Updated: November 22, 2000 (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?

 4. Running Problems
  4.1 Problems connecting to SSL servers.
  4.2 Why do I get problems when I use & 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!

 5. libcurl Issues
  5.1 Is libcurl thread safe?
  5.2 How can I receive all data into a large memory chunk?

 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 and FILE.

  1.2 What is libcurl?

  libcurl is the engine inside curl that does all the work. curl is more or
  less the command line interface that converts the given options into libcurl
  function invokes. libcurl is a reliable, higly portable multiprotocol file
  transfer library.

  Any application is free to use libcurl, even commercial or closed-source
  ones. Just make sure changes to the lib itself are made public.

  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,
  Windows, Amiga, BeOS, OS/2, OS X, QNX etc.

  1.4 When will you make curl do XXXX ?

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

  * It 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 one single tool that does every
    imaginable thing. That's also one of the great advantages of having the
    core of curl as a library: libcurl.

  * I 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.

  * I 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 me to make all the work while you wait for me to implement it
    for you, that is not a very friendly attitude. I spend a considerable time
    already on maintaining and developing curl. In order to get more out of
    me, I trust you will offer 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 I doubt there would be much
  problems using a different library. I just don't know any other free one and
  that has limited my possibilities to develop against anything else.

  If anyone does "port" curl to use a commercial SSL library, I am 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 page 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. I would appriciate patches that
  brings 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 resume both ways on FTP, download ways on HTTP.

  Try the -c and -C options.

  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', than 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'.

  I have described this 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. I would also suggest that you read 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

4. Running Problems

  4.1. Problems connecting to SSL servers.

  It took a very long time before I 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).

  I have also seen 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 & 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'

  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'

  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. I'll make a short transcript
  here. 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 is currently no nice way for
  curl to abort from such a condition and that's why it gets this undocumented
  error. This should be changed in releases after 7.4.1.

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

  I see this problem as two parts:

  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.

  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 or mail a detailed bug description
  to curl-bug@haxx.se.

  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.


5. libcurl Issues

  5.1. Is libcurl thread safe?

  We have attempted to write the entire code adjusted for multi-threaded
  programs. If your system has such, curl will attempt to use threadsafe
  functions instead of non-safe ones.

  I am very interested in once and for all getting some kind of report or
  README file from those who have used libcurl in a threaded environment,
  since I haven't and I get this question more and more frequently!

  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;
        }


6. License Issues

  Curl and libcurl are released under the MPL, the Mozilla Public License. To
  get a really good answer to your license conflict questions, you should
  study the MPL license 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?

  No, unfortunately you cannot distribute the (lib)curl code with your code.
  According to both the Free Software Foundation and the Mozilla
  Organization, MPL and GPL are not compatible, because GPL requires of
  other licenses that they do "not impose any further restrictions on the
  recipients' exercise of the rights granted herein" [GPL paragraph 6] (One
  could argue about which license that actually imposes restrictions, but
  that'll be left for you to ponder about.)

  However, you have two alternative options:

    1) Nothing prevents you from distributing your program separately from
       curl and libcurl, and request that the user receives it separately.
       The incompability issues between MPL and GPL only applies to the
       distribution.

    2) Add the following exception to your distribution (you must have the
       consent of all the copyright holders of the source code in your
       project in order to do this) 

         As a special exception, you have permission to link this program
         with the curl library and distribute executables, as long as you
         follow the requirements of the GNU GPL in regard to all of the
         software in the executable aside from curl.

       This exception must be added to your GPL covered source code files
       immediately after the notification mentioned in the appendix of GPL.
       The wording was originally suggested by the Free Software Foundation
       in relationship with the Qt library. We have changed the wording to
       apply to curl.

  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. If you end up doing changes to the library, only those changes must
  be made available, not the ones to your program.

  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. If you end up doing changes to the library, only those changes must
  be made available, not the ones to your program.

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

  Yes you can. LGPL libraries don't spread to other libraries the same way GPL
  ones do.

  However, when you read paragraph (3) of the LGPL license, you'll see that
  anyone - at will - may at any time convert that LGPL program into GPL. And
  GPL programs can't be distributed together with MPL programs, neither with
  (lib)curl source code and not as a binary.

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

  No, you're not allowed to do that.

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

  No. We carefully picked this license years ago 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.