Updated: March 13, 2001 (http://curl.haxx.se/docs/faq.shtml)
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FAQ

 1. Philosophy
  1.1 What is cURL?
  1.2 What is libcurl?
  1.3 What is cURL not?
  1.4 When will you make curl do XXXX ?

 2. Install Related Problems
  2.1 configure doesn't find OpenSSL even when it is installed
   2.1.1. native linker doesn't find openssl
   2.1.2. only the libssl lib is missing
  2.2 Does curl work/build with other SSL libraries?
  2.3 Where can I find a copy of LIBEAY32.DLL?
  2.4 Does cURL support Socks (RFC 1928) ?

 3. Usage Problems
  3.1 curl: (1) SSL is disabled, https: not supported
  3.2 How do I tell curl to resume a transfer?
  3.3 Why doesn't my posting using -F work?
  3.4 How do I tell curl to run custom FTP commands?
  3.5 How can I disable the Pragma: nocache header?
  3.6 Does curl support javascript, ASP, XML, XHTML or HTML version Y?
  3.7 Can I use curl to delete/rename a file through FTP?
  3.8 How do I tell curl to follow HTTP redirects?
  3.9 How do I use curl in PHP?
  3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?

 4. Running Problems
  4.1 Problems connecting to SSL servers.
  4.2 Why do I get problems when I use & or % in the URL?
  4.3 How can I use {, }, [ or ] to specify multiple URLs?
  4.4 Why do I get downloaded data even though the web page doesn't exist?
  4.5 Why do I get return code XXX from a HTTP server?
   4.5.1 "400 Bad Request"
   4.5.2 "401 Unauthorized"
   4.5.3 "403 Forbidden"
   4.5.4 "404 Not Found"
   4.5.5 "405 Method Not Allowed"
  4.6 Can you tell me what error code 142 means?
  4.7 How do I keep usernames and passwords secret in Curl command lines?
  4.8 I found a bug!
  4.9 Curl can't authenticate to the server that requires NTLM?

 5. libcurl Issues
  5.1 Is libcurl thread-safe?
  5.2 How can I receive all data into a large memory chunk?
  5.3 How do I fetch multiple files with libcurl?
  5.4 Does libcurl do Winsock initing on win32 systems?
  5.5 Does CURLOPT_FILE and CURLOPT_INFILE work on win32 ?
  5.6 What about Keep-Alive or persistant connections?

 6. License Issues
  6.1 I have a GPL program, can I use the libcurl library?
  6.2 I have a closed-source program, can I use the libcurl library?
  6.3 I have a BSD licensed program, can I use the libcurl library?
  6.4 I have a program that uses LGPL libraries, can I use libcurl?
  6.5 Can I modify curl/libcurl for my program and keep the changes secret?
  6.6 Can you please change the curl/libcurl license to XXXX?

==============================================================================

1. Philosophy

  1.1 What is cURL?

  cURL (or simply just 'curl') is a command line tool for getting or sending
  files using URL syntax. The name is a play on 'Client for URLs', originally
  with URL spelled in uppercase to make it obvious it deals with URLs. The
  fact it can also be pronounced 'see URL' also helped.

  Curl supports a range of common internet protocols, currently including
  HTTP, HTTPS, FTP, GOPHER, LDAP, DICT, TELNET and FILE.

  We spell it cURL or just curl.

  1.2 What is libcurl?

  libcurl is a reliable, higly portable multiprotocol file transfer library.

  Any application is free to use libcurl, even commercial or closed-source
  ones.

  1.3 What is cURL not?
  
  Curl is *not*, I repeat, *not* a wget clone even though that is a very
  common misconception. Never, during curl's development, have I intended curl
  to replace wget or compete on its market. Curl is targeted at single-shot
  file transfers.

  Curl is not a web site mirroring program. If you wanna use curl to mirror
  something: fine, go ahead and write a script that wraps around curl to make
  it reality (like curlmirror.pl does).

  Curl is not an ftp site mirroring program. Sure, get and send ftp with curl
  but if you want systematic and sequential behaviour you should write a
  script (or write a new program that interfaces libcurl) and do it.

  Curl is not a PHP tool, even though it works perfectly well when used from
  or with PHP.

  Curl is not a single-OS program. Curl exists, compiles, builds and runs
  under a wide range of operating systems, including all modern Unixes (and a
  bunch of older ones too), Windows, Amiga, BeOS, OS/2, OS X, QNX etc.

  1.4 When will you make curl do XXXX ?

  We love suggestions of what to change in order to make curl and libcurl
  better. We do however believe in a few rules when it comes to the future of
  curl:

  * Curl is to remain a command line tool. If you want GUIs or fancy scripting
    capabilities, you're free to write another tool that uses libcurl and that
    offers this. There's no point in having a single tool that does every
    imaginable thing. That's also one of the great advantages of having the
    core of curl as a library.

  * We do not add things to curl that other small and available tools already
    do very fine at the side. Curl's output is fine to pipe into another
    program or redirect to another file for the next program to interpret.

  * We focus on protocol related issues and improvements. If you wanna do more
    magic with the supported protocols than curl currently does, chances are
    big I will agree. If you wanna add more protocols, I may very well
    agree.

  * If you want someone else to make all the work while you wait for us to
    implement it for you, that is not a very friendly attitude. We spend a
    considerable time already on maintaining and developing curl. In order to
    get more out of us, you should consider trading in some of your time and
    efforts in return.

  * If you write the code, chances are bigger that it will get into curl
    faster.

2. Install Related Problems

  2.1. configure doesn't find OpenSSL even when it is installed

  This may be because of several reasons.

    2.1.1. native linker doesn't find openssl

    Affected platforms:
      Solaris (native cc compiler)
      HPUX (native cc compiler)
      SGI IRIX (native cc compiler)
      SCO UNIX (native cc compiler)

    When configuring curl, I specify --with-ssl. OpenSSL is installed in
    /usr/local/ssl Configure reports SSL in /usr/local/ssl, but fails to find
    CRYPTO_lock in -lcrypto

    Cause: The cc for this test places the -L/usr/local/ssl/lib AFTER
    -lcrypto, so ld can't find the library. This is due to a bug in the GNU
    autoconf tool.

    Workaround: Specifying "LDFLAGS=-L/usr/local/ssl/lib" in front of
    ./configure places the -L/usr/local/ssl/lib early enough in the command
    line to make things work

    Solution submitted by: Bob Allison <allisonb@users.sourceforge.net>

    2.1.2. only the libssl lib is missing

    If all include files and the libcrypto lib is present, with only the
    libssl being missing according to configure, this is mostly likely because
    a few functions are left out from the libssl.

    If the function names missing include RSA or RSAREF you can be certain
    that this is because libssl requires the RSA and RSASEF libs to build.

    See the INSTALL file section that explains how to add those libs to
    configure. Make sure that you remove the config.cache file before you
    rerun configure with the new flags.

  2.2. Does curl work/build with other SSL libraries?

  Curl has been written to use OpenSSL, although there should not be much
  problems using a different library. If anyone does "port" curl to use a
  different SSL library, we are of course very interested in getting the
  patch!

  2.3. Where can I find a copy of LIBEAY32.DLL?

  That is an OpenSSL binary built for Windows.

  Curl uses OpenSSL to do the SSL stuff. The LIBEAY32.DLL is what curl needs
  on a windows machine to do https://. Check out the curl web site to find
  accurate and up-to-date pointers to recent OpenSSL DDLs and other binary
  packages.

  2.4. Does cURL support Socks (RFC 1928) ?

  No. Nobody has wanted it that badly yet. We appriciate patches that bring
  this functionality.


3. Usage problems

  3.1. curl: (1) SSL is disabled, https: not supported

  If you get this output when trying to get anything from a https:// server,
  it means that the configure script couldn't find all libs and include files
  it requires for SSL to work. If the configure script fails to find them,
  curl is simply built without SSL support.

  To get the https:// support into a curl that was previously built but that
  reports that https:// is not supported, you should dig through the document
  and logs and check out why the configure script doesn't find the SSL libs
  and/or include files.

  Also, check out the other paragraph in this FAQ labeled "configure doesn't
  find OpenSSL even when it is installed".

  3.2. How do I tell curl to resume a transfer?

  Curl supports resumed transfers both ways on both FTP and HTTP.

  Try the -C option.

  3.3. Why doesn't my posting using -F work?

  You can't simply use -F or -d at your choice. The web server that will
  receive your post assumes one of the formats. If the form you're trying to
  "fake" sets the type to 'multipart/form-data', then and only then you must
  use the -F type. In all the most common cases, you should use -d which then
  causes a posting with the type 'application/x-www-form-urlencoded'.

  This is described in some detail in the README.curl file, and if you don't
  understand it the first time, read it again before you post questions about
  this to the mailing list. Also, try reading through the mailing list
  archives for old postings and questions regarding this.

  3.4. How do I tell curl to run custom FTP commands?

  You can tell curl to perform optional commands both before and/or after a
  file transfer. Study the -Q/--quote option.

  Since curl is used for file transfers, you don't use curl to just perform
  ftp commands without transfering anything. Therefore you must always specify
  a URL to transfer to/from even when doing custom FTP commands.

  3.5. How can I disable the Pragma: nocache header?

  You can change all internally generated headers by adding a replacement with
  the -H/--header option. By adding a header with empty contents you safely
  disable that one. Use -H "Pragma:" to disable that specific header.

  3.6. Does curl support javascript, ASP, XML, XHTML or HTML version Y?

  To curl, all contents are alike. It doesn't matter how the page was
  generated. It may be ASP, PHP, perl, shell-script, SSI or plain
  HTML-files. There's no difference to curl and it doesn't even know what kind
  of language that generated the page.

  Javascript is slightly different since that is code embedded in the HTML
  that is sent for the client to interpret and curl has no javascript
  interpreter.

  3.7. Can I use curl to delete/rename a file through FTP?

  Yes. You specify custom ftp commands with -Q/--quote.

  One example would be to delete a file after you have downloaded it:

     curl -O ftp://download.com/coolfile -Q '-DELE coolfile'

  3.8 How do I tell curl to follow HTTP redirects?

  Curl does not follow so-called redirects by default. The Location: header
  that informs the client about this is only interpreted if you're using the
  -L/--location option. As in:

     curl -L http://redirector.com

  3.9 How do I use curl in PHP?

  PHP4 has the ability to use libcurl as an internal module if built with that
  option enabled. You then get a set of extra functions that can be used
  within your PHP programs. You find all details about those functions in the
  curl section in the PHP manual, see the online version at:

        http://www.php.net/manual/ref.curl.php

  PHP also offers the option to run a command line, and then you can of course
  invoke the curl tool using a command line. This is the way to use curl if
  you're using PHP3 or PHP4 built without curl module support.

  3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?

  Curl adheres to the HTTP spec, which basically means you can play with *any*
  protocol that is built ontop of HTTP. Protocols such as SOAP, WEBDAV and
  XML-RPC are all such ones. You can use -X to set custom requests and -H to
  set custom headers (or replace internally generated ones).

  Using libcurl or PHP's curl modules is just as fine and you'd just use the
  proper library options to do the same.

4. Running Problems

  4.1. Problems connecting to SSL servers.

  It took a very long time before we could sort out why curl had problems to
  connect to certain SSL servers when using SSLeay or OpenSSL v0.9+.  The
  error sometimes showed up similar to:

  16570:error:1407D071:SSL routines:SSL2_READ:bad mac decode:s2_pkt.c:233:

  It turned out to be because many older SSL servers don't deal with SSLv3
  requests properly. To correct this problem, tell curl to select SSLv2 from
  the command line (-2/--sslv2).

  There has also been examples where the remote server didn't like the SSLv2
  request and instead you had to force curl to use SSLv3 with -3/--sslv3.

  4.2. Why do I get problems when I use & or % in the URL?

  In general unix shells, the & letter is treated special and when used, it
  runs the specified command in the background. To safely send the & as a part
  of a URL, you should qoute the entire URL by using single (') or double (")
  quotes around it.

  An example that would invoke a remote CGI that uses &-letters could be:

     curl 'http://www.altavista.com/cgi-bin/query?text=yes&q=curl'

  In win32, the standard DOS shell treats the %-letter specially and you may
  need to quote the string properly when % is used in it.

  Also note that if you want the literal %-letter to be part of the data you
  pass in a POST using -d/--data you must encode it as '%25'.

  4.3. How can I use {, }, [ or ] to specify multiple URLs?

  Because those letters have a special meaning to the shell, and to be used in
  a URL specified to curl you must quote them.

  An example that downloads two URLs (sequentially) would do:

    curl '{curl,www}.haxx.se'

  To be able to use those letters as actual parts of the URL (without using
  them for the curl URL "globbing" system), use the -g/--globoff option (curl
  7.6 and later):

    curl -g 'www.site.com/weirdname[].html'

  4.4. Why do I get downloaded data even though the web page doesn't exist?

  Curl asks remote servers for the page you specify. If the page doesn't exist
  at the server, the HTTP protocol defines how the server should respond and
  that means that headers and a "page" will be returned. That's simply how
  HTTP works.

  By using the --fail option you can tell curl explicitly to not get any data
  if the HTTP return code doesn't say success.

  4.5 Why do I get return code XXX from a HTTP server?

  RFC2616 clearly explains the return codes. This is a short transcript. Go
  read the RFC for exact details:

    4.5.1 "400 Bad Request"

    The request could not be understood by the server due to malformed
    syntax. The client SHOULD NOT repeat the request without modifications.

    4.5.2 "401 Unauthorized"

    The request requires user authentication.

    4.5.3 "403 Forbidden"

    The server understood the request, but is refusing to fulfill it.
    Authorization will not help and the request SHOULD NOT be repeated.

    4.5.4 "404 Not Found"

    The server has not found anything matching the Request-URI. No indication
    is given of whether the condition is temporary or permanent.

    4.5.5 "405 Method Not Allowed"

    The method specified in the Request-Line is not allowed for the resource
    identified by the Request-URI. The response MUST include an Allow header
    containing a list of valid methods for the requested resource.

  4.6. Can you tell me what error code 142 means?

  All error codes that are larger than the highest documented error code means
  that curl has existed due to a timeout. There was no nice way for curl to
  abort from such a condition and that's why it got this undocumented
  error. This should not occur in releases after 7.4.1.

  4.7. How do I keep usernames and passwords secret in Curl command lines?

  This problem has two sides:

  The first part is to avoid having clear-text passwords in the command line
  so that they don't appear in 'ps' outputs and similar. That is easily
  avoided by using the "-K" option tho tell curl to read parameters from a
  file or stdin to which you can pass the secret info.

  To keep the passwords in your account secret from the rest of the world is
  not a task that curl addresses. You could of course encrypt them somehow to
  at least hide them from being read by human eyes, but that is not what
  anyone would call security.

  Also note that regular HTTP and FTP passwords are sent in clear across the
  network. All it takes for anyone to fetch them is to listen on the network.
  Evesdropping is very easy.

  4.8 I found a bug!

  It is not a bug if the behaviour is documented. Read the docs first.

  If it is a problem with a binary you've downloaded or a package for your
  particular platform, try contacting the person who built the package/archive
  you have.

  If there is a bug, post a bug report in the Curl Bug Track System over at
  http://sourceforge.net/bugs/?group_id=976

  Always include as many details you can think of, including curl version,
  operating system name and version and complete instructions how to repeat
  the bug.

  4.9. Curl can't authenticate to the server that requires NTLM?

  NTLM is a Microsoft proprietary protocol. Unfortunately, curl does not
  currently support that. Proprietary formats are evil. You should not use
  such ones.

5. libcurl Issues

  5.1. Is libcurl thread-safe?

  Yes.

  We have written the libcurl code specificly adjusted for multi-threaded
  programs. libcurl will use thread-safe functions instead of non-safe ones if
  your system has such.

  We would appriciate some kind of report or README file from those who have
  used libcurl in a threaded environment.

  5.2 How can I receive all data into a large memory chunk?

  You are in full control of the callback function that gets called every time
  there is data received from the remote server. You can make that callback do
  whatever you want. You do not have to write the receivied data to a file.

  One solution to this problem could be to have a pointer to a struct that you
  pass to the callback function. You set the pointer using the
  curl_easy_setopt(CURLOPT_FILE) function. Then that pointer will be passed to
  the callback instead of a FILE * to a file:

        /* imaginary struct */
        struct MemoryStruct {
          char *memory;
          size_t size;
        };

        /* imaginary callback function */
        size_t
        WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data)
        {
          register int realsize = size * nmemb;
          struct MemoryStruct *mem = (struct MemoryStruct *)data;
        
          mem->memory = (char *)realloc(mem->memory, mem->size + realsize + 1);
          if (mem->memory) {
            memcpy(&(mem->memory[mem->size]), ptr, realsize);
            mem->size += realsize;
            mem->memory[mem->size] = 0;
          }
          return realsize;
        }

  5.3 How do I fetch multiple files with libcurl?

  Starting with version 7.7, curl and libcurl will have excellent support for
  transferring multiple files. You should just repeatedly set new URLs with
  curl_easy_setopt() and then transfer it with curl_easy_perform(). The handle
  you get from curl_easy_init() is not only reusable starting with libcurl
  7.7, but also you're encouraged to reuse it if you can, as that will enable
  libcurl to use persistant connections.

  For libcurl prior to 7.7, there was no multiple file support. The only
  available way to do multiple requests was to init/perform/cleanup for each
  transfer.

  5.4 Does libcurl do Winsock initing on win32 systems?

  No.

  On win32 systems, you need to init the winsock stuff manually, libcurl will
  not do that for you. WSAStartup() and WSACleanup() should be used
  accordingly. The reason for this is of course that a single application may
  use several different libraries and parts, and there's no reason for every
  single library to do this.

  5.5 Does CURLOPT_FILE and CURLOPT_INFILE work on win32 ?

  Yes, but you cannot open a FILE * and pass the pointer to a DLL and have
  that DLL use the FILE *. If you set CURLOPT_FILE you must also use
  CURLOPT_WRITEFUNCTION as well to set a function that writes the file, even
  if that simply writes the data to the specified FILE*. Similarly, if you use
  CURLOPT_INFILE you must also specify CURLOPT_READFUNCTION.

  (Provided by Joel DeYoung and Bob Schader)

  5.6 What about Keep-Alive or persistant connections?

  Starting with version 7.7, curl and libcurl will have excellent support for
  persistant connections when transferring several files from the same server.
  Curl will attempt to reuse connections for all URLs specified on the same
  command line/config file, and libcurl will reuse connections for all
  transfers that are made using the same libcurl handle.

  Previous versions had no persistant connection support.

6. License Issues

  Curl and libcurl are released under a MIT/X derivate license *or* the MPL,
  the Mozilla Public License. To get a really good answer to your license
  conflict questions, you should study the MPL and MIT/X licenses and the
  license you are about to use and check for clashes yourself. This section is
  just a brief summary for the cases we get the most questions. (Parts of this
  section was much enhanced by Bjorn Reese.)

  6.1. I have a GPL program, can I use the libcurl library?

  Yes!

  Since libcurl may be distributed under the MIT/X derivate license, it can be
  used together with GPL in any software.

  6.2. I have a closed-source program, can I use the libcurl library?

  Yes!

  libcurl does not put any restrictions on the program that uses the library.

  6.3. I have a BSD licensed program, can I use the libcurl library?

  Yes!

  libcurl does not put any restrictions on the program that uses the library.

  6.4. I have a program that uses LGPL libraries, can I use libcurl?

  Yes!

  The LGPL license doesn't clash with other licenses.

  6.5. Can I modify curl/libcurl for my program and keep the changes secret?

  Yes!

  The MIT/X derivate license practically allows you to do almost anything with
  the sources, on the condition that the copyright texts in the sources are
  left intact.

  6.6. Can you please change the curl/libcurl license to XXXX?

  No.

  We have carefully picked this license after years of development and
  discussions and a large amount of people have contributed with source code
  knowing that this is the license we use. This license puts the restrictions
  we want on curl/libcurl and it does not spread to other programs or
  libraries that use it. The recent dual license modification should make it
  possible for everyone to use libcurl or curl in their projects, no matter
  what license they already have in use.