TheArtOfHttpScripting

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The Art Of Scripting HTTP Requests Using Curl

 1. HTTP Scripting
 1.1 Background
 1.2 The HTTP Protocol
 1.3 See the Protocol
 1.4 See the Timing
 1.5 See the Response
 2. URL
 2.1 Spec
 2.2 Host
 2.3 Port number
 2.4 User name and password
 2.5 Path part
 3. Fetch a page
 3.1 GET
 3.2 HEAD
 4. HTML forms
 4.1 Forms explained
 4.2 GET
 4.3 POST
 4.4 File Upload POST
 4.5 Hidden Fields
 4.6 Figure Out What A POST Looks Like
 5. HTTP upload
 5.1 PUT
 6. HTTP Authentication
 6.1 Basic Authentication
 6.2 Other Authentication
 6.3 Proxy Authentication
 6.4 Hiding credentials
 7. More HTTP Headers
 7.1 Referer
 7.2 User Agent
 8. Redirects
 8.1 Location header
 8.2 Other redirects
 9. Cookies
 9.1 Cookie Basics
 9.2 Cookie options
 10. HTTPS
 10.1 HTTPS is HTTP secure
 10.2 Certificates
 11. Custom Request Elements
 11.1 Modify method and headers
 11.2 More on changed methods
 12. Web Login
 12.1 Some login tricks
 13. Debug
 13.1 Some debug tricks
 14. References
 14.1 Standards
 14.2 Sites

==============================================================================

1. HTTP Scripting

 1.1 Background

 This document assumes that you're familiar with HTML and general networking.

 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.

 1.2 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.

 1.3 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/

 1.4 See the Timing

  Many times you may wonder what exactly is taking all the time, or you just
  want to know the amount of milliseconds between two points in a
  transfer. For those, and other similar situations, the --trace-time option
  is what you need. It'll prepend the time to each trace output line:

      curl --trace-ascii d.txt --trace-time http://example.com/

 1.5 See the Response

  By default curl sends the response to stdout. You need to redirect it
  somewhere to avoid that, most often that is done with -o or -O.

2. URL

 2.1 Spec

 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. RFC 3986 is the
 canonical spec.

 2.2 Host

 The host name is usually resolved using DNS or your /etc/hosts file to an IP
 address and that's what curl will communicate with. Alternatively you specify
 the IP address directly in the URL instead of a name.

 For development and other trying out situation, you can point out a different
 IP address for a host name than what would otherwise be used, by using curl's
 --resolve option:

      curl --resolve www.example.org:80:127.0.0.1 http://www.example.org/
 
 2.3 Port number

 Each protocol curl supports operate on a default port number, be it over TCP
 or in some cases UDP. Normally you don't have to take that into
 consideration, but at times you run test servers on other ports or
 similar. Then you can specify the port number in the URL with a colon and a
 number immediately following the host name. Like when doing HTTP to port
 1234:

      curl http://www.example.org:1234/

 The port number you specify in the URL is the number that the server uses to
 offer its services. Sometimes you may use a local proxy, and then you may
 need to specify that proxy's port number separate on what curl needs to
 connect to locally. Like when using a HTTP proxy on port 4321:

      curl --proxy http://proxy.example.org:4321 http://remote.example.org/

 2.4 User name and password

 Some services are setup to require HTTP authentication and then you need to
 provide name and password which then is transferred to the remote site in
 various ways depending on the exact authentication protocol used.

 You can opt to either insert the user and password in the URL or you can
 provide them separately:

      curl http://user:password@example.org/

 or

      curl -u user:password http://example.org/

 You need to pay attention that this kind of HTTP authentication is not what
 is usually done and requested by user-oriented web sites these days. They
 tend to use forms and cookies instead.

 2.5 Path part

 The path part is just sent off to the server to request that it sends back
 the associated response. The path is what is to the right side of the slash
 that follows the host name and possibly port number.


3. Fetch a page

 3.1 GET

 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.

 3.2 HEAD

 You can ask the remote server for ONLY the headers by using the --head (-I)
 option which will make curl issue a HEAD request. In some special cases
 servers deny the HEAD method while others still work, which is a particular
 kind of annoyance.

 The HEAD method is defined and made so that the server returns the headers
 exactly the way it would do for a GET, but without a body. It means that you
 may see a Content-Length: in the response headers, but there must not be an
 actual body in the HEAD response.

4. HTML forms

 4.1 Forms explained

 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.

 4.2 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"

 4.3 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

 4.4 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]

 4.5 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]

 4.6 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. HTTP upload

 5.1 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

 6.1 Basic 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

 6.2 Other Authentication

 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.

 6.3 Proxy Authentication

 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.

 6.4 Hiding credentials

 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. More HTTP Headers

 7.1 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

 7.2 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]

8. Redirects

 8.1 Location header

 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.

 8.2 Other redirects

 Browser typically support at least two other ways of redirects that curl
 doesn't: first the html may contain a meta refresh tag that asks the browser
 to load a specific URL after a set number of seconds, or it may use
 javascript to do it.

9. Cookies

 9.1 Cookie Basics

 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.

 9.2 Cookie options

 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 hand-crafted 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 once used. It is a convenient way to share
 cookies between scripts or invokes. 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

10. HTTPS

 10.1 HTTPS is HTTP secure

 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 when built to use a TLS library and it can be
 built to use one out of a fairly large set of libraries - "curl -V" will show
 which one your curl was built to use (if any!). To get a page from a HTTPS
 server, simply run curl like:

        curl https://secure.example.com

 10.2 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

11. Custom Request Elements

11.1 Modify method and headers

 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

 11.2 More on changed methods

 It should be noted that curl selects which methods to use on its own
 depending on what action to ask for. -d will do POST, -I will do HEAD and so
 on. If you use the --request / -X option you can change the method keyword
 curl selects, but you will not modify curl's behavior. This means that if you
 for example use -d "data" to do a POST, you can modify the method to a
 PROPFIND with -X and curl will still think it sends a POST. You can change
 the normal GET to a POST method by simply adding -X POST in a command line
 like:

        curl -X POST http://example.org/

 ... but curl will still think and act as if it sent a GET so it won't send any
 request body etc.


12. Web Login

 12.1 Some login tricks

 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 browsers 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.

13. Debug

 13.1 Some debug tricks

 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.

 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). Chrome features similar functionality out of the box
 among the developer's tools.

 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.)

14. References

 14.1 Standards

 RFC 2616 is a must to read if you want in-depth understanding of the HTTP
 protocol

 RFC 3986 explains the URL syntax

 RFC 1867 defines the HTTP post upload format

 RFC 6525 defines how HTTP cookies work

 14.2 Sites

 http://curl.haxx.se is the home of the cURL project