domain name of the server Host:
If-Match Conditional execution of an action, depending on the matching of a transmitted code If-Match: „ft678iujhnjio90’pöl”
If-Modified-Since Send only if the requested content has been modified since the specified time IF-Modified-Since: Mon 2 Mar 2020 1:00:00 GMT
If-None-Match As above, but specified via an ETag (entity tag, see below) If-None-Match: „cxdrt5678iujhgbvb”
If-Range Requests only the part of the content that was changed or is missing in the client cache If-Range: Mon 2 Mar 2020 1:00:00 GMT
If-Unmodified-Since Analog IF-Modified-Since If-Modified-Since: Mon 2 Mar 2020 1:00:00 GMT
Max-Forwards Defines the maximum number of times the server response may be forwarded Max-Forwards: 12
Proxy-Authorization Used to authenticate the client to a proxy server Proxy-Authorization: Basic WjbU7D25zTAlV2tZ7==
Range Specifies a portion of the requested content Range: bytes=0-9999
Referrer URL of the resource from which the request comes (i.e. from which the link was made) Referrer:
TE Accepted extension transfer coding TE: gzip, deflate
User-Agent User-Agent of the client (simply put: the browser) Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/80.0.3987.132 Safari/537.36
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HTTP-Response (Server Response)

Header Field Meaning Example
Accept-Ranges Which units the server accepts for the range specifications (see above) Accept-Ranges: bytes
Age Number of seconds the object has been in the cache Age: 2300
Allow Permitted request types for a specific resource Allow: GET, POST, HEAD
Cache-Control Whether and how long the object may be kept in the cache Cache-Control: max-age=4800
Connection Preferred type of connection Connection: close
Content-Encoding Type of compression Content-Encoding: deflate
Content-Language Language of the resource Content-Language: en-US
Content-Length Size of the body in bytes Content-Length: 135674
Content-Location Location of the file if it comes from a different location than the one requested (e.g. CDN) Content-Location: /
Content-Security-Policy Security concepts of the server Content-Security-Policy: frame-src 'none‘; object-src 'none‘
Content-Type MIME type of the requested file Content-Type: text/tml; charset=utf-8
Date Time of the response Date: Mon 2 Mar 2020 1:00:00 GMT
ETag Marks a specific version of the file ETag: „vt6789oi8uztgfvbn”
Expires When the file should be considered obsolete Expires: Tue 3 Mar 2020 1:00:00 GMT
Last-Modified Time of the last modification of the file Last-Modified: Mon 2 Mar 2020 1:00:00 GMT
Location Identifies the location to which the request was forwarded Location:
Proxy-Authenticate Says if and how the client must authenticate to the proxy Proxy-Authenticate: Basic
Retry-After Sets from when the client should request again if the resource is temporarily unavailable (date or seconds) Retry-After: 300
Server Identification of the server Server: Apache
Set-Cookie Sets a cookie at the client Set-Cookie: UserID=XY; Max-Age=3800; Version=1
Transfer-Encoding Compression method Transfer-Encoding: gpzip
Vary Sets which header fields should be considered as varying if a file is requested from the cache. Vary: User-Agent (= the server holds different file versions depending on the user agent)
Via Which proxies the response was sent through. Via:
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