148 lines
5.6 KiB
Markdown
148 lines
5.6 KiB
Markdown
# HTTP Cookies
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## Cookie overview
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Cookies are `name=contents` pairs that an HTTP server tells the client to
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hold and then the client sends back those to the server on subsequent
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requests to the same domains and paths for which the cookies were set.
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Cookies are either "session cookies" which typically are forgotten when the
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session is over which is often translated to equal when browser quits, or
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the cookies are not session cookies they have expiration dates after which
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the client will throw them away.
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Cookies are set to the client with the Set-Cookie: header and are sent to
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servers with the Cookie: header.
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For a long time, the only spec explaining how to use cookies was the
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original [Netscape spec from 1994](https://curl.se/rfc/cookie_spec.html).
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In 2011, [RFC 6265](https://www.ietf.org/rfc/rfc6265.txt) was finally
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published and details how cookies work within HTTP. In 2016, an update which
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added support for prefixes was
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[proposed](https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-cookie-prefixes-00),
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and in 2017, another update was
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[drafted](https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-cookie-alone-01)
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to deprecate modification of 'secure' cookies from non-secure origins. Both
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of these drafts have been incorporated into a proposal to
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[replace](https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-rfc6265bis-11)
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RFC 6265. Cookie prefixes and secure cookie modification protection has been
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implemented by curl.
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curl considers `http://localhost` to be a *secure context*, meaning that it
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will allow and use cookies marked with the `secure` keyword even when done
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over plain HTTP for this host. curl does this to match how popular browsers
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work with secure cookies.
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## Cookies saved to disk
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Netscape once created a file format for storing cookies on disk so that they
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would survive browser restarts. curl adopted that file format to allow
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sharing the cookies with browsers, only to see browsers move away from that
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format. Modern browsers no longer use it, while curl still does.
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The Netscape cookie file format stores one cookie per physical line in the
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file with a bunch of associated meta data, each field separated with
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TAB. That file is called the cookie jar in curl terminology.
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When libcurl saves a cookie jar, it creates a file header of its own in
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which there is a URL mention that will link to the web version of this
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document.
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## Cookie file format
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The cookie file format is text based and stores one cookie per line. Lines
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that start with `#` are treated as comments. An exception is lines that
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start with `#HttpOnly_`, which is a prefix for cookies that have the
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`HttpOnly` attribute set.
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Each line that specifies a single cookie consists of seven text fields
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separated with TAB characters. A valid line must end with a newline
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character.
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### Fields in the file
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Field number, what type and example data and the meaning of it:
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0. string `example.com` - the domain name
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1. boolean `FALSE` - include subdomains
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2. string `/foobar/` - path
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3. boolean `TRUE` - send/receive over HTTPS only
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4. number `1462299217` - expires at - seconds since Jan 1st 1970, or 0
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5. string `person` - name of the cookie
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6. string `daniel` - value of the cookie
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## Cookies with curl the command line tool
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curl has a full cookie "engine" built in. If you just activate it, you can
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have curl receive and send cookies exactly as mandated in the specs.
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Command line options:
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`-b, --cookie`
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tell curl a file to read cookies from and start the cookie engine, or if it
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is not a file it will pass on the given string. `-b name=var` works and so
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does `-b cookiefile`.
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`-j, --junk-session-cookies`
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when used in combination with -b, it will skip all "session cookies" on load
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so as to appear to start a new cookie session.
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`-c, --cookie-jar`
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tell curl to start the cookie engine and write cookies to the given file
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after the request(s)
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## Cookies with libcurl
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libcurl offers several ways to enable and interface the cookie engine. These
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options are the ones provided by the native API. libcurl bindings may offer
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access to them using other means.
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`CURLOPT_COOKIE`
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Is used when you want to specify the exact contents of a cookie header to
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send to the server.
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`CURLOPT_COOKIEFILE`
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Tell libcurl to activate the cookie engine, and to read the initial set of
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cookies from the given file. Read-only.
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`CURLOPT_COOKIEJAR`
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Tell libcurl to activate the cookie engine, and when the easy handle is
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closed save all known cookies to the given cookie jar file. Write-only.
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`CURLOPT_COOKIELIST`
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Provide detailed information about a single cookie to add to the internal
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storage of cookies. Pass in the cookie as an HTTP header with all the
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details set, or pass in a line from a Netscape cookie file. This option can
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also be used to flush the cookies etc.
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`CURLOPT_COOKIESESSION`
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Tell libcurl to ignore all cookies it is about to load that are session
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cookies.
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`CURLINFO_COOKIELIST`
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Extract cookie information from the internal cookie storage as a linked
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list.
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## Cookies with JavaScript
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These days a lot of the web is built up by JavaScript. The web browser loads
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complete programs that render the page you see. These JavaScript programs
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can also set and access cookies.
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Since curl and libcurl are plain HTTP clients without any knowledge of or
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capability to handle JavaScript, such cookies will not be detected or used.
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Often, if you want to mimic what a browser does on such websites, you can
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record web browser HTTP traffic when using such a site and then repeat the
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cookie operations using curl or libcurl.
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