Http tutorial

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HTTP TUTORIAL
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ABOUT THE TUTORIAL
HTTP tutorial
The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia
information systems. This is the foundation for data communication for the World Wide Web (ie. internet) since 1990.
HTTP is a generic and stateless protocol which can be used for other purposes as well using extension of its request
methods, error codes and headers.
This tutorial is based on RFC-2616 specification, which defines the protocol referred to as HTTP/1.1. HTTP/1.1 is a
revision of the original HTTP (HTTP/1.0) and a major difference between HTTP/1.0 and HTTP/1.1 is that HTTP/1.0
uses a new connection for each request/response exchange where as HTTP/1.1 connection may be used for one or
more request/response exchanges.
Audience
This reference has been prepared for the computer science graduates and web developers to help them understand
the basic to advanced concepts related to Hypertext Transfer Protocol (HTTP). After completing this tutorial you will
find yourself at a moderate level of expertise in HTTP from where you can take yourself to next levels.
Prerequisites
Before proceeding with this tutorial, its good if you have basic understanding of web concepts, web browsers, web
servers, client and server architecture based softwares.

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Table of Content
HTTP tutorial............................................................................ 1
Audience.................................................................................. 1
Prerequisites............................................................................ 1
HTTP – Overview..................................................................... 6
Basic Features......................................................................... 6
Basic Architecture.................................................................... 6
Client ....................................................................................... 7
Server ...................................................................................... 7
HTTP – Parameters ................................................................. 8
HTTP Version .......................................................................... 8
Example................................................................................... 8
Uniform Resource Identifiers (URI) .......................................... 8
Example................................................................................... 8
Date/Time Formats .................................................................. 9
Character Sets......................................................................... 9
Example................................................................................... 9
Content Encodings................................................................... 9
Example................................................................................... 9
Media Types .......................................................................... 10
Example................................................................................. 10
Language Tags...................................................................... 10
Example................................................................................. 10
HTTP – Messages ................................................................. 11
Message Start-Line................................................................ 11
Header Fields......................................................................... 12
Message Body ....................................................................... 12
HTTP – Requests .................................................................. 14
Message Request-Line .......................................................... 14
Request Method..................................................................... 14

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Request-URI .......................................................................... 15
Request Header Fields .......................................................... 16
Request Message Examples.................................................. 16
HTTP – Responses................................................................ 18
Message Status-Line ............................................................. 18
HTTP Version ........................................................................ 18
Status Code........................................................................... 18
Response Header Fields........................................................ 19
Response Message Examples............................................... 19
HTTP – Methods.................................................................... 22
GET Method .......................................................................... 22
HEAD Method........................................................................ 23
PUT Method........................................................................... 25
DELETE Method.................................................................... 25
CONNECT Method ................................................................ 26
OPTIONS Method.................................................................. 26
TRACE Method...................................................................... 27
HTTP - Status Codes............................................................. 28
1xx: Information ..................................................................... 28
2xx: Successful...................................................................... 28
3xx: Redirection ..................................................................... 29
4xx: Client Error ..................................................................... 29
5xx: Server Error.................................................................... 30
HTTP - Header Fields ............................................................ 31
General Headers.................................................................... 31
Connection............................................................................. 32
Date....................................................................................... 32
Pragma .................................................................................. 33
Trailer .................................................................................... 33
Transfer-Encoding ................................................................. 33
Upgrade................................................................................. 33
Via ......................................................................................... 33
Warning ................................................................................. 34
Client Request Headers......................................................... 34
Accept.................................................................................... 34
Accept-Charset ...................................................................... 34
Accept-Encoding.................................................................... 34
Accept-Language................................................................... 35
Authorization.......................................................................... 35

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Cookie.................................................................................... 35
Expect.................................................................................... 35
From ...................................................................................... 35
Host ....................................................................................... 35
If-Match.................................................................................. 36
If-Modified-Since.................................................................... 36
If-None-Match ........................................................................ 36
If-Range................................................................................. 36
If-Unmodified-Since ............................................................... 37
Max-Forwards........................................................................ 37
Proxy-Authorization................................................................ 37
Range .................................................................................... 37
Referer................................................................................... 38
TE.......................................................................................... 38
User-Agent............................................................................. 39
Server Response Headers..................................................... 39
Accept-Ranges ...................................................................... 39
Age ........................................................................................ 39
ETag ...................................................................................... 39
Location ................................................................................. 39
Proxy-Authenticate................................................................. 40
Retry-After ............................................................................. 40
Server .................................................................................... 40
Set-Cookie............................................................................. 40
Vary ....................................................................................... 41
WWW-Authenticate................................................................ 41
Entity Headers ....................................................................... 41
Allow ...................................................................................... 41
Content-Encoding .................................................................. 41
Content-Language ................................................................. 41
Content-Length ...................................................................... 42
Content-Location.................................................................... 42
Content-MD5 ......................................................................... 42
Content-Range....................................................................... 42
Content-Type ......................................................................... 43
Expires................................................................................... 43
Last-Modified ......................................................................... 43
HTTP – Caching .................................................................... 44
HTTP - URL Encoding ........................................................... 46

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HTTP – Security..................................................................... 49
Personal Information leakage................................................. 49
File and path names based attack ......................................... 49
DNS Spoofing ........................................................................ 49
Location Headers and Spoofing............................................. 50
Authentication Credentials ..................................................... 50
Proxies and Caching.............................................................. 50
HTTP - Message Examples ................................................... 51
Example 1.............................................................................. 51
Client request......................................................................... 51
Server response..................................................................... 51
Example 2.............................................................................. 51
Client request......................................................................... 51
Server response..................................................................... 52
Example 3.............................................................................. 52
Client request......................................................................... 52
Server response..................................................................... 53
Example 4.............................................................................. 53
Client request......................................................................... 53
Server response..................................................................... 54

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HTTP – Overview
The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia
information systems. This is the foundation for data communication for the World Wide Web (ie. internet) since 1990.
HTTP is a generic and stateless protocol which can be used for other purposes as well using extension of its request
methods, error codes and headers.
Basically, HTTP is an TCP/IP based communication protocol, which is used to deliver data (HTML files, image files,
query results etc) on the World Wide Web. The default port is TCP 80, but other ports can be used. It provides a
standardized way for computers to communicate with each other. HTTP specification specifies how clients request
data will be constructed and sent to the serve, and how servers respond to these requests.
Basic Features
There are following three basic features which makes HTTP a simple but powerful protocol:
 HTTP is connectionless: The HTTP client ie. browser initiates an HTTP request and after a request is made,
the client disconnects from the server and waits for a response. The server process the request and re-establish
the connection with the client to send response back.
 HTTP is media independent: This means, any type of data can be sent by HTTP as long as both the client and
server know how to handle the data content. This is required for client as well as server to specify the content
type using appropriate MIME-type.
 HTTP is stateless: As mentioned above, HTTP is a connectionless and this is a direct result that HTTP is a
stateless protocol. The server and client are aware of each other only during a current request. Afterwards, both
of them forget about each other. Due to this nature of the protocol, neither the client nor the browser can retain
information between different request across the web pages.
HTTP/1.0 uses a new connection for each request/response exchange where as HTTP/1.1 connection may be used
for one or more request/response exchanges.
Basic Architecture
Following diagram shows a very basic architecture of a web application and depicts where HTTP sits:
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The HTTP protocol is a request/response protocol based on client/server based architecture where web browser,
robots and search engines, etc. act like HTTP clients and Web server acts as server.
Client
The HTTP client sends a request to the server in the form of a request method, URI, and protocol version, followed by
a MIME-like message containing request modifiers, client information, and possible body content over a TCP/IP
connection.
Server
The HTTP server responds with a status line, including the message's protocol version and a success or error code,
followed by a MIME-like message containing server information, entity metainformation, and possible entity-body
content.

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HTTP – Parameters
This chapter is going to list down few of the important HTTP Protocol Parameters and their syntax in a way they
are used in the communication. For example, format for date, format of URL etc. This will help you in constructing your
request and response messages while writing HTTP client or server programs. You will see complete usage of these
parameters in subsequent chapters while explaining message structure for HTTP requests and responses.
HTTP Version
HTTP uses a <major>.<minor> numbering scheme to indicate versions of the protocol. The version of an HTTP
message is indicated by an HTTP-Version field in the first line. Here is the general syntax of specifying HTTP version
number:
HTTP-Version = "HTTP" "/" 1*DIGIT "." 1*DIGIT
Example
HTTP/1.0
or
HTTP/1.1
Uniform Resource Identifiers (URI)
Uniform Resource Identifiers (URI) is simply formatted, case-insensitive string containing name, location etc to identify
a resource, for example a website, a web service etc. A general syntax of URI used for HTTP is as follows:
URI = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]]
Here if the port is empty or not given, port 80 is assumed for HTTP and an empty abs_path is equivalent to an
abs_path of "/". The characters other than those in the reserved and unsafe sets are equivalent to their ""%" HEX
HEX" encoding.
Example
Following two URIs are equivalent:
http://abc.com:80/~smith/home.html
http://ABC.com/%7Esmith/home.html
http://ABC.com:/%7esmith/home.html
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Date/Time Formats
All HTTP date/time stamps MUST be represented in Greenwich Mean Time (GMT), without exception. HTTP
applications are allowed to use any of the following three representations of date/time stamps:
Sun, 06 Nov 1994 08:49:37 GMT ; RFC 822, updated by RFC 1123
Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
Character Sets
You use character set to specify the character sets that the client prefers. Multiple character sets can be listed
separated by commas. If a value is not specified, the default is US-ASCII.
Example
Following are valid character sets:
US-ASCII
or
ISO-8859-1
or
ISO-8859-7
Content Encodings
A content ecoding values indicate an encoding algorithm has been used to encode the content before passing it over
the network. Content codings are primarily used to allow a document to be compressed or otherwise usefully
transformed without losing the identity.
All content-coding values are case-insensitive. HTTP/1.1 uses content-coding values in the Accept-Encoding and
Content-Encoding header fields which we will see in subsequent chapters.
Example
Following are valid encoding schemes:
Accept-encoding: gzip
or
Accept-encoding: compress
or
Accept-encoding: deflate

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Media Types
HTTP uses Internet Media Types in the Content-Type and Accept header fields in order to provide open and
extensible data typing and type negotiation. All the Media-type values are registered with the Internet Assigned
Number Authority ((IANA). Following is a general syntax to specify media type:
media-type = type "/" subtype *( ";" parameter )
The type, subtype, and parameter attribute names are case- insensitive.
Example
Accept: image/gif
Language Tags
HTTP uses language tags within the Accept-Language and Content-Language fields. A language tag is composed
of 1 or more parts: A primary language tag and a possibly empty series of subtags:
language-tag = primary-tag *( "-" subtag )
White space is not allowed within the tag and all tags are case- insensitive.
Example
Example tags include:
en, en-US, en-cockney, i-cherokee, x-pig-latin
Where any two-letter primary-tag is an ISO-639 language abbreviation and any two-letter initial subtag is an
ISO-3166 country code.

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HTTP – Messages
HTTP is based on client-server architecture model and a stateless request/response protocol that operates by
exchanging messages across a reliable TCP/IP connection.
An HTTP "client" is a program (Web browser or any other client) that establishes a connection to a server for the
purpose of sending one or more HTTP request messages. An HTTP "server" is a program ( generally a web server
like Apache Web Server or Internet Information Services IIS etc. ) that accepts connections in order to serve HTTP
requests by sending HTTP response messages.
HTTP makes use of the Uniform Resource Identifier (URI) to identify a given resource and to establish a connection.
Once connection is established, HTTP messages are passed in a format similar to that used by Internet mail
[RFC5322] and the Multipurpose Internet Mail Extensions (MIME) [RFC2045]. These messages are consisted of
requests from client to server and responses from server to client which will have following format:
HTTP-message = <Request> | <Response> ; HTTP/1.1 messages
HTTP request and HTTP response use a generic message format of RFC 822 for transferring the required data. This
generic message format consists of following four items.
 A Start-line
 Zero or more header fields followed by CRLF
 An empty line (i.e., a line with nothing preceding the CRLF) indicating the end of the
header fields
 Optionally a message-body
Following section will explain each of the entities used in HTTP message.
Message Start-Line
A start-line will have following generic syntax:
start-line = Request-Line | Status-Line
We will discuss Request-Line and Status-Line while discussing HTTP Request and HTTP Response messages
respectively. For now let's see the examples of start line in case of request and response:
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GET /hello.htm HTTP/1.1 (This is Request-Line sent by the client)
HTTP/1.1 200 OK (This is Status-Line sent by the server)
Header Fields
HTTP deader fields provide required information about the request or response, or about the object sent in the
message body. There are following four types of HTTP message headers:
 General-header: These header fields have general applicability for both request and response messages.
 Request-header: These header fields are applicability only for request messages.
 Response-header: These header fields are applicability only for response messages.
 Entity-header: These header fields define metainformation about the entity-body or, if no body is present
All the above mentioned headers follow the same generic format and each of the header field consists of a name
followed by a colon (:) and the field value as follows:
message-header = field-name ":" [ field-value ]
Following are the examples of various header fields:
User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
Host: www.example.com
Accept-Language: en, mi
Date: Mon, 27 Jul 2009 12:28:53 GMT
Server: Apache
Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
ETag: "34aa387-d-1568eb00"
Accept-Ranges: bytes
Content-Length: 51
Vary: Accept-Encoding
Content-Type: text/plain
Message Body
The message body part is optional for an HTTP message but if it is available then it is used to carry the entity-body
associated with the request or response. If entity body is associated then usually Content-Type and Content-Length
headers lines specify the nature of the body associated.
A message body is the one which carries actual HTTP request data (including form data and uploaded etc.) and
HTTP response data from the server ( including files, images etc). Following is a simple content of a message body:

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<html>
<body>
<h1>Hello, World!</h1>
</body>
</html>
Next two chapters will make use of above explained concepts to prepare HTTP Requests and HTTP Responses.

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HTTP – Requests
An HTTP client sends an HTTP request to a server in the form of a request message which includes following
format:
 A Request-line
 Zero or more header (General|Request|Entity) fields followed by CRLF
 An empty line (i.e., a line with nothing preceding the CRLF) indicating the end of the header
fields
Message Request-Line
The Request-Line begins with a method token, followed by the Request-URI and the protocol version, and ending
with CRLF. The elements are separated by space SP characters.
Request-Line = Method SP Request-URI SP HTTP-Version CRLF
Let's discuss each of the part mentioned in Request-Line.
Request Method
The request Method indicates the method to be performed on the resource identified by the given Request-URI. The
method is case-sensitive ans should always be mentioned uppercase. Following are supported methods in HTTP/1.1
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S.N. Method and Description
1
GET
The GET method is used to retrieve information from the given server using a given URI. Requests
using GET should only retrieve data and should have no other effect on the data.
2
HEAD
Same as GET, but only transfer the status line and header section.
3
POST
A POST request is used to send data to the server, for example customer information, file upload etc
using HTML forms.
4
PUT
Replace all current representations of the target resource with the uploaded content.
5
DELETE
Remove all current representations of the target resource given by URI.
6
CONNECT
Establish a tunnel to the server identified by a given URI.
7
OPTIONS
Describe the communication options for the target resource.
8
TRACE
Perform a message loop-back test along the path to the target resource.
Request-URI
The Request-URI is a Uniform Resource Identifier and identifies the resource upon which to apply the request.
Following are the most commonly used forms to specify an URI:
Request-URI = "*" | absoluteURI | abs_path | authority
1
The asterisk * is used when HTTP request does not apply to a particular resource, but to the server
itself, and is only allowed when the method used does not necessarily apply to a resource. For
example:
OPTIONS * HTTP/1.1
2
The absoluteURI is used when HTTP request is being made to a proxy. The proxy is requested to
forward the request or service it from a valid cache, and return the response. For example:
GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1
3
The most common form of Request-URI is that used to identify a resource on an origin server or
gateway. For example, a client wishing to retrieve the resource above directly from the origin server
would create a TCP connection to port 80 of the host "www.w3.org" and send the lines:
GET /pub/WWW/TheProject.html HTTP/1.1
Host: www.w3.org
Note that the absolute path cannot be empty; if none is present in the original URI, it MUST be given as
"/" (the server root)

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Request Header Fields
We will study General-header and Entity-header in a separate chapter when we will learn HTTP header fields. For
now let's check what are Request header fields.
The request-header fields allow the client to pass additional information about the request, and about the client itself,
to the server. These fields act as request modifiers and there are following important Request-header fields available
which can be used based on requirement.
 Accept-Charset
 Accept-Encoding
 Accept-Language
 Authorization
 Expect
 From
 Host
 If-Match
 If-Modified-Since
 If-None-Match
 If-Range
 If-Unmodified-Since
 Max-Forwards
 Proxy-Authorization
 Range
 Referer
 TE
 User-Agent
You can introduce your custom fields in case you are going to write your own custom Client and Web Server.
Request Message Examples
Now let's put it all together to form an HTTP request to fetch hello.htm page from the web server running on
tutorialspoint.com
GET /hello.htm HTTP/1.1
User-Agent: Mozilla/4.0 (compatible; MSIE5.01; Windows NT)
Host: www.tutorialspoint.com
Accept-Language: en-us
Accept-Encoding: gzip, deflate
Connection: Keep-Alive
Here we are not sending any request data to the server because we are fetching a plan HTML page from the server.
Connection is a general-header used here and rest of the headers are request headers. Following is one more
example where we send form data to the server using request message body:

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POST /cgi-bin/process.cgi HTTP/1.1
Content-Type: application/x-www-form-urlencoded
Content-Length: length
licenseID=string&content=string&/paramsXML=string
Here given URL /cgi-bin/process.cgi will be used to process the passed data and accordingly a response will be
retuned. Here content-type tells the server that passed data is simple web form data and length will be actual length
of the data put in the message body. Following example shows how you can pass plan XML to your web server:
Content-Type: text/xml; charset=utf-8
Content-Length: length
<?xml version="1.0" encoding="utf-8"?>
<string xmlns="http://clearforest.com/">string</string>

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HTTP – Responses
After receiving and interpreting a request message, a server responds with an HTTP response message:
 A Status-line
 Zero or more header (General|Response|Entity) fields followed by CRLF
 An empty line (i.e., a line with nothing preceding the CRLF) indicating the end of the header
fields
Message Status-Line
The Status-Line consisting of the protocol version followed by a numeric status code and its associated textual
phrase. The elements are separated by space SP characters.
Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
Let's discuss each of the part mentioned in Status-Line.
HTTP Version
A server supporting HTTP version 1.1 will return following version information:
HTTP-Version = HTTP/1.1
Status Code
The Status-Code element is a 3-digit integer where first digit of the Status-Code defines the class of response and the
last two digits do not have any categorization role. There are 5 values for the first digit:
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S.N. Code and Description
1
1xx: Informational
This means request received and continuing process.
2
2xx: Success
This means the action was successfully received, understood, and accepted.
3
3xx: Redirection
This means further action must be taken in order to complete the request.
4
4xx: Client Error
This means the request contains bad syntax or cannot be fulfilled
5
5xx: Server Error
The server failed to fulfill an apparently valid request
HTTP status codes are extensible and HTTP applications are not required to understand the meaning of all registered
status codes. A list of all the status code has been given in a separate chapter for you reference.
Response Header Fields
We will study General-header and Entity-header in a separate chapter when we will learn HTTP header fields. For
now let's check what are Response header fields.
The response-header fields allow the server to pass additional information about the response which cannot be
placed in the Status- Line. These header fields give information about the server and about further access to the
resource identified by the Request-URI.
 Accept-Ranges
 Age
 ETag
 Location
 Proxy-Authenticate
 Retry-After
 Server
 Vary
 WWW-Authenticate
You can introduce your custom fields in case you are going to write your own custom Web Client and Server.
Response Message Examples
Now let's put it all together to form an HTTP response for a request to fetch hello.htm page from the web server
running on tutorialspoint.com

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HTTP/1.1 200 OK
Date: Mon, 27 Jul 2009 12:28:53 GMT
Server: Apache/2.2.14 (Win32)
Content-Length: 88
Content-Type: text/html
Connection: Closed
<html>
<body>
</body>
</html>
Following is an example of HTTP response message showing error condition when web server could not find
requested page:
HTTP/1.1 404 Not Found
Date: Sun, 18 Oct 2012 10:36:20 GMT
Content-Length: 230
Connection: Closed
Content-Type: text/html; charset=iso-8859-1
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html>
<head>
<title>404 Not Found</title>
</head>
<body>
<h1>Not Found</h1>
<p>The requested URL /t.html was not found on this server.</p>
</body>
</html>

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Following is an example of HTTP response message showing error condition when web server encountered a wrong
HTTP version in given HTTP request:
HTTP/1.1 400 Bad Request
Date: Sun, 18 Oct 2012 10:36:20 GMT
Content-Length: 230
Connection: Closed
<html>
<head>
<title>400 Bad Request</title>
</head>
<body>
<h1>Bad Request</h1>
<p>Your browser sent a request that this server could not understand.<p>
<p>The request line contained invalid characters following the protocol string.<p>
</body>
</html>

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HTTP – Methods
The set of common methods for HTTP/1.1 is defined below and this set can be expanded based on requirement.
These method names are case sensitive and they must be used in uppercase.
1
GET
The GET method is used to retrieve information from the given server using a given URI. Requests
using GET should only retrieve data and should have no other effect on the data.
2
HEAD
Same as GET, but only transfer the status line and header section.
3
POST
A POST request is used to send data to the server, for example customer information, file upload etc
using HTML forms.
4
PUT
Replace all current representations of the target resource with the uploaded content.
5
DELETE
Remove all current representations of the target resource given by URI.
6
CONNECT
Establish a tunnel to the server identified by a given URI.
7
OPTIONS
Describe the communication options for the target resource.
8
TRACE
Perform a message loop-back test along the path to the target resource.
GET Method
A GET request retrieves data from a web server by specifying parameters in the URL portion of the request. This is
the main method used for document retrieval. Following is a simple example which makes use of GET method to
fetch hello.htm:
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Following will be a server response against the above GET request:
HTTP/1.1 200 OK
Date: Mon, 27 Jul 2009 12:28:53 GMT
ETag: "34aa387-d-1568eb00"
Vary: Authorization,Accept
Content-Length: 88
Connection: Closed
<html>
<body>
</body>
</html>
HEAD Method
The HEAD method is functionally like GET, except that the server replies with a response line and headers, but no
entity-body. Following is a simple example which makes use of HEAD method to fetch header information about
hello.htm:
HEAD /hello.htm HTTP/1.1
Following will be a server response against the above GET request:
HTTP/1.1 200 OK
Date: Mon, 27 Jul 2009 12:28:53 GMT
ETag: "34aa387-d-1568eb00"
Content-Length: 88
Connection: Closed

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You can notice that here server does not send any data after header.
POST Method
The POST method is used when you want to send some data to the server, for example file update, form data etc.
Following is a simple example which makes use of POST method to send a form data to the server which will be
processed by a process.cgi and finally a response will be returned:
Content-Length: 88
<?xml version="1.0" encoding="utf-8"?>
<string xmlns="http://clearforest.com/">string</string>
Server side script process.cgi process the passed data and send following response:
HTTP/1.1 200 OK
Date: Mon, 27 Jul 2009 12:28:53 GMT
ETag: "34aa387-d-1568eb00"
Content-Length: 88
Connection: Closed
<html>
<body>
<h1>Request Processed Successfully</h1>
</body>
</html>

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PUT Method
The PUT method is used to request the server to store the included entity-body at a location specified by the given
URL. The following example request server to save the given entity-boy in hello.htm at the root of the server:
PUT /hello.htm HTTP/1.1
Content-type: text/html
Content-Length: 182
<html>
<body>
</body>
</html>
The server will store given entity-body in hello.htm file and will send following response back to the client:
HTTP/1.1 201 Created
Date: Mon, 27 Jul 2009 12:28:53 GMT
Content-length: 30
Connection: Closed
<html>
<body>
<h1>The file was created.</h1>
</body>
</html>
DELETE Method
The DELETE method is used to request the server to delete file at a location specified by the given URL. The
following example request server to delete the given file hello.htm at the root of the server:
DELETE /hello.htm HTTP/1.1

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The server will delete mentioned file hello.htm and will send following response back to the client:
HTTP/1.1 200 OK
Date: Mon, 27 Jul 2009 12:28:53 GMT
Content-length: 30
Connection: Closed
<html>
<body>
<h1>URL deleted.</h1>
</body>
</html>
CONNECT Method
The CONNECT method is used by the client to establish a network connection to a web server over HTTP. The
following example request a connection with a web server running on host tutorialspoint.com:
CONNECT www.tutorialspoint.com HTTP/1.1
The connection is established with the server and following response is sent back to the client:
HTTP/1.1 200 Connection established
Date: Mon, 27 Jul 2009 12:28:53 GMT
OPTIONS Method
The OPTIONS method is used by the client to find out what are the HTTP methods and other options supported by a
web server. The client can specify a URL for the OPTIONS method, or an asterisk (*) to refer to the entire server. The
following example request a list of methods supported by a web server running on tutorialspoint.com:
OPTIONS * HTTP/1.1
The server will send information based on the current configuration of the server, for example:
HTTP/1.1 200 OK
Date: Mon, 27 Jul 2009 12:28:53 GMT
Allow: GET,HEAD,POST,OPTIONS,TRACE
Content-Type: httpd/unix-directory

TUTORIALS POINT
TRACE Method
The TRACE method is used to eacho the contents of an HTTP Request back to the requester which can be used for
debugging purpose at the time of development. The following example shows the usage of TRACE method:
TRACE / HTTP/1.1
The server will send following message in response of the above request:
HTTP/1.1 200 OK
Date: Mon, 27 Jul 2009 12:28:53 GMT
Connection: close
Content-Type: message/http
Content-Length: 39
TRACE / HTTP/1.1

TUTORIALS POINT
HTTP - Status Codes
The Status-Code element in a server response, is a 3-digit integer where first digit of the Status-Code defines the
class of response and the last two digits do not have any categorization role. There are 5 values for the first digit:
S.N. Code and Description
1
1xx: Informational
This means request received and continuing process.
2
2xx: Success
This means the action was successfully received, understood, and accepted.
3
3xx: Redirection
This means further action must be taken in order to complete the request.
4
4xx: Client Error
This means the request contains bad syntax or cannot be fulfilled
5
5xx: Server Error
The server failed to fulfill an apparently valid request
HTTP status codes are extensible and HTTP applications are not required to understand the meaning of all registered
status codes. Following is a list of all the status code.
1xx: Information
Message: Description:
100 Continue
Only a part of the request has been received by the server, but
as long as it has not been rejected, the client should continue
with the request
101 Switching Protocols The server switches protocol
2xx: Successful
200 OK The request is OK
201 Created The request is complete, and a new resource is created
202 Accepted
The request is accepted for processing, but the processing is not
complete
203 Non-authoritative Information
The information in the entity header is from a local or third-party
copy, not from the original server.
204 No Content
A status code and header are given in the response, but there is
no entity-body in the reply.
205 Reset Content
The browser should clear the form used for this transaction for
additional input.
206 Partial Content
The server is returning partial data of the size requested. Used in
response to a request specifying a Range header. The server
must specify the range included in the response with the Content-
Range header.
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3xx: Redirection
300 Multiple Choices
A link list. The user can select a link and go to that location.
Maximum five addresses
301 Moved Permanently The requested page has moved to a new url
302 Found The requested page has moved temporarily to a new url
303 See Other The requested page can be found under a different url
304 Not Modified
This is the response code to an If-Modified-Since or If-None-
Match header, where the URL has not been modified since the
specified date.
305 Use Proxy
The requested URL must be accessed through the proxy
mentioned in the Location header.
306 Unused
This code was used in a previous version. It is no longer used,
but the code is reserved
307 Temporary Redirect The requested page has moved temporarily to a new url
4xx: Client Error
400 Bad Request The server did not understand the request
401 Unauthorized The requested page needs a username and a password
402 Payment Required You can not use this code yet
403 Forbidden Access is forbidden to the requested page
404 Not Found The server can not find the requested page
405 Method Not Allowed The method specified in the request is not allowed
406 Not Acceptable
The server can only generate a response that is not accepted by
the client
407 Proxy Authentication Required
You must authenticate with a proxy server before this request
can be served
408 Request Timeout The request took longer than the server was prepared to wait
409 Conflict The request could not be completed because of a conflict
410 Gone The requested page is no longer available
411 Length Required
The "Content-Length" is not defined. The server will not accept
the request without it
412 Precondition Failed
The precondition given in the request evaluated to false by the
server
413 Request Entity Too Large
The server will not accept the request, because the request entity
is too large
414 Request-url Too Long
The server will not accept the request, because the url is too
long. Occurs when you convert a "post" request to a "get"
request with a long query information
415 Unsupported Media Type
The server will not accept the request, because the media type is
not supported
416 Requested Range Not Satisfiable The requested byte range is not available and is out of bounds.
417 Expectation Failed
The expectation given in an Expect request-header field could
not be met by this server.

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5xx: Server Error
500 Internal Server Error
The request was not completed. The server met an unexpected
condition
501 Not Implemented
The request was not completed. The server did not support the
functionality required
502 Bad Gateway
The request was not completed. The server received an invalid
response from the upstream server
503 Service Unavailable
The request was not completed. The server is temporarily
overloading or down
504 Gateway Timeout The gateway has timed out
505 HTTP Version Not Supported The server does not support the "http protocol" version

TUTORIALS POINT
HTTP - Header Fields
HTTP deader fields provide required information about the request or response, or about the object sent in the
message body. There are following four types of HTTP message headers:
 General-header: These header fields have general applicability for both request and response messages.
 Client Request-header: These header fields are applicability only for request messages.
 Server Response-header: These header fields are applicability only for response messages.
 Entity-header: These header fields define metainformation about the entity-body or, if no body is present
General Headers
Cache-control
The Cache-Control general-header field is used to specify directives that MUST be obeyed by all caching system.
Following is the syntax:
Cache-Control : cache-request-directive|cache-response-directive
An HTTP clients or servers can use the Cache-control general header to specify parameters for the cache or to
request certain kinds of documents from the cache. The caching directives are specified in a comma-separated list.
For example:
Cache-control: no-cache
There are following important cache request directives which can be used by the client in its HTTP request:
S.N. Cache Request Directive and Description
1
no-cache
A cache must not use the response to satisfy a subsequent request without successful revalidation with
the origin server.
2
no-store
The cache should not store anything about the client request or server response.
3
max-age = seconds
Indicates that the client is willing to accept a response whose age is no greater than the specified time
in seconds.
4
max-stale [ = seconds ]
Indicates that the client is willing to accept a response that has exceeded its expiration time. If seconds
are given, it must not be expired by more than that time.
5
min-fresh = seconds
Indicates that the client is willing to accept a response whose freshness lifetime is no less than its
current age plus the specified time in seconds.
6
no-transform
Do not convert the entity-body.
7
only-if-cached
Do not retrieve new data. The cache can send a document only if it is in the cache, and should not
contact the origin-server to see if a newer copy exists.
There are following important cache response directives which can be used by the server in its HTTP response:
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1
public
Indicates that the response may be cached by any cache.
2
private
Indicates that all or part of the response message is intended for a single user and must not be cached
by a shared cache.
3
no-cache
the origin server.
4
no-store
5
no-transform
6
must-revalidate
The cache must verify the status of stale documents before using it and expired one should not be
used.
7
proxy-revalidate
The proxy-revalidate directive has the same meaning as the must- revalidate directive, except that it
does not apply to non-shared user agent caches.
8
max-age = seconds
in seconds.
9
s-maxage = seconds
The maximum age specified by this directive overrides the maximum age specified by either the max-
age directive or the Expires header. The s-maxage directive is always ignored by a private cache.
Connection
The Connection general-header field allows the sender to specify options that are desired for that particular
connection and must not be communicated by proxies over further connections. Following is the simple syntax of
using connection header:
Connection : "Connection"
HTTP/1.1 defines the "close" connection option for the sender to signal that the connection will be closed after
completion of the response. For example:
Connection: close
By default, HTTP 1.1 uses persistent connections, where the connection does not automatically close after a
transaction. HTTP 1.0, on the other hand, does not have persistent connections by default. If a 1.0 client wishes to
use persistent connections, it uses the keep-alive parameter as follows:
Connection: keep-alive
Date
All HTTP date/time stamps MUST be represented in Greenwich Mean Time (GMT), without exception. HTTP
applications are allowed to use any of the following three representations of date/time stamps:
Sun, 06 Nov 1994 08:49:37 GMT ; RFC 822, updated by RFC 1123
Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
Here first format is the most preferred one.

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Pragma
The Pragma general-header field is used to include implementation- specific directives that might apply to any
recipient along the request/response chain. For example:
Pragma: no-cache
The only directive defined in HTTP/1.0 is the no-cache directive and is maintained in HTTP 1.1 for backward
compatibility. No new Pragma directives will be defined in the future.
Trailer
The Trailer general field value indicates that the given set of header fields is present in the trailer of a message
encoded with chunked transfer-coding. Following is the syntax of Trailer header field:
Trailer : field-name
Message header fields listed in the Trailer header field must not include the following header fields:
 Transfer-Encoding
 Content-Length
 Trailer
Transfer-Encoding
The Transfer-Encoding general-header field indicates what type of transformation has been applied to the message
body in order to safely transfer it between the sender and the recipient. This is not the same as content-encoding
because transfer-encodings are a property of the message, not of the entity-body. Following is the syntax of Transfer-
Encoding header field:
Transfer-Encoding: chunked
All transfer-coding values are case-insensitive.
Upgrade
The Upgrade general-header allows the client to specify what additional communication protocols it supports and
would like to use if the server finds it appropriate to switch protocols. For example:
Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
The Upgrade header field is intended to provide a simple mechanism for transition from HTTP/1.1 to some other,
incompatible protocol
Via
The Via general-header must be used by gateways and proxies to indicate the intermediate protocols and recipients.
For example, a request message could be sent from an HTTP/1.0 user agent to an internal proxy code-named "fred",
which uses HTTP/1.1 to forward the request to a public proxy at nowhere.com, which completes the request by
forwarding it to the origin server at www.ics.uci.edu. The request received by www.ics.uci.edu would then have the
following Via header field:
Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)
The Upgrade header field is intended to provide a simple mechanism for transition from HTTP/1.1 to some other,
incompatible protocol

TUTORIALS POINT
Warning
The Warning general-header is used to carry additional information about the status or transformation of a message
which might not be reflected in the message. A response may carry more than one Warning header.
Warning : warn-code SP warn-agent SP warn-text SP warn-date
Client Request Headers
Accept
The Accept request-header field can be used to specify certain media types which are acceptable for the response.
Following is the general syntax:
Accept: type/subtype [q=qvalue]
Multiple media types can be listed separated by commas and the optional qvalue represents an acceptable quality
level for accept types on a scale of 0 to 1. Following is an example:
Accept: text/plain; q=0.5, text/html, text/x-dvi; q=0.8, text/x-c
This would be interpreted as text/html and text/x-c are the preferred media types, but if they do not exist, then send
the text/x-dvi entity, and if that does not exist, send the text/plain entity.
Accept-Charset
The Accept-Charset request-header field can be used to indicate what character sets are acceptable for the response.
Accept-Charset: character_set [q=qvalue]
Multiple character sets can be listed separated by commas and the optional qvalue represents an acceptable quality
level for nonpreferred character sets on a scale of 0 to 1. Following is an example:
Accept-Charset: iso-8859-5, unicode-1-1; q=0.8
The special value "*", if present in the Accept-Charset field, matches every character set and if no Accept-Charset
header is present, the default is that any character set is acceptable.
Accept-Encoding
The Accept-Encoding request-header field is similar to Accept, but restricts the content-codings that are acceptable in
the response. Following is the general syn tax:
Accept-Encoding: encoding types
Following are examples:
Accept-Encoding: compress, gzip
Accept-Encoding:
Accept-Encoding: *
Accept-Encoding: compress;q=0.5, gzip;q=1.0
Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0

TUTORIALS POINT
Accept-Language
The Accept-Language request-header field is similar to Accept, but restricts the set of natural languages that are
preferred as a response to the request. Following is the general syntax:
Accept-Language: language [q=qvalue]
Multiple languages can be listed separated by commas and the optional qvalue represents an acceptable quality level
for nonpreferred languages on a scale of 0 to 1. Following is an example:
Accept-Language: da, en-gb;q=0.8, en;q=0.7
Authorization
The Authorization request-header field value consists of credentials containing the authentication information of the
user agent for the realm of the resource being requested. Following is the general syntax:
Authorization : credentials
The HTTP/1.0 specification defines the BASIC authorization scheme, where the authorization parameter is the string
of username:password encoded in base 64. Following is an example:
Authorization: BASIC Z3Vlc3Q6Z3Vlc3QxMjM=
The value decodes into is guest:guest123 where guest is user ID and guest123 is the password.
Cookie
The Cookie request-header field value contains a name/value pair of information stored for that URL. Following is the
general syntax:
Cookie: name=value
Multiple cookies can be specified separated by semicolons as follows:
Cookie: name1=value1:name2=value2:name3=value3
Expect
The Expect request-header field is used to indicate that particular server behaviors are required by the client.
Expect : 100-continue | expectation-extension
If a server receives a request containing an Expect field that includes an expectation-extension that it does not
support, it must respond with a 417 (Expectation Failed) status.
From
The From request-header field contains an Internet e-mail address for the human user who controls the requesting
user agent. Following is a simple example:
From: webmaster@w3.org
This header field may be used for logging purposes and as a means for identifying the source
of invalid or unwanted requests.
Host
The Host request-header field is used to specify the Internet host and port number of the
resource being requested. Following is the general syntax:

TUTORIALS POINT
Host : "Host" ":" host [ ":" port ] ;
A host without any trailing port information implies the default port, which is 80. For example, a request on the origin
server for http://www.w3.org/pub/WWW/ would be:
GET /pub/WWW/ HTTP/1.1
Host: www.w3.org
If-Match
The If-Match request-header field is used with a method to make it conditional. This header request the server to
perform the requested method only if given value in this tag matches the given entity tags represented by ETag.
If-Match : entity-tag
An asterisk (*) matches any entity, and the transaction continues only if the entity exists. Following are possible
examples:
If-Match: "xyzzy"
If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-Match: *
If none of the entity tags match, or if "*" is given and no current entity exists, the server must not perform the requested
method, and must return a 412 (Precondition Failed) response.
If-Modified-Since
The If-Modified-Since request-header field is used with a method to make it conditional. If the requested URL has not
been modified since the time specified in this field, an entity will not be returned from the server; instead, a 304 (not
modified) response will be returned without any message-body. Following is the general syntax:
If-Modified-Since : HTTP-date
An example of the field is:
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
If none of the entity tags match, or if "*" is given and no current entity exists, the server must not perform the requested
method, and must return a 412 (Precondition Failed) response.
If-None-Match
The If-None-Match request-header field is used with a method to make it conditional. This header request the server
to perform the requested method only if one of the given value in this tag matches the given entity tags represented by
ETag. Following is the general syntax:
If-None-Match : entity-tag
An asterisk (*) matches any entity, and the transaction continues only if the entity does not exist. Following are
possible examples:
If-None-Match: "xyzzy"
If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-None-Match: *
If-Range
The If-Range request-header field can be used with a conditional GET to request only the portion of the entity that is
missing, if it has not been changed, and the entire entity if it has changed. Following is the general syntax:

TUTORIALS POINT
If-Range : entity-tag | HTTP-date
Either an entity tag or a date can be used to identify the partial entity already received. For example:
If-Range: Sat, 29 Oct 1994 19:43:31 GMT
Here if the document has not been modified since the given date, the server returns the byte range given by the
Range header otherwise, it returns all of the new document.
If-Unmodified-Since
The If-Unmodified-Since request-header field is used with a method to make it conditional. Following is the general
syntax:
If-Unmodified-Since : HTTP-date
If the requested resource has not been modified since the time specified in this field, the server should perform the
requested operation as if the If-Unmodified-Since header were not present. For example:
If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
If the request normally would result in anything other than a 2xx or 412 status, the If-Unmodified-Since header should
be ignored.
Max-Forwards
The Max-Forwards request-header field provides a mechanism with the TRACE and OPTIONS methods to limit the
number of proxies or gateways that can forward the request to the next inbound server. Following is the general
syntax:
Max-Forwards : n
The Max-Forwards value is a decimal integer indicating the remaining number of times this request message may be
forwarded. This is useful for debugging with the TRACE method, avoiding infinite loops. For example:
Max-Forwards : 5
The Max-Forwards header field may be ignored for all other methods defined in HTTP specification.
Proxy-Authorization
The Proxy-Authorization request-header field allows the client to identify itself (or its user) to a proxy which requires
authentication. Following is the general syntax:
The Proxy-Authorization field value consists of credentials containing the authentication information of the user agent
for the proxy and/or realm of the resource being requested.
Range
The Range request-header field specifies the partial range(s) of the content requested from the document. Following
is the general syntax:
Range: bytes-unit=first-byte-pos "-" [last-byte-pos]
The first-byte-pos value in a byte-range-spec gives the byte-offset of the first byte in a range. The last-byte-pos value
gives the byte-offset of the last byte in the range; that is, the byte positions specified are inclusive. You can specify a
byte-unit as bytes Byte offsets start at zero. Following are a simple examples:

TUTORIALS POINT
- The first 500 bytes
Range: bytes=0-499
- The second 500 bytes
Range: bytes=500-999
- The final 500 bytes
Range: bytes=-500
- The first and last bytes only
Range: bytes=0-0,-1
Multiple ranges can be listed, separated by commas. If the first digit in the comma-separated byte range(s) is missing,
the range is assumed to count from the end of the document. If the second digit is missing, the range is byte n to the
end of the document.
Referer
The Referer request-header field allows the client to specify the address (URI) of the resource from which the URL
has been requested. Following is the general syntax:
Referer : absoluteURI | relativeURI
Following is a simple example:
Referer: http://www.tutorialspoint.org/http/index.htm
If the field value is a relative URI, it should be interpreted relative to the Request-URI.
TE
The TE request-header field indicates what extension transfer-coding it is willing to accept in the response and
whether or not it is willing to accept trailer fields in a chunked transfer-coding. Following is the general syntax:
TE : t-codings
The presence of the keyword "trailers" indicates that the client is willing to accept trailer fields in a chunked transfer-
coding and it is specified either of the ways:
TE: deflate
TE:
TE: trailers, deflate;q=0.5
If the TE field-value is empty or if no TE field is present, the only transfer-coding is chunked. A message with no
transfer-coding is always acceptable.

TUTORIALS POINT
User-Agent
The User-Agent request-header field contains information about the user agent originating the request. Following is
the general syntax:
User-Agent : product | comment
Example:
Server Response Headers
Accept-Ranges
The Accept-Ranges response-header field allows the server to indicate its acceptance of range requests for a
resource. Following is the general syntax:
Accept-Ranges : range-unit | none
For example a server that accept byte-range requests may send
Servers that do not accept any kind of range request for a resource may send:
Accept-Ranges: none
This will advise the client not to attempt a range request.
Age
The Age response-header field conveys the sender's estimate of the amount of time since the response (or its
revalidation) was generated at the origin server. Following is the general syntax:
Age : delta-seconds
Age values are non-negative decimal integers, representing time in seconds. Following is a simple example:
Age: 1030
An HTTP/1.1 server that includes a cache must include an Age header field in every response generated from its own
cache.
ETag
The ETag response-header field provides the current value of the entity tag for the requested variant. Following is the
general syntax:
ETag : entity-tag
Following are simple examples:
ETag: "xyzzy"
ETag: W/"xyzzy"
ETag: ""
Location
The Location response-header field is used to redirect the recipient to a location other than the Request-URI for
completion. Following is the general syntax:
Location : absoluteURI

TUTORIALS POINT
Location: http://www.tutorialspoint.org/http/index.htm
The Content-Location header field differs from Location in that the Content-Location identifies the original location
of the entity enclosed in the request.
Proxy-Authenticate
The Proxy-Authenticate response-header field must be included as part of a 407 (Proxy Authentication Required)
response. Following is the general syntax:
Proxy-Authenticate : challenge
Retry-After
The Retry-After response-header field can be used with a 503 (Service Unavailable) response to indicate how
long the service is expected to be unavailable to the requesting client. Following is the general syntax:
Retry-After : HTTP-date | delta-seconds
Following are two simple examples:
Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
Retry-After: 120
In the latter example, the delay is 2 minutes.
Server
The Server response-header field contains information about the software used by the origin server to handle the
request. Following is the general syntax:
Server : product | comment
If the response is being forwarded through a proxy, the proxy application must not modify the Server response-
header.
Set-Cookie
The Set-Cookie response-header field contains a name/value pair of information to retain for this URL. Following
is the general syntax:
Set-Cookie: NAME=VALUE; OPTIONS
Set-Cookie response header comprises the token Set-Cookie:, followed by a comma-separated list of one or
more cookies. Here are possible values you can specify as options:
S.N. Options and Description
1
Comment=comment
This option can be used to specify any comment associated with the cookie.
2
Domain=domain
The Domain attribute specifies the domain for which the cookie is valid.
3
Expires=Date-time
The date the cookie will expire. If this is blank, the cookie will expire when the visitor quits the browser
4
Path=path
The Path attribute specifies the subset of URLs to which this cookie applies.
5
Secure
This instructs the user agent to return the cookie only under a secure connection.

TUTORIALS POINT
Following is an example of a simple cookie header generated by the server:
Set-Cookie: name1=value1,name2=value2; Expires=Wed, 09 Jun 2021 10:18:14 GMT
Vary
The Vary response-header field specifies that the entity has multiple sources and may therefore vary according to
specified list of request header(s). Following is the general syntax:
Vary : field-name
You can specify multiple headers separated by commas and a value of asterisk "*" signals that unspecified
parameters not limited to the request-headers. Following is a simple example:
Vary: Accept-Language, Accept-Encoding
Here field names are case-insensitive.
WWW-Authenticate
The WWW-Authenticate response-header field must be included in 401 (Unauthorized) response messages. The field
value consists of at least one challenge that indicates the authentication scheme(s) and parameters applicable to the
Request-URI. Following is the general syntax:
WWW-Authenticate : challenge
WWW- Authenticate field value as it might contain more than one challenge, or if more than one WWW-Authenticate
header field is provided, the contents of a challenge itself can contain a comma-separated list of authentication
parameters. Following is a simple example:
WWW-Authenticate: BASIC realm="Admin"
Entity Headers
Allow
The Allow entity-header field lists the set of methods supported by the resource identified by the Request-URI.
Allow : Method
You can specify multiple method separated by commas. Following is a simple example:
Allow: GET, HEAD, PUT
This field cannot prevent a client from trying other methods.
Content-Encoding
The Content-Encoding entity-header field is used as a modifier to the media-type. Following is the general
syntax:
Content-Encoding : content-coding
The content-coding is a characteristic of the entity identified by the Request-URI. Following is a simple example:
Content-Encoding: gzip
If the content-coding of an entity in a request message is not acceptable to the origin server, the server should
respond with a status code of 415 (Unsupported Media Type).
Content-Language
The Content-Language entity-header field describes the natural language(s) of the intended audience for the
enclosed entity. Following is the general syntax:

TUTORIALS POINT
Content-Language : language-tag
Multiple languages may be listed for content that is intended for multiple audiences. Following is a simple
example:
Content-Language: mi, en
The primary purpose of Content-Language is to allow a user to identify and differentiate entities according to the
user's own preferred language.
Content-Length
The Content-Length entity-header field indicates the size of the entity-body, in decimal number of OCTETs, sent
to the recipient or, in the case of the HEAD method, the size of the entity-body that would have been sent had the
request been a GET. Following is the general syntax:
Content-Length : DIGITS
Content-Length: 3495
Any Content-Length greater than or equal to zero is a valid value.
Content-Location
The Content-Location entity-header field may be used to supply the resource location for the entity enclosed in
the message when that entity is accessible from a location separate from the requested resource's URI.
Content-Location: absoluteURI | relativeURI
Content-Location: http://www.tutorialspoint.org/http/index.htm
The value of Content-Location also defines the base URI for the entity.
Content-MD5
The Content-MD5 entity-header field may be used to supply an MD5 digest of the entity, for checking the integrity
of the message upon receipt. Following is the general syntax:
Content-MD5 : md5-digest using base64 of 128 bit MD5 digest as per RFC 1864
Content-MD5 : 8c2d46911f3f5a326455f0ed7a8ed3b3
The MD5 digest is computed based on the content of the entity-body, including any content-coding that has been
applied, but not including any transfer-encoding applied to the message-body.
Content-Range
The Content-Range entity-header field is sent with a partial entity-body to specify where in the full entity-body the
partial body should be applied. Following is the general syntax:
Content-Range : bytes-unit SP first-byte-pos "-" last-byte-pos
Examples of byte-content-range-spec values, assuming that the entity contains a total of 1234 bytes:
- The first 500 bytes:
Content-Range : bytes 0-499/1234
- The second 500 bytes:

TUTORIALS POINT
- All except for the first 500 bytes:
- The last 500 bytes:
When an HTTP message includes the content of a single range, this content is transmitted with a Content-Range
header, and a Content-Length header showing the number of bytes actually transferred. For example,
HTTP/1.1 206 Partial content
Date: Wed, 15 Nov 1995 06:25:24 GMT
Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
Content-Range: bytes 21010-47021/47022
Content-Length: 26012
Content-Type: image/gif
Content-Type
The Content-Type entity-header field indicates the media type of the entity-body 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. Following is the general
syntax:
Content-Type : media-type
Following is an example:
Content-Type: text/html; charset=ISO-8859-4
Expires
The Expires entity-header field gives the date/time after which the response is considered stale. Following is the
general syntax:
Expires : HTTP-date
Expires: Thu, 01 Dec 1994 16:00:00 GMT
Last-Modified
The Last-Modified entity-header field indicates the date and time at which the origin server believes the variant was
last modified. Following is the general syntax:
Last-Modified: HTTP-date
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT

TUTORIALS POINT
HTTP – Caching
HTTP is typically used for distributed information systems, where performance can be improved by the use of
response caches. The HTTP/1.1 protocol includes a number of elements intended to make caching work.
The goal of caching in HTTP/1.1 is to eliminate the need to send requests in many cases, and to eliminate the need to
send full responses in many other cases.
The basic cache mechanisms in HTTP/1.1 are implicit directives to caches where server-specifies expiration times
and validators. We use the Cache-Control header for this purpose.
The Cache-Control header allows a client or server to transmit a variety of directives in either requests or responses.
These directives typically override the default caching algorithms. The caching directives are specified in a comma-
separated list. For example:
Cache-control: no-cache
There are following important cache request directives which can be used by the client in its HTTP request:
1
no-cache
the origin server.
2
no-store
3
max-age = seconds
in seconds.
4
max-stale [ = seconds ]
Indicates that the client is willing to accept a response that has exceeded its expiration time. If seconds
are given, it must not be expired by more than that time.
5
min-fresh = seconds
Indicates that the client is willing to accept a response whose freshness lifetime is no less than its
current age plus the specified time in seconds.
6
no-transform
7
only-if-cached
Do not retrieve new data. The cache can send a document only if it is in the cache, and should not
contact the origin-server to see if a newer copy exists.
There are following important cache response directives which can be used by the server in its HTTP response:
CHAPTER
9

TUTORIALS POINT
1
public
Indicates that the response may be cached by any cache.
2
private
Indicates that all or part of the response message is intended for a single user and must not be cached
by a shared cache.
3
no-cache
the origin server.
4
no-store
5
no-transform
6
must-revalidate
The cache must verify the status of stale documents before using it and expired one should not be
used.
7
proxy-revalidate
The proxy-revalidate directive has the same meaning as the must- revalidate directive, except that it
does not apply to non-shared user agent caches.
8
max-age = seconds
in seconds.
9
s-maxage = seconds
The maximum age specified by this directive overrides the maximum age specified by either the max-
age directive or the Expires header. The s-maxage directive is always ignored by a private cache.

TUTORIALS POINT
HTTP - URL Encoding
HTTP URLs can only be sent over the Internet using the ASCII character-set, which often contain characters
outside the ASCII set. So these unsafe characters must be replaced with a % followed by two hexadecimal digits.
Following table shows ASCII symbol of the character and its equal Symbol and finally its replacement which can be
used in URL before passing it to the server:
ASCII Symbol Replacement
< 32 Encode with %xx where xx is the hexadecimal representation of the character.
32 space + or %20
33 ! %21
34 " %22
35 # %23
36 $ %24
37 % %25
38 & %26
39 ' %27
40 ( %28
41 ) %29
42 * *
43 + %2B
44 , %2C
45 - -
46 . .
47 / %2F
48 0 0
49 1 1
50 2 2
51 3 3
52 4 4
53 5 5
54 6 6
55 7 7
56 8 8
57 9 9
58 : %3A
59 ; %3B
60 < %3C
61 = %3D
62 > %3E
63 ? %3F
CHAPTER
10

TUTORIALS POINT
64 @ %40
65 A A
66 B B
67 C C
68 D D
69 E E
70 F F
71 G G
72 H H
73 I I
74 J J
75 K K
76 L L
77 M M
78 N N
79 O O
80 P P
81 Q Q
82 R R
83 S S
84 T T
85 U U
86 V V
87 W W
88 X X
89 Y Y
90 Z Z
91 [ %5B
92 %5C
93 ] %5D
94 ^ %5E
95 _ _
96 ` %60
97 a a
98 b b
99 c c
100 d d
101 e e
102 f f
103 g g
104 h h
105 i i
106 j j
107 k k
108 l l
109 m m
110 n n
111 o o
112 p p
113 q q
114 r r
115 s s
116 t t
117 u u
118 v v
119 w w
120 x x

TUTORIALS POINT
121 y y
122 z z
123 { %7B
124 | %7C
125 } %7D
126 ~ %7E
127 %7F
> 127 Encode with %xx where xx is the hexadecimal representation of the character

TUTORIALS POINT
HTTP – Security
HTTP is used for a communication over the internet, so application developers, information providers, and users
should be aware of the security limitations in HTTP/1.1. This discussion does not include definitive solutions to the
problems mentioned here but it does make some suggestions for reducing security risks.
Personal Information leakage
HTTP clients are often privy to large amounts of personal information such as the user's name, location, mail address,
passwords, encryption keys, etc. So you should be very careful to prevent unintentional leakage of this information via
the HTTP protocol to other sources.
 All the confidential information should be stored at server side in encrypted form.
 Revealing the specific software version of the server might allow the server machine to become more
vulnerable to attacks against software that is known to contain security holes.
 Proxies which serve as a portal through a network firewall should take special precautions regarding the
transfer of header information that identifies the hosts behind the firewall.
 The information sent in the From field might conflict with the user's privacy interests or their site's security
policy, and hence it should not be transmitted without the user being able to disable, enable, and modify the
contents of the field.
 Clients should not include a Referer header field in a (non-secure) HTTP request if the referring page was
transferred with a secure protocol.
 Authors of services which use the HTTP protocol should not use GET based forms for the submission of
sensitive data, because this will cause this data to be encoded in the Request-URI
File and path names based attack
The document should be restricted to the documents returned by HTTP requests to be only those that were intended
by the server administrators.
For example, UNIX, Microsoft Windows, and other operating systems use .. as a path component to indicate a
directory level above the current one. On such a system, an HTTP server MUST disallow any such construct in the
Request-URI if it would otherwise allow access to a resource outside those intended to be accessible via the HTTP
server.
DNS Spoofing
Clients using HTTP rely heavily on the Domain Name Service, and are thus generally prone to security attacks based
on the deliberate mis-association of IP addresses and DNS names. So clients need to be cautious in assuming the
continuing validity of an IP number/DNS name association.
If HTTP clients cache the results of host name lookups in order to achieve a performance improvement, they must
observe the TTL information reported by DNS. If HTTP clients do not observe this rule, they could be spoofed when a
previously-accessed server's IP address changes.
CHAPTER
11

TUTORIALS POINT
Location Headers and Spoofing
If a single server supports multiple organizations that do not trust one another, then it MUST check the values of
Location and Content- Location headers in responses that are generated under control of said organizations to make
sure that they do not attempt to invalidate resources over which they have no authority.
Authentication Credentials
Existing HTTP clients and user agents typically retain authentication information indefinitely. HTTP/1.1. does not
provide a method for a server to direct clients to discard these cached credentials which is a big security risk.
There are a number of work- arounds to parts of this problem, and so its is recommended to make the use of
password protection in screen savers, idle time-outs, and other methods which mitigate the security problems inherent
in this problem.
Proxies and Caching
HTTP proxies are men-in-the-middle, and represent an opportunity for man-in-the-middle attacks. Proxies have
access to security-related information, personal information about individual users and organizations, and proprietary
information belonging to users and content providers.
Proxy operators should protect the systems on which proxies run as they would protect any system that contains or
transports sensitive information.
Caching proxies provide additional potential vulnerabilities, since the contents of the cache represent an attractive
target for malicious exploitation. Therefore, cache contents should be protected as sensitive information.

TUTORIALS POINT
HTTP - Message Examples
Example 1
HTTP request to fetch hello.htm page from the web server running on tutorialspoint.com
Client request
Server response
HTTP/1.1 200 OK
Date: Mon, 27 Jul 2009 12:28:53 GMT
Content-Length: 88
Connection: Closed
<html>
<body>
</body>
</html>
Example 2
HTTP request to fetch t.html page which does not exist on the web server running on tutorialspoint.com
Client request
CHAPTER
12

TUTORIALS POINT
GET /t.html HTTP/1.1
Server response
HTTP/1.1 404 Not Found
Date: Sun, 18 Oct 2012 10:36:20 GMT
Content-Length: 230
Connection: Closed
<html>
<head>
<title>404 Not Found</title>
</head>
<body>
<h1>Not Found</h1>
<p>The requested URL /t.html was not found on this server.</p>
</body>
</html>
Example 3
HTTP request to fetch hello.htm page from the web server running on tutorialspoint.com, but request goes with
wrong HTTP version:
Client request
GET /hello.htm HTTP1

TUTORIALS POINT
Server response
HTTP/1.1 400 Bad Request
Date: Sun, 18 Oct 2012 10:36:20 GMT
Content-Length: 230
Connection: Closed
<html>
<head>
<title>400 Bad Request</title>
</head>
<body>
<h1>Bad Request</h1>
<p>Your browser sent a request that this server could not understand.<p>
<p>The request line contained invalid characters following the protocol string.<p>
</body>
</html>
Example 4
HTTP request to post form data to process.cgi CGI page on a web server running on tutorialspoint.com. Server
returns passed name after setting them as cookies:
Client request
Content-Length: 60
first=Zara&last=Ali

TUTORIALS POINT
Server response
HTTP/1.1 200 OK
Date: Mon, 27 Jul 2009 12:28:53 GMT
Content-Length: 88
Set-Cookie: first=Zara,last=Ali;domain=tutorialspoint.com;Expires=Mon, 19-
Nov-2010 04:38:14 GMT;Path=/
Connection: Closed
<html>
<body>
<h1>Hello Zara Ali</h1>
</body>
</html>

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