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
