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

       libcurl-tutorial - libcurl programming tutorial

       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

       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

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

       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

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‐

       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


       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:

		     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.

		     which only does anything on libcurls compiled  and	 built
		     SSL-enabled.  On  these  systems,	this will make libcurl
		     initialize the SSL library properly for this application.
		     This  only	 needs to be done once for each application so
		     if your program or another	 library  already  does	 this,
		     this bit should not be needed.

       libcurl	 has   a   default   protection	  mechanism  that  detects  if
       curl_global_init(3) hasn't been called by the time curl_easy_perform(3)
       is  called  and	if  that is the case, libcurl runs the function itself
       with a guessed bit pattern. Please note that depending solely  on  this
       is not considered nice nor very good.

       When   the   program   no   longer   uses   libcurl,   it  should  call
       curl_global_cleanup(3), which is the opposite of the init call. It will
       then   do   the	reversed  operations  to  cleanup  the	resources  the
       curl_global_init(3) call initialized.

       Repeated calls to curl_global_init(3) and curl_global_cleanup(3) should
       be avoided. They should only be called once each.

Features libcurl Provides
       It  is  considered  best-practice to determine libcurl features at run-
       time rather than at build-time (if  possible  of	 course).  By  calling
       curl_version_info(3)  and  checking  out	 the  details  of the returned
       struct, your program can figure out exactly what the currently  running
       libcurl supports.

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

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

       To  use the easy interface, you must first create yourself an easy 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

       Get an easy handle with

	easyhandle = curl_easy_init();

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

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

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

	curl_easy_setopt(handle, CURLOPT_URL, "");

       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

	size_t	write_data(void	 *buffer,  size_t  size,  size_t  nmemb,  void

       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.

       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

       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, it will crash. You should therefore avoid this
       to make your program run fine virtually everywhere.

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

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

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

	success = curl_easy_perform(easyhandle);

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

       When the transfer is complete, the function returns a return code  that
       informs	you  if	 it  succeeded in its mission or not. If a return code
       isn't enough for you, you can  use  the	CURLOPT_ERRORBUFFER  to	 point
       libcurl	to  a buffer of yours where it'll store a human readable error
       message as well.

       If you then want to transfer another file, the handle is	 ready	to  be
       used  again. Mind you, it is even preferred that you re-use an existing
       handle if you intend  to	 make  another	transfer.  libcurl  will  then
       attempt to re-use the previous connection.

       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 han‐
       dle 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 prop‐
       erly. For all details, see this:




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


	Required actions unknown.


	Required actions unknown.


	Required actions unknown.

       Secure Transport

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

       When  using multiple threads you should set the CURLOPT_NOSIGNAL option
       to 1 for all handles. Everything will or might work  fine  except  that
       timeouts	 are  not  honored  during the DNS lookup - which you can work
       around by building libcurl with c-ares support.	c-ares	is  a  library
       that  provides  asynchronous  name resolves. On some platforms, libcurl
       simply will not function properly multi-threaded unless this option  is

       Also, note that CURLOPT_DNS_USE_GLOBAL_CACHE is not thread-safe.

When It Doesn't Work
       There will always be times when the transfer fails for some reason. You
       might have set the wrong	 libcurl  option  or  misunderstood  what  the
       libcurl	option	actually  does, or the remote server might return non-
       standard replies that confuse the library which then confuses your pro‐

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

       Of  course,  there are bugs left. We need to know about them to be able
       to fix them, so we're quite dependent on your bug reports! When you  do
       report suspected bugs in libcurl, please include as many details as you
       possibly can: a protocol dump that  CURLOPT_VERBOSE  produces,  library
       version,	 as much as possible of your code that uses libcurl, operating
       system name and version, compiler name and version etc.

       If CURLOPT_VERBOSE is not enough, you increase the level of debug  data
       your application receive by using the CURLOPT_DEBUGFUNCTION.

       Getting	some  in-depth knowledge about the protocols involved is never
       wrong, and if you're trying to do funny things,	you  might  very  well
       understand  libcurl and how to use it better if you study the 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

       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  for	 all  known  file  sizes  like

	/* 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.

       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:


       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‐

       libcurl	also  provides options to set various passwords. The user name
       and password as shown embedded in the URL can instead get set with  the
       CURLOPT_USERPWD option. The argument passed to libcurl should be a char
       * to a string in the format "user:password". In a manner like this:

	curl_easy_setopt(easyhandle, CURLOPT_USERPWD, "myname:thesecret");

       Another case where name and password might be needed at times,  is  for
       those  users  who  need to authenticate themselves to a proxy they use.
       libcurl offers another option for this, the CURLOPT_PROXYUSERPWD. It is
       used quite similar to the CURLOPT_USERPWD option like this:

	curl_easy_setopt(easyhandle,	CURLOPT_PROXYUSERPWD,	"myname:these‐

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

	curl_easy_setopt(easyhandle, CURLOPT_NETRC, 1L);

       And a very basic example of how such a .netrc file may look like:

	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‐

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

	curl_easy_setopt(easyhandle, CURLOPT_HTTPAUTH, CURLAUTH_DIGEST);

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

	curl_easy_setopt(easyhandle, CURLOPT_PROXYAUTH, CURLAUTH_NTLM);

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

	curl_easy_setopt(easyhandle, CURLOPT_HTTPAUTH,

       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.

       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, "");

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

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

       Ok,  so	what if you want to post binary data that also requires you to
       set the Content-Type: header of the post? Well,	binary	posts  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

	struct curl_httppost *post=NULL;
	struct curl_httppost *last=NULL;
	curl_formadd(&post, &last,
		     CURLFORM_COPYNAME, "name",
	curl_formadd(&post, &last,
		     CURLFORM_COPYNAME, "project",
	curl_formadd(&post, &last,
		     CURLFORM_COPYNAME, "logotype-image",

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

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

	/* free the post data again */

       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",

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

	curl_formfree(post); /* free post */
	curl_slist_free_all(headers); /* free custom header list */

       Since all options on an easyhandle are "sticky", they remain  the  same
       until changed even if you do call curl_easy_perform(3), you may need to
       tell curl to go back to a plain GET request if you intend to do one  as
       your  next  request. You force an easyhandle to go back to GET by using
       the CURLOPT_HTTPGET option:

	curl_easy_setopt(easyhandle, CURLOPT_HTTPGET, 1L);

       Just setting CURLOPT_POSTFIELDS to "" or NULL will *not*	 stop  libcurl
       from doing a POST. It will just make it POST without any data to send!

Showing Progress
       For historical and traditional reasons, libcurl has a built-in progress
       meter that can be switched on and then  makes  it  present  a  progress
       meter in your terminal.

       Switch	on   the   progress  meter  by,	 oddly	enough,	 setting  CUR‐
       LOPT_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 CUR‐
       LOPT_PROGRESSDATA. libcurl won't touch it.

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

       The callbacks CANNOT be non-static class member functions

       Example C++ code:

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

       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-");

	      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‐

	      If  you  want to, you can specify the host name only in the CUR‐
	      LOPT_PROXY option, and set the port number separately with  CUR‐

	      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,	 CURL‐

       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

	      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.

       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

	      - 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‐

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 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 to 1. In a similar spirit, you  can  also	forbid
       the  upcoming  request  to  be  "lying" around and possibly get re-used
       after the request by setting CURLOPT_FORBID_REUSE to 1.

HTTP Headers Used by libcurl
       When you use libcurl to do HTTP requests, it'll pass along a series  of
       headers	automatically. It might be good for you to know and understand
       these. You can replace or remove them by using  the  CURLOPT_HTTPHEADER

       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

       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.

	      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

	       curl_easy_setopt(easyhandle,  CURLOPT_CUSTOMREQUEST,  "MYOWNRE‐

	      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:");

       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,

       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 CURLOPT_POSTQUOTE
	      and used the exact same way.

	      The custom FTP command will be issued to the server in the  same
	      order they are added to the list, and if a command gets an error
	      code returned back from the server, no  more  commands  will  be
	      issued   and   libcurl   will   bail  out	 with  an  error  code
	      (CURLE_QUOTE_ERROR). Note that if you use CURLOPT_QUOTE to  send
	      commands before a transfer, no transfer will actually take place
	      when a quote command has failed.

	      If you set the CURLOPT_HEADER to 1, you will tell libcurl to get
	      information about the target file and output "headers" about it.
	      The headers will be in "HTTP-style", looking  like  they	do  in

	      The  option  to enable headers or to run custom FTP commands may
	      be useful to combine with CURLOPT_NOBODY. If this option is set,
	      no actual file content transfer will be performed.

	      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 "ses‐

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

       To  just	 send  whatever	 cookie you want to a server, you can use CUR‐
       LOPT_COOKIE to set a cookie string like this:

	curl_easy_setopt(easyhandle,	   CURLOPT_COOKIE,	  "name1=var1;

       In  many	 cases, that is not enough. You might want to dynamically save
       whatever cookies the remote server passes to you, and make  sure	 those
       cookies are then used accordingly on later requests.

       One  way to do this, is to save all headers you receive in a plain file
       and when you make a request, you tell  libcurl  to  read	 the  previous
       headers to figure out which cookies to use. Set the header file to read
       cookies from with CURLOPT_COOKIEFILE.

       The CURLOPT_COOKIEFILE option also  automatically  enables  the	cookie
       parser in libcurl. Until the cookie parser is enabled, libcurl will not
       parse or understand incoming cookies and they  will  just  be  ignored.
       However,	 when the parser is enabled the cookies will be understood and
       the cookies will be kept in memory  and	used  properly	in  subsequent
       requests	 when  the same handle is used. Many times this is enough, and
       you may not have to save the cookies to disk at all. Note that the file
       you  specify  to CURLOPT_COOKIEFILE doesn't have to exist to enable the
       parser, so a common way to just enable the  parser  and	not  read  any
       cookies is to use the name of a file you know doesn't exist.

       If  you	would  rather  use  existing  cookies  that  you've previously
       received with your Netscape or Mozilla browsers, you can	 make  libcurl
       use  that cookie file as input. The CURLOPT_COOKIEFILE is used for that
       too, as libcurl will automatically find out what kind of file it is and
       act accordingly.

       Perhaps	the  most  advanced cookie operation libcurl offers, is saving
       the entire internal cookie state back into a Netscape/Mozilla formatted
       cookie file. We call that the cookie-jar. When you set a file name with
       CURLOPT_COOKIEJAR, that file name will  be  created  and	 all  received
       cookies	will be stored in it when curl_easy_cleanup(3) is called. This
       enables cookies to get passed  on  properly  between  multiple  handles
       without any information getting lost.

FTP Peculiarities We Need
       FTP  transfers  use  a  second TCP/IP connection for the data transfer.
       This is usually a fact you can forget and ignore but at times this fact
       will  come  back to haunt you. libcurl offers several different ways to
       customize how the second connection is being made.

       libcurl can either connect to the server a  second  time	 or  tell  the
       server to connect back to it. The first option is the default and it is
       also what works best for all the people behind firewalls, NATs  or  IP-
       masquerading  setups.   libcurl	then tells the server to open up a new
       port and wait for a second connection. This  is	by  default  attempted
       with  EPSV first, and if that doesn't work it tries PASV instead. (EPSV
       is an extension to the original FTP spec and does not exist nor work on
       all FTP servers.)

       You  can	 prevent libcurl from first trying the EPSV command by setting
       CURLOPT_FTP_USE_EPSV to zero.

       In some cases, you will prefer to have the server connect back  to  you
       for  the	 second	 connection.  This might be when the server is perhaps
       behind a firewall or something and only allows connections on a	single
       port.  libcurl then informs the remote server which IP address and port
       number to connect to.  This is made with the CURLOPT_FTPPORT option. If
       you set it to "-", libcurl will use your system's "default IP address".
       If you want to use a particular IP, you can set the full IP address,  a
       host name to resolve to an IP address or even a local network interface
       name that libcurl will get the IP address from.

       When doing the "PORT" approach, libcurl will attempt to	use  the  EPRT
       and  the LPRT before trying PORT, as they work with more protocols. You
       can disable this behavior by setting CURLOPT_FTP_USE_EPRT to zero.

Headers Equal Fun
       Some protocols provide "headers", meta-data separated from  the	normal
       data.  These  headers  are  by  default not included in the normal data
       stream, but you can make them appear in the data stream by setting CUR‐
       LOPT_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,

       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
       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 pro‐

	      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 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.

	      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 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  and	 CURLOPT_NETRC	options can be
	      used to mitigate against this kind of attack.

	      A redirect can also specify a location  available	 only  on  the
	      machine running libcurl, including servers hidden behind a fire‐
	      wall   from   the	  attacker.    e.g.    or
	      http://intranet/delete-stuff.cgi?delete=all   or	 tftp://bootp-

	      Apps can mitigate against this by disabling  CURLOPT_FOLLOWLOCA‐
	      TION  and	 handling  redirects itself, sanitizing URLs as neces‐
	      sary. Alternately, an  app  could	 leave	CURLOPT_FOLLOWLOCATION
	      enabled  but  set	 CURLOPT_REDIR_PROTOCOLS  and  install	a CUR‐
	      LOPT_OPENSOCKETFUNCTION callback function in which addresses are
	      sanitized before use.

       Private Resources
	      A	 user  who can control the DNS server of a domain being passed
	      in within a URL can change the address of the host to  a	local,
	      private  address	which  a server-side libcurl-using application
	      could then use. e.g. the innocuous URL http://fuzzybunnies.exam‐	could  actually	 resolve to the IP address of a server
	      behind a firewall, such as or	 Apps can mit‐
	      igate  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 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 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

       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 isn't suffi‐
	      cient--the   equivalent	IPv6  addresses	 ::1,  ::,  0:00::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.

	      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.

	      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

	      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.

	      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‐ Apps	can  mitigate  against
	      this by disabling cookies or clearing them between requests.

       Dangerous URLs
	      SCP  URLs can contain raw commands within the scp: URL, which is
	      a side  effect  of  how  the  SCP	 protocol  is  designed.  e.g.
	      scp://user:pass@host/a;date  >/tmp/test;	Apps  must  not	 allow
	      unsanitized SCP: URLs to be passed in for downloads.

       Denial of Service
	      A malicious server could cause libcurl to	 effectively  hang  by
	      sending  a  trickle  of data through, or even no data at all but
	      just keeping the TCP connection open.  This could	 result	 in  a
	      denial-of-service	  attack.   The	 CURLOPT_TIMEOUT  and/or  CUR‐
	      LOPT_LOW_SPEED_LIMIT options can be  used	 to  mitigate  against

	      A	 malicious  server  could cause libcurl to effectively hang by
	      starting to send data,  then  severing  the  connection  without
	      cleanly  closing	the  TCP  connection.  The app could install a
	      CURLOPT_SOCKOPTFUNCTION  callback	 function  and	set  the   TCP
	      SO_KEEPALIVE  option  to	mitigate against this.	Setting one of
	      the timeout options would also work against this attack.

	      A malicious server could cause libcurl to download  an  infinite
	      amount  of data, potentially causing all of memory or disk to be
	      filled. Setting the CURLOPT_MAXFILESIZE_LARGE option is not suf‐
	      ficient  to guard against this.  Instead, the app should monitor
	      the amount of data received within the write or  progress	 call‐
	      back 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
	      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

       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,	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.

Multiple Transfers Using the multi Interface
       The easy interface as described in detail in this document  is  a  syn‐
       chronous interface that transfers one file at a time and doesn't return
       until it is done.

       The multi interface, on the other hand, allows your program to transfer
       multiple files in both directions at the same time, without forcing you
       to use multiple threads.	 The name might make it seem  that  the	 multi
       interface  is  for multi-threaded programs, but the truth is almost the
       reverse.	 The multi interface can allow a  single-threaded  application
       to  perform  the	 same  kinds  of multiple, simultaneous transfers that
       multi-threaded programs can perform.  It allows many of the benefits of
       multi-threaded  transfers  without  the complexity of managing and syn‐
       chronizing 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) 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  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

       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

       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

       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

       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.

       [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.

libcurl				  4 Mar 2009		   libcurl-tutorial(3)

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