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 sum­mar­ised 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 www.example.com is opened, the web server not only opens the website itself, but also sends out – invisible to users – the following header:

Image: Screenshot of HTTP Response Header
The server returns this header together with the website www.example.com.

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, apĀ­plicĀ­aĀ­tion/xml
Accept-Charset Which character sets the client can display. Accept-Charset: utf-8
Accept-Encoding Which comĀ­pressed formats the client supports. Accept-Encoding: gzip
Accept-Language Requested language version Accept-Language: en-UK
AuĀ­thorĀ­izĀ­aĀ­tion AuĀ­thenĀ­ticĀ­aĀ­tion 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: apĀ­plicĀ­aĀ­tion/x_222-form-urĀ­lenĀ­coded
Date Date and time of the request Date: Mon, 9 March 2020 09:02:22 GMT
Expect Sends an exĀ­pectĀ­aĀ­tion 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: example.com
If-Match ConĀ­diĀ­tionĀ­al execution of an action, depending on the matching of a transĀ­mitĀ­ted code If-Match: ā€žft678iujhnjio90ā€™pƶlā€
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: ā€žcxdrt5678iujhgbvbā€
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-UnĀ­modĀ­iĀ­fied-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-AuĀ­thorĀ­izĀ­aĀ­tion Used to auĀ­thenĀ­ticĀ­ate the client to a proxy server Proxy-AuĀ­thorĀ­izĀ­aĀ­tion: 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: https://example.com/index.html
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) ApĀ­pleWebĀ­Kit/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 speĀ­cificĀ­aĀ­tions (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
ConĀ­necĀ­tion Preferred type of conĀ­necĀ­tion ConĀ­necĀ­tion: close
Content-Encoding Type of comĀ­presĀ­sion Content-Encoding: deflate
Content-Language Language of the resource Content-Language: en-UK
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: /example.com
Content-Security-Policy Security concepts of the server Content-Security-Policy: frame-src 'noneā€˜; object-src 'noneā€˜
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: ā€žvt6789oi8uztgfvbnā€
Expires When the file should be conĀ­sidered obsolete Expires: Tue 3 Mar 2020 1:00:00 GMT
Last-Modified Time of the last modiĀ­ficĀ­aĀ­tion of the file Last-Modified: Mon 2 Mar 2020 1:00:00 GMT
Location IdenĀ­tiĀ­fies the location to which the request was forwarded Location: https://www.example.com
Proxy-AuĀ­thenĀ­ticĀ­ate Says if and how the client must auĀ­thenĀ­ticĀ­ate to the proxy Proxy-AuĀ­thenĀ­ticĀ­ate: Basic
Retry-After Sets from when the client should request again if the resource is temĀ­porĀ­arĀ­ily unĀ­availĀ­able (date or seconds) Retry-After: 300
Server IdenĀ­tiĀ­ficĀ­aĀ­tion of the server Server: Apache
Set-Cookie Sets a cookie at the client Set-Cookie: UserID=XY; Max-Age=3800; Version=1
Transfer-Encoding ComĀ­presĀ­sion method Transfer-Encoding: gpzip
Vary Sets which header fields should be conĀ­sidered 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: 1.1www.example.com
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