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+Online:  http://curl.haxx.se/docs/httpscripting.html
+Date:    Jan 19, 2011
+
+                The Art Of Scripting HTTP Requests Using Curl
+                =============================================
+
+ This document will assume that you're familiar with HTML and general
+ networking.
+
+ The possibility to write scripts is essential to make a good computer
+ system. Unix' capability to be extended by shell scripts and various tools to
+ run various automated commands and scripts is one reason why it has succeeded
+ so well.
+
+ The increasing amount of applications moving to the web has made "HTTP
+ Scripting" more frequently requested and wanted. To be able to automatically
+ extract information from the web, to fake users, to post or upload data to
+ web servers are all important tasks today.
+
+ Curl is a command line tool for doing all sorts of URL manipulations and
+ transfers, but this particular document will focus on how to use it when
+ doing HTTP requests for fun and profit. I'll assume that you know how to
+ invoke 'curl --help' or 'curl --manual' to get basic information about it.
+
+ Curl is not written to do everything for you. It makes the requests, it gets
+ the data, it sends data and it retrieves the information. You probably need
+ to glue everything together using some kind of script language or repeated
+ manual invokes.
+
+1. The HTTP Protocol
+
+ HTTP is the protocol used to fetch data from web servers. It is a very simple
+ protocol that is built upon TCP/IP. The protocol also allows information to
+ get sent to the server from the client using a few different methods, as will
+ be shown here.
+
+ HTTP is plain ASCII text lines being sent by the client to a server to
+ request a particular action, and then the server replies a few text lines
+ before the actual requested content is sent to the client.
+
+ The client, curl, sends a HTTP request. The request contains a method (like
+ GET, POST, HEAD etc), a number of request headers and sometimes a request
+ body. The HTTP server responds with a status line (indicating if things went
+ well), response headers and most often also a response body. The "body" part
+ is the plain data you requested, like the actual HTML or the image etc.
+
+ 1.1 See the Protocol
+
+  Using curl's option --verbose (-v as a short option) will display what kind
+  of commands curl sends to the server, as well as a few other informational
+  texts.
+
+  --verbose is the single most useful option when it comes to debug or even
+  understand the curl<->server interaction.
+
+  Sometimes even --verbose is not enough. Then --trace and --trace-ascii offer
+  even more details as they show EVERYTHING curl sends and receives. Use it
+  like this:
+
+      curl --trace-ascii debugdump.txt http://www.example.com/
+
+2. URL
+
+ The Uniform Resource Locator format is how you specify the address of a
+ particular resource on the Internet. You know these, you've seen URLs like
+ http://curl.haxx.se or https://yourbank.com a million times.
+
+3. GET a page
+
+ The simplest and most common request/operation made using HTTP is to get a
+ URL. The URL could itself refer to a web page, an image or a file. The client
+ issues a GET request to the server and receives the document it asked for.
+ If you issue the command line
+
+        curl http://curl.haxx.se
+
+ you get a web page returned in your terminal window. The entire HTML document
+ that that URL holds.
+
+ All HTTP replies contain a set of response headers that are normally hidden,
+ use curl's --include (-i) option to display them as well as the rest of the
+ document. You can also ask the remote server for ONLY the headers by using
+ the --head (-I) option (which will make curl issue a HEAD request).
+
+4. Forms
+
+ Forms are the general way a web site can present a HTML page with fields for
+ the user to enter data in, and then press some kind of 'OK' or 'submit'
+ button to get that data sent to the server. The server then typically uses
+ the posted data to decide how to act. Like using the entered words to search
+ in a database, or to add the info in a bug track system, display the entered
+ address on a map or using the info as a login-prompt verifying that the user
+ is allowed to see what it is about to see.
+
+ Of course there has to be some kind of program in the server end to receive
+ the data you send. You cannot just invent something out of the air.
+
+ 4.1 GET
+
+  A GET-form uses the method GET, as specified in HTML like:
+
+        <form method="GET" action="junk.cgi">
+          <input type=text name="birthyear">
+          <input type=submit name=press value="OK">
+        </form>
+
+  In your favorite browser, this form will appear with a text box to fill in
+  and a press-button labeled "OK". If you fill in '1905' and press the OK
+  button, your browser will then create a new URL to get for you. The URL will
+  get "junk.cgi?birthyear=1905&press=OK" appended to the path part of the
+  previous URL.
+
+  If the original form was seen on the page "www.hotmail.com/when/birth.html",
+  the second page you'll get will become
+  "www.hotmail.com/when/junk.cgi?birthyear=1905&press=OK".
+
+  Most search engines work this way.
+
+  To make curl do the GET form post for you, just enter the expected created
+  URL:
+
+        curl "http://www.hotmail.com/when/junk.cgi?birthyear=1905&press=OK"
+
+ 4.2 POST
+
+  The GET method makes all input field names get displayed in the URL field of
+  your browser. That's generally a good thing when you want to be able to
+  bookmark that page with your given data, but it is an obvious disadvantage
+  if you entered secret information in one of the fields or if there are a
+  large amount of fields creating a very long and unreadable URL.
+
+  The HTTP protocol then offers the POST method. This way the client sends the
+  data separated from the URL and thus you won't see any of it in the URL
+  address field.
+
+  The form would look very similar to the previous one:
+
+        <form method="POST" action="junk.cgi">
+          <input type=text name="birthyear">
+          <input type=submit name=press value=" OK ">
+        </form>
+
+  And to use curl to post this form with the same data filled in as before, we
+  could do it like:
+
+        curl --data "birthyear=1905&press=%20OK%20" \
+        http://www.example.com/when.cgi
+
+  This kind of POST will use the Content-Type
+  application/x-www-form-urlencoded and is the most widely used POST kind.
+
+  The data you send to the server MUST already be properly encoded, curl will
+  not do that for you. For example, if you want the data to contain a space,
+  you need to replace that space with %20 etc. Failing to comply with this
+  will most likely cause your data to be received wrongly and messed up.
+
+  Recent curl versions can in fact url-encode POST data for you, like this:
+
+        curl --data-urlencode "name=I am Daniel" http://www.example.com
+
+ 4.3 File Upload POST
+
+  Back in late 1995 they defined an additional way to post data over HTTP. It
+  is documented in the RFC 1867, why this method sometimes is referred to as
+  RFC1867-posting.
+
+  This method is mainly designed to better support file uploads. A form that
+  allows a user to upload a file could be written like this in HTML:
+
+    <form method="POST" enctype='multipart/form-data' action="upload.cgi">
+      <input type=file name=upload>
+      <input type=submit name=press value="OK">
+    </form>
+
+  This clearly shows that the Content-Type about to be sent is
+  multipart/form-data.
+
+  To post to a form like this with curl, you enter a command line like:
+
+        curl --form upload=@localfilename --form press=OK [URL]
+
+ 4.4 Hidden Fields
+
+  A very common way for HTML based application to pass state information
+  between pages is to add hidden fields to the forms. Hidden fields are
+  already filled in, they aren't displayed to the user and they get passed
+  along just as all the other fields.
+
+  A similar example form with one visible field, one hidden field and one
+  submit button could look like:
+
+    <form method="POST" action="foobar.cgi">
+      <input type=text name="birthyear">
+      <input type=hidden name="person" value="daniel">
+      <input type=submit name="press" value="OK">
+    </form>
+
+  To post this with curl, you won't have to think about if the fields are
+  hidden or not. To curl they're all the same:
+
+        curl --data "birthyear=1905&press=OK&person=daniel" [URL]
+
+ 4.5 Figure Out What A POST Looks Like
+
+  When you're about fill in a form and send to a server by using curl instead
+  of a browser, you're of course very interested in sending a POST exactly the
+  way your browser does.
+
+  An easy way to get to see this, is to save the HTML page with the form on
+  your local disk, modify the 'method' to a GET, and press the submit button
+  (you could also change the action URL if you want to).
+
+  You will then clearly see the data get appended to the URL, separated with a
+  '?'-letter as GET forms are supposed to.
+
+5. PUT
+
+ The perhaps best way to upload data to a HTTP server is to use PUT. Then
+ again, this of course requires that someone put a program or script on the
+ server end that knows how to receive a HTTP PUT stream.
+
+ Put a file to a HTTP server with curl:
+
+        curl --upload-file uploadfile http://www.example.com/receive.cgi
+
+6. HTTP Authentication
+
+ HTTP Authentication is the ability to tell the server your username and
+ password so that it can verify that you're allowed to do the request you're
+ doing. The Basic authentication used in HTTP (which is the type curl uses by
+ default) is *plain* *text* based, which means it sends username and password
+ only slightly obfuscated, but still fully readable by anyone that sniffs on
+ the network between you and the remote server.
+
+ To tell curl to use a user and password for authentication:
+
+        curl --user name:password http://www.example.com
+
+ The site might require a different authentication method (check the headers
+ returned by the server), and then --ntlm, --digest, --negotiate or even
+ --anyauth might be options that suit you.
+
+ Sometimes your HTTP access is only available through the use of a HTTP
+ proxy. This seems to be especially common at various companies. A HTTP proxy
+ may require its own user and password to allow the client to get through to
+ the Internet. To specify those with curl, run something like:
+
+        curl --proxy-user proxyuser:proxypassword curl.haxx.se
+
+ If your proxy requires the authentication to be done using the NTLM method,
+ use --proxy-ntlm, if it requires Digest use --proxy-digest.
+
+ If you use any one these user+password options but leave out the password
+ part, curl will prompt for the password interactively.
+
+ Do note that when a program is run, its parameters might be possible to see
+ when listing the running processes of the system. Thus, other users may be
+ able to watch your passwords if you pass them as plain command line
+ options. There are ways to circumvent this.
+
+ It is worth noting that while this is how HTTP Authentication works, very
+ many web sites will not use this concept when they provide logins etc. See
+ the Web Login chapter further below for more details on that.
+
+7. Referer
+
+ A HTTP request may include a 'referer' field (yes it is misspelled), which
+ can be used to tell from which URL the client got to this particular
+ resource. Some programs/scripts check the referer field of requests to verify
+ that this wasn't arriving from an external site or an unknown page. While
+ this is a stupid way to check something so easily forged, many scripts still
+ do it. Using curl, you can put anything you want in the referer-field and
+ thus more easily be able to fool the server into serving your request.
+
+ Use curl to set the referer field with:
+
+        curl --referer http://www.example.come http://www.example.com
+
+8. User Agent
+
+ Very similar to the referer field, all HTTP requests may set the User-Agent
+ field. It names what user agent (client) that is being used. Many
+ applications use this information to decide how to display pages. Silly web
+ programmers try to make different pages for users of different browsers to
+ make them look the best possible for their particular browsers. They usually
+ also do different kinds of javascript, vbscript etc.
+
+ At times, you will see that getting a page with curl will not return the same
+ page that you see when getting the page with your browser. Then you know it
+ is time to set the User Agent field to fool the server into thinking you're
+ one of those browsers.
+
+ To make curl look like Internet Explorer 5 on a Windows 2000 box:
+
+  curl --user-agent "Mozilla/4.0 (compatible; MSIE 5.01; Windows NT 5.0)" [URL]
+
+ Or why not look like you're using Netscape 4.73 on an old Linux box:
+
+  curl --user-agent "Mozilla/4.73 [en] (X11; U; Linux 2.2.15 i686)" [URL]
+
+9. Redirects
+
+ When a resource is requested from a server, the reply from the server may
+ include a hint about where the browser should go next to find this page, or a
+ new page keeping newly generated output. The header that tells the browser
+ to redirect is Location:.
+
+ Curl does not follow Location: headers by default, but will simply display
+ such pages in the same manner it display all HTTP replies. It does however
+ feature an option that will make it attempt to follow the Location: pointers.
+
+ To tell curl to follow a Location:
+
+        curl --location http://www.example.com
+
+ If you use curl to POST to a site that immediately redirects you to another
+ page, you can safely use --location (-L) and --data/--form together. Curl will
+ only use POST in the first request, and then revert to GET in the following
+ operations.
+
+10. Cookies
+
+ The way the web browsers do "client side state control" is by using
+ cookies. Cookies are just names with associated contents. The cookies are
+ sent to the client by the server. The server tells the client for what path
+ and host name it wants the cookie sent back, and it also sends an expiration
+ date and a few more properties.
+
+ When a client communicates with a server with a name and path as previously
+ specified in a received cookie, the client sends back the cookies and their
+ contents to the server, unless of course they are expired.
+
+ Many applications and servers use this method to connect a series of requests
+ into a single logical session. To be able to use curl in such occasions, we
+ must be able to record and send back cookies the way the web application
+ expects them. The same way browsers deal with them.
+
+ The simplest way to send a few cookies to the server when getting a page with
+ curl is to add them on the command line like:
+
+        curl --cookie "name=Daniel" http://www.example.com
+
+ Cookies are sent as common HTTP headers. This is practical as it allows curl
+ to record cookies simply by recording headers. Record cookies with curl by
+ using the --dump-header (-D) option like:
+
+        curl --dump-header headers_and_cookies http://www.example.com
+
+ (Take note that the --cookie-jar option described below is a better way to
+ store cookies.)
+
+ Curl has a full blown cookie parsing engine built-in that comes to use if you
+ want to reconnect to a server and use cookies that were stored from a
+ previous connection (or handicrafted manually to fool the server into
+ believing you had a previous connection). To use previously stored cookies,
+ you run curl like:
+
+        curl --cookie stored_cookies_in_file http://www.example.com
+
+ Curl's "cookie engine" gets enabled when you use the --cookie option. If you
+ only want curl to understand received cookies, use --cookie with a file that
+ doesn't exist. Example, if you want to let curl understand cookies from a
+ page and follow a location (and thus possibly send back cookies it received),
+ you can invoke it like:
+
+        curl --cookie nada --location http://www.example.com
+
+ Curl has the ability to read and write cookie files that use the same file
+ format that Netscape and Mozilla do. It is a convenient way to share cookies
+ between browsers and automatic scripts. The --cookie (-b) switch
+ automatically detects if a given file is such a cookie file and parses it,
+ and by using the --cookie-jar (-c) option you'll make curl write a new cookie
+ file at the end of an operation:
+
+        curl --cookie cookies.txt --cookie-jar newcookies.txt \
+        http://www.example.com
+
+11. HTTPS
+
+ There are a few ways to do secure HTTP transfers. The by far most common
+ protocol for doing this is what is generally known as HTTPS, HTTP over
+ SSL. SSL encrypts all the data that is sent and received over the network and
+ thus makes it harder for attackers to spy on sensitive information.
+
+ SSL (or TLS as the latest version of the standard is called) offers a
+ truckload of advanced features to allow all those encryptions and key
+ infrastructure mechanisms encrypted HTTP requires.
+
+ Curl supports encrypted fetches thanks to the freely available OpenSSL
+ libraries. To get a page from a HTTPS server, simply run curl like:
+
+        curl https://secure.example.com
+
+ 11.1 Certificates
+
+  In the HTTPS world, you use certificates to validate that you are the one
+  you claim to be, as an addition to normal passwords. Curl supports client-
+  side certificates. All certificates are locked with a pass phrase, which you
+  need to enter before the certificate can be used by curl. The pass phrase
+  can be specified on the command line or if not, entered interactively when
+  curl queries for it. Use a certificate with curl on a HTTPS server like:
+
+        curl --cert mycert.pem https://secure.example.com
+
+  curl also tries to verify that the server is who it claims to be, by
+  verifying the server's certificate against a locally stored CA cert
+  bundle. Failing the verification will cause curl to deny the connection. You
+  must then use --insecure (-k) in case you want to tell curl to ignore that
+  the server can't be verified.
+
+  More about server certificate verification and ca cert bundles can be read
+  in the SSLCERTS document, available online here:
+
+        http://curl.haxx.se/docs/sslcerts.html
+
+12. Custom Request Elements
+
+ Doing fancy stuff, you may need to add or change elements of a single curl
+ request.
+
+ For example, you can change the POST request to a PROPFIND and send the data
+ as "Content-Type: text/xml" (instead of the default Content-Type) like this:
+
+         curl --data "<xml>" --header "Content-Type: text/xml" \
+              --request PROPFIND url.com
+
+ You can delete a default header by providing one without content. Like you
+ can ruin the request by chopping off the Host: header:
+
+        curl --header "Host:" http://www.example.com
+
+ You can add headers the same way. Your server may want a "Destination:"
+ header, and you can add it:
+
+        curl --header "Destination: http://nowhere" http://example.com
+
+13. Web Login
+
+ While not strictly just HTTP related, it still cause a lot of people problems
+ so here's the executive run-down of how the vast majority of all login forms
+ work and how to login to them using curl.
+
+ It can also be noted that to do this properly in an automated fashion, you
+ will most certainly need to script things and do multiple curl invokes etc.
+
+ First, servers mostly use cookies to track the logged-in status of the
+ client, so you will need to capture the cookies you receive in the
+ responses. Then, many sites also set a special cookie on the login page (to
+ make sure you got there through their login page) so you should make a habit
+ of first getting the login-form page to capture the cookies set there.
+
+ Some web-based login systems features various amounts of javascript, and
+ sometimes they use such code to set or modify cookie contents. Possibly they
+ do that to prevent programmed logins, like this manual describes how to...
+ Anyway, if reading the code isn't enough to let you repeat the behavior
+ manually, capturing the HTTP requests done by your browers and analyzing the
+ sent cookies is usually a working method to work out how to shortcut the
+ javascript need.
+
+ In the actual <form> tag for the login, lots of sites fill-in random/session
+ or otherwise secretly generated hidden tags and you may need to first capture
+ the HTML code for the login form and extract all the hidden fields to be able
+ to do a proper login POST. Remember that the contents need to be URL encoded
+ when sent in a normal POST.
+
+14. Debug
+
+ Many times when you run curl on a site, you'll notice that the site doesn't
+ seem to respond the same way to your curl requests as it does to your
+ browser's.
+
+ Then you need to start making your curl requests more similar to your
+ browser's requests:
+
+ * Use the --trace-ascii option to store fully detailed logs of the requests
+   for easier analyzing and better understanding
+
+ * Make sure you check for and use cookies when needed (both reading with
+   --cookie and writing with --cookie-jar)
+
+ * Set user-agent to one like a recent popular browser does
+
+ * Set referer like it is set by the browser
+
+ * If you use POST, make sure you send all the fields and in the same order as
+   the browser does it. (See chapter 4.5 above)
+
+ A very good helper to make sure you do this right, is the LiveHTTPHeader tool
+ that lets you view all headers you send and receive with Mozilla/Firefox
+ (even when using HTTPS).
+
+ A more raw approach is to capture the HTTP traffic on the network with tools
+ such as ethereal or tcpdump and check what headers that were sent and
+ received by the browser. (HTTPS makes this technique inefficient.)
+
+15. References
+
+ RFC 2616 is a must to read if you want in-depth understanding of the HTTP
+ protocol.
+
+ RFC 3986 explains the URL syntax.
+
+ RFC 2109 defines how cookies are supposed to work.
+
+ RFC 1867 defines the HTTP post upload format.
+
+ http://curl.haxx.se is the home of the cURL project