FAQ

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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 ?
  1.5 Who makes curl?
  1.6 What do you get for making curl?
  1.7 What about CURL from curl.com?
  1.8 I have a problem who do I mail?
  1.9 Where do I buy commercial support for curl?
  1.10 How many are using curl?
  1.11 Why don't you update ca-bundle.crt
  1.12 I have a problem who can I chat with?
  1.13 curl's ECCN number?
  1.14 How do I submit my patch?
  1.15 How do I port libcurl to my OS?

 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) ?
  2.5 Install libcurl for both 32bit and 64bit?

 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 Accept: */* header?
  3.6 Does curl support 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 my favorite programming language?
  3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
  3.11 How do I POST with a different Content-Type?
  3.12 Why do FTP specific features over HTTP proxy fail?
  3.13 Why does my single/double quotes fail?
  3.14 Does curl support Javascript or PAC (automated proxy config)?
  3.15 Can I do recursive fetches with curl?
  3.16 What certificates do I need when I use SSL?
  3.17 How do I list the root dir of an FTP server?
  3.18 Can I use curl to send a POST/PUT and not wait for a response?
  3.19 How do I get HTTP from a host using a specific IP address?
  3.20 How to SFTP from my user's home directory?
  3.21 Protocol xxx not supported or disabled in libcurl
  3.22 curl -X gives me HTTP problems

 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"
   4.5.6 "301 Moved Permanently"
  4.6 Can you tell me what error code 142 means?
  4.7 How do I keep user names 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?
  4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
  4.11 Why does my HTTP range requests return the full document?
  4.12 Why do I get "certificate verify failed" ?
  4.13 Why is curl -R on Windows one hour off?
  4.14 Redirects work in browser but not with curl!
  4.15 FTPS doesn't work
  4.16 My HTTP POST or PUT requests are slow!
  4.17 Non-functional connect timeouts on Windows
  4.18 file:// URLs containing drive letters (Windows, NetWare)
  4.19 Why doesn't curl return an error when the network cable is unplugged?
  4.20 curl doesn't return error for HTTP non-200 responses!
  4.21 Why is there a HTTP/1.1 in my HTTP/2 request?

 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_WRITEDATA and CURLOPT_READDATA work on win32 ?
  5.6 What about Keep-Alive or persistent connections?
  5.7 Link errors when building libcurl on Windows!
  5.8 libcurl.so.X: open failed: No such file or directory
  5.9 How does libcurl resolve host names?
  5.10 How do I prevent libcurl from writing the response to stdout?
  5.11 How do I make libcurl not receive the whole HTTP response?
  5.12 Can I make libcurl fake or hide my real IP address?
  5.13 How do I stop an ongoing transfer?
  5.14 Using C++ non-static functions for callbacks?
  5.15 How do I get an FTP directory listing?
  5.16 I want a different time-out!
  5.17 Can I write a server with libcurl?
  5.18 Does libcurl use threads?

 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?
  6.7 What are my obligations when using libcurl in my commercial apps?

 7. PHP/CURL Issues
  7.1 What is PHP/CURL?
  7.2 Who wrote PHP/CURL?
  7.3 Can I perform multiple requests using the same handle?

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

1. Philosophy

  1.1 What is cURL?

  cURL is the name of the project. 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, it works as
  an abbreviation for "Client URL Request Library" or why not the recursive
  version: "Curl URL Request Library".

  The cURL project produces two products:

  libcurl

    A free and easy-to-use client-side URL transfer library, supporting DICT,
    FILE, FTP, FTPS, GOPHER, HTTP, HTTPS, IMAP, IMAPS, LDAP, LDAPS, POP3,
    POP3S, RTMP, RTSP, SCP, SFTP, SMB, SMBS, SMTP, SMTPS, TELNET and TFTP.

    libcurl supports HTTPS certificates, HTTP POST, HTTP PUT, FTP uploading,
    Kerberos, SPNEGO, HTTP form based upload, proxies, cookies, user+password
    authentication, file transfer resume, http proxy tunneling and more!

    libcurl is highly portable, it builds and works identically on numerous
    platforms, including Solaris, NetBSD, FreeBSD, OpenBSD, Darwin, HP-UX,
    IRIX, AIX, Tru64, Linux, UnixWare, HURD, Windows, Amiga, OS/2, BeOS, Mac
    OS X, Ultrix, QNX, OpenVMS, RISC OS, Novell NetWare, DOS, Symbian, OSF,
    Android, Minix, IBM TPF and more...

    libcurl is free, thread-safe, IPv6 compatible, feature rich, well
    supported and fast.

  curl

    A command line tool for getting or sending files using URL syntax.

    Since curl uses libcurl, curl supports the same wide range of common
    Internet protocols that libcurl does.

  We pronounce curl with an initial k sound. It rhymes with words like girl
  and earl. This is a short WAV file to help you:

     http://media.merriam-webster.com/soundc11/c/curl0001.wav

  There are numerous sub-projects and related projects that also use the word
  curl in the project names in various combinations, but you should take
  notice that this FAQ is directed at the command-line tool named curl (and
  libcurl the library), and may therefore not be valid for other curl-related
  projects. (There is however a small section for the PHP/CURL in this FAQ.)

  1.2 What is libcurl?

  libcurl is a reliable and portable library which provides you with an easy
  interface to a range of common Internet protocols.

  You can use libcurl for free in your application, be it open source,
  commercial or closed-source.

  libcurl is most probably the most portable, most powerful and most often
  used C-based multi-platform file transfer library on this planet - be it
  open source or commercial.

  1.3 What is curl not?

  Curl is not a wget clone. That is a common misconception.  Never, during
  curl's development, have we 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 want to 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 behavior 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 (when using the PHP/CURL module).

  Curl is not a program for a single operating system. 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 -- the command line tool -- is to remain a non-graphical command line
  tool. If you want GUIs or fancy scripting capabilities, you should look for
  another tool that uses libcurl.

  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
  we will agree. If you wanna add more protocols, we 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. Simply go to the GitHub repo which resides at
  https://github.com/curl/curl, fork the project, and create pull requests
  with your proposed changes.

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

  1.5 Who makes curl?

  curl and libcurl are not made by any single individual. Daniel Stenberg is
  project leader and main developer, but other persons' submissions are
  important and crucial. Anyone can contribute and post their changes and
  improvements and have them inserted in the main sources (of course on the
  condition that developers agree on that the fixes are good).

  The full list of all contributors is found in the docs/THANKS file.

  curl is developed by a community, with Daniel at the wheel.

  1.6 What do you get for making curl?

  Project cURL is entirely free and open. No person gets paid for developing
  curl on full time. We do this voluntarily, mostly on spare time.
  Occasionally companies pay individual developers to work on curl, but that's
  up to each company and developer. It is not controlled by nor supervised in
  any way by the project.

  We still get help from companies. Haxx provides web site, bandwidth, mailing
  lists etc, sourceforge.net hosts project services we take advantage from,
  like the bug tracker, and GitHub hosts the primary git repository at
  https://github.com/curl/curl. Also again, some companies have sponsored
  certain parts of the development in the past and I hope some will continue to
  do so in the future.

  If you want to support our project, consider a donation or a banner-program
  or even better: by helping us coding, documenting, testing etc.

  1.7 What about CURL from curl.com?

  During the summer 2001, curl.com was busy advertising their client-side
  programming language for the web, named CURL.

  We are in no way associated with curl.com or their CURL programming
  language.

  Our project name curl has been in effective use since 1998. We were not the
  first computer related project to use the name "curl" and do not claim any
  rights to the name.

  We recognize that we will be living in parallel with curl.com and wish them
  every success.

  1.8 I have a problem who do I mail?

  Please do not mail any single individual unless you really need to. Keep
  curl-related questions on a suitable mailing list. All available mailing
  lists are listed in the MANUAL document and online at
  https://curl.haxx.se/mail/

  Keeping curl-related questions and discussions on mailing lists allows
  others to join in and help, to share their ideas, contribute their
  suggestions and spread their wisdom. Keeping discussions on public mailing
  lists also allows for others to learn from this (both current and future
  users thanks to the web based archives of the mailing lists), thus saving us
  from having to repeat ourselves even more. Thanks for respecting this.

  If you have found or simply suspect a security problem in curl or libcurl,
  mail curl-security at haxx.se (closed list of receivers, mails are not
  disclosed) and tell. Then we can produce a fix in a timely manner before the
  flaw is announced to the world, thus lessen the impact the problem will have
  on existing users.

  1.9 Where do I buy commercial support for curl?

  curl is fully open source. It means you can hire any skilled engineer to fix
  your curl-related problems.

  We list available alternatives on the curl web site:
  https://curl.haxx.se/support.html

  1.10 How many are using curl?

  It is impossible to tell.

  We don't know how many users that knowingly have installed and use curl.

  We don't know how many users that use curl without knowing that they are in
  fact using it.

  We don't know how many users that downloaded or installed curl and then
  never use it.

  In May 2012 Daniel did a counting game and came up with a number that may
  be completely wrong or somewhat accurate. Over 500 million!

  See https://daniel.haxx.se/blog/2012/05/16/300m-users/

  1.11 Why don't you update ca-bundle.crt

  The ca cert bundle that used to shipped with curl was very outdated and must
  be replaced with an up-to-date version by anyone who wants to verify
  peers. It is no longer provided by curl. The last curl release ever that
  shipped a ca cert bundle was curl 7.18.0.

  In the cURL project we've decided not to attempt to keep this file updated
  (or even present anymore) since deciding what to add to a ca cert bundle is
  an undertaking we've not been ready to accept, and the one we can get from
  Mozilla is perfectly fine so there's no need to duplicate that work.

  Today, with many services performed over HTTPS, every operating system
  should come with a default ca cert bundle that can be deemed somewhat
  trustworthy and that collection (if reasonably updated) should be deemed to
  be a lot better than a private curl version.

  If you want the most recent collection of ca certs that Mozilla Firefox
  uses, we recommend that you extract the collection yourself from Mozilla
  Firefox (by running 'make ca-bundle), or by using our online service setup
  for this purpose: https://curl.haxx.se/docs/caextract.html

  1.12 I have a problem who can I chat with?

  There's a bunch of friendly people hanging out in the #curl channel on the
  IRC network irc.freenode.net. If you're polite and nice, chances are big
  that you can get -- or provide -- help instantly.

  1.13 curl's ECCN number?

  The US government restricts exports of software that contains or uses
  cryptography. When doing so, the Export Control Classification Number (ECCN)
  is used to identify the level of export control etc.

  Apache Software Foundation gives a good explanation of ECCNs at
  https://www.apache.org/dev/crypto.html

  We believe curl's number might be ECCN 5D002, another possibility is
  5D992. It seems necessary to write them (the authority that administers ECCN
  numbers), asking to confirm.

  Comprehensible explanations of the meaning of such numbers and how to obtain
  them (resp.) are here

  http://www.bis.doc.gov/licensing/exportingbasics.htm
  http://www.bis.doc.gov/licensing/do_i_needaneccn.html

  An incomprehensible description of the two numbers above is here
  http://www.access.gpo.gov/bis/ear/pdf/ccl5-pt2.pdf

  1.14 How do I submit my patch?

  When you have made a patch or a change of whatever sort, and want to submit
  that to the project, there are a few different ways we prefer:

  o send a patch to the curl-library mailing list. We're many subscribers
    there and there are lots of people who can review patches, comment on them
    and "receive" them properly.

  o if your patch changes or fixes a bug, you can also opt to submit a bug
    report in the bug tracker and attach your patch there. There are less
    people involved there.

  Lots of more details are found in the CONTRIBUTE and INTERNALS docs.

  1.15 How do I port libcurl to my OS?

  Here's a rough step-by-step:

  1. copy a suitable lib/config-*.h file as a start to lib/config-[youros].h

  2. edit lib/config-[youros].h to match your OS and setup

  3. edit lib/curl_setup.h to include config-[youros].h when your OS is
     detected by the preprocessor, in the style others already exist

  4. compile lib/*.c and make them into a library


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

    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 RSAREF 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 a generic SSL function layer internally, and
  that SSL functionality can then be provided by one out of many different SSL
  backends.

  curl can be built to use one of the following SSL alternatives: OpenSSL,
  GnuTLS, yassl, NSS, PolarSSL, axTLS, Secure Transport (native iOS/OS X),
  WinSSL (native Windows) or GSKit (native IBM i). They all have their pros
  and cons, and we try to maintain a comparison of them here:
  https://curl.haxx.se/docs/ssl-compared.html

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

  That is an OpenSSL binary built for Windows.

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

  2.4 Does curl support SOCKS (RFC 1928) ?

  Yes, SOCKS 4 and 5 are supported.

  2.5 Install libcurl for both 32bit and 64bit?

  In curl's configure procedure one of the regular include files get created
  with platform specific information. The file 'curl/curlbuild.h' in the
  installed libcurl file tree is therefore somewhat tied to that particular
  platform.

  To allow applications to get built for either 32bit or 64bit you need to
  install libcurl headers for both setups and unfortunately curl doesn't do
  this automatically.

  A commonly used procedure is this:

     $ ./configure [32bit platform]
     $ mv curl/curlbuild.h curl/curlbuild-32bit.h
     $ ./configure [64bit platform]
     $ mv curl/curlbuild.h curl/curlbuild-64bit.h

  Then you make a toplevel curl/curlbuild.h replacement that only does this:

     #ifdef IS_32BIT
     #include "curlbuild-32bit.h"
     else
     #include "curlbuild-64bit.h"
     #endif


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 instance of curl/libcurl that you're using was built
  without support for this protocol.

  This could've happened if the configure script that was run at build time
  couldn't find all libs and include files curl 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 labelled "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 expects one of the formats. If the form you're trying to
  submit uses the type '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 MANUAL and TheArtOfHttpScripting
  documents, 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 normally use curl to
  perform FTP commands without transferring anything. Therefore you must
  always specify a URL to transfer to/from even when doing custom FTP
  commands, or use -I which implies the "no body" option sent to libcurl.

  3.5 How can I disable the Accept: */* 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 "Accept:" to disable that specific header.

  3.6 Does curl support 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.

  See also item 3.14 regarding javascript.

  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'

  or rename a file after upload:

     curl -T infile ftp://upload.com/dir/ -Q "-RNFR infile" -Q "-RNTO newname"

  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

  Not all redirects are HTTP ones, see 4.14

  3.9 How do I use curl in my favorite programming language?

  There exist many language interfaces/bindings for curl that integrates it
  better with various languages. If you are fluid in a script language, you
  may very well opt to use such an interface instead of using the command line
  tool.

  Find out more about which languages that support curl directly, and how to
  install and use them, in the libcurl section of the curl web site:
  https://curl.haxx.se/libcurl/

  All the various bindings to libcurl are made by other projects and people,
  outside of the cURL project. The cURL project itself only produces libcurl
  with its plain C API. If you don't find anywhere else to ask you can ask
  about bindings on the curl-library list too, but be prepared that people on
  that list may not know anything about bindings.

  In October 2009, there were interfaces available for the following
  languages: Ada95, Basic, C, C++, Ch, Cocoa, D, Dylan, Eiffel, Euphoria,
  Ferite, Gambas, glib/GTK+, Haskell, ILE/RPG, Java, Lisp, Lua, Mono, .NET,
  Object-Pascal, O'Caml, Pascal, Perl, PHP, PostgreSQL, Python, R, Rexx, Ruby,
  Scheme, S-Lang, Smalltalk, SP-Forth, SPL, Tcl, Visual Basic, Visual FoxPro,
  Q, wxwidgets and XBLite. By the time you read this, additional ones may have
  appeared!

  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 on top 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 is of course just as fine and you'd just use the proper
  library options to do the same.

  3.11 How do I POST with a different Content-Type?

  You can always replace the internally generated headers with -H/--header.
  To make a simple HTTP POST with text/xml as content-type, do something like:

        curl -d "datatopost" -H "Content-Type: text/xml" [URL]

  3.12 Why do FTP specific features over HTTP proxy fail?

  Because when you use a HTTP proxy, the protocol spoken on the network will
  be HTTP, even if you specify a FTP URL. This effectively means that you
  normally can't use FTP specific features such as FTP upload and FTP quote
  etc.

  There is one exception to this rule, and that is if you can "tunnel through"
  the given HTTP proxy. Proxy tunneling is enabled with a special option (-p)
  and is generally not available as proxy admins usually disable tunneling to
  other ports than 443 (which is used for HTTPS access through proxies).

  3.13 Why does my single/double quotes fail?

  To specify a command line option that includes spaces, you might need to
  put the entire option within quotes. Like in:

   curl -d " with spaces " url.com

  or perhaps

   curl -d ' with spaces ' url.com

  Exactly what kind of quotes and how to do this is entirely up to the shell
  or command line interpreter that you are using. For most unix shells, you
  can more or less pick either single (') or double (") quotes. For
  Windows/DOS prompts I believe you're forced to use double (") quotes.

  Please study the documentation for your particular environment. Examples in
  the curl docs will use a mix of both these ones as shown above. You must
  adjust them to work in your environment.

  Remember that curl works and runs on more operating systems than most single
  individuals have ever tried.

  3.14 Does curl support Javascript or PAC (automated proxy config)?

  Many web pages do magic stuff using embedded Javascript. Curl and libcurl
  have no built-in support for that, so it will be treated just like any other
  contents.

  .pac files are a netscape invention and are sometimes used by organizations
  to allow them to differentiate which proxies to use. The .pac contents is
  just a Javascript program that gets invoked by the browser and that returns
  the name of the proxy to connect to. Since curl doesn't support Javascript,
  it can't support .pac proxy configuration either.

  Some workarounds usually suggested to overcome this Javascript dependency:

  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 language.

  Implement a Javascript interpreter, people have successfully used the
  Mozilla Javascript engine in the past.

  Ask your admins to stop this, for a static proxy setup or similar.

  3.15 Can I do recursive fetches with curl?

  No. curl itself has no code that performs recursive operations, such as
  those performed by wget and similar tools.

  There exist wrapper scripts with that functionality (for example the
  curlmirror perl script), and you can write programs based on libcurl to do
  it, but the command line tool curl itself cannot.

  3.16 What certificates do I need when I use SSL?

  There are three different kinds of "certificates" to keep track of when we
  talk about using SSL-based protocols (HTTPS or FTPS) using curl or libcurl.

  CLIENT CERTIFICATE

  The server you communicate may require that you can provide this in order to
  prove that you actually are who you claim to be.  If the server doesn't
  require this, you don't need a client certificate.

  A client certificate is always used together with a private key, and the
  private key has a pass phrase that protects it.

  SERVER CERTIFICATE

  The server you communicate with has a server certificate. You can and should
  verify this certificate to make sure that you are truly talking to the real
  server and not a server impersonating it.

  CERTIFICATE AUTHORITY CERTIFICATE ("CA cert")

  You often have several CA certs in a CA cert bundle that can be used to
  verify a server certificate that was signed by one of the authorities in the
  bundle. curl does not come with a CA cert bundle but most curl installs
  provide one. You can also override the default.

  The server certificate verification process is made by using a Certificate
  Authority certificate ("CA cert") that was used to sign the server
  certificate. Server certificate verification is enabled by default in curl
  and libcurl and is often the reason for problems as explained in FAQ entry
  4.12 and the SSLCERTS document
  (https://curl.haxx.se/docs/sslcerts.html). Server certificates that are
  "self-signed" or otherwise signed by a CA that you do not have a CA cert
  for, cannot be verified. If the verification during a connect fails, you are
  refused access. You then need to explicitly disable the verification to
  connect to the server.

  3.17 How do I list the root dir of an FTP server?

  There are two ways. The way defined in the RFC is to use an encoded slash
  in the first path part. List the "/tmp" dir like this:

     curl ftp://ftp.sunet.se/%2ftmp/

  or the not-quite-kosher-but-more-readable way, by simply starting the path
  section of the URL with a slash:

     curl ftp://ftp.sunet.se//tmp/

  3.18 Can I use curl to send a POST/PUT and not wait for a response?

  No.

  But you could easily write your own program using libcurl to do such stunts.

  3.19 How do I get HTTP from a host using a specific IP address?

  For example, you may be trying out a web site installation that isn't yet in
  the DNS. Or you have a site using multiple IP addresses for a given host
  name and you want to address a specific one out of the set.

  Set a custom Host: header that identifies the server name you want to reach
  but use the target IP address in the URL:

    curl --header "Host: www.example.com" http://127.0.0.1/

  You can also opt to add faked host name entries to curl with the --resolve
  option. That has the added benefit that things like redirects will also work
  properly. The above operation would instead be done as:

    curl --resolve www.example.com:80:127.0.0.1 http://www.example.com/

  3.20 How to SFTP from my user's home directory?

  Contrary to how FTP works, SFTP and SCP URLs specify the exact directory to
  work with. It means that if you don't specify that you want the user's home
  directory, you get the actual root directory.

  To specify a file in your user's home directory, you need to use the correct
  URL syntax which for sftp might look similar to:

    curl -O -u user:password sftp://example.com/~/file.txt

  and for SCP it is just a different protocol prefix:

    curl -O -u user:password scp://example.com/~/file.txt

  3.21 Protocol xxx not supported or disabled in libcurl

  When passing on a URL to curl to use, it may respond that the particular
  protocol is not supported or disabled. The particular way this error message
  is phrased is because curl doesn't make a distinction internally of whether
  a particular protocol is not supported (i.e. never got any code added that
  knows how to speak that protocol) or if it was explicitly disabled. curl can
  be built to only support a given set of protocols, and the rest would then
  be disabled or not supported.

  Note that this error will also occur if you pass a wrongly spelled protocol
  part as in "htpt://example.com" or as in the less evident case if you prefix
  the protocol part with a space as in " http://example.com/".

  3.22 curl -X gives me HTTP problems

  In normal circumstances, -X should hardly ever be used.

  By default you use curl without explicitly saying which request method to
  use when the URL identifies a HTTP transfer. If you just pass in a URL like
  "curl http://example.com" it will use GET. If you use -d or -F curl will use
  POST, -I will cause a HEAD and -T will make it a PUT.

  If for whatever reason you're not happy with these default choices that curl
  does for you, you can override those request methods by specifying -X
  [WHATEVER]. This way you can for example send a DELETE by doing "curl -X
  DELETE [URL]".

  It is thus pointless to do "curl -XGET [URL]" as GET would be used
  anyway. In the same vein it is pointless to do "curl -X POST -d data
  [URL]"... But you can make a fun and somewhat rare request that sends a
  request-body in a GET request with something like "curl -X GET -d data
  [URL]"

  Note that -X doesn't actually change curl's behavior as it only modifies the
  actual string sent in the request, but that may of course trigger a
  different set of events.

  Accordingly, by using -XPOST on a command line that for example would follow
  a 303 redirect, you will effectively prevent curl from behaving
  correctly. Be aware.


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 have 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 & symbol is treated specially and when used, it
  runs the specified command in the background. To safely send the & as a part
  of a URL, you should quote the entire URL by using single (') or double (")
  quotes around it. Similar problems can also occur on some shells with other
  characters, including ?*!$~(){}<>\|;`.  When in doubt, quote the URL.

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

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

  In Windows, the standard DOS shell treats the percent sign specially and you
  need to use TWO percent signs for each single one you want to use in the
  URL.

  If you want a literal percent sign to be part of the data you pass in a POST
  using -d/--data you must encode it as '%25' (which then also needs the
  percent sign doubled on Windows machines).

  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 -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 fulfil 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.

    4.5.6 "301 Moved Permanently"

    If you get this return code and an HTML output similar to this:

       <H1>Moved Permanently</H1> The document has moved <A
       HREF="http://same_url_now_with_a_trailing_slash/">here</A>.

    it might be because you request a directory URL but without the trailing
    slash. Try the same operation again _with_ the trailing URL, or use the
    -L/--location option to follow the redirection.

  4.6 Can you tell me what error code 142 means?

  All curl error codes are described at the end of the man page, in the
  section called "EXIT CODES".

  Error codes that are larger than the highest documented error code means
  that curl has exited due to a crash. This is a serious error, and we
  appreciate a detailed bug report from you that describes how we could go
  ahead and repeat this!

  4.7 How do I keep user names 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 to tell curl to read parameters from a file
  or stdin to which you can pass the secret info. curl itself will also
  attempt to "hide" the given password by blanking out the option - this
  doesn't work on all platforms.

  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 (using Basic authentication) and FTP passwords
  are sent in clear across the network. All it takes for anyone to fetch them
  is to listen on the network.  Eavesdropping is very easy. Use more secure
  authentication methods (like Digest, Negotiate or even NTLM) or consider the
  SSL-based alternatives HTTPS and FTPS.

  4.8 I found a bug!

  It is not a bug if the behavior is documented. Read the docs first.
  Especially check out the KNOWN_BUGS file, it may be a documented bug!

  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, read the BUGS document first. Then report it as described
  in there.

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

  NTLM support requires OpenSSL, GnuTLS, mbedTLS, NSS, Secure Transport, or
  Microsoft Windows libraries at build-time to provide this functionality.

  NTLM is a Microsoft proprietary protocol. Proprietary formats are evil. You
  should not use such ones.

  4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!

  Many web servers allow or demand that the administrator configures the
  server properly for these requests to work on the web server.

  Some servers seem to support HEAD only on certain kinds of URLs.

  To fully grasp this, try the documentation for the particular server
  software you're trying to interact with. This is not anything curl can do
  anything about.

  4.11 Why does my HTTP range requests return the full document?

  Because the range may not be supported by the server, or the server may
  choose to ignore it and return the full document anyway.

  4.12 Why do I get "certificate verify failed" ?

  You invoke curl 7.10 or later to communicate on a https:// URL and get an
  error back looking something similar to this:

      curl: (35) SSL: error:14090086:SSL routines:
      SSL3_GET_SERVER_CERTIFICATE:certificate verify failed