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Updated: July 3, 2012 (
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HTTP Cookies
1. HTTP Cookies
1.1 Cookie overview
1.2 Cookies saved to disk
1.3 Cookies with curl the command line tool
1.4 Cookies with libcurl
1.5 Cookies with javascript
1. HTTP Cookies
1.1 Cookie overview
HTTP cookies are pieces of 'name=contents' snippets that a server tells the
client to hold and then the client sends back those the server on subsequent
requests to the same domains/paths for which the cookies were set.
Cookies are either "session cookies" which typically are forgotten when the
session is over which is often translated to equal when browser quits, or
the cookies aren't session cookies they have expiration dates after which
the client will throw them away.
Cookies are set to the client with the Set-Cookie: header and are sent to
servers with the Cookie: header.
For a very long time, the only spec explaining how to use cookies was the
original Netscape spec from 1994:
In 2011, RFC6265 ( was finally published
and details how cookies work within HTTP.
1.2 Cookies saved to disk
Netscape once created a file format for storing cookies on disk so that they
would survive browser restarts. curl adopted that file format to allow
sharing the cookies with browsers, only to see browsers move away from that
format. Modern browsers no longer use it, while curl still does.
The netscape cookie file format stores one cookie per physical line in the
file with a bunch of associated meta data, each field separated with
TAB. That file is called the cookiejar in curl terminology.
When libcurl saves a cookiejar, it creates a file header of its own in which
there is a URL mention that will link to the web version of this document.
1.3 Cookies with curl the command line tool
curl has a full cookie "engine" built in. If you just activate it, you can
have curl receive and send cookies exactly as mandated in the specs.
Command line options:
-b, --cookie
tell curl a file to read cookies from and start the cookie engine, or if
it isn't a file it will pass on the given string. -b name=var works and so
does -b cookiefile.
-j, --junk-session-cookies
when used in combination with -b, it will skip all "session cookies" on
load so as to appear to start a new cookie session.
-c, --cookie-jar
tell curl to start the cookie engine and write cookies to the given file
after the request(s)
1.4 Cookies with libcurl
libcurl offers several ways to enable and interface the cookie engine. These
options are the ones provided by the native API. libcurl bindings may offer
access to them using other means.
Is used when you want to specify the exact contents of a cookie header to
send to the server.
Tell libcurl to activate the cookie engine, and to read the initial set of
cookies from the given file. Read-only.
Tell libcurl to activate the cookie engine, and when the easy handle is
closed save all known cookies to the given cookiejar file. Write-only.
Provide detailed information about a single cookie to add to the internal
storage of cookies. Pass in the cookie as a HTTP header with all the
details set, or pass in a line from a netscape cookie file. This option
can also be used to flush the cookies etc.
Extract cookie information from the internal cookie storage as a linked
1.5 Cookies with javascript
These days a lot of the web is built up by javascript. The webbrowser loads
complete programs that render the page you see. These javascript programs
can also set and access cookies.
Since curl and libcurl are plain HTTP clients without any knowledge of or
capability to handle javascript, such cookies will not be detected or used.
Often, if you want to mimic what a browser does on such web sites, you can
record web browser HTTP traffic when using such a site and then repeat the
cookie operations using curl or libcurl.
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