The Art Of Scripting HTTP Requests Using Curl

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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.

Using curl’s option -v will display what kind of commands curl sends to the
server, as well as a few other informational texts. -v is the single most
useful option when it comes to debug or even understand the curl<->server
interaction.

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 headers that are normally hidden, use
curl’s -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 -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 “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 -d “birthyear=1905&press=%20OK%20″ www.hotmail.com/when/junk.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.

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 -F upload=@localfilename -F 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 -d “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 -T uploadfile www.uploadhttp.com/receive.cgi

6. Authentication

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 -u name:password www.secrets.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 -U 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.

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 -e http://curl.haxx.se daniel.haxx.se

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 on a Windows 2000 box:

curl -A “Mozilla/4.0 (compatible; MSIE 5.01; Windows NT 5.0)” [URL]

Or why not look like you’re using Netscape 4.73 on a Linux (PIII) box:

curl -A “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 -L www.sitethatredirects.com

If you use curl to POST to a site that immediately redirects you to another
page, you can safely use -L and -d/-F 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 -b “name=Daniel” www.cookiesite.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 -D option like:

curl -D headers_and_cookies www.cookiesite.com

(Take note that the -c 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 -b stored_cookies_in_file www.cookiesite.com

Curl’s “cookie engine” gets enabled when you use the -b option. If you only
want curl to understand received cookies, use -b 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 -b nada -L www.cookiesite.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 -b switch automatically detects
if a given file is such a cookie file and parses it, and by using the
-c/–cookie-jar option you’ll make curl write a new cookie file at the end of
an operation:

curl -b cookies.txt -c newcookies.txt www.cookiesite.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://that.secure.server.com

11.1 Certificates

In the HTTPS world, you use certificates to validate that you are the one
you 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 -E mycert.pem https://that.secure.server.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 -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 -d “<xml>” -H “Content-Type: text/xml” -X 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 -H “Host:” http://mysite.com

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

curl -H “Destination: http://moo.com/nowhere” http://url.com

13. 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 -b
and writing with -c)

* 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.)

14. References

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

RFC 2396 explains the URL syntax.

RFC 2109 defines how cookies are supposed to work.

RFC 1867 defines the HTTP post upload format.

http://www.openssl.org is the home of the OpenSSL project

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