HTTP Header Fields
An HTTP transaction consists of a header followed optionally by an empty
line and some data. The header will specify such things as the action
required of the server, or the type of data being returned,
or a status code.
The header lines received from the client, if any,
are placed by the server into the CGI environment variables
with the prefix HTTP_ followed by the header name.
Any - characters in the header name are changed to _ characters.
The server may exclude any headers which it has already processed,
such as Authorization, Content-type, and Content-length.
-
HTTP_ACCEPT
The MIME types which the client will accept, as given by HTTP
headers. Other protocols may need to get this information from
elsewhere. Each item in this list should be separated by commas as
per the HTTP spec.
Format: type/subtype, type/subtype
-
HTTP_USER_AGENT
The browser the client is using to send the request.
General format: software/version library/version.
The server sends back to the client:
- A status code that indicates whether the request was successful or not.
Typical error codes indicate that the requested file was not found,
that the request was malformed, or that authentication is required
to access the file.
- The data itself. Since HTTP is liberal about sending documents
of any format, it is ideal for transmitting multimedia such as
graphics, audio, and video files.
-
It also sends back information about the object being returned.
Indicates the media type of the data sent to the recipient
or, in the case of the HEAD method,
the media type that would have been sent had the request
been a GET.
Content-Type: text/html
The date and time at which the message was originated.
Date: Tue, 15 Nov 1994 08:12:31 GMT
The date after which the information in the document ceases to
be valid.
Caching clients, including proxies, must not cache this copy
of the resource beyond the date given, unless its
status has been updated by a later check of the origin server.
Expires: Thu, 01 Dec 1994 16:00:00 GMT
An Internet e-mail address for the human user
who controls the requesting user agent.
From: Stars@WDVL.com
The request is being performed on behalf of the person
given, who accepts responsibility for the method performed.
Robot agents should include this header so that the person
responsible for running the robot can be contacted if
problems occur on the receiving end.
Used with the
GET method to make it conditional: if the
requested resource has not been modified since the time
specified in this field, a copy of the
resource will not be returned from the server;
instead, a 304 (not modified)
response will be returned without any data.
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
Indicates the date and time at which the sender believes the
resource was last modified.
Useful for clients that eliminate
unnecessary transfers by using caching.
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
The Location response header field defines the exact location of the
resource that was identified by the request URI.
If the value is a full URL, the server returns a "redirect" to the
client to retrieve the specified object directly.
Location: http://WWW.Stars.com/Tutorial/HTTP/index.html
If you want to reference another file on your own server, you
should output a partial URL, such as the following:
Location: /Tutorial/HTTP/index.html
Allows the client to specify, for the server's benefit,
the address (URI) of the resource from which the request URI
was obtained. This allows a server to
generate lists of back-links to resources for interest,
logging, optimized caching, etc. It also
allows obsolete or mistyped links to be traced for maintenance.
Referer: http://WWW.Stars.com/index.html
The Server response header field contains
information about the software used by the origin
server to handle the request.
Server: CERN/3.0 libwww/2.17
Information about the user agent originating the request.
This is for statistical purposes, the tracing of protocol
violations, and automated recognition of user agents for the
sake of tailoring responses to avoid particular user agent
limitations - such as inability to support HTML tables.
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
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