curl/docs/FAQ

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FAQ

 1. Install Related Problems
  1.1 configure doesn't find OpenSSL even when it is installed
   1.1.1. native linker doesn't find openssl
   1.1.2. only the libssl lib is missing
  1.2 Does curl work/build with other SSL libraries?
  1.3 Where can I find a copy of LIBEAY32.DLL?
  1.4 Does cURL support Socks (RFC 1928) ?

 2. Usage problems
  2.1 curl: (1) SSL is disabled, https: not supported
  2.2 How do I tell curl to resume a transfer?
  2.3 Why doesn't my posting using -F work?
  2.4 How do I tell curl to run custom FTP commands?
  2.5 How can I disable the Pragma: nocache header?
  2.6 Does curl support javascript, ASP, XML, XHTML or HTML version Y?
  2.7 Can I use curl to delete/rename a file through FTP?

 3. Running Problems
  3.1 Problems connecting to SSL servers.
  3.2 Why do I get problems when I use & in the URL?
  3.3 How can I use {, }, [ or ] to specify multiple URLs?
  3.4 Why do I get downloaded data even though the web page doesn't exist?
  3.5 Why do I get "HTTP/1.1 403 Forbidden" from a http server?
  3.6 Can you tell me what error code 142 means?

 4. libcurl issues
  4.1 Is libcurl thread safe?

 5. license issues
  5.1 I have a GPL program, can I use the libcurl library?
  5.2 I have a closed-source program, can I use the libcurl library?
  5.3 I have a program that uses LGPL libraries, can I use libcurl?
  5.4 Can I modify curl/libcurl for my program and keep the changes secret?
  5.5 Can you please change the curl/libcurl license to XXXX?

  

1. Install Related Problems

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

  This may be because of several reasons.

  1.1.1. native linker doesn't find openssl

    Platforms: Solaris (native cc compiler) and HPUX (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

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

  1.1.2. 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.

  1.2. 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!

  1.3. 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.

  1.4. Does cURL support Socks (RFC 1928) ?

  No. Nobody has wanted it that badly yet. I would appriciate patches that
  brings this functionality.



2. Usage problems

  2.1. 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".

  2.2. 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.

  2.3. 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.

  2.4. 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.

  2.5. 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.

  2.6. 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.

  2.7. 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. Running Problems

  3.1. 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.

  3.2. 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'

  3.3. 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'

  3.4. 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.

  3.5. Why do I get "HTTP/1.1 403 Forbidden" from a http server?

  RFC2616 clearly explains this return code:

  "10.4.4 403 Forbidden"

  The server understood the request, but is refusing to fulfill it.
  Authorization will not help and the request SHOULD NOT be repeated.  If the
  request method was not HEAD and the server wishes to make public why the
  request has not been fulfilled, it SHOULD describe the reason for the
  refusal in the entity.  If the server does not wish to make this information
  available to the client, the status code 404 (Not Found) can be used
  instead.
 
  3.6. 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 currentl no nice way for
  curl to abort from such a condition and that's why it gets this undocumented
  error. This is planned to change in a future release.

  3.7. 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. libcurl issues

  4.1. Is libcurl thread safe?

  As version seven is slowly marching in as the libcurl version to use, we
  have made a serious attempt to address all places in the code where we could
  forsee problems for multi-threaded programs. If your system has them, 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. license issues

  Curl and libcurl are released under the MPL, the Mozilla Public License. To
  get a really good answer to this or other licensing questions, you should
  study the MPL license and the license you are about to use and check for
  clashes yourself. This is a brief summary for a few cases for which we get
  questions:

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

    No. GPL'd software requires all parts of the final executable to be
    licensed under GPL.

  5.2. 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.

  5.3. 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.

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

    No, you're not allowed to do that.

  5.5. 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.