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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.

Handle the Easy libcurl

       libcurl first introduced the  so  called  easy  interface.  All  operations  in  the  easy
       interface are prefixed with 'curl_easy'.

       Recent libcurl versions also offer the multi interface. More about that interface, what it
       is targeted for and how to use it is detailed in a  separate  chapter  further  down.  You
       still  need  to understand the easy interface first, so please continue reading for better
       understanding.

       To use the easy interface, you must first create yourself an easy  handle.  You  need  one
       handle for each easy session you want to perform. Basically, you should use one handle for
       every thread you plan to use for transferring. You must never share  the  same  handle  in
       multiple threads.

       Get an easy handle with

        easyhandle = curl_easy_init();

       It  returns  an  easy  handle.  Using  that  you proceed to the next step: setting up your
       preferred actions. A handle is just a logic entity for the upcoming transfer or series  of
       transfers.

       You set properties and options for this handle using curl_easy_setopt(3). They control how
       the subsequent transfer or transfers will be made. Options remain set in the handle  until
       set  again  to something different. Alas, multiple requests using the same handle will use
       the same options.

       Many of the options you set in libcurl are "strings", pointers to data terminated  with  a
       zero  byte.  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 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.

       libcurl offers its own default internal callback that will take care of the  data  if  you
       don't set the callback with CURLOPT_WRITEFUNCTION. It will then simply output the received
       data to stdout. You can have the default callback write  the  data  to  a  different  file
       handle  by  passing  a  'FILE  *'  to a file opened for writing with the CURLOPT_WRITEDATA
       option.

       Now, we need to take a step back and  have  a  deep  breath.  Here's  one  of  those  rare
       platform-dependent  nitpicks. Did you spot it? On some platforms[2], libcurl won't be able
       to operate on files opened by the program. Thus, if you use the default callback and  pass
       in  an open file with CURLOPT_WRITEDATA, it will crash. You should therefore avoid this to
       make your program run fine virtually everywhere.

       (CURLOPT_WRITEDATA was formerly known as CURLOPT_FILE. Both names still work  and  do  the
       same thing).

       If  you're using libcurl as a win32 DLL, you MUST use the CURLOPT_WRITEFUNCTION if you set
       CURLOPT_WRITEDATA - or you will experience crashes.

       There are of course many more options you can set, and we'll get back to  a  few  of  them
       later. Let's instead continue to the actual transfer:

        success = curl_easy_perform(easyhandle);

       curl_easy_perform(3)  will  connect  to  the  remote  site,  do the necessary commands and
       receive the transfer. Whenever it  receives  data,  it  calls  the  callback  function  we
       previously  set.  The function may get one byte at a time, or it may get many kilobytes at
       once. libcurl delivers as much as possible as often as possible.  Your  callback  function
       should  return the number of bytes it "took care of". If that is not the exact same amount
       of bytes that was passed to it, libcurl will abort the operation and return with an  error
       code.

       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.

       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

       The first basic rule is that you must never simultaneously share a libcurl handle  (be  it
       easy  or multi or whatever) between multiple threads. Only use one handle in one thread at
       any time. You can pass the handles around among threads, but you must never use  a  single
       handle from more than one thread at any given time.

       libcurl  is  completely  thread safe, except for two issues: signals and SSL/TLS handlers.
       Signals are used for timing out 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 the underlying SSL library multi-threaded  and  those  libs  might  have  their  own
       requirements  on  this issue. Basically, you need to provide one or two functions to allow
       it to function properly. For all details, see this:

       OpenSSL

        http://www.openssl.org/docs/crypto/threads.html#DESCRIPTION

       GnuTLS

        http://www.gnu.org/software/gnutls/manual/html_node/Multi_002dthreaded-applications.html

       NSS

        is claimed to be thread-safe already without anything required.

       PolarSSL

        Required actions unknown.

       yassl

        Required actions unknown.

       axTLS

        Required actions unknown.

       Secure Transport

        The engine is fully thread-safe, and no additional steps are required.

       When using multiple threads you should set  the  CURLOPT_NOSIGNAL  option  to  1  for  all
       handles.  Everything  will  or might 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. On some platforms, libcurl simply
       will not function properly multi-threaded unless this option is set.

       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
       program.

       There's  one  golden  rule  when  these things occur: set the CURLOPT_VERBOSE 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 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
       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 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
       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  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:thesecret");

       There's a long time UNIX "standard" way of storing ftp user names and passwords, namely in
       the $HOME/.netrc file. The file should be made private so that only the user may  read  it
       (see  also  the  "Security  Considerations"  chapter), as it might contain the password in
       plain text. libcurl has the ability to use this file to figure out what set of  user  name
       and  password  to  use for a particular host. As an extension to the normal functionality,
       libcurl also supports this file for non-FTP protocols such as HTTP. To make curl use  this
       file, use the CURLOPT_NETRC option:

        curl_easy_setopt(easyhandle, CURLOPT_NETRC, 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,
       GSS-Negotiate and SPNEGO. You can tell libcurl which one to use with  CURLOPT_HTTPAUTH  as
       in:

        curl_easy_setopt(easyhandle, CURLOPT_HTTPAUTH, CURLAUTH_DIGEST);

       And when you send authentication to a proxy, you can also set authentication type the same
       way but instead with CURLOPT_PROXYAUTH:

        curl_easy_setopt(easyhandle, CURLOPT_PROXYAUTH, CURLAUTH_NTLM);

       Both these options allow you to set multiple types  (by  ORing  them  together),  to  make
       libcurl pick the most secure one out of the types the server/proxy claims to support. This
       method does however add a round-trip since libcurl must  first  ask  the  server  what  it
       supports:

        curl_easy_setopt(easyhandle, CURLOPT_HTTPAUTH,
        CURLAUTH_DIGEST|CURLAUTH_BASIC);

       For  convenience,  you  can use the 'CURLAUTH_ANY' define (instead of a list with specific
       types) which allows libcurl to use whatever method it wants.

       When asking for multiple types, libcurl will pick the available one it considers "best" in
       its own internal order of preference.

HTTP POSTing

       We  get many questions regarding how to issue HTTP POSTs with libcurl the proper way. This
       chapter will thus include examples using both different versions of HTTP POST that libcurl
       supports.

       The  first version is the simple POST, the most common version, that most HTML pages using
       the <form> tag uses. We provide a pointer to the data and tell libcurl to post it  all  to
       the remote site:

           char *data="name=daniel&project=curl";
           curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDS, data);
           curl_easy_setopt(easyhandle, CURLOPT_URL, "http://posthere.com/");

           curl_easy_perform(easyhandle); /* post away! */

       Simple  enough,  huh?  Since  you  set  the POST options with the CURLOPT_POSTFIELDS, this
       automatically switches the handle to use POST in the upcoming request.

       Ok, so what if you want to post binary data that also requires you  to  set  the  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 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 uploads the whole thing.

        struct curl_httppost *post=NULL;
        struct curl_httppost *last=NULL;
        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "name",
                     CURLFORM_COPYCONTENTS, "daniel", CURLFORM_END);
        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "project",
                     CURLFORM_COPYCONTENTS, "curl", CURLFORM_END);
        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "logotype-image",
                     CURLFORM_FILECONTENT, "curl.png", CURLFORM_END);

        /* Set the form info */
        curl_easy_setopt(easyhandle, CURLOPT_HTTPPOST, post);

        curl_easy_perform(easyhandle); /* post away! */

        /* free the post data again */
        curl_formfree(post);

       Multipart  formposts are chains of parts using MIME-style separators and headers. It means
       that each one of these separate parts get a few headers set that describe  the  individual
       content-type,  size etc. To enable your application to handicraft this formpost even more,
       libcurl allows you to supply your own set of custom headers to  such  an  individual  form
       part.  You  can  of  course  supply  headers to as many parts 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 option:

        curl_easy_setopt(easyhandle, CURLOPT_HTTPGET, 1L);

       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 present a progress meter in your terminal.

       Switch on the progress meter by, oddly enough, setting CURLOPT_NOPROGRESS  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.  And  pass  a  pointer  to  a
       function that matches this prototype:

        int progress_callback(void *clientp,
                              double dltotal,
                              double dlnow,
                              double ultotal,
                              double ulnow);

       If  any  of  the  input  arguments is unknown, a 0 will be passed. The first argument, the
       'clientp' is the pointer you pass to  libcurl  with  CURLOPT_PROGRESSDATA.  libcurl  won't
       touch it.

libcurl with C++

       There's  basically  only  one  thing  to  keep  in  mind  when using C++ instead of C when
       interfacing libcurl:

       The callbacks CANNOT be non-static class member functions

       Example C++ code:

       class AClass {
           static size_t write_data(void *ptr, size_t size, size_t nmemb,
                                    void *ourpointer)
           {
             /* do what you want with the data */
           }
        }

Proxies

       What "proxy" means according to Merriam-Webster: "a person authorized to act for  another"
       but  also  "the  agency,  function,  or  office  of  a deputy who acts as a 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 option, and
              set the port number separately with CURLOPT_PROXYPORT.

              Tell  libcurl  what  kind  of  proxy  it is with CURLOPT_PROXYTYPE (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.

       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 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 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 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 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 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 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  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 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. 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  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 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.

       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  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  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, 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
       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  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 zero.

Headers Equal Fun

       Some  protocols provide "headers", meta-data separated from the normal data. These headers
       are by default not included in the normal data stream, but you can make them appear in the
       data stream by setting CURLOPT_HEADER to 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_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

       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 http://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 HTTP authentication  methods  or  other  protocols  that
              don't let snoopers see your password: HTTP with Digest, NTLM or GSS authentication,
              HTTPS, FTPS, SCP, SFTP and FTP-Kerberos are a few examples.

       Redirects
              The CURLOPT_FOLLOWLOCATION option automatically follows HTTP redirects  sent  by  a
              remote  server.   These  redirects  can refer to any kind of URL, not just HTTP.  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 and CURLOPT_NETRC  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  and  handling
              redirects  itself,  sanitizing  URLs  as necessary. Alternately, an app could leave
              CURLOPT_FOLLOWLOCATION  enabled  but  set  CURLOPT_REDIR_PROTOCOLS  and  install  a
              CURLOPT_OPENSOCKETFUNCTION  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  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 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
              option or CURLOPT_FTPPORT.

       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 addressses
              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 should not be used on  uploads.   Instead,  the  app  should
              handle redirects itself, sanitizing each URL first.

       Authentication
              Use  of CURLOPT_UNRESTRICTED_AUTH could cause authentication information to be sent
              to an  unknown  second  server.   Apps  can  mitigate  against  this  by  disabling
              CURLOPT_FOLLOWLOCATION and handling redirects itself, sanitizing where necessary.

              Use  of  the  CURLAUTH_ANY option to CURLOPT_HTTPAUTH 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 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  and/or
              CURLOPT_LOW_SPEED_LIMIT 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  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 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 option.

       Arbitrary Headers
              User-supplied data must be sanitized when used in options  like  CURLOPT_USERAGENT,
              CURLOPT_HTTPHEADER,  CURLOPT_POSTFIELDS  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 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.

Multiple Transfers Using the multi Interface

       The easy interface as described in detail in this document is a synchronous interface that
       transfers one file at a time and doesn't return until 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 can allow 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 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 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 calling 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.

       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), 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

       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.