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libcurl-tutorial(3)           libcurl programming          libcurl-tutorial(3)




NAME

       libcurl-tutorial - libcurl programming tutorial


Objective

       This  document  attempts  to  describe  the general principles and some
       basic approaches to consider when programming with  libcurl.  The  text
       will  focus  mainly  on  the C interface but might apply fairly well on
       other interfaces as well as  they  usually  follow  the  C  one  pretty
       closely.

       This document will refer to 'the user' as the person writing the source
       code that uses libcurl. That would probably be you or someone  in  your
       position.   What will be generally referred to as 'the program' will be
       the collected source code that you write  that  is  using  libcurl  for
       transfers. The program is outside libcurl and libcurl is outside of the
       program.

       To get more details on all  options  and  functions  described  herein,
       please refer to their respective man pages.



Building

       There  are  many  different ways to build C programs. This chapter will
       assume a Unix style build process. If you use a different build system,
       you  can  still  read this to get general information that may apply to
       your environment as well.

       Compiling the Program
              Your compiler needs  to  know  where  the  libcurl  headers  are
              located.  Therefore you must set your compiler's include path to
              point to the directory where you installed them. The  'curl-con-
              fig'[3] tool can be used to get this information:

              $ curl-config --cflags


       Linking the Program with libcurl
              When  having  compiled the program, you need to link your object
              files to create a single executable. For that  to  succeed,  you
              need to link with libcurl and possibly also with other libraries
              that libcurl itself depends on. Like the OpenSSL libraries,  but
              even  some  standard  OS  libraries may be needed on the command
              line. To figure out which flags to use, once  again  the  'curl-
              config' tool comes to the rescue:

              $ curl-config --libs


       SSL or Not
              libcurl  can  be  built  and customized in many ways. One of the
              things that varies from different libraries and  builds  is  the
              support  for SSL-based transfers, like HTTPS and FTPS. If a sup-
              ported 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-con-
              fig' 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 envi-
       ronments.

       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 prop-
                     erly, your  program  cannot  use  sockets  properly.  You
                     should only do this once for each application, so if your
                     program already does this or of another  library  in  use
                     does  it, you should not tell libcurl to do this as well.

              CURL_GLOBAL_SSL
                     which only does anything on libcurls compiled  and  built
                     SSL-enabled.  On  these  systems,  this will make libcurl
                     initialize 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  han-
       dle.  You  need  one  handle for each easy session you want to perform.
       Basically, you should use one handle for every thread you plan  to  use
       for  transferring.  You  must  never  share the same handle in multiple
       threads.

       Get an easy handle with

        easyhandle = curl_easy_init();

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

       You   set   properties   and   options   for    this    handle    using
       curl_easy_setopt(3). They control how the subsequent transfer or trans-
       fers will be made. Options remain set in the handle until set again  to
       something  different. 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 sim-
       ilar to:

        curl_easy_setopt(handle, CURLOPT_URL, "http://domain.com/");

       Let's assume for a while that you want to receive data as the URL iden-
       tifies  a  remote resource you want to get here. Since you write a sort
       of application that needs this transfer, I assume that you  would  like
       to  get  the  data  passed to you directly instead of simply getting it
       passed to stdout. So, you write your own  function  that  matches  this
       prototype:

        size_t  write_data(void  *buffer,  size_t  size,  size_t  nmemb,  void
       *userp);

       You tell libcurl to pass all data to this function by issuing  a  func-
       tion similar to this:

        curl_easy_setopt(easyhandle, CURLOPT_WRITEFUNCTION, write_data);

       You  can  control  what  data your 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 appli-
       cation  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 pass-
       ing a 'FILE *' to a file opened for  writing  with  the  CURLOPT_WRITE-
       DATA(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  plat-
       forms[2],  libcurl won't be able to operate on files opened by the pro-
       gram. Thus, if you use the default callback and pass in  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_WRITE-
       FUNCTION(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 pro-
       gram.

       There's one golden rule when these things occur: set  the  CURLOPT_VER-
       BOSE(3)  option  to  1.  It'll cause the library to spew out the entire
       protocol details it sends, some internal info and some received  proto-
       col  data  as  well  (especially when using FTP). If you're using HTTP,
       adding the headers in the received output to study is also a clever way
       to  get  a better understanding why the server behaves the way it does.
       Include headers in the normal body output with CURLOPT_HEADER(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 appropri-
       ate RFC documents at least briefly.



Upload Data to a Remote Site

       libcurl tries to keep a protocol independent approach  to  most  trans-
       fers,  thus uploading to a remote FTP site is very similar to uploading
       data to a HTTP server with a PUT request.

       Of course, first you either create an easy handle  or  you  re-use  one
       existing one. Then you set the URL to operate on just like before. This
       is the remote URL, that we now will upload.

       Since we write an application, we most likely want libcurl to  get  the
       upload  data  by  asking us for it. To make it do that, we set the read
       callback and the custom pointer libcurl will pass to our read callback.
       The read callback should have a prototype similar to:

        size_t   function(char  *bufptr,  size_t  size,  size_t  nitems,  void
       *userp);

       Where bufptr is the pointer to a buffer we fill in with data to  upload
       and  size*nitems is the size of the buffer and therefore also the maxi-
       mum amount of data we can return to libcurl in this call.  The  'userp'
       pointer  is  the  custom pointer we set to point to a struct of ours to
       pass private data between the application and the callback.

        curl_easy_setopt(easyhandle, CURLOPT_READFUNCTION, read_function);

        curl_easy_setopt(easyhandle, CURLOPT_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 num-
       ber of bytes it wrote in the buffer. Returning 0 will signal the end of
       the upload.



Passwords

       Many protocols use or even require that user name and password are pro-
       vided to be able to download or upload the data of your choice. libcurl
       offers several ways to specify them.

       Most  protocols  support  that you specify the name and password in the
       URL itself. libcurl will detect this and use them accordingly. This  is
       written like this:

        protocol://user:password@example.com/path/

       If  you  need any odd letters in your user name or password, you should
       enter them URL encoded, as %XX where XX is a two-digit hexadecimal num-
       ber.

       libcurl  also  provides options to set various passwords. The user name
       and password as shown embedded in the URL can instead get set with  the
       CURLOPT_USERPWD(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:these-
       cret");

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

       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 get-
       ting URLs that require authentication. When using  the  HTTP  protocol,
       there are many different ways a client can provide those credentials to
       the server and you can control 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 authenti-
       cation 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 dif-
       ferent versions of HTTP POST that libcurl supports.

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

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

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

       Simple enough, huh? Since you  set  the  POST  options  with  the  CUR-
       LOPT_POSTFIELDS(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. Set-
       ting headers in libcurl requests are done in a generic way, by building
       a list of our own headers and then passing that list to libcurl.

        struct curl_slist *headers=NULL;
        headers = curl_slist_append(headers, "Content-Type: text/xml");

        /* post binary data */
        curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDS, binaryptr);

        /* set the size of the postfields data */
        curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDSIZE, 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 form-
       post 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 head-
       ers set that describe the individual content-type, size etc. To  enable
       your  application to handicraft this formpost even more, libcurl allows
       you to supply your own set of custom headers to such an individual form
       part.  You  can  of course supply headers to as many parts 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!



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  CUR-
       LOPT_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 CUR-
       LOPT_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:pass-
              word");

              If  you  want to, you can specify the host name only in the CUR-
       LOPT_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,      CURL-
              PROXY_SOCKS4);


       Environment Variables

              libcurl automatically checks and uses a set of environment vari-
              ables to know what proxies to use  for  certain  protocols.  The
              names  of  the variables are following an ancient de facto stan-
              dard and are built up as "[protocol]_proxy" (note the lower cas-
              ing).  Which  makes the variable 'http_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 proto-
              col:// part is simply ignored if present  (so  http://proxy  and
              bluerk://proxy  will  do  the same) and the optional port number
              specifies on which port the proxy operates on the host.  If  not
              specified,  the  internal  default  port number will be used and
              that is most likely *not* the one you would like it to be.

              There are two special environment variables. 'all_proxy' is what
              sets  proxy  for  any URL in case the protocol specific variable
              wasn't set, and 'no_proxy' defines a list of hosts  that  should
              not use a proxy even though a variable may say so. If 'no_proxy'
              is a plain asterisk ("*") it matches all hosts.

              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 sim-
              ilar.



Persistence Is The Way to Happiness

       Re-cycling  the  same  easy  handle  several  times when doing multiple
       requests is the way to go.

       After each single curl_easy_perform(3) operation, libcurl will keep the
       connection  alive  and  open.  A subsequent request using the same easy
       handle to the same host might just be able to use the already open con-
       nection! This reduces network impact a lot.

       Even if the connection is dropped, all connections involving SSL to the
       same host again, will benefit from  libcurl's  session  ID  cache  that
       drastically reduces re-connection time.

       FTP connections that are kept alive 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  previ-
       ously looked up name a lot faster.

       Other  interesting  details  that  improve  performance  for subsequent
       requests may also be added in the future.

       Each easy handle will attempt to keep the last  few  connections  alive
       for  a while in case they are to be used again. You can set the size of
       this "cache" with the  CURLOPT_MAXCONNECTS(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  connec-
       tion  (it will even close one first if there happens to be one alive to
       the same host you're about to operate on), you can do that  by  setting
       CURLOPT_FRESH_CONNECT(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_HTTP-
       HEADER(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-con-
              tinue"  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,  "MYOWNRE-
              QUEST");

              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-Encod-
              ing: chunked" when doing a non-GET HTTP operation, libcurl  will
              switch  over  to  "chunked"  upload, even though the size of the
              data to upload might  be  known.  By  default,  libcurl  usually
              switches over to chunked upload automatically if the upload data
              size is unknown.


       HTTP Version

              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  opera-
              tions) to happen _after_ the data transfer took place the option
              to   curl_easy_setopt(3)   would   instead   be   called    CUR-
       LOPT_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  "ses-
       sions".

       Cookies are sent from server to clients with the header Set-Cookie: and
       they're sent from clients to servers with the Cookie: header.

       To just send whatever cookie you want to a server,  you  can  use  CUR-
       LOPT_COOKIE(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  net-
       work 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.



Headers Equal Fun

       Some  protocols  provide "headers", meta-data separated from the normal
       data. These headers are by default not  included  in  the  normal  data
       stream, but you can make them appear in the data stream by setting CUR-
       LOPT_HEADER(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  comprehen-
       sive, but shows classes of attacks that robust applications should con-
       sider.    The    Common     Weakness     Enumeration     project     at
       https://cwe.mitre.org/  is a good reference for many of these and simi-
       lar 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 pro-
              cesses.

              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,  TEL-
              NET  etc).  It is very easy for anyone on your network or a net-
              work nearby yours to just fire up a network  analyzer  tool  and
              eavesdrop  on your passwords. Don't let the fact that HTTP Basic
              uses base64 encoded passwords fool you. They may not look  read-
              able  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 appli-
              cation 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://local-
              host/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 con-
              figured 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 fire-
              wall    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_FOLLOWLOCA-
       TION(3) and handling redirects itself, sanitizing URLs as neces-
              sary.  Alternately, an app could leave CURLOPT_FOLLOWLOCATION(3)
              enabled but set CURLOPT_REDIR_PROTOCOLS(3) and  install  a  CUR-
       LOPT_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.exam-
              ple.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 mit-
              igate 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  spec-
              ify 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 redirec-
              tions.

              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 addressses like 127.0.0.1 isn't suffi-
              cient--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 unde-
              sired 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_IPRE-
              SOLVE_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  redi-
              rects 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 authentica-
              tion  is  already  stored  in  a  cookie. e.g. http://mail.exam-
              ple.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 CUR-
       LOPT_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 moni-
              tor 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_POST-
       FIELDS(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  sup-
              plying  one  like "/etc/passwd", "\autoexec.bat", "prn:" or even
              ".bashrc".


       Server Certificates
              A secure application  should  never  use  the  CURLOPT_SSL_VERI-
       FYPEER(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  cer-
              tificates 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, every-
              thing you reveal in order to get best possible help  might  also
              impose  certain  security related risks. Host names, user names,
              paths, operating system specifics, etc. (not  to  mention  pass-
              words  of course) may in fact be used by intruders to gain addi-
              tional 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 candi-
              dates 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 syn-
       chronous 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  syn-
       chronizing 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 descrip-
       tion 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 trans-
       fers are completed.

       The best usage of this interface is when you do a select() on all  pos-
       sible  file  descriptors or sockets to know when to call libcurl again.
       This also makes it easy for you to wait and respond to actions on  your
       own  application's sockets/handles. You figure out what to select() for
       by using curl_multi_fdset(3), that fills in a set of  fd_set  variables
       for  you  with  the  particular  file  descriptors libcurl uses for the
       moment.

       When you then call select(), it'll return when one of the file  handles
       signal  action and you then call curl_multi_perform(3) to allow libcurl
       to do what it wants to do. Take note that  libcurl  does  also  feature
       some  time-out code so we advise you to never use very long timeouts on
       select() before you call curl_multi_perform(3) again.  curl_multi_time-
       out(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 sim-
              ilar  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)



libcurl                           19 Sep 2014              libcurl-tutorial(3)

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