Provided by: libcurl4-doc_7.58.0-2ubuntu3.24_all 
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 more details on all options and functions described herein, please refer to their respective man
pages.
Building
There are many different ways to build C programs. This chapter will assume a Unix style build process.
If you use a different build system, you can still read this to get general information that may apply to
your environment as well.
Compiling the Program
Your compiler needs to know where the libcurl headers are located. Therefore you must set your
compiler's include path to point to the directory where you installed them. The 'curl-config'[3]
tool can be used to get this 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 a supported
SSL library 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 could be either on or off on for different libcurls.
See also the "Features libcurl Provides" further down.
autoconf macro
When you write your configure script to detect libcurl and setup variables accordingly, we offer a
prewritten macro that probably does everything you need in this area. See docs/libcurl/libcurl.m4
file - it includes docs on how to use it.
Portable Code in a Portable World
The people behind libcurl have put a considerable effort to make libcurl work on a large amount of
different operating systems and environments.
You program libcurl the same way on all platforms that libcurl runs on. There are only very few minor
considerations that differ. 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 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 the SSL library properly for this application. This only needs
to be done 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 at run-time rather than at build-time (if
possible of course). By calling curl_version_info(3) and checking out the details of the returned struct,
your program can figure out exactly what the currently running libcurl supports.
Two Interfaces
libcurl first introduced the so called easy interface. All operations in the easy interface are prefixed
with 'curl_easy'. The easy interface lets you do single transfers with a synchronous and blocking
function call.
libcurl also offers another interface that allows multiple simultaneous transfers in a single thread, the
so called multi interface. More about that interface is detailed in a separate chapter further down. You
still need to understand the easy interface first, so please continue reading for better understanding.
Handle the Easy libcurl
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. They are sticky. Multiple requests using the same handle will use the same options.
If you at any point would like to blank all previously set options for a single easy handle, you can call
curl_easy_reset(3) and you can also make a clone of an easy handle (with all its set options) using
curl_easy_duphandle(3).
Many of the options you set in libcurl are "strings", pointers to data terminated with a zero byte. When
you set strings with curl_easy_setopt(3), libcurl makes its own copy so that they don't need to be kept
around in your application after being set[4].
One of the most basic properties to set in the handle is the URL. You set your preferred URL to transfer
with CURLOPT_URL(3) in a manner similar to:
curl_easy_setopt(handle, CURLOPT_URL, "http://domain.com/");
Let's assume for a while that you want to receive data as the URL identifies a remote resource you want
to get here. Since you write a sort of application that needs this transfer, I assume that you would like
to get the data passed to you directly instead of simply getting it passed to stdout. So, you write your
own function that matches this prototype:
size_t write_data(void *buffer, size_t size, size_t nmemb, void *userp);
You tell libcurl to pass all data to this function by issuing a function similar to this:
curl_easy_setopt(easyhandle, CURLOPT_WRITEFUNCTION, write_data);
You can control what data your callback function gets in the fourth argument by setting another property:
curl_easy_setopt(easyhandle, CURLOPT_WRITEDATA, &internal_struct);
Using that property, you can easily pass local data between your application and the function that gets
invoked by libcurl. libcurl itself won't touch the data you pass with CURLOPT_WRITEDATA(3).
libcurl offers its own default internal callback that will take care of the data if you don't set the
callback with CURLOPT_WRITEFUNCTION(3). 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(3) 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 an open file with CURLOPT_WRITEDATA(3), it
will crash. You should therefore avoid this to make your program run fine virtually everywhere.
(CURLOPT_WRITEDATA(3) 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(3) if you set
CURLOPT_WRITEDATA(3) - 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(3) 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.
For some protocols, downloading a file can involve a complicated process of logging in, setting the
transfer mode, changing the current directory and finally transferring the file data. libcurl takes care
of all that complication for you. Given simply the URL to a file, libcurl will take care of all the
details needed to get the file moved from one machine to another.
Multi-threading Issues
libcurl is thread safe but there are a few exceptions. Refer to libcurl-thread(3) for more information.
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 program.
There's one golden rule when these things occur: set the CURLOPT_VERBOSE(3) option to 1. 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(3) set 1.
Of course, there are bugs left. We need 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 many
details as you possibly can: a protocol dump that CURLOPT_VERBOSE(3) 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(3) is not enough, you increase the level of debug data your application receive by
using the CURLOPT_DEBUGFUNCTION(3).
Getting some in-depth knowledge about the protocols involved is never wrong, and if you're trying to do
funny things, you might very well understand libcurl and how to use it better if you study the
appropriate RFC documents at least briefly.
Upload Data to a Remote Site
libcurl tries to keep a protocol independent approach to most transfers, thus uploading to a remote FTP
site is very similar to uploading data to a HTTP server with a PUT request.
Of course, first you either create an easy handle or you re-use one existing one. Then you set the URL to
operate on just like before. This is the remote URL, that we now will upload.
Since we write an application, we most likely want libcurl to get the upload data by asking us for it. To
make it do that, we set the read callback and the custom pointer libcurl will pass to our read callback.
The read callback should have a prototype similar to:
size_t function(char *bufptr, size_t size, size_t nitems, void *userp);
Where bufptr is the pointer to a buffer we fill in with data to upload and size*nitems is the size of the
buffer and therefore also the maximum amount of data we can return to libcurl in this call. The 'userp'
pointer is the custom pointer we set to point to a struct of ours to pass private data between the
application and the callback.
curl_easy_setopt(easyhandle, CURLOPT_READFUNCTION, read_function);
curl_easy_setopt(easyhandle, CURLOPT_READDATA, &filedata);
Tell libcurl that we want to upload:
curl_easy_setopt(easyhandle, CURLOPT_UPLOAD, 1L);
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(3) for all known file sizes
like this[1]:
/* in this example, file_size must be an curl_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 number of bytes it wrote in the buffer. Returning 0 will signal
the end of the upload.
Passwords
Many protocols use or even require that user name and password are provided to be able to download or
upload the data of your choice. libcurl offers several ways to specify them.
Most protocols support that you specify the name and password in the URL itself. libcurl will detect this
and use them accordingly. This is written like this:
protocol://user:password@example.com/path/
If you need any odd letters in your user name or password, you should enter them URL encoded, as %XX
where XX is a two-digit hexadecimal number.
libcurl also provides options to set various passwords. The user name and password as shown embedded in
the URL can instead get set with the CURLOPT_USERPWD(3) 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(3). It is used quite similar to the CURLOPT_USERPWD(3) 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(3) option:
curl_easy_setopt(easyhandle, CURLOPT_NETRC, 1L);
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_KEYPASSWD, "keypassword");
HTTP Authentication
The previous chapter showed how to set user name and password for getting URLs that require
authentication. When using the HTTP protocol, there are many different ways a client can provide those
credentials to the server and you can control which way libcurl will (attempt to) use them. 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 (SPNEGO). You
can tell libcurl which one to use with CURLOPT_HTTPAUTH(3) 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(3):
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 different versions of HTTP POST that libcurl supports.
The first version is the simple POST, the most common version, that most HTML pages using the <form> tag
uses. We provide a pointer to the data and tell libcurl to post it all to the remote site:
char *data="name=daniel&project=curl";
curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDS, data);
curl_easy_setopt(easyhandle, CURLOPT_URL, "http://posthere.com/");
curl_easy_perform(easyhandle); /* post away! */
Simple enough, huh? Since you set the POST options with the CURLOPT_POSTFIELDS(3), 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 prevent libcurl from being able to do strlen() on the data to figure out the
size, so therefore we must tell libcurl the size of the post data. Setting headers in libcurl requests
are done in a generic way, by building a list of our own headers and then passing that list to libcurl.
struct curl_slist *headers=NULL;
headers = curl_slist_append(headers, "Content-Type: text/xml");
/* post binary data */
curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDS, binaryptr);
/* set the size of the postfields data */
curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDSIZE, 23L);
/* 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 were first documented in the RFC1867 (updated in RFC2388). They're
called multi-part because they're built by a chain of parts, each part being a single unit of data. Each
part has its own name and contents. You can in fact create and post a multi-part formpost with the
regular libcurl POST support described above, but that would require that you build a formpost yourself
and provide to libcurl. To make that easier, libcurl provides a MIME API consisting in several functions:
using those, you can create and fill a multi-part form. Function curl_mime_init(3) creates a multi-part
body; you can then append new parts to a multi-part body using curl_mime_addpart(3). There are three
possible data sources for a part: memory using curl_mime_data(3), file using curl_mime_filedata(3) and
user-defined data read callback using curl_mime_data_cb(3). curl_mime_name(3) sets a part's (i.e.: form
field) name, while curl_mime_filename(3) fills in the remote file name. With curl_mime_type(3), you can
tell the MIME type of a part, curl_mime_headers(3) allows defining the part's headers. When a multi-part
body is no longer needed, you can destroy it using curl_mime_free(3).
The following example sets two simple text parts with plain textual contents, and then a file with binary
contents and uploads the whole thing.
curl_mime *multipart = curl_mime_init(easyhandle);
curl_mimepart *part = curl_mime_addpart(mutipart);
curl_mime_name(part, "name");
curl_mime_data(part, "daniel", CURL_ZERO_TERMINATED);
part = curl_mime_addpart(mutipart);
curl_mime_name(part, "project");
curl_mime_data(part, "curl", CURL_ZERO_TERMINATED);
part = curl_mime_addpart(mutipart);
curl_mime_name(part, "logotype-image");
curl_mime_filedata(part, "curl.png");
/* Set the form info */
curl_easy_setopt(easyhandle, CURLOPT_MIMEPOST, multipart);
curl_easy_perform(easyhandle); /* post away! */
/* free the post data again */
curl_mime_free(multipart);
To post multiple files for a single form field, you must supply each file in a separate part, all with
the same field name. Although function curl_mime_subparts(3) implements nested muti-parts, this way of
multiple files posting is deprecated by RFC 7578, chapter 4.3.
To set the data source from an already opened FILE pointer, use:
curl_mime_data_cb(part, filesize, (curl_read_callback) fread,
(curl_seek_callback) fseek, NULL, filepointer);
A deprecated curl_formadd(3) function is still supported in libcurl. It should however not be used
anymore for new designs and programs using it ought to be converted to the MIME API. It is however
described here as an aid to conversion.
Using curl_formadd, you add parts to the form. When you're done adding parts, you post the whole form.
The MIME API example above is expressed as follows using this function:
struct curl_httppost *post=NULL;
struct curl_httppost *last=NULL;
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "name",
CURLFORM_COPYCONTENTS, "daniel", CURLFORM_END);
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "project",
CURLFORM_COPYCONTENTS, "curl", CURLFORM_END);
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "logotype-image",
CURLFORM_FILECONTENT, "curl.png", CURLFORM_END);
/* Set the form info */
curl_easy_setopt(easyhandle, CURLOPT_HTTPPOST, post);
curl_easy_perform(easyhandle); /* post away! */
/* free the post data again */
curl_formfree(post);
Multipart formposts are chains of parts using MIME-style separators and headers. It means that each one
of these separate parts get a few headers set that describe the individual content-type, size etc. To
enable your application to handicraft this formpost even more, libcurl allows you to supply your own set
of custom headers to such an individual form part. You can of course supply headers to as many parts as
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(headers); /* 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 one
as your next request. You force an easyhandle to go back to GET by using the CURLOPT_HTTPGET(3) option:
curl_easy_setopt(easyhandle, CURLOPT_HTTPGET, 1L);
Just setting CURLOPT_POSTFIELDS(3) to "" or NULL will *not* stop libcurl from doing a POST. It will just
make it POST without any data to send!
Converting from deprecated form API to MIME API
Four rules have to be respected in building the multi-part:
- The easy handle must be created before building the multi-part.
- The multi-part is always created by a call to curl_mime_init(easyhandle).
- Each part is created by a call to curl_mime_addpart(multipart).
- When complete, the multi-part must be bound to the easy handle using CURLOPT_MIMEPOST(3) instead of
CURLOPT_HTTPPOST(3).
Here are some example of curl_formadd calls to MIME API sequences:
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "id",
CURLFORM_COPYCONTENTS, "daniel", CURLFORM_END);
CURLFORM_CONTENTHEADER, headers,
CURLFORM_END);
becomes:
part = curl_mime_addpart(multipart);
curl_mime_name(part, "id");
curl_mime_data(part, "daniel", CURL_ZERO_TERMINATED);
curl_mime_headers(part, headers, FALSE);
Setting the last curl_mime_headers argument to TRUE would have caused the headers to be automatically
released upon destroyed the multi-part, thus saving a clean-up call to curl_slist_free_all(3).
curl_formadd(&post, &last,
CURLFORM_PTRNAME, "logotype-image",
CURLFORM_FILECONTENT, "-",
CURLFORM_END);
becomes:
part = curl_mime_addpart(multipart);
curl_mime_name(part, "logotype-image");
curl_mime_data_cb(part, (curl_off_t) -1, fread, fseek, NULL, stdin);
curl_mime_name always copies the field name. The special file name "-" is not supported by
curl_mime_file: to read an open file, use a callback source using fread(). The transfer will be chunked
since the data size is unknown.
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "datafile[]",
CURLFORM_FILE, "file1",
CURLFORM_FILE, "file2",
CURLFORM_END);
becomes:
part = curl_mime_addpart(multipart);
curl_mime_name(part, "datafile[]");
curl_mime_filedata(part, "file1");
part = curl_mime_addpart(multipart);
curl_mime_name(part, "datafile[]");
curl_mime_filedata(part, "file2");
The deprecated multipart/mixed implementation of multiple files field is translated to two distinct parts
with the same name.
curl_easy_setopt(easyhandle, CURLOPT_READFUNCTION, myreadfunc);
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "stream",
CURLFORM_STREAM, arg,
CURLFORM_CONTENTLEN, (curl_off_t) datasize,
CURLFORM_FILENAME, "archive.zip",
CURLFORM_CONTENTTYPE, "application/zip",
CURLFORM_END);
becomes:
part = curl_mime_addpart(multipart);
curl_mime_name(part, "stream");
curl_mime_data_cb(part, (curl_off_t) datasize,
myreadfunc, NULL, NULL, arg);
curl_mime_filename(part, "archive.zip");
curl_mime_type(part, "application/zip");
CURLOPT_READFUNCTION callback is not used: it is replace by directly setting the part source data from
the callback read function.
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "memfile",
CURLFORM_BUFFER, "memfile.bin",
CURLFORM_BUFFERPTR, databuffer,
CURLFORM_BUFFERLENGTH, (long) sizeof databuffer,
CURLFORM_END);
becomes:
part = curl_mime_addpart(multipart);
curl_mime_name(part, "memfile");
curl_mime_data(part, databuffer, (curl_off_t) sizeof databuffer);
curl_mime_filename(part, "memfile.bin");
curl_mime_data always copies the initial data: data buffer is thus free for immediate reuse.
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "message",
CURLFORM_FILECONTENT, "msg.txt",
CURLFORM_END);
becomes:
part = curl_mime_addpart(multipart);
curl_mime_name(part, "message");
curl_mime_filedata(part, "msg.txt");
curl_mime_filename(part, NULL);
Use of curl_mime_filedata sets the remote file name as a side effect: it is therefore necessary to clear
it for CURLFORM_FILECONTENT emulation.
Showing Progress
For historical and traditional reasons, libcurl has a built-in progress meter that can be switched on and
then makes it present a progress meter in your terminal.
Switch on the progress meter by, oddly enough, setting CURLOPT_NOPROGRESS(3) to zero. This option is set
to 1 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(3). And pass a pointer to a function that
matches this prototype:
int progress_callback(void *clientp,
double dltotal,
double dlnow,
double ultotal,
double ulnow);
If any of the input arguments is unknown, a 0 will be passed. The first argument, the 'clientp' is the
pointer you pass to libcurl with CURLOPT_PROGRESSDATA(3). 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 proxies. Network clients or user-agents ask the proxy for documents, the proxy does the
actual request and then it returns them.
libcurl supports SOCKS and HTTP proxies. 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.
If you're using a SOCKS proxy, you may find that libcurl doesn't quite support all operations through it.
For HTTP proxies: 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 use the HTTP
protocol. For example, you can't invoke your own custom FTP commands or even proper FTP directory
listings.
Proxy Options
To tell libcurl to use a proxy at a given port number:
curl_easy_setopt(easyhandle, CURLOPT_PROXY, "proxy-host.com:8080");
Some proxies require user authentication before allowing a request, and you pass that information
similar to this:
curl_easy_setopt(easyhandle, CURLOPT_PROXYUSERPWD, "user:password");
If you want to, you can specify the host name only in the CURLOPT_PROXY(3) option, and set the
port number separately with CURLOPT_PROXYPORT(3).
Tell libcurl what kind of proxy it is with CURLOPT_PROXYTYPE(3) (if not, it will default to assume
a HTTP proxy):
curl_easy_setopt(easyhandle, CURLOPT_PROXYTYPE, CURLPROXY_SOCKS4);
Environment Variables
libcurl automatically checks and uses a set of environment variables to know what proxies to use
for certain protocols. The names of the variables are following an ancient de facto standard and
are built up as "[protocol]_proxy" (note the lower casing). Which makes the variable 'http_proxy'
checked for a name of a proxy to use when the input URL is 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 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.
To explicitly disable libcurl's checking for and using the proxy environment variables, set the
proxy name to "" - an empty string - with CURLOPT_PROXY(3).
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, 1L);
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 interpreter; people have successfully used the Mozilla Javascript engine
in the past.
- Ask your admins to stop this, for a static proxy setup or similar.
Persistence Is The Way to Happiness
Re-cycling the same easy handle several times when doing multiple requests is the way to go.
After each single curl_easy_perform(3) operation, libcurl will keep the connection alive and open. A
subsequent request using the same easy handle to the same host might just be able to use the already open
connection! This reduces network impact a lot.
Even if the connection is dropped, all connections involving SSL to the same host again, will benefit
from libcurl's session ID cache that drastically reduces re-connection time.
FTP connections that are kept alive save a lot of time, as the command- response round-trips are skipped,
and also you don't risk getting blocked without permission to login again like on many FTP servers only
allowing N persons to be logged in at the same time.
libcurl caches DNS name resolving results, to make lookups of a previously looked up name a lot faster.
Other interesting details that improve performance for subsequent requests may also be added in the
future.
Each easy handle will attempt to keep the last few connections alive for a while in case they are to be
used again. You can set the size of this "cache" with the CURLOPT_MAXCONNECTS(3) option. Default is 5.
There 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.
To force your upcoming request to not use an already existing connection (it will even close one first if
there happens to be one alive to the same host you're about to operate on), you can do that by setting
CURLOPT_FRESH_CONNECT(3) to 1. 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(3) to 1.
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. You can replace or remove them by using the
CURLOPT_HTTPHEADER(3) option.
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.
Accept "*/*".
Expect When doing POST requests, libcurl sets this header to "100-continue" to ask the server for an "OK"
message before it proceeds with sending the data part of the post. If the POSTed data amount is
deemed "small", libcurl will not use this header.
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 has
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(3) is there for you. It is very simple to use:
curl_easy_setopt(easyhandle, CURLOPT_CUSTOMREQUEST, "MYOWNREQUEST");
When using the custom request, you change the request keyword of the actual request you are
performing. Thus, by default you make a 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 is 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. For instance, if you want to completely prevent the "Accept:" header from being 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
All HTTP requests includes the version number to tell the server which version we support. libcurl
speaks 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 require data interchange and thus need a
data-connection must be left to libcurl's own judgement. Also be aware that libcurl will do its
very best to change directory to the target directory before doing any transfer, so if you change
directory (with CWD or similar) you might confuse libcurl and then it might not attempt to
transfer the file in the correct remote directory.
A little example that deletes a given file before an operation:
headers = curl_slist_append(headers, "DELE file-to-remove");
/* pass the list of custom commands to the handle */
curl_easy_setopt(easyhandle, CURLOPT_QUOTE, headers);
curl_easy_perform(easyhandle); /* transfer ftp data! */
curl_slist_free_all(headers); /* free the header list */
If you would instead want this operation (or chain of operations) to happen _after_ the data
transfer took place the option to curl_easy_setopt(3) would instead be called CURLOPT_POSTQUOTE(3)
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_QUOTE_ERROR). Note that if you use
CURLOPT_QUOTE(3) to send commands before a transfer, no transfer will actually take place when a
quote command has failed.
If you set the CURLOPT_HEADER(3) to 1, 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(3). If this option is set, no actual file content transfer will be performed.
FTP Custom CUSTOMREQUEST
If you do want to list the contents of a FTP directory using your own defined FTP command,
CURLOPT_CUSTOMREQUEST(3) 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 ones 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(3) 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 used 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 the header file to read
cookies from with CURLOPT_COOKIEFILE(3).
The CURLOPT_COOKIEFILE(3) 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(3) doesn't have to exist to enable the parser, so a common way to just enable the
parser and not read any cookies is to use the name of a file you know doesn't exist.
If you would 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(3) is used for that
too, as libcurl will automatically find out what kind of file it is and act accordingly.
Perhaps the 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(3), that file name will be created and all received cookies will be stored in it
when curl_easy_cleanup(3) is called. This enables 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
customize 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(3) to zero.
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(3) 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(3) to zero.
MIME API revisited for SMTP and IMAP
In addition to support HTTP multi-part form fields, the MIME API can be used to build structured e-mail
messages and send them via SMTP or append such messages to IMAP directories.
A structured e-mail message may contain several parts: some are displayed inline by the MUA, some are
attachments. Parts can also be structured as multi-part, for example to include another e-mail message or
to offer several text formats alternatives. This can be nested to any level.
To build such a message, you prepare the nth-level multi-part and then include it as a source to the
parent multi-part using function curl_mime_subparts(3). Once it has been bound to its parent multi-part,
a nth-level multi-part belongs to it and should not be freed explicitly.
E-mail messages data is not supposed to be non-ascii and line length is limited: fortunately, some
transfer encodings are defined by the standards to support the transmission of such incompatible data.
Function curl_mime_encoder(3) tells a part that its source data must be encoded before being sent. It
also generates the corresponding header for that part. If the part data you want to send is already
encoded in such a scheme, do not use this function (this would over-encode it), but explicitly set the
corresponding part header.
Upon sending such a message, libcurl prepends it with the header list set with CURLOPT_HTTPHEADER(3), as
0th-level mime part headers.
Here is an example building an e-mail message with an inline plain/html text alternative and a file
attachment encoded in base64:
curl_mime *message = curl_mime_init(easyhandle);
/* The inline part is an alternative proposing the html and the text
versions of the e-mail. */
curl_mime *alt = curl_mime_init(easyhandle);
/* HTML message. */
curl_mimepart *part = curl_mime_addpart(alt);
curl_mime_data(part, "<html><body><p>This is HTML</p></body></html>",
CURL_ZERO_TERMINATED);
curl_mime_type(part, "text/html");
/* Text message. */
part = curl_mime_addpart(alt);
curl_mime_data(part, "This is plain text message",
CURL_ZERO_TERMINATED);
/* Create the inline part. */
part = curl_mime_addpart(message);
curl_mime_subparts(part, alt);
curl_mime_type(part, "multipart/alternative");
struct curl_slist *headers = curl_slist_append(NULL,
"Content-Disposition: inline");
curl_mime_headers(part, headers, TRUE);
/* Add the attachment. */
part = curl_mime_addpart(message);
curl_mime_filedata(part, "manual.pdf");
curl_mime_encoder(part, "base64");
/* Build the mail headers. */
headers = curl_slist_append(NULL, "From: me@example.com");
headers = curl_slist_append(headers, "To: you@example.com");
/* Set these into the easy handle. */
curl_easy_setopt(easyhandle, CURLOPT_HTTPHEADER, headers);
curl_easy_setopt(easyhandle, CURLOPT_MIMEPOST, mime);
It should be noted that appending a message to an IMAP directory requires the message size to be known
prior upload. It is therefore not possible to include parts with unknown data size in this context.
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(3) to 1.
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_HEADERDATA(3).
Or, you can set an entirely separate function to receive the headers, by using CURLOPT_HEADERFUNCTION(3).
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
The libcurl project takes security seriously. The library is written with caution and precautions are
taken to mitigate many kinds of risks encountered while operating with potentially malicious servers on
the Internet. It is a powerful library, however, which allows application writers to make trade offs
between ease of writing and exposure to potential risky operations. If used the right way, you can use
libcurl to transfer data pretty safely.
Many applications are used in closed networks where users and servers can be trusted, but many others are
used on arbitrary servers and are fed input from potentially untrusted users. Following is a discussion
about some risks in the ways in which applications commonly use libcurl and potential mitigations of
those risks. It is by no means comprehensive, but shows classes of attacks that robust applications
should consider. The Common Weakness Enumeration project at https://cwe.mitre.org/ is a good reference
for many of these and similar types of weaknesses of which application writers should be aware.
Command Lines
If you use a command line tool (such as curl) that uses libcurl, and you give options 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. Write
them to a protected file and use the -K option to avoid this.
.netrc .netrc is a pretty handy file/feature that allows you to login quickly and automatically to
frequently visited sites. The file contains passwords in clear text and is a real security risk.
In some cases, your .netrc is also stored in a home directory that is NFS mounted or used on
another network based file system, so the clear text password will fly through your network every
time anyone reads that file!
To avoid this problem, don't use .netrc files and never store passwords in plain text anywhere.
Clear Text Passwords
Many of the protocols libcurl supports send name and password unencrypted as clear text (HTTP
Basic authentication, FTP, TELNET etc). It is very easy for anyone on your network or a network
nearby yours to just fire up a network analyzer tool and eavesdrop on your passwords. Don't let
the fact that HTTP Basic 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 an authentication mechanism or other protocol that doesn't let snoopers
see your password: Digest, CRAM-MD5, Kerberos, SPNEGO or NTLM authentication, HTTPS, FTPS, SCP and
SFTP are a few examples.
Redirects
The CURLOPT_FOLLOWLOCATION(3) option automatically follows HTTP redirects sent by a remote server.
These redirects can refer to any kind of URL, not just HTTP. By default libcurl will allow all
protocols on redirect except several disabled for security reasons: Since 7.19.4 FILE and SCP are
disabled, and since 7.40.0 SMB and SMBS are also disabled.
A redirect to a file: URL would cause the libcurl to read (or write) arbitrary files from the
local filesystem. If the application returns the data back to the user (as would happen in some
kinds of CGI scripts), an attacker could leverage this to read otherwise forbidden data (e.g.
file://localhost/etc/passwd).
If authentication credentials are stored in the ~/.netrc file, or Kerberos is in use, any other
URL type (not just file:) that requires authentication is also at risk. A redirect such as
ftp://some-internal-server/private-file would then return data even when the server is password
protected.
In the same way, if an unencrypted SSH private key has been configured for the user running the
libcurl application, SCP: or SFTP: URLs could access password or private-key protected resources,
e.g. sftp://user@some-internal-server/etc/passwd
The CURLOPT_REDIR_PROTOCOLS(3) and CURLOPT_NETRC(3) options can be used to mitigate against this
kind of attack.
A redirect can also specify a location available only on the machine running libcurl, including
servers hidden behind a firewall from the attacker. e.g. http://127.0.0.1/ or
http://intranet/delete-stuff.cgi?delete=all or tftp://bootp-server/pc-config-data
Apps can mitigate against this by disabling CURLOPT_FOLLOWLOCATION(3) and handling redirects
itself, sanitizing URLs as necessary. Alternately, an app could leave CURLOPT_FOLLOWLOCATION(3)
enabled but set CURLOPT_REDIR_PROTOCOLS(3) and install a CURLOPT_OPENSOCKETFUNCTION(3) callback
function in which addresses are sanitized before use.
Private Resources
A user who can control the DNS server of a domain being passed in within a URL can change the
address of the host to a local, private address which a server-side libcurl-using application
could then use. e.g. the innocuous URL http://fuzzybunnies.example.com/ could actually resolve to
the IP address of a server behind a firewall, such as 127.0.0.1 or 10.1.2.3. Apps can mitigate
against this by setting a CURLOPT_OPENSOCKETFUNCTION(3) and checking the address before a
connection.
All the malicious scenarios regarding redirected URLs apply just as well to non-redirected URLs,
if the user is allowed to specify an arbitrary URL that could point to a private resource. For
example, a web app providing a translation service might happily translate
file://localhost/etc/passwd and display the result. Apps can mitigate against this with the
CURLOPT_PROTOCOLS(3) option as well as by similar mitigation techniques for redirections.
A malicious FTP server could in response to the PASV command return an IP address and port number
for a server local to the app running libcurl but behind a firewall. Apps can mitigate against
this by using the CURLOPT_FTP_SKIP_PASV_IP(3) option or CURLOPT_FTPPORT(3).
IPv6 Addresses
libcurl will normally handle IPv6 addresses transparently and just as easily as IPv4 addresses.
That means that a sanitizing function that filters out addresses like 127.0.0.1 isn't
sufficient--the equivalent IPv6 addresses ::1, ::, 0:00::0:1, ::127.0.0.1 and ::ffff:7f00:1
supplied somehow by an attacker would all bypass a naive filter and could allow access to
undesired local resources. IPv6 also has special address blocks like link-local and site-local
that generally shouldn't be accessed by a server-side libcurl-using application. A poorly-
configured firewall installed in a data center, organization or server may also be configured to
limit IPv4 connections but leave IPv6 connections wide open. In some cases, the CURL_IPRESOLVE_V4
option can be used to limit resolved addresses to IPv4 only and bypass these issues.
Uploads
When uploading, a redirect can cause a local (or remote) file to be overwritten. Apps must not
allow any unsanitized URL to be passed in for uploads. Also, CURLOPT_FOLLOWLOCATION(3) should not
be used on uploads. Instead, the app should handle redirects itself, sanitizing each URL first.
Authentication
Use of CURLOPT_UNRESTRICTED_AUTH(3) could cause authentication information to be sent to an
unknown second server. Apps can mitigate against this by disabling CURLOPT_FOLLOWLOCATION(3) and
handling redirects itself, sanitizing where necessary.
Use of the CURLAUTH_ANY option to CURLOPT_HTTPAUTH(3) could result in user name and password being
sent in clear text to an HTTP server. Instead, use CURLAUTH_ANYSAFE which ensures that the
password is encrypted over the network, or else fail the request.
Use of the CURLUSESSL_TRY option to CURLOPT_USE_SSL(3) could result in user name and password
being sent in clear text to an FTP server. Instead, use CURLUSESSL_CONTROL to ensure that an
encrypted connection is used or else fail the request.
Cookies
If cookies are enabled and cached, then a user could craft a URL which performs some malicious
action to a site whose authentication is already stored in a cookie. e.g.
http://mail.example.com/delete-stuff.cgi?delete=all Apps can mitigate against this by disabling
cookies or clearing them between requests.
Dangerous URLs
SCP URLs can contain raw commands within the scp: URL, which is a side effect of how the SCP
protocol is designed. e.g. scp://user:pass@host/a;date >/tmp/test; Apps must not allow
unsanitized SCP: URLs to be passed in for downloads.
Denial of Service
A malicious server could cause libcurl to effectively hang by sending a trickle of data through,
or even no data at all but just keeping the TCP connection open. This could result in a denial-
of-service attack. The CURLOPT_TIMEOUT(3) and/or CURLOPT_LOW_SPEED_LIMIT(3) options can be used to
mitigate against this.
A malicious server could cause libcurl to effectively hang by starting to send data, then severing
the connection without cleanly closing the TCP connection. The app could install a
CURLOPT_SOCKOPTFUNCTION(3) callback function and set the TCP SO_KEEPALIVE option to mitigate
against this. Setting one of the timeout options would also work against this attack.
A malicious server could cause libcurl to download an infinite amount of data, potentially causing
all of memory or disk to be filled. Setting the CURLOPT_MAXFILESIZE_LARGE(3) option is not
sufficient to guard against this. Instead, the app should monitor the amount of data received
within the write or progress callback and abort once the limit is reached.
A malicious HTTP server could cause an infinite redirection loop, causing a denial-of-service.
This can be mitigated by using the CURLOPT_MAXREDIRS(3) option.
Arbitrary Headers
User-supplied data must be sanitized when used in options like CURLOPT_USERAGENT(3),
CURLOPT_HTTPHEADER(3), CURLOPT_POSTFIELDS(3) and others that are used to generate structured data.
Characters like embedded carriage returns or ampersands could allow the user to create additional
headers or fields that could cause malicious transactions.
Server-supplied Names
A server can supply data which the application may, in some cases, use as a file name. The curl
command-line tool does this with --remote-header-name, using the Content-disposition: header to
generate a file name. An application could also use CURLINFO_EFFECTIVE_URL to generate a file
name from a server-supplied redirect URL. Special care must be taken to sanitize such names to
avoid the possibility of a malicious server supplying one like "/etc/passwd", "\autoexec.bat",
"prn:" or even ".bashrc".
Server Certificates
A secure application should never use the CURLOPT_SSL_VERIFYPEER(3) option to disable certificate
validation. There are numerous attacks that are enabled by apps that fail to properly validate
server TLS/SSL certificates, thus enabling a malicious server to spoof a legitimate one. HTTPS
without validated certificates is potentially as insecure as a plain HTTP connection.
Showing What You Do
On a related issue, be aware that even in situations like when you have problems with libcurl and
ask someone for help, everything you reveal in order to get best possible help might also impose
certain security related risks. Host names, user names, paths, operating system specifics, etc.
(not to mention passwords of course) may in fact be used by intruders to gain additional
information of a potential target.
Be sure to limit access to application logs if they could hold private or security-related data.
Besides the obvious candidates like user names and passwords, things like URLs, cookies or even
file names could also hold sensitive data.
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.
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 it is 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. The name might make it seem that the
multi interface is for multi-threaded programs, but the truth is almost the reverse. The multi interface
allows a single-threaded application to perform the same kinds of multiple, simultaneous transfers that
multi-threaded programs can perform. It allows many of the benefits of multi-threaded transfers without
the complexity of managing and synchronizing many threads.
To complicate matters somewhat more, there are even two versions of the multi interface. The event based
one, also called multi_socket and the "normal one" designed for using with select(). See the libcurl-
multi.3 man page for details on the multi_socket event based API, this description here is for the
select() oriented one.
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 into a "multi stack".
You create the easy handles you want, one for each concurrent transfer, and you set all the options just
like you learned 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 calling curl_multi_perform(3).
curl_multi_perform(3) is asynchronous. It will only perform what can be done now and then return back
control to your program. It is designed to never block. You need to keep calling the function until all
transfers are completed.
The best usage of this interface is when you do a select() on all possible file descriptors or sockets to
know when to call libcurl again. This also makes it easy for you to wait and respond to actions on your
own application's sockets/handles. You figure out what to select() for by using curl_multi_fdset(3), that
fills in a set of fd_set variables for you with the particular file descriptors libcurl uses for the
moment.
When you then call select(), it'll return when one of the file handles signal action and you then call
curl_multi_perform(3) to allow libcurl to do what it wants to do. Take note that libcurl does also
feature some time-out code so we advise you to never use very long timeouts on select() before you call
curl_multi_perform(3) again. curl_multi_timeout(3) is provided to help you get a suitable timeout period.
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 in any curl function invoke.
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
You can share some data between easy handles when the easy interface is used, and some data is share
automatically when you use the multi interface.
When you add easy handles to a multi handle, these easy handles will automatically share a lot of the
data that otherwise would be kept on a per-easy handle basis when the easy interface is used.
The DNS cache is shared between handles within a multi handle, making subsequent name resolving faster,
and the connection pool that is kept to better allow persistent connections and connection re-use is also
shared. If you're using the easy interface, you can still share these between specific easy handles by
using the share interface, see libcurl-share(3).
Some things are never shared automatically, not within multi handles, like for example cookies so the
only way to share that is with the share interface.
Footnotes
[1] libcurl 7.10.3 and later have the ability to switch over to chunked Transfer-Encoding in cases
where 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.
[4] This behavior was different in versions before 7.17.0, where strings had to remain valid past the
end of the curl_easy_setopt(3) call.
SEE ALSO
libcurl-errors(3), libcurl-multi(3), libcurl-easy(3)