TheArtOfHttpScripting   [plain text]

Date:    Jan 19, 2011

                The Art Of Scripting HTTP Requests Using Curl

 This document will assume that you're familiar with HTML and general

 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

  --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

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 or 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


 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">

  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 "",
  the second page you'll get will become

  Most search engines work this way.

  To make curl do the GET form post for you, just enter the expected created

        curl ""

 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 ">

  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" \

  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"

 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

  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">

  This clearly shows that the Content-Type about to be sent is

  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">

  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

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

 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

 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

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

 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

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"

 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

 (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

 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

 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 \


 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:


 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

  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:

12. Custom Request Elements

 Doing fancy stuff, you may need to add or change elements of a single curl

 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

 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:"

 You can add headers the same way. Your server may want a "Destination:"
 header, and you can add it:

        curl --header "Destination: http://nowhere"

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

 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

 RFC 3986 explains the URL syntax.

 RFC 2109 defines how cookies are supposed to work.

 RFC 1867 defines the HTTP post upload format. is the home of the cURL project