Updated: February 11, 2007 (http://curl.haxx.se/docs/faq.html)
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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?

 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 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?

 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!

 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.3: 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?

 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 commerical apps?

 7. PHP/CURL Issues
  7.1 What is PHP/CURL?
  7.2 Who write 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 FTP,
    FTPS, HTTP, HTTPS, SCP, SFTP, TFTP, TELNET, DICT, FILE and LDAP.  libcurl
    supports HTTPS certificates, HTTP POST, HTTP PUT, FTP uploading, kerberos,
    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, HPUX,
    IRIX, AIX, Tru64, Linux, UnixWare, HURD, Windows, Amiga, OS/2, BeOs, Mac
    OS X, Ultrix, QNX, OpenVMS, RISC OS, Novell NetWare, DOS 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, it supports a range of common Internet protocols,
    currently including HTTP, HTTPS, FTP, FTPS, SCP, SFTP, TFTP, LDAP, DICT,
    TELNET and FILE.

  We pronounce curl and cURL with an initial k sound: [kurl].

  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.

  * 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 the more than 530 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
  (lib)curl. We do this voluntarily on our spare time.

  We get some help from companies. Contactor Data hosts the curl web site,
  Haxx owns the curl web site's domain and sourceforge.net hosts project
  services we take advantage from, like the bug tracker. Also, 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
  first-hand 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
  http://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:
  http://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.

  Some facts to use as input to the math:

  curl packages are downloaded from the curl.haxx.se and mirrors almost one
  million times per year. curl is installed by default with most Linux
  distributions. curl is installed by default with Mac OS X. curl and libcurl
  as used by numerous applications that include libcurl binaries in their
  distribution packages (like Adobe Acrobat Reader and Google Earth).

  More than 60 known named companies use curl in commercial environments and
  products. More than 100 known named open source projects depend on
  (lib)curl.

  In a poll on the curl web site mid-2005, more than 50% of the 300+ answers
  estimated a user base of one million users or more.

  In March 2005, the "Linux Counter project" estimated a total Linux user base
  of some 29 millions, while Netcraft detected some 4 million "active" Linux
  based web servers. A guess is that a fair amount of these Linux
  installations have curl installed.

  All this taken together, there is no doubt that there are millions of
  (lib)curl users.

  http://curl.haxx.se/docs/companies.html
  http://curl.haxx.se/docs/programs.html
  http://curl.haxx.se/libcurl/using/apps.html
  http://counter.li.org/estimates.php
  http://news.netcraft.com/archives/2005/03/14/fedora_makes_rapid_progress.html


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 OpenSSL, GnuTLS, yassl or NSS, although there
  should not be many 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 DLLs and other binary
  packages.

  2.4 Does curl support Socks (RFC 1928) ?

  Yes, SOCKS5 is supported.


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 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 use curl to just perform
  FTP commands without transferring 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 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'

  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:
  http://curl.haxx.se/libcurl/

  In February 2007, there are interfaces available for the following
  languages: Ada95, Basic, C, C++, Ch, Cocoa, D, Dylan, Euphoria, Ferite,
  Gambas, glib/GTK+, Java, Lisp, Lua, Mono, .NET, Object-Pascal, O'Caml,
  Pascal, Perl, PHP, PostgreSQL, Python, R, Rexx, Ruby, Scheme, S-Lang,
  Smalltalk, SPL, Tcl, Visual Basic, 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 work-arounds 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.

  - 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 comes with a
    default CA cert bundle. You can 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
    (http://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.


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 & 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 quote 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 Windows, the standard DOS shell treats the %-letter specially and you
  need to use TWO %-letters for each single one you want to use in the URL.

  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' (which then also
  needs the %-letter 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 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.

    4.5.6 "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?

  This is supported in curl 7.10.6 or later. No earlier curl version knows
  of this magic. Later versions require the OpenSSL or Microsoft Windows 
  libraries to provide this functionality. Using GnuTLS or NSS libraries will 
  not provide NTLM authentication functionality in curl.

  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

  Then it means that curl couldn't verify that the server's certificate was
  good. Curl verifies the certificate using the CA cert bundle that comes with
  the curl installation.

  To disable the verification (which makes it act like curl did before 7.10),
  use -k. This does however enable man-in-the-middle attacks.

  If you get this failure but are having a CA cert bundle installed and used,
  the server's certificate is not signed by one of the CA's in the bundle. It
  might for example be self-signed. You then correct this problem by obtaining
  a valid CA cert for the server. Or again, decrease the security by disabling
  this check.

  Details are also in the SSLCERTS file in the release archives, found online
  here: http://curl.haxx.se/docs/sslcerts.html

  4.13 Why is curl -R on Windows one hour off?

  During daylight savings time, when -R is used, curl will set a time that
  appears one hour off. This happens due to a flaw in how Windows stores and
  uses file modification times and it is not easily worked around. For details
  on this problem, read this: http://www.codeproject.com/datetime/dstbugs.asp

  4.14 Redirects work in browser but not with curl!

  curl supports HTTP redirects fine (see item 3.8). Browsers generally support
  at least two other ways to perform directs that curl does not:

  - Meta tags. You can write a HTML tag that will cause the browser to
    redirect to another given URL after a certain time.

  - Javascript. You can write a javascript program embeded in a HTML page
    that redirects the browser to another given URL.

  There is no way to make curl follow these redirects. You must either
  manually figure out what the page is set to do, or you write a script that
  parses the results and fetches the new URL.


5. libcurl Issues

  5.1 Is libcurl thread-safe?

  Yes.

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

  If you use a OpenSSL-powered libcurl in a multi-threaded environment, you
  need to provide one or two locking functions:

    http://www.openssl.org/docs/crypto/threads.html#DESCRIPTION

  If you use a GnuTLS-powered libcurl in a multi-threaded environment, you
  need to provide locking function(s) for libgcrypt (which is used by GnuTLS
  for the crypto functions).

    http://www.gnu.org/software/gnutls/manual/html_node/Multi_002dthreaded-applications.html

  No special locking is needed with a NSS-powered libcurl. NSS is thread-safe.

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

  [ See also the examples/getinmemory.c source ]

  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 received 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
  CURLOPT_WRITEDATA option. 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)
        {
          size_t 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?

  libcurl has 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, but you're even encouraged to reuse it if you can, as that
  will enable libcurl to use persistent connections.

  5.4 Does libcurl do Winsock initialization on win32 systems?

  Yes, if told to in the curl_global_init() call.

  5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?

  Yes, but you cannot open a FILE * and pass the pointer to a DLL and have
  that DLL use the FILE * (as the DLL and the client application cannot access
  each others' variable memory areas). If you set CURLOPT_WRITEDATA 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_READDATA you must also specify
  CURLOPT_READFUNCTION.

  5.6 What about Keep-Alive or persistent connections?

  curl and libcurl have excellent support for persistent 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.

  5.7 Link errors when building libcurl on Windows!

  You need to make sure that your project, and all the libraries (both static
  and dynamic) that it links against, are compiled/linked against the same run
  time library.

  This is determined by the /MD, /ML, /MT (and their corresponding /M?d)
  options to the command line compiler. /MD (linking against MSVCRT dll) seems
  to be the most commonly used option.

  When building an application that uses the static libcurl library, you must
  add -DCURL_STATICLIB to your CFLAGS. Otherwise the linker will look for
  dynamic import symbols. If you get linker error like "unknown symbol
  __imp__curl_easy_init ..." you have linked against the wrong (static)
  library.  If you want to use the libcurl.dll and import lib, you don't need
  any extra CFLAGS, but use one of the import libraries below. These are the
  libraries produced by the various lib/Makefile.* files:

  Target:          static lib.   import lib for libcurl*.dll.
  -----------------------------------------------------------
  MingW:           libcurl.a     libcurldll.a
  MSVC (release):  libcurl.lib   libcurl_imp.lib
  MSVC (debug):    libcurld.lib  libcurld_imp.lib
  Borland:         libcurl.lib   libcurl_imp.lib


  5.8 libcurl.so.3: open failed: No such file or directory

  This is an error message you might get when you try to run a program linked
  with a shared version of libcurl and your run-time linker (ld.so) couldn't
  find the shared library named libcurl.so.3.

  You need to make sure that ld.so finds libcurl.so.3. You can do that
  multiple ways, and it differs somewhat between different operating systems,
  but they are usually:

  * Add an option to the linker command line that specify the hard-coded path
    the run-time linker should check for the lib (usually -R)

  * Set an environment variable (LD_LIBRARY_PATH for example) where ld.so
    should check for libs

  * Adjust the system's config to check for libs in the directory where you've
    put the dir (like Linux's /etc/ld.so.conf)

  'man ld.so' and 'man ld' will tell you more details

  5.9 How does libcurl resolve host names?

  libcurl supports a large a number of different name resolve functions. One
  of them is picked at build-time and will be used unconditionally. Thus, if
  you want to change name resolver function you must rebuild libcurl and tell
  it to use a different function.

  - The non-ipv6 resolver that can use one out of four host name resolve calls
    (depending on what your system supports):

    A - gethostbyname()
    B - gethostbyname_r() with 3 arguments
    C - gethostbyname_r() with 5 arguments
    D - gethostbyname_r() with 6 arguments

  - The ipv6-resolver that uses getaddrinfo()

  - The c-ares based name resolver that uses the c-ares library for resolves.
    Using this offers asynchronous name resolves but it currently has no IPv6
    support.

  - The Windows threaded resolver. It use:

    A - gethostbyname() on plain ipv4 windows hosts
    B - getaddrinfo() on ipv6-enabled windows hosts

  Also note that libcurl never resolves or reverse-lookups addresses given as
  pure numbers, such as 127.0.0.1 or ::1.

  5.10 How do I prevent libcurl from writing the response to stdout?

  libcurl provides a default built-in write function that writes received data
  to stdout. Set the CURLOPT_WRITEFUNCTION to receive the data, or possibly
  set CURLOPT_WRITEDATA to a different FILE * handle.

  5.11 How do I make libcurl not receive the whole HTTP response?

  You make the write callback (or progress callback) return an error and
  libcurl will then abort the transfer.

  5.12 Can I make libcurl fake or hide my real IP address?

  No. libcurl operates on a higher level than so. Besides, faking IP address
  would imply sending IP packages with a made-up source address, and then you
  normally get a problem with intercepting the packages sent back as they
  would then not be routed to you!

  If you use a proxy to access remote sites, the sites will not see your local
  IP address but instead the address of the proxy.

  Also note that on many networks NATs or other IP-munging techniques are used
  that makes you see and use a different IP address locally than what the
  remote server will see you coming from.


6. License Issues

  Curl and libcurl are released under a MIT/X derivate license. The license is
  very liberal and should not impose a problem for your project. 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.)

  We are not lawyers and this is not legal advice. You should probably consult
  one if you want true and accurate legal insights without our prejudice.

  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. It should be possible for everyone to use libcurl or
  curl in their projects, no matter what license they already have in use.

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

  Next to none. All you need to adhere to is the MIT-style license (stated in
  the COPYING file) which basically says you have to include the copyright
  notice in "all copies" and that you may not use the copyright holder's name
  when promoting your software.

  You do not have to release any of your source code.

  You do not have to reveal or make public any changes to the libcurl source
  code.

  You do not have to reveal or make public that you are using libcurl within
  your app.

  As can be seen here: http://curl.haxx.se/docs/companies.html and
  elsewhere, more and more companies are dicovering the power
  of libcurl and take advantage of it even in commercial environments.


7. PHP/CURL Issues

  7.1 What is PHP/CURL?

  The module for PHP that makes it possible for PHP programs to access curl-
  functions from within PHP.

  In the cURL project we call this module PHP/CURL to differentiate it from
  curl the command line tool and libcurl the library. The PHP team however
  does not refer to it like this (for unknown reasons). They call it plain
  CURL (often using all caps) which causes much confusion to users which in
  turn gives us a higher question load.

  7.2 Who write PHP/CURL?

  PHP/CURL is a module that comes with the regular PHP package. It depends and
  uses libcurl, so you need to have libcurl installed properly first before
  PHP/CURL can be used. PHP/CURL is written by Sterling Hughes.

  7.3 Can I perform multiple requests using the same handle?

  Yes - at least in PHP version 4.3.8 and later (this has been known to not
  work in earlier versions, but the exact version when it started to work is
  unknown to me).

  After a transfer, you just set new options in the handle and make another
  transfer. This will make libcurl to re-use the same connection if it can.