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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 gen-
eral 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 information:
$ curl-config --cflags
Linking the Program with libcurl
When having compiled the program, you need to link your
object files to create 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 fig-
ure out which flags to use, once again the 'curl-
config' tool comes to the rescue:
$ curl-config --libs
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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, possibly 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 lib-
curl runs on. There are only very few minor considerations
that differs. If you just make sure to write your code port-
able enough, you may very well create yourself a very port-
able program. libcurl shouldn't stop you from that.
Global Preparation
The program must initialize some of the libcurl functional-
ity 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 initialize. Using CURL_GLOBAL_ALL will make
it initialize all known internal sub modules, and might be a
good default option. The current two bits that are specified
are:
CURL_GLOBAL_WIN32
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which only does anything on Windows machines. When
used on a Windows machine, it'll make libcurl ini-
tialize the win32 socket stuff. Without 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 application 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, lib-
curl 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
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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 control 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 lib-
curl 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 preferred 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 issu-
ing a function similar to this:
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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 lib-
curl. 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 kilobytes 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.
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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
connection.
Multi-threading Issues
The first basic rule is that you must never share a libcurl
handle (be it easy or multi or whatever) between multiple
threads. Only use one handle in one thread at a time.
libcurl is completely thread safe, except for two issues:
signals and SSL/TLS handlers. Signals are used timeouting
name resolves (during DNS lookup) - when built without c-
ares support and not on Windows..
If you are accessing HTTPS or FTPS URLs in a multi-threaded
manner, you are then of course using OpenSSL/GnuTLS multi-
threaded and those libs have their own requirements on this
issue. Basically, you need to provide one or two functions
to allow it to function properly. For all details, see this:
OpenSSL
http://www.openssl.org/docs/crypto/threads.html#DESCRIPTION
GnuTLS
http://www.gnu.org/software/gnutls/manual/html_node/Multi_002dthreaded-
applications.html
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 lib-
curl 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 Doesn't Work
There will always be times when the transfer fails for some
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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 lib-
curl, please include as much details you possibly can: a
protocol dump that CURLOPT_VERBOSE produces, library ver-
sion, 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 application 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 lib-
curl 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,
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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 CURLOPT_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 operations 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 per-
form as fast as possible. The callback 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 pass-
word 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 pass-
word in the URL itself. libcurl will detect this and use
them accordingly. This is written like this:
protocol://user:password@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
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user name and password as shown embedded in the URL can
instead get set with the CURLOPT_USERPWD option. The argu-
ment 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 them-
selves to a proxy they use. libcurl offers another option
for this, the CURLOPT_PROXYUSERPWD. It is used quite similar
to the CURLOPT_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 contain 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 extension
to the normal functionality, libcurl also supports this file
for non-FTP protocols 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 pass-
word for getting URLs that require authentication. When
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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, Negotiate, GSS-Negotiate and SPNEGO.
You can tell libcurl which one to use with CURLOPT_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 specific types) which allows libcurl
to use whatever method it wants.
When asking for multiple types, libcurl will pick the avail-
able 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 exam-
ples using both different versions of HTTP POST that libcurl
supports.
The first version is the simple POST, the most common ver-
sion, 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";
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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 Content-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 lib-
curl.
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 sin-
gle unit. Each part has its own name and contents. You can
in fact create and post a multi-part formpost with the regu-
lar 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.
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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 indi-
vidual 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:
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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
information about the current transfer.
Set the progress callback by using CURLOPT_PROGRESSFUNCTION.
And pass a pointer to a function that matches this proto-
type:
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 */
}
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}
Proxies
What "proxy" means according to Merriam-Webster: "a person
authorized to act for another" but also "the agency, func-
tion, or office of a deputy who acts as a substitute 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 identified in the URL.
The fact that the proxy is a HTTP proxy puts certain res-
trictions on what can actually 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 tran-
sparently, and an application may not need to know. I say
"may", because at times it is very important to understand
that all operations over a HTTP proxy is using the HTTP pro-
tocol. 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 allow-
ing 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
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environment variables to know what proxies to use for
certain protocols. The names of the variables are fol-
lowing an ancient de facto standard and are built up as
"[protocol]_proxy" (note the lower casing). Which makes
the variable HTTP. Following the same rule, the vari-
able named 'ftp_proxy' is checked for FTP URLs. Again,
the proxies are always HTTP proxies, the different
names of the variables simply allows different HTTP
proxies to be used.
The proxy environment variable contents should be in
the format
"[protocol://][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 encryption 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, con-
nect 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).
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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 appli-
cation to the remote machine, it suddenly also re-
introduces the ability to do non-HTTP operations 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 javas-
cript and thus it doesn't support this. If you get
yourself in a position where you face this nasty inven-
tion, 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 exe-
cute that.
- Read the javascript code and rewrite the same logic
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in another language.
- Implement a javascript interpreted, people have suc-
cessfully 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 ses-
sion 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 sub-
sequent requests may also be added in the future.
Each easy handle will attempt to keep the last few connec-
tions alive for a while in case they are to be used again.
You can set the size of this "cache" with the
CURLOPT_MAXCONNECTS option. Default is 5. It is very seldom
any point in changing this value, and if you think of chang-
ing 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 pol-
icy" 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
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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 neces-
sary 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 exist-
ing connection (it will even close one first if there hap-
pens to be one alive to the same host you're about to oper-
ate on), you can do that by setting CURLOPT_FRESH_CONNECT to
TRUE. In a similar spirit, you can also forbid the upcoming
request to be "lying" around and possibly 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-continue" to ask the server for an "OK"
message before it proceeds with sending the data part
of the post.
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libcurl-tutorial(3) libcurl programming libcurl-tutorial(3)Customizing Operations
There is an ongoing development today where more and more
protocols are built upon HTTP for transport. This has obvi-
ous benefits as HTTP is a tested and reliable protocol 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 tradi-
tional 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:
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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 con-
tents, you will prevent 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 mentioned how to modify: the version
field. All HTTP requests includes the version 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 commands exactly as the FTP server
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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 inter-
change and thus needs a data-connection 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 lib-
curl 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
CURLOPT_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 information 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 com-
mands 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
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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 dynami-
cally 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 cook-
ies 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.
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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 con-
nection is being made.
libcurl can either connect to the server a second time or
tell the server to connect back to it. The first option is
the default and it is also what works best for all the peo-
ple behind firewalls, NATs or IP-masquerading setups. lib-
curl 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 CURLOPT_FTP_USE_EPSV to FALSE.
In some cases, you will prefer to have the server connect
back to you for the second connection. This might be when
the server is perhaps behind a firewall or something 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 CURLOPT_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
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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 sep-
arate 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 CURLOPT_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 res-
ponses. They aren't actually 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
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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 net-
work 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 authen-
tication, 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 pass-
words. 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 any-
one within seconds.
To avoid this problem, use protocols that don't let
snoopers see your password: HTTPS, FTPS and FTP-
kerberos are a few examples. HTTP Digest authentication
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 some-
one 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 com-
mon sense. Often, you can just edit out the sensitive
data or just search/replace your true information with
faked data.
Multiple Transfers Using the multi Interface
The easy interface as described in detail in this document
is a synchronous interface that transfers one file at a time
and doesn't return until its done.
The multi interface on the other hand, allows your program
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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 lib-
curl 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.
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When a transfer within the multi stack has finished, the
counter of running transfers (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 Transfer-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.
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