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.
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
This string is a sequence of characters excluding semicolons, commas, 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.
The expires attribute specifies a date string that defines the valid lifetime 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.
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.
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 falls 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.
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.
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.
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 ...
Servers should not expect clients to be able to exceed these limits. When the 300-cookie limit or the 20-cookie-per-server limit is exceeded, clients should delete the least recently used cookie. When a cookie larger than 4 kilobytes is encountered the cookie should be trimmed to fit, but the name should remain intact as long as it is less than 4 kilobytes.
Similarly, if a client request contains a Cookie: header, it should be forwarded through a proxy, even if the conditional If-modified-since request is being made.