This is the original spec once found on netscape.com but since they decided to not keep the original URL alive and working, we host it here and point to this URL instead. Go to curl.haxx.se for more curl info

PERSISTENT CLIENT STATE
HTTP COOKIES

Preliminary Specification - Use with caution


INTRODUCTION

Cookies are a general mechanism which server side connections (such as CGI scripts) can use to both store and retrieve information on the client side of the connection. The addition of a simple, persistent, client-side state significantly extends the capabilities of Web-based client/server applications.

OVERVIEW

A server, when returning an HTTP object to a client, may also send a piece of state information which the client will store. Included in that state object is a description of the range of URLs for which that state is valid. Any future HTTP requests made by the client which fall in that range will include a transmittal of the current value of the state object from the client back to the server. The state object is called a cookie, for no compelling reason.

This simple mechanism provides a powerful new tool which enables a host of new types of applications to be written for web-based environments. Shopping applications can now store information about the currently selected items, for fee services can send back registration information and free the client from retyping a user-id on next connection, sites can store per-user preferences on the client, and have the client supply those preferences every time that site is connected to.

SPECIFICATION

A cookie is introduced to the client by including a Set-Cookie header as part of an HTTP response, typically this will be generated by a CGI script.

Syntax of the Set-Cookie HTTP Response Header

This is the format a CGI script would use to add to the HTTP headers a new piece of data which is to be stored by the client for later retrieval.
Set-Cookie: NAME=VALUE; expires=DATE;
path=PATH; domain=DOMAIN_NAME; secure
NAME=VALUE
This string is a sequence of characters excluding semi-colon, comma and white space. If there is a need to place such data in the name or value, some encoding method such as URL style %XX encoding is recommended, though no encoding is defined or required.

This is the only required attribute on the Set-Cookie header.

expires=DATE
The expires attribute specifies a date string that defines the valid life time of that cookie. Once the expiration date has been reached, the cookie will no longer be stored or given out.

The date string is formatted as:

Wdy, DD-Mon-YYYY HH:MM:SS GMT
This is based on RFC 822, RFC 850, RFC 1036, and RFC 1123, with the variations that the only legal time zone is GMT and the separators between the elements of the date must be dashes.

expires is an optional attribute. If not specified, the cookie will expire when the user's session ends.

Note: There is a bug in Netscape Navigator version 1.1 and earlier. Only cookies whose path attribute is set explicitly to "/" will be properly saved between sessions if they have an expires attribute.

domain=DOMAIN_NAME
When searching the cookie list for valid cookies, a comparison of the domain attributes of the cookie is made with the Internet domain name of the host from which the URL will be fetched. If there is a tail match, then the cookie will go through path matching to see if it should be sent. "Tail matching" means that domain attribute is matched against the tail of the fully qualified domain name of the host. A domain attribute of "acme.com" would match host names "anvil.acme.com" as well as "shipping.crate.acme.com".

Only hosts within the specified domain can set a cookie for a domain and domains must have at least two (2) or three (3) periods in them to prevent domains of the form: ".com", ".edu", and "va.us". Any domain that fails within one of the seven special top level domains listed below only require two periods. Any other domain requires at least three. The seven special top level domains are: "COM", "EDU", "NET", "ORG", "GOV", "MIL", and "INT".

The default value of domain is the host name of the server which generated the cookie response.

path=PATH
The path attribute is used to specify the subset of URLs in a domain for which the cookie is valid. If a cookie has already passed domain matching, then the pathname component of the URL is compared with the path attribute, and if there is a match, the cookie is considered valid and is sent along with the URL request. The path "/foo" would match "/foobar" and "/foo/bar.html". The path "/" is the most general path.

If the path is not specified, it as assumed to be the same path as the document being described by the header which contains the cookie.

secure
If a cookie is marked secure, it will only be transmitted if the communications channel with the host is a secure one. Currently this means that secure cookies will only be sent to HTTPS (HTTP over SSL) servers.

If secure is not specified, a cookie is considered safe to be sent in the clear over unsecured channels.

Syntax of the Cookie HTTP Request Header

When requesting a URL from an HTTP server, the browser will match the URL against all cookies and if any of them match, a line containing the name/value pairs of all matching cookies will be included in the HTTP request. Here is the format of that line:
Cookie: NAME1=OPAQUE_STRING1; NAME2=OPAQUE_STRING2 ...

Additional Notes

EXAMPLES

Here are some sample exchanges which are designed to illustrate the use of cookies.

First Example transaction sequence:

Client requests a document, and receives in the response:
Set-Cookie: CUSTOMER=WILE_E_COYOTE; path=/; expires=Wednesday, 09-Nov-99 23:12:40 GMT
When client requests a URL in path "/" on this server, it sends:
Cookie: CUSTOMER=WILE_E_COYOTE
Client requests a document, and receives in the response:
Set-Cookie: PART_NUMBER=ROCKET_LAUNCHER_0001; path=/
When client requests a URL in path "/" on this server, it sends:
Cookie: CUSTOMER=WILE_E_COYOTE; PART_NUMBER=ROCKET_LAUNCHER_0001
Client receives:
Set-Cookie: SHIPPING=FEDEX; path=/foo
When client requests a URL in path "/" on this server, it sends:
Cookie: CUSTOMER=WILE_E_COYOTE; PART_NUMBER=ROCKET_LAUNCHER_0001
When client requests a URL in path "/foo" on this server, it sends:
Cookie: CUSTOMER=WILE_E_COYOTE; PART_NUMBER=ROCKET_LAUNCHER_0001; SHIPPING=FEDEX

Second Example transaction sequence:

Assume all mappings from above have been cleared.

Client receives:
Set-Cookie: PART_NUMBER=ROCKET_LAUNCHER_0001; path=/
When client requests a URL in path "/" on this server, it sends:
Cookie: PART_NUMBER=ROCKET_LAUNCHER_0001
Client receives:
Set-Cookie: PART_NUMBER=RIDING_ROCKET_0023; path=/ammo
When client requests a URL in path "/ammo" on this server, it sends:
Cookie: PART_NUMBER=RIDING_ROCKET_0023; PART_NUMBER=ROCKET_LAUNCHER_0001
NOTE: There are two name/value pairs named "PART_NUMBER" due to the inheritance of the "/" mapping in addition to the "/ammo" mapping.