On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2009-01-09 09:42 @38.103.63.58 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)
libcurl-tutorial(3) libcurl programming libcurl-tutorial(3)
NAME
libcurl-tutorial - libcurl programming tutorial
Objective
This document attempts to describe the general principles and some basic approaches
to consider when programming with libcurl. The text will focus mainly on the C
interface but might apply fairly well on other interfaces as well as they usually
follow the C one pretty closely.
This document will refer to ’the user’ as the person writing the source code that
uses libcurl. That would probably be you or someone in your position. What will be
generally referred to as ’the program’ will be the collected source code that you
write that is using libcurl for transfers. The program is outside libcurl and
libcurl is outside of the program.
To get the more details on all options and functions described herein, please refer
to their respective man pages.
Building
There are many different ways to build C programs. This chapter will assume a unix-
style build process. If you use a different build system, you can still read this
to get general information that may apply to your environment as well.
Compiling the Program
Your compiler needs to know where the libcurl headers are located. Therefore
you must set your compiler’s include path to point to the directory where
you installed them. The ’curl-config’[3] tool can be used to get this infor-
mation:
$ curl-config --cflags
Linking the Program with libcurl
When having compiled the program, you need to link your object files to cre-
ate a single executable. For that to succeed, you need to link with libcurl
and possibly also with other libraries that libcurl itself depends on. Like
the OpenSSL libraries, but even some standard OS libraries may be needed on
the command line. To figure out which flags to use, once again the ’curl-
config’ tool comes to the rescue:
$ curl-config --libs
SSL or Not
libcurl can be built and customized in many ways. One of the things that
varies from different libraries and builds is the support for SSL-based
transfers, like HTTPS and FTPS. If OpenSSL was detected properly at build-
time, libcurl will be built with SSL support. To figure out if an installed
libcurl has been built with SSL support enabled, use ’curl-config’ like
this:
$ curl-config --feature
And if SSL is supported, the keyword ’SSL’ will be written to stdout, possi-
bly together with a few other features that can be on and off on different
libcurls.
See also the "Features libcurl Provides" further down.
autoconf macro
When you write your configure script to detect libcurl and setup variables
accordingly, we offer a prewritten macro that probably does everything you
need in this area. See docs/libcurl/libcurl.m4 file - it includes docs on
how to use it.
Portable Code in a Portable World
The people behind libcurl have put a considerable effort to make libcurl work on a
large amount of different operating systems and environments.
You program libcurl the same way on all platforms that libcurl runs on. There are
only very few minor considerations that differs. If you just make sure to write
your code portable enough, you may very well create yourself a very portable pro-
gram. libcurl shouldn’t stop you from that.
Global Preparation
The program must initialize some of the libcurl functionality globally. That means
it should be done exactly once, no matter how many times you intend to use the
library. Once for your program’s entire life time. This is done using
curl_global_init()
and it takes one parameter which is a bit pattern that tells libcurl what to ini-
tialize. Using CURL_GLOBAL_ALL will make it initialize all known internal sub mod-
ules, and might be a good default option. The current two bits that are specified
are:
CURL_GLOBAL_WIN32
which only does anything on Windows machines. When used on a Windows
machine, it’ll make libcurl initialize the win32 socket stuff. With-
out having that initialized properly, your program cannot use sockets
properly. You should only do this once for each application, so if
your program already does this or of another library in use does it,
you should not tell libcurl to do this as well.
CURL_GLOBAL_SSL
which only does anything on libcurls compiled and built SSL-enabled.
On these systems, this will make libcurl initialize OpenSSL properly
for this application. This is only needed to do once for each appli-
cation so if your program or another library already does this, this
bit should not be needed.
libcurl has a default protection mechanism that detects if curl_global_init(3)
hasn’t been called by the time curl_easy_perform(3) is called and if that is the
case, libcurl runs the function itself with a guessed bit pattern. Please note that
depending solely on this is not considered nice nor very good.
When the program no longer uses libcurl, it should call curl_global_cleanup(3),
which is the opposite of the init call. It will then do the reversed operations to
cleanup the resources the curl_global_init(3) call initialized.
Repeated calls to curl_global_init(3) and curl_global_cleanup(3) should be avoided.
They should only be called once each.
Features libcurl Provides
It is considered best-practice to determine libcurl features run-time rather than
build-time (if possible of course). By calling curl_version_info() and checking
tout he details of the returned struct, your program can figure out exactly what
the currently running libcurl supports.
Handle the Easy libcurl
libcurl first introduced the so called easy interface. All operations in the easy
interface are prefixed with ’curl_easy’.
Recent libcurl versions also offer the multi interface. More about that interface,
what it is targeted for and how to use it is detailed in a separate chapter further
down. You still need to understand the easy interface first, so please continue
reading for better understanding.
To use the easy interface, you must first create yourself an easy handle. You need
one handle for each easy session you want to perform. Basically, you should use one
handle for every thread you plan to use for transferring. You must never share the
same handle in multiple threads.
Get an easy handle with
easyhandle = curl_easy_init();
It returns an easy handle. Using that you proceed to the next step: setting up your
preferred actions. A handle is just a logic entity for the upcoming transfer or
series of transfers.
You set properties and options for this handle using curl_easy_setopt(3). They con-
trol how the subsequent transfer or transfers will be made. Options remain set in
the handle until set again to something different. Alas, multiple requests using
the same handle will use the same options.
Many of the options you set in libcurl are "strings", pointers to data terminated
with a zero byte. Keep in mind that when you set strings with curl_easy_setopt(3),
libcurl will not copy the data. It will merely point to the data. You MUST make
sure that the data remains available for libcurl to use until finished or until you
use the same option again to point to something else.
One of the most basic properties to set in the handle is the URL. You set your pre-
ferred URL to transfer with CURLOPT_URL in a manner similar to:
curl_easy_setopt(handle, CURLOPT_URL, "http://domain.com/");
Let’s assume for a while that you want to receive data as the URL identifies a
remote resource you want to get here. Since you write a sort of application that
needs this transfer, I assume that you would like to get the data passed to you
directly instead of simply getting it passed to stdout. So, you write your own
function that matches this prototype:
size_t write_data(void *buffer, size_t size, size_t nmemb, void *userp);
You tell libcurl to pass all data to this function by issuing a function similar to
this:
curl_easy_setopt(easyhandle, CURLOPT_WRITEFUNCTION, write_data);
You can control what data your function get in the forth argument by setting
another property:
curl_easy_setopt(easyhandle, CURLOPT_WRITEDATA, &internal_struct);
Using that property, you can easily pass local data between your application and
the function that gets invoked by libcurl. libcurl itself won’t touch the data you
pass with CURLOPT_WRITEDATA.
libcurl offers its own default internal callback that’ll take care of the data if
you don’t set the callback with CURLOPT_WRITEFUNCTION. It will then simply output
the received data to stdout. You can have the default callback write the data to a
different file handle by passing a ’FILE *’ to a file opened for writing with the
CURLOPT_WRITEDATA option.
Now, we need to take a step back and have a deep breath. Here’s one of those rare
platform-dependent nitpicks. Did you spot it? On some platforms[2], libcurl won’t
be able to operate on files opened by the program. Thus, if you use the default
callback and pass in a an open file with CURLOPT_WRITEDATA, it will crash. You
should therefore avoid this to make your program run fine virtually everywhere.
(CURLOPT_WRITEDATA was formerly known as CURLOPT_FILE. Both names still work and do
the same thing).
If you’re using libcurl as a win32 DLL, you MUST use the CURLOPT_WRITEFUNCTION if
you set CURLOPT_WRITEDATA - or you will experience crashes.
There are of course many more options you can set, and we’ll get back to a few of
them later. Let’s instead continue to the actual transfer:
success = curl_easy_perform(easyhandle);
curl_easy_perform(3) will connect to the remote site, do the necessary commands and
receive the transfer. Whenever it receives data, it calls the callback function we
previously set. The function may get one byte at a time, or it may get many kilo-
bytes at once. libcurl delivers as much as possible as often as possible. Your
callback function should return the number of bytes it "took care of". If that is
not the exact same amount of bytes that was passed to it, libcurl will abort the
operation and return with an error code.
When the transfer is complete, the function returns a return code that informs you
if it succeeded in its mission or not. If a return code isn’t enough for you, you
can use the CURLOPT_ERRORBUFFER to point libcurl to a buffer of yours where it’ll
store a human readable error message as well.
If you then want to transfer another file, the handle is ready to be used again.
Mind you, it is even preferred that you re-use an existing handle if you intend to
make another transfer. libcurl will then attempt to re-use the previous
Multi-threading Issues
libcurl is completely thread safe, except for two issues: signals and alarm han-
dlers. Signals are needed for a SIGPIPE handler, and the alarm() call is used to
deal with timeouts (during DNS lookup).
If you are accessing HTTPS or FTPS URLs in a multi-threaded manner, you are then of
course using OpenSSL multi-threaded and it has itself a few requirements on this.
Basically, you need to provide one or two functions to allow it to function prop-
erly. For all details, see this:
http://www.openssl.org/docs/crypto/threads.html#DESCRIPTION
When using multiple threads you should set the CURLOPT_NOSIGNAL option to TRUE for
all handles. Everything will work fine except that timeouts are not honored during
the DNS lookup - which you can work around by building libcurl with c-ares support.
c-ares is a library that provides asynchronous name resolves. Unfortunately, c-ares
does not yet support IPv6.
Also, note that CURLOPT_DNS_USE_GLOBAL_CACHE is not thread-safe.
When It Doesnâ€â€™t Work
There will always be times when the transfer fails for some reason. You might have
set the wrong libcurl option or misunderstood what the libcurl option actually
does, or the remote server might return non-standard replies that confuse the
library which then confuses your program.
There’s one golden rule when these things occur: set the CURLOPT_VERBOSE option to
TRUE. It’ll cause the library to spew out the entire protocol details it sends,
some internal info and some received protocol data as well (especially when using
FTP). If you’re using HTTP, adding the headers in the received output to study is
also a clever way to get a better understanding why the server behaves the way it
does. Include headers in the normal body output with CURLOPT_HEADER set TRUE.
Of course there are bugs left. We need to get to know about them to be able to fix
them, so we’re quite dependent on your bug reports! When you do report suspected
bugs in libcurl, please include as much details you possibly can: a protocol dump
that CURLOPT_VERBOSE produces, library version, as much as possible of your code
that uses libcurl, operating system name and version, compiler name and version
etc.
If CURLOPT_VERBOSE is not enough, you increase the level of debug data your appli-
cation receive by using the CURLOPT_DEBUGFUNCTION.
Getting some in-depth knowledge about the protocols involved is never wrong, and if
you’re trying to do funny things, you might very well understand libcurl and how to
use it better if you study the appropriate RFC documents at least briefly.
Upload Data to a Remote Site
libcurl tries to keep a protocol independent approach to most transfers, thus
uploading to a remote FTP site is very similar to uploading data to a HTTP server
with a PUT request.
Of course, first you either create an easy handle or you re-use one existing one.
Then you set the URL to operate on just like before. This is the remote URL, that
we now will upload.
Since we write an application, we most likely want libcurl to get the upload data
by asking us for it. To make it do that, we set the read callback and the custom
pointer libcurl will pass to our read callback. The read callback should have a
prototype similar to:
size_t function(char *bufptr, size_t size, size_t nitems, void *userp);
Where bufptr is the pointer to a buffer we fill in with data to upload and
size*nitems is the size of the buffer and therefore also the maximum amount of data
we can return to libcurl in this call. The ’userp’ pointer is the custom pointer we
set to point to a struct of ours to pass private data between the application and
the callback.
curl_easy_setopt(easyhandle, CURLOPT_READFUNCTION, read_function);
curl_easy_setopt(easyhandle, CURLOPT_INFILE, &filedata);
Tell libcurl that we want to upload:
curl_easy_setopt(easyhandle, CURLOPT_UPLOAD, TRUE);
A few protocols won’t behave properly when uploads are done without any prior
knowledge of the expected file size. So, set the upload file size using the CUR-
LOPT_INFILESIZE_LARGE for all known file sizes like this[1]:
/* in this example, file_size must be an off_t variable */
curl_easy_setopt(easyhandle, CURLOPT_INFILESIZE_LARGE, file_size);
When you call curl_easy_perform(3) this time, it’ll perform all the necessary oper-
ations and when it has invoked the upload it’ll call your supplied callback to get
the data to upload. The program should return as much data as possible in every
invoke, as that is likely to make the upload perform as fast as possible. The call-
back should return the number of bytes it wrote in the buffer. Returning 0 will
signal the end of the upload.
Passwords
Many protocols use or even require that user name and password are provided to be
able to download or upload the data of your choice. libcurl offers several ways to
specify them.
Most protocols support that you specify the name and password in the URL itself.
libcurl will detect this and use them accordingly. This is written like this:
protocol://user:password AT example.com/path/
If you need any odd letters in your user name or password, you should enter them
URL encoded, as %XX where XX is a two-digit hexadecimal number.
libcurl also provides options to set various passwords. The user name and password
as shown embedded in the URL can instead get set with the CURLOPT_USERPWD option.
The argument passed to libcurl should be a char * to a string in the format
"user:password:". In a manner like this:
curl_easy_setopt(easyhandle, CURLOPT_USERPWD, "myname:thesecret");
Another case where name and password might be needed at times, is for those users
who need to authenticate themselves to a proxy they use. libcurl offers another
option for this, the CURLOPT_PROXYUSERPWD. It is used quite similar to the CUR-
LOPT_USERPWD option like this:
curl_easy_setopt(easyhandle, CURLOPT_PROXYUSERPWD, "myname:thesecret");
There’s a long time unix "standard" way of storing ftp user names and passwords,
namely in the $HOME/.netrc file. The file should be made private so that only the
user may read it (see also the "Security Considerations" chapter), as it might con-
tain the password in plain text. libcurl has the ability to use this file to figure
out what set of user name and password to use for a particular host. As an exten-
sion to the normal functionality, libcurl also supports this file for non-FTP pro-
tocols such as HTTP. To make curl use this file, use the CURLOPT_NETRC option:
curl_easy_setopt(easyhandle, CURLOPT_NETRC, TRUE);
And a very basic example of how such a .netrc file may look like:
machine myhost.mydomain.com
login userlogin
password secretword
All these examples have been cases where the password has been optional, or at
least you could leave it out and have libcurl attempt to do its job without it.
There are times when the password isn’t optional, like when you’re using an SSL
private key for secure transfers.
To pass the known private key password to libcurl:
curl_easy_setopt(easyhandle, CURLOPT_SSLKEYPASSWD, "keypassword");
HTTP Authentication
The previous chapter showed how to set user name and password for getting URLs that
require authentication. When using the HTTP protocol, there are many different ways
a client can provide those credentials to the server and you can control what way
libcurl will (attempt to) use. The default HTTP authentication method is called
’Basic’, which is sending the name and password in clear-text in the HTTP request,
base64-encoded. This is insecure.
At the time of this writing libcurl can be built to use: Basic, Digest, NTLM, Nego-
tiate, GSS-Negotiate and SPNEGO. You can tell libcurl which one to use with CUR-
LOPT_HTTPAUTH as in:
curl_easy_setopt(easyhandle, CURLOPT_HTTPAUTH, CURLAUTH_DIGEST);
And when you send authentication to a proxy, you can also set authentication type
the same way but instead with CURLOPT_PROXYAUTH:
curl_easy_setopt(easyhandle, CURLOPT_PROXYAUTH, CURLAUTH_NTLM);
Both these options allow you to set multiple types (by ORing them together), to
make libcurl pick the most secure one out of the types the server/proxy claims to
support. This method does however add a round-trip since libcurl must first ask the
server what it supports:
curl_easy_setopt(easyhandle, CURLOPT_HTTPAUTH,
CURLAUTH_DIGEST|CURLAUTH_BASIC);
For convenience, you can use the ’CURLAUTH_ANY’ define (instead of a list with spe-
cific types) which allows libcurl to use whatever method it wants.
When asking for multiple types, libcurl will pick the available one it considers
"best" in its own internal order of preference.
HTTP POSTing
We get many questions regarding how to issue HTTP POSTs with libcurl the proper
way. This chapter will thus include examples using both different versions of HTTP
POST that libcurl supports.
The first version is the simple POST, the most common version, that most HTML pages
using the <form> tag uses. We provide a pointer to the data and tell libcurl to
post it all to the remote site:
char *data="name=daniel&project=curl";
curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDS, data);
curl_easy_setopt(easyhandle, CURLOPT_URL, "http://posthere.com/");
curl_easy_perform(easyhandle); /* post away! */
Simple enough, huh? Since you set the POST options with the CURLOPT_POSTFIELDS,
this automatically switches the handle to use POST in the upcoming request.
Ok, so what if you want to post binary data that also requires you to set the Con-
tent-Type: header of the post? Well, binary posts prevents libcurl from being able
to do strlen() on the data to figure out the size, so therefore we must tell
libcurl the size of the post data. Setting headers in libcurl requests are done in
a generic way, by building a list of our own headers and then passing that list to
libcurl.
struct curl_slist *headers=NULL;
headers = curl_slist_append(headers, "Content-Type: text/xml");
/* post binary data */
curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDS, binaryptr);
/* set the size of the postfields data */
curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDSIZE, 23);
/* pass our list of custom made headers */
curl_easy_setopt(easyhandle, CURLOPT_HTTPHEADER, headers);
curl_easy_perform(easyhandle); /* post away! */
curl_slist_free_all(headers); /* free the header list */
While the simple examples above cover the majority of all cases where HTTP POST
operations are required, they don’t do multi-part formposts. Multi-part formposts
were introduced as a better way to post (possibly large) binary data and was first
documented in the RFC1867. They’re called multi-part because they’re built by a
chain of parts, each being a single unit. Each part has its own name and contents.
You can in fact create and post a multi-part formpost with the regular libcurl POST
support described above, but that would require that you build a formpost yourself
and provide to libcurl. To make that easier, libcurl provides curl_formadd(3).
Using this function, you add parts to the form. When you’re done adding parts, you
post the whole form.
The following example sets two simple text parts with plain textual contents, and
then a file with binary contents and upload the whole thing.
struct curl_httppost *post=NULL;
struct curl_httppost *last=NULL;
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "name",
CURLFORM_COPYCONTENTS, "daniel", CURLFORM_END);
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "project",
CURLFORM_COPYCONTENTS, "curl", CURLFORM_END);
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "logotype-image",
CURLFORM_FILECONTENT, "curl.png", CURLFORM_END);
/* Set the form info */
curl_easy_setopt(easyhandle, CURLOPT_HTTPPOST, post);
curl_easy_perform(easyhandle); /* post away! */
/* free the post data again */
curl_formfree(post);
Multipart formposts are chains of parts using MIME-style separators and headers. It
means that each one of these separate parts get a few headers set that describe the
individual content-type, size etc. To enable your application to handicraft this
formpost even more, libcurl allows you to supply your own set of custom headers to
such an individual form part. You can of course supply headers to as many parts you
like, but this little example will show how you set headers to one specific part
when you add that to the post handle:
struct curl_slist *headers=NULL;
headers = curl_slist_append(headers, "Content-Type: text/xml");
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "logotype-image",
CURLFORM_FILECONTENT, "curl.xml",
CURLFORM_CONTENTHEADER, headers,
CURLFORM_END);
curl_easy_perform(easyhandle); /* post away! */
curl_formfree(post); /* free post */
curl_slist_free_all(post); /* free custom header list */
Since all options on an easyhandle are "sticky", they remain the same until changed
even if you do call curl_easy_perform(3), you may need to tell curl to go back to a
plain GET request if you intend to do such a one as your next request. You force an
easyhandle to back to GET by using the CURLOPT_HTTPGET option:
curl_easy_setopt(easyhandle, CURLOPT_HTTPGET, TRUE);
Just setting CURLOPT_POSTFIELDS to "" or NULL will *not* stop libcurl from doing a
POST. It will just make it POST without any data to send!
Showing Progress
For historical and traditional reasons, libcurl has a built-in progress meter that
can be switched on and then makes it presents a progress meter in your terminal.
Switch on the progress meter by, oddly enough, set CURLOPT_NOPROGRESS to FALSE.
This option is set to TRUE by default.
For most applications however, the built-in progress meter is useless and what
instead is interesting is the ability to specify a progress callback. The function
pointer you pass to libcurl will then be called on irregular intervals with infor-
mation about the current transfer.
Set the progress callback by using CURLOPT_PROGRESSFUNCTION. And pass a pointer to
a function that matches this prototype:
int progress_callback(void *clientp,
double dltotal,
double dlnow,
double ultotal,
double ulnow);
If any of the input arguments is unknown, a 0 will be passed. The first argument,
the ’clientp’ is the pointer you pass to libcurl with CURLOPT_PROGRESSDATA. libcurl
won’t touch it.
libcurl with C++
There’s basically only one thing to keep in mind when using C++ instead of C when
interfacing libcurl:
The callbacks CANNOT be non-static class member functions
Example C++ code:
class AClass {
static size_t write_data(void *ptr, size_t size, size_t nmemb,
void *ourpointer)
{
/* do what you want with the data */
}
}
Proxies
What "proxy" means according to Merriam-Webster: "a person authorized to act for
another" but also "the agency, function, or office of a deputy who acts as a sub-
stitute for another".
Proxies are exceedingly common these days. Companies often only offer Internet
access to employees through their HTTP proxies. Network clients or user-agents ask
the proxy for documents, the proxy does the actual request and then it returns
them.
libcurl has full support for HTTP proxies, so when a given URL is wanted, libcurl
will ask the proxy for it instead of trying to connect to the actual host identi-
fied in the URL.
The fact that the proxy is a HTTP proxy puts certain restrictions on what can actu-
ally happen. A requested URL that might not be a HTTP URL will be still be passed
to the HTTP proxy to deliver back to libcurl. This happens transparently, and an
application may not need to know. I say "may", because at times it is very impor-
tant to understand that all operations over a HTTP proxy is using the HTTP proto-
col. For example, you can’t invoke your own custom FTP commands or even proper FTP
directory listings.
Proxy Options
To tell libcurl to use a proxy at a given port number:
curl_easy_setopt(easyhandle, CURLOPT_PROXY, "proxy-host.com:8080");
Some proxies require user authentication before allowing a request, and you
pass that information similar to this:
curl_easy_setopt(easyhandle, CURLOPT_PROXYUSERPWD, "user:password");
If you want to, you can specify the host name only in the CURLOPT_PROXY
option, and set the port number separately with CURLOPT_PROXYPORT.
Environment Variables
libcurl automatically checks and uses a set of environment variables to know
what proxies to use for certain protocols. The names of the variables are
following an ancient de facto standard and are built up as "[proto-
col]_proxy" (note the lower casing). Which makes the variable HTTP. Follow-
ing the same rule, the variable named ’ftp_proxy’ is checked for FTP URLs.
Again, the proxies are always HTTP proxies, the different names of the vari-
ables simply allows different HTTP proxies to be used.
The proxy environment variable contents should be in the format "[proto-
col://][user:password@]machine[:port]". Where the protocol:// part is simply
ignored if present (so http://proxy and bluerk://proxy will do the same) and
the optional port number specifies on which port the proxy operates on the
host. If not specified, the internal default port number will be used and
that is most likely *not* the one you would like it to be.
There are two special environment variables. ’all_proxy’ is what sets proxy
for any URL in case the protocol specific variable wasn’t set, and
’no_proxy’ defines a list of hosts that should not use a proxy even though a
variable may say so. If ’no_proxy’ is a plain asterisk ("*") it matches all
hosts.
SSL and Proxies
SSL is for secure point-to-point connections. This involves strong encryp-
tion and similar things, which effectively makes it impossible for a proxy
to operate as a "man in between" which the proxy’s task is, as previously
discussed. Instead, the only way to have SSL work over a HTTP proxy is to
ask the proxy to tunnel trough everything without being able to check or
fiddle with the traffic.
Opening an SSL connection over a HTTP proxy is therefor a matter of asking
the proxy for a straight connection to the target host on a specified port.
This is made with the HTTP request CONNECT. ("please mr proxy, connect me to
that remote host").
Because of the nature of this operation, where the proxy has no idea what
kind of data that is passed in and out through this tunnel, this breaks some
of the very few advantages that come from using a proxy, such as caching.
Many organizations prevent this kind of tunneling to other destination port
numbers than 443 (which is the default HTTPS port number).
Tunneling Through Proxy
As explained above, tunneling is required for SSL to work and often even
restricted to the operation intended for SSL; HTTPS.
This is however not the only time proxy-tunneling might offer benefits to
you or your application.
As tunneling opens a direct connection from your application to the remote
machine, it suddenly also re-introduces the ability to do non-HTTP opera-
tions over a HTTP proxy. You can in fact use things such as FTP upload or
FTP custom commands this way.
Again, this is often prevented by the administrators of proxies and is
rarely allowed.
Tell libcurl to use proxy tunneling like this:
curl_easy_setopt(easyhandle, CURLOPT_HTTPPROXYTUNNEL, TRUE);
In fact, there might even be times when you want to do plain HTTP operations
using a tunnel like this, as it then enables you to operate on the remote
server instead of asking the proxy to do so. libcurl will not stand in the
way for such innovative actions either!
Proxy Auto-Config
Netscape first came up with this. It is basically a web page (usually using
a .pac extension) with a javascript that when executed by the browser with
the requested URL as input, returns information to the browser on how to
connect to the URL. The returned information might be "DIRECT" (which means
no proxy should be used), "PROXY host:port" (to tell the browser where the
proxy for this particular URL is) or "SOCKS host:port" (to direct the
browser to a SOCKS proxy).
libcurl has no means to interpret or evaluate javascript and thus it doesn’t
support this. If you get yourself in a position where you face this nasty
invention, the following advice have been mentioned and used in the past:
- Depending on the javascript complexity, write up a script that translates
it to another language and execute that.
- Read the javascript code and rewrite the same logic in another language.
- Implement a javascript interpreted, people have successfully used the
Mozilla javascript engine in the past.
- Ask your admins to stop this, for a static proxy setup or similar.
Persistence Is The Way to Happiness
Re-cycling the same easy handle several times when doing multiple requests is the
way to go.
After each single curl_easy_perform(3) operation, libcurl will keep the connection
alive and open. A subsequent request using the same easy handle to the same host
might just be able to use the already open connection! This reduces network impact
a lot.
Even if the connection is dropped, all connections involving SSL to the same host
again, will benefit from libcurl’s session ID cache that drastically reduces re-
connection time.
FTP connections that are kept alive saves a lot of time, as the command- response
round-trips are skipped, and also you don’t risk getting blocked without permission
to login again like on many FTP servers only allowing N persons to be logged in at
the same time.
libcurl caches DNS name resolving results, to make lookups of a previously looked
up name a lot faster.
Other interesting details that improve performance for subsequent requests may also
be added in the future.
Each easy handle will attempt to keep the last few connections alive for a while in
case they are to be used again. You can set the size of this "cache" with the CUR-
LOPT_MAXCONNECTS option. Default is 5. It is very seldom any point in changing this
value, and if you think of changing this it is often just a matter of thinking
again.
When the connection cache gets filled, libcurl must close an existing connection in
order to get room for the new one. To know which connection to close, libcurl uses
a "close policy" that you can affect with the CURLOPT_CLOSEPOLICY option. There’s
only two polices implemented as of this writing (libcurl 7.9.4) and they are:
CURLCLOSEPOLICY_LEAST_RECENTLY_USED
simply close the one that hasn’t been used for the longest time. This
is the default behavior.
CURLCLOSEPOLICY_OLDEST
closes the oldest connection, the one that was created the longest
time ago.
There are, or at least were, plans to support a close policy that would call a
user-specified callback to let the user be able to decide which connection to dump
when this is necessary and therefor is the CURLOPT_CLOSEFUNCTION an existing option
still today. Nothing ever uses this though and this will not be used within the
foreseeable future either.
To force your upcoming request to not use an already existing connection (it will
even close one first if there happens to be one alive to the same host you’re about
to operate on), you can do that by setting CURLOPT_FRESH_CONNECT to TRUE. In a sim-
ilar spirit, you can also forbid the upcoming request to be "lying" around and pos-
sibly get re-used after the request by setting CURLOPT_FORBID_REUSE to TRUE.
HTTP Headers Used by libcurl
When you use libcurl to do HTTP requests, it’ll pass along a series of headers
automatically. It might be good for you to know and understand these ones.
Host This header is required by HTTP 1.1 and even many 1.0 servers and should be
the name of the server we want to talk to. This includes the port number if
anything but default.
Pragma "no-cache". Tells a possible proxy to not grab a copy from the cache but to
fetch a fresh one.
Accept "*/*".
Expect:
When doing multi-part formposts, libcurl will set this header to "100-con-
tinue" to ask the server for an "OK" message before it proceeds with sending
the data part of the post.
Customizing Operations
There is an ongoing development today where more and more protocols are built upon
HTTP for transport. This has obvious benefits as HTTP is a tested and reliable pro-
tocol that is widely deployed and have excellent proxy-support.
When you use one of these protocols, and even when doing other kinds of programming
you may need to change the traditional HTTP (or FTP or...) manners. You may need
to change words, headers or various data.
libcurl is your friend here too.
CUSTOMREQUEST
If just changing the actual HTTP request keyword is what you want, like when
GET, HEAD or POST is not good enough for you, CURLOPT_CUSTOMREQUEST is there
for you. It is very simple to use:
curl_easy_setopt(easyhandle, CURLOPT_CUSTOMREQUEST, "MYOWNRUQUEST");
When using the custom request, you change the request keyword of the actual
request you are performing. Thus, by default you make GET request but you
can also make a POST operation (as described before) and then replace the
POST keyword if you want to. You’re the boss.
Modify Headers
HTTP-like protocols pass a series of headers to the server when doing the
request, and you’re free to pass any amount of extra headers that you think
fit. Adding headers are this easy:
struct curl_slist *headers=NULL; /* init to NULL is important */
headers = curl_slist_append(headers, "Hey-server-hey: how are you?");
headers = curl_slist_append(headers, "X-silly-content: yes");
/* pass our list of custom made headers */
curl_easy_setopt(easyhandle, CURLOPT_HTTPHEADER, headers);
curl_easy_perform(easyhandle); /* transfer http */
curl_slist_free_all(headers); /* free the header list */
... and if you think some of the internally generated headers, such as
Accept: or Host: don’t contain the data you want them to contain, you can
replace them by simply setting them too:
headers = curl_slist_append(headers, "Accept: Agent-007");
headers = curl_slist_append(headers, "Host: munged.host.line");
Delete Headers
If you replace an existing header with one with no contents, you will pre-
vent the header from being sent. Like if you want to completely prevent the
"Accept:" header to be sent, you can disable it with code similar to this:
headers = curl_slist_append(headers, "Accept:");
Both replacing and canceling internal headers should be done with careful
consideration and you should be aware that you may violate the HTTP protocol
when doing so.
Enforcing chunked transfer-encoding
By making sure a request uses the custom header "Transfer-Encoding: chunked"
when doing a non-GET HTTP operation, libcurl will switch over to "chunked"
upload, even though the size of the data to upload might be known. By
default, libcurl usually switches over to chunked upload automatically if
the upload data size is unknown.
HTTP Version
There’s only one aspect left in the HTTP requests that we haven’t yet men-
tioned how to modify: the version field. All HTTP requests includes the ver-
sion number to tell the server which version we support. libcurl speak HTTP
1.1 by default. Some very old servers don’t like getting 1.1-requests and
when dealing with stubborn old things like that, you can tell libcurl to use
1.0 instead by doing something like this:
curl_easy_setopt(easyhandle, CURLOPT_HTTP_VERSION, CURLHTTP_VERSION_1_0);
FTP Custom Commands
Not all protocols are HTTP-like, and thus the above may not help you when
you want to make for example your FTP transfers to behave differently.
Sending custom commands to a FTP server means that you need to send the com-
mands exactly as the FTP server expects them (RFC959 is a good guide here),
and you can only use commands that work on the control-connection alone. All
kinds of commands that requires data interchange and thus needs a data-con-
nection must be left to libcurl’s own judgment. Also be aware that libcurl
will do its very best to change directory to the target directory before
doing any transfer, so if you change directory (with CWD or similar) you
might confuse libcurl and then it might not attempt to transfer the file in
the correct remote directory.
A little example that deletes a given file before an operation:
headers = curl_slist_append(headers, "DELE file-to-remove");
/* pass the list of custom commands to the handle */
curl_easy_setopt(easyhandle, CURLOPT_QUOTE, headers);
curl_easy_perform(easyhandle); /* transfer ftp data! */
curl_slist_free_all(headers); /* free the header list */
If you would instead want this operation (or chain of operations) to happen
_after_ the data transfer took place the option to curl_easy_setopt(3) would
instead be called CURLOPT_POSTQUOTE and used the exact same way.
The custom FTP command will be issued to the server in the same order they
are added to the list, and if a command gets an error code returned back
from the server, no more commands will be issued and libcurl will bail out
with an error code (CURLE_FTP_QUOTE_ERROR). Note that if you use CUR-
LOPT_QUOTE to send commands before a transfer, no transfer will actually
take place when a quote command has failed.
If you set the CURLOPT_HEADER to true, you will tell libcurl to get informa-
tion about the target file and output "headers" about it. The headers will
be in "HTTP-style", looking like they do in HTTP.
The option to enable headers or to run custom FTP commands may be useful to
combine with CURLOPT_NOBODY. If this option is set, no actual file content
transfer will be performed.
FTP Custom CUSTOMREQUEST
If you do what list the contents of a FTP directory using your own defined
FTP command, CURLOPT_CUSTOMREQUEST will do just that. "NLST" is the default
one for listing directories but you’re free to pass in your idea of a good
alternative.
Cookies Without Chocolate Chips
In the HTTP sense, a cookie is a name with an associated value. A server sends the
name and value to the client, and expects it to get sent back on every subsequent
request to the server that matches the particular conditions set. The conditions
include that the domain name and path match and that the cookie hasn’t become too
old.
In real-world cases, servers send new cookies to replace existing one to update
them. Server use cookies to "track" users and to keep "sessions".
Cookies are sent from server to clients with the header Set-Cookie: and they’re
sent from clients to servers with the Cookie: header.
To just send whatever cookie you want to a server, you can use CURLOPT_COOKIE to
set a cookie string like this:
curl_easy_setopt(easyhandle, CURLOPT_COOKIE, "name1=var1; name2=var2;");
In many cases, that is not enough. You might want to dynamically save whatever
cookies the remote server passes to you, and make sure those cookies are then use
accordingly on later requests.
One way to do this, is to save all headers you receive in a plain file and when you
make a request, you tell libcurl to read the previous headers to figure out which
cookies to use. Set header file to read cookies from with CURLOPT_COOKIEFILE.
The CURLOPT_COOKIEFILE option also automatically enables the cookie parser in
libcurl. Until the cookie parser is enabled, libcurl will not parse or understand
incoming cookies and they will just be ignored. However, when the parser is enabled
the cookies will be understood and the cookies will be kept in memory and used
properly in subsequent requests when the same handle is used. Many times this is
enough, and you may not have to save the cookies to disk at all. Note that the file
you specify to CURLOPT_COOKIEFILE doesn’t have to exist to enable the parser, so a
common way to just enable the parser and not read able might be to use a file name
you know doesn’t exist.
If you rather use existing cookies that you’ve previously received with your
Netscape or Mozilla browsers, you can make libcurl use that cookie file as input.
The CURLOPT_COOKIEFILE is used for that too, as libcurl will automatically find out
what kind of file it is and act accordingly.
The perhaps most advanced cookie operation libcurl offers, is saving the entire
internal cookie state back into a Netscape/Mozilla formatted cookie file. We call
that the cookie-jar. When you set a file name with CURLOPT_COOKIEJAR, that file
name will be created and all received cookies will be stored in it when
curl_easy_cleanup(3) is called. This enabled cookies to get passed on properly
between multiple handles without any information getting lost.
FTP Peculiarities We Need
FTP transfers use a second TCP/IP connection for the data transfer. This is usually
a fact you can forget and ignore but at times this fact will come back to haunt
you. libcurl offers several different ways to custom how the second connection is
being made.
libcurl can either connect to the server a second time or tell the server to con-
nect back to it. The first option is the default and it is also what works best for
all the people behind firewalls, NATs or IP-masquerading setups. libcurl then
tells the server to open up a new port and wait for a second connection. This is by
default attempted with EPSV first, and if that doesn’t work it tries PASV instead.
(EPSV is an extension to the original FTP spec and does not exist nor work on all
FTP servers.)
You can prevent libcurl from first trying the EPSV command by setting CUR-
LOPT_FTP_USE_EPSV to FALSE.
In some cases, you will prefer to have the server connect back to you for the sec-
ond connection. This might be when the server is perhaps behind a firewall or some-
thing and only allows connections on a single port. libcurl then informs the remote
server which IP address and port number to connect to. This is made with the CUR-
LOPT_FTPPORT option. If you set it to "-", libcurl will use your system’s "default
IP address". If you want to use a particular IP, you can set the full IP address, a
host name to resolve to an IP address or even a local network interface name that
libcurl will get the IP address from.
When doing the "PORT" approach, libcurl will attempt to use the EPRT and the LPRT
before trying PORT, as they work with more protocols. You can disable this behavior
by setting CURLOPT_FTP_USE_EPRT to FALSE.
Headers Equal Fun
Some protocols provide "headers", meta-data separated from the normal data. These
headers are by default not included in the normal data stream, but you can make
them appear in the data stream by setting CURLOPT_HEADER to TRUE.
What might be even more useful, is libcurl’s ability to separate the headers from
the data and thus make the callbacks differ. You can for example set a different
pointer to pass to the ordinary write callback by setting CURLOPT_WRITEHEADER.
Or, you can set an entirely separate function to receive the headers, by using CUR-
LOPT_HEADERFUNCTION.
The headers are passed to the callback function one by one, and you can depend on
that fact. It makes it easier for you to add custom header parsers etc.
"Headers" for FTP transfers equal all the FTP server responses. They aren’t actu-
ally true headers, but in this case we pretend they are! ;-)
Post Transfer Information
[ curl_easy_getinfo ]
Security Considerations
libcurl is in itself not insecure. If used the right way, you can use libcurl to
transfer data pretty safely.
There are of course many things to consider that may loosen up this situation:
Command Lines
If you use a command line tool (such as curl) that uses libcurl, and you
give option to the tool on the command line those options can very likely
get read by other users of your system when they use ’ps’ or other tools to
list currently running processes.
To avoid this problem, never feed sensitive things to programs using command
line options.
.netrc .netrc is a pretty handy file/feature that allows you to login quickly and
automatically to frequently visited sites. The file contains passwords in
clear text and is a real security risk. In some cases, your .netrc is also
stored in a home directory that is NFS mounted or used on another network
based file system, so the clear text password will fly through your network
every time anyone reads that file!
To avoid this problem, don’t use .netrc files and never store passwords in
plain text anywhere.
Clear Text Passwords
Many of the protocols libcurl supports send name and password unencrypted as
clear text (HTTP Basic authentication, FTP, TELNET etc). It is very easy for
anyone on your network or a network nearby yours, to just fire up a network
analyzer tool and eavesdrop on your passwords. Don’t let the fact that HTTP
uses base64 encoded passwords fool you. They may not look readable at a
first glance, but they very easily "deciphered" by anyone within seconds.
To avoid this problem, use protocols that don’t let snoopers see your pass-
word: HTTPS, FTPS and FTP-kerberos are a few examples. HTTP Digest authenti-
cation allows this too, but isn’t supported by libcurl as of this writing.
Showing What You Do
On a related issue, be aware that even in situations like when you have
problems with libcurl and ask someone for help, everything you reveal in
order to get best possible help might also impose certain security related
risks. Host names, user names, paths, operating system specifics etc (not to
mention passwords of course) may in fact be used by intruders to gain addi-
tional information of a potential target.
To avoid this problem, you must of course use your common sense. Often, you
can just edit out the sensitive data or just search/replace your true infor-
mation with faked data.
Multiple Transfers Using the multi Interface
The easy interface as described in detail in this document is a synchronous inter-
face that transfers one file at a time and doesn’t return until its done.
The multi interface on the other hand, allows your program to transfer multiple
files in both directions at the same time, without forcing you to use multiple
threads.
To use this interface, you are better off if you first understand the basics of how
to use the easy interface. The multi interface is simply a way to make multiple
transfers at the same time, by adding up multiple easy handles in to a "multi
stack".
You create the easy handles you want and you set all the options just like you have
been told above, and then you create a multi handle with curl_multi_init(3) and add
all those easy handles to that multi handle with curl_multi_add_handle(3).
When you’ve added the handles you have for the moment (you can still add new ones
at any time), you start the transfers by call curl_multi_perform(3).
curl_multi_perform(3) is asynchronous. It will only execute as little as possible
and then return back control to your program. It is designed to never block. If it
returns CURLM_CALL_MULTI_PERFORM you better call it again soon, as that is a signal
that it still has local data to send or remote data to receive.
The best usage of this interface is when you do a select() on all possible file
descriptors or sockets to know when to call libcurl again. This also makes it easy
for you to wait and respond to actions on your own application’s sockets/handles.
You figure out what to select() for by using curl_multi_fdset(3), that fills in a
set of fd_set variables for you with the particular file descriptors libcurl uses
for the moment.
When you then call select(), it’ll return when one of the file handles signal
action and you then call curl_multi_perform(3) to allow libcurl to do what it wants
to do. Take note that libcurl does also feature some time-out code so we advice you
to never use very long timeouts on select() before you call curl_multi_perform(3),
which thus should be called unconditionally every now and then even if none of its
file descriptors have signaled ready. Another precaution you should use: always
call curl_multi_fdset(3) immediately before the select() call since the current set
of file descriptors may change when calling a curl function.
If you want to stop the transfer of one of the easy handles in the stack, you can
use curl_multi_remove_handle(3) to remove individual easy handles. Remember that
easy handles should be curl_easy_cleanup(3)ed.
When a transfer within the multi stack has finished, the counter of running trans-
fers (as filled in by curl_multi_perform(3)) will decrease. When the number reaches
zero, all transfers are done.
curl_multi_info_read(3) can be used to get information about completed transfers.
It then returns the CURLcode for each easy transfer, to allow you to figure out
success on each individual transfer.
SSL, Certificates and Other Tricks
[ seeding, passwords, keys, certificates, ENGINE, ca certs ]
Sharing Data Between Easy Handles
[ fill in ]
Footnotes
[1] libcurl 7.10.3 and later have the ability to switch over to chunked Trans-
fer-Encoding in cases were HTTP uploads are done with data of an unknown
size.
[2] This happens on Windows machines when libcurl is built and used as a DLL.
However, you can still do this on Windows if you link with a static library.
[3] The curl-config tool is generated at build-time (on unix-like systems) and
should be installed with the ’make install’ or similar instruction that
installs the library, header files, man pages etc.
libcurl 25 Jan 2005 libcurl-tutorial(3)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2009-01-09 09:42 @38.103.63.58 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)