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-con-
fig'[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 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, 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 envi-
ronments.
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 program. 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 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
which only does anything on Windows machines. When used
on a Windows machine, it'll make libcurl initialize the
win32 socket stuff. Without having that initialized prop-
erly, 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, 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_ver-
sion_info() and checking tout he details of the returned struct, your
program can figure out exactly what the currently running libcurl sup-
ports.
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 han-
dle. 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 trans-
fers 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 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 iden-
tifies 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 func-
tion 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 appli-
cation 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 plat-
forms[2], libcurl won't be able to operate on files opened by the pro-
gram. 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_WRITE-
FUNCTION 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.
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 pro-
vide 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/man-
ual/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 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 Doesn'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 pro-
gram.
There's one golden rule when these things occur: set the CURLOPT_VER-
BOSE option to TRUE. It'll cause the library to spew out the entire
protocol details it sends, some internal info and some received proto-
col 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 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 appropri-
ate RFC documents at least briefly.
Upload Data to a Remote Site
libcurl tries to keep a protocol independent approach to most trans-
fers, 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 maxi-
mum 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 perform as fast as possible. The callback should return the num-
ber 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 pro-
vided 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@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 num-
ber.
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 CURLOPT_USERPWD option like this:
curl_easy_setopt(easyhandle, CURLOPT_PROXYUSERPWD, "myname:these-
cret");
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 Con-
siderations" 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 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 trans-
fers.
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 get-
ting 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, 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 authenti-
cation 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 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 dif-
ferent 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 CUR-
LOPT_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. Set-
ting 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 form-
post yourself and provide to libcurl. To make that easier, libcurl pro-
vides 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 head-
ers 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 information 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 CUR-
LOPT_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 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 restrictions 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 transparently, 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 protocol. For example,
you can't invoke your own custom FTP commands or even proper FTP direc-
tory 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:pass-
word");
If you want to, you can specify the host name only in the CUR-
LOPT_PROXY option, and set the port number separately with CUR-
LOPT_PROXYPORT.
Environment Variables
libcurl automatically checks and uses a set of environment vari-
ables to know what proxies to use for certain protocols. The
names of the variables are following an ancient de facto stan-
dard and are built up as "[protocol]_proxy" (note the lower cas-
ing). Which makes the variable HTTP. Following 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
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 proto-
col:// 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, 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 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 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 sim-
ilar.
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 con-
nection! 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 com-
mand- 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 previ-
ously 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 CURLOPT_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 connec-
tion 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 long-
est 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 CUR-
LOPT_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 connec-
tion (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 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.
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 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 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, "MYOWN-
RUQUEST");
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 prevent the header from being sent. Like if you want to
completely prevent the "Accept:" header to be sent, you can dis-
able 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-Encod-
ing: 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, CURL-
HTTP_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 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-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 direc-
tory 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 opera-
tions) 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 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 "ses-
sions".
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 CUR-
LOPT_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 accord-
ingly.
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 connect 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
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 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 CUR-
LOPT_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 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 responses. 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 pro-
cesses.
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, TEL-
NET etc). It is very easy for anyone on your network or a net-
work 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 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 someone for help, every-
thing 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 additional
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 information with faked data.
Multiple Transfers Using the multi Interface
The easy interface as described in detail in this document is a syn-
chronous 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 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 multi-
ple 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 pos-
sible 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 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 sim-
ilar instruction that installs the library, header files, man
pages etc.
libcurl 9 May 2005 libcurl-tutorial(3)
Man(1) output converted with
man2html