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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  pro-gram' program'
       gram'  will be the collected source code that you write that is using libcurl for transfers. The pro-gram program
       gram 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] 'curl-config'[3]
              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 exe-cutable. executable.
              cutable. 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 supported
              ported  SSL  library  was detected properly at build-time, libcurl will be built with SSL sup-port. support.
              port. 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  prop-erly, properly,
                     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.


Handle the Easy libcurl
       libcurl first introduced the so called easy interface. All operations in the easy interface are  pre-fixed prefixed
       fixed with 'curl_easy'.

       Recent  libcurl  versions  also offer the multi interface. More about that interface, what it is tar-geted targeted
       geted for and how to use it is detailed in a separate chapter further down. You still need to  under-stand understand
       stand 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 subse-quent subsequent
       quent  transfer  or transfers will be made. Options remain set in the handle until set again to some-thing something
       thing 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 prop-erty: property:
       erty:

        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 CUR-LOPT_WRITEDATA CURLOPT_WRITEDATA
       LOPT_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 possi-ble possible
       ble 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. Every-thing Everything
       thing 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 asynchro-nous asynchronous
       nous 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 proto-col protocol
       col  data  as  well  (especially  when  using  FTP).  If you're using HTTP, adding the headers in the
       received output to study is also a clever way to get a better understanding why  the  server  behaves
       the way it does. Include headers in the normal body output with CURLOPT_HEADER set 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 ver-sion version
       sion 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 authen-ticate authenticate
       ticate themselves to a proxy they use. libcurl offers another option for this, the CURLOPT_PROXYUSER-PWD. CURLOPT_PROXYUSERPWD.
       PWD. 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 proto-cols protocols
       cols 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 authenti-cation. authentication.
       cation. When using the HTTP protocol, there are many different ways a client can provide  those  cre-dentials credentials
       dentials  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-Negoti-ate GSS-Negotiate
       ate 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 form-post formpost
       post 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 CUR-LOPT_HTTPGET CURLOPT_HTTPGET
       LOPT_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 actu-ally actually
       ally 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 informa-tion information
              tion 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  stan-dard standard
              dard  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:pass-word@]machine[:port]". "[protocol://][user:password@]machine[:port]".
              word@]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 applica-tion. application.
              tion.

              As tunneling opens a direct connection from your application to the remote  machine,  it  sud-denly suddenly
              denly  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 lan-guage language
              guage 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  CUR-LOPT_HTTPHEADER CURLOPT_HTTPHEADER
       LOPT_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  trans-port. transport.
       port. 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 nonGET
              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   CUR-LOPT_POSTQUOTE CURLOPT_POSTQUOTE
              LOPT_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 CUR-LOPT_NOBODY. CURLOPT_NOBODY.
              LOPT_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  differ-ent different
       ent 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_HEADERFUNC-TION. CURLOPT_HEADERFUNCTION.
       TION.

       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 mit-igations mitigations
       igations 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  any-where. anywhere.
              where.


       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  net-work network
              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
              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,  includ-ing including
              ing   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 the libcurl application will  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.


       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.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 unsan-itized unsanitized
              itized 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 sev-ering severing
              ering the connection without cleanly closing the TCP connection.  The app could install a CUR-LOPT_SOCKOPTFUNCTION CURLOPT_SOCKOPTFUNCTION
              LOPT_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, CUR-LOPT_HTTPHEADER, CURLOPT_HTTPHEADER,
              LOPT_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 addi-tional additional
              tional 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  gen-erate generate
              erate  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",
              "utoexec.bat" 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.

              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 direc-tions directions
       tions  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, simul-taneous simultaneous
       taneous transfers that multi-threaded programs can perform.  It allows many of the benefits of multi-threaded multithreaded
       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 sock-ets sockets
       ets 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 resolvings
       faster and the connection pool that is kept to better allow persistent connections and connection re-use reuse
       use  is  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.



libcurl                                          4 Mar 2009                              libcurl-tutorial(3)

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