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460 lines
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460 lines
19 KiB
Plaintext
Updated: August 22, 2000 (http://curl.haxx.se/docs/faq.shtml)
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\___|\___/|_| \_\_____|
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FAQ
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1. Philosophy
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1.1 What is cURL?
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1.2 What is libcurl?
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1.3 What is cURL not?
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1.4 When will you make curl do XXXX ?
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2. Install Related Problems
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2.1 configure doesn't find OpenSSL even when it is installed
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2.1.1. native linker doesn't find openssl
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2.1.2. only the libssl lib is missing
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2.2 Does curl work/build with other SSL libraries?
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2.3 Where can I find a copy of LIBEAY32.DLL?
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2.4 Does cURL support Socks (RFC 1928) ?
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3. Usage Problems
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3.1 curl: (1) SSL is disabled, https: not supported
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3.2 How do I tell curl to resume a transfer?
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3.3 Why doesn't my posting using -F work?
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3.4 How do I tell curl to run custom FTP commands?
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3.5 How can I disable the Pragma: nocache header?
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3.6 Does curl support javascript, ASP, XML, XHTML or HTML version Y?
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3.7 Can I use curl to delete/rename a file through FTP?
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3.8 How do I tell curl to follow HTTP redirects?
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4. Running Problems
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4.1 Problems connecting to SSL servers.
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4.2 Why do I get problems when I use & in the URL?
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4.3 How can I use {, }, [ or ] to specify multiple URLs?
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4.4 Why do I get downloaded data even though the web page doesn't exist?
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4.5 Why do I get return code XXX from a HTTP server?
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4.5.1 "400 Bad Request"
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4.5.2 "401 Unauthorized"
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4.5.3 "403 Forbidden"
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4.5.4 "404 Not Found"
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4.5.5 "405 Method Not Allowed"
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4.6 Can you tell me what error code 142 means?
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5. libcurl Issues
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5.1 Is libcurl thread safe?
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6. License Issues
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6.1 I have a GPL program, can I use the libcurl library?
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6.2 I have a closed-source program, can I use the libcurl library?
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6.3 I have a BSD licensed program, can I use the libcurl library?
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6.4 I have a program that uses LGPL libraries, can I use libcurl?
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6.5 Can I modify curl/libcurl for my program and keep the changes secret?
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6.6 Can you please change the curl/libcurl license to XXXX?
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==============================================================================
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1. Philosophy
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1.1 What is cURL?
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cURL (or simply just 'curl') is a command line tool for getting or sending
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files using URL syntax. The name is a play on 'Client for URLs', originally
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with URL spelled in uppercase to make it obvious it deals with URLs.
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Curl supports a range of common internet protocols, currently including
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HTTP, HTTPS, FTP, GOPHER, LDAP, DICT and FILE.
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1.2 What is libcurl?
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libcurl is the engine inside curl that does all the work. curl is more or
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less the command line interface that converts the given options into libcurl
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function invokes. libcurl is a reliable, higly portable multiprotocol file
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transfer library.
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Any application is free to use libcurl, even commercial or closed-source
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ones. Just make sure changes to the lib itself is made public.
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1.3 What is cURL not?
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Curl is *not*, I repeat, *not* a wget clone even though that is a very
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common misconception. Never, during curl's development, have I intended curl
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to replace wget or compete on its market. Curl is targeted at single-shot
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file transfers.
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Curl is not a web site mirroring program. If you wanna use curl to mirror
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something: fine, go ahead and write a script that wraps around curl to make
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it reality (like curlmirror.pl does).
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Curl is not an ftp site mirroring program. Sure, get and send ftp with curl
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but if you want systematic and sequential behaviour you should write a
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script (or write a new program that interfaces libcurl) and do it.
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Curl is not a PHP tool, even though it works perfectly well when used from
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or with PHP.
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Curl is not a single-OS program. Curl exists, compiles, builds and runs
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under a long range of operating systems, including all modern Unixes,
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Windows, Amiga, BeOS, OS/2, OS X, QNX etc.
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1.4 When will you make curl do XXXX ?
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I love suggestions of what to change in order to make curl and libcurl
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better. I do however believe in a few rules when it comes to the future of
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curl:
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* It is to remain a command line tool. If you want GUIs or fancy scripting
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capabilities, you're free to write another tool that uses libcurl and that
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offers this. There's no point in having one single tool that does every
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imaginable thing. That's also one of the great advantages of having the
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core of curl as a library: libcurl.
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* I do not add things to curl that other small and available tools already
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do very fine at the side. Curl's output is fine to pipe into another
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program or redirect to another file for the next program to interpret.
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* I focus on protocol related issues and improvements. If you wanna do more
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magic with the supported protocols than curl currently does, changes are
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big I will agree. If you wanna add more protocols, I may very well
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agree.
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* If you want me to make all the work while you wait for me to implement it
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for you, that is not a very friendly attitude. I spend a considerable time
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already on maintaining and developing curl. In order to get more out of
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me, I trust you will offer some of your time and efforts in return.
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* If you write the code, chances are bigger that it will get into curl
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faster.
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2. Install Related Problems
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2.1. configure doesn't find OpenSSL even when it is installed
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This may be because of several reasons.
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2.1.1. native linker doesn't find openssl
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Affected platforms:
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Solaris (native cc compiler)
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HPUX (native cc compiler)
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SGI IRIX (native cc compiler)
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SCO UNIX (native cc compiler)
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When configuring curl, I specify --with-ssl. OpenSSL is installed in
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/usr/local/ssl Configure reports SSL in /usr/local/ssl, but fails to find
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CRYPTO_lock in -lcrypto
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Cause: The cc for this test places the -L/usr/local/ssl/lib AFTER
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-lcrypto, so ld can't find the library. This is due to a bug in the GNU
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autoconf tool.
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Workaround: Specifying "LDFLAGS=-L/usr/local/ssl/lib" in front of
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./configure places the -L/usr/local/ssl/lib early enough in the command
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line to make things work
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Solution submitted by: Bob Allison <allisonb@users.sourceforge.net>
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2.1.2. only the libssl lib is missing
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If all include files and the libcrypto lib is present, with only the
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libssl being missing according to configure, this is mostly likely because
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a few functions are left out from the libssl.
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If the function names missing include RSA or RSAREF you can be certain
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that this is because libssl requires the RSA and RSASEF libs to build.
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See the INSTALL file section that explains how to add those libs to
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configure. Make sure that you remove the config.cache file before you
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rerun configure with the new flags.
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2.2. Does curl work/build with other SSL libraries?
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Curl has been written to use OpenSSL, although I doubt there would be much
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problems using a different library. I just don't know any other free one and
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that has limited my possibilities to develop against anything else.
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If anyone does "port" curl to use a commercial SSL library, I am of course
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very interested in getting the patch!
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2.3. Where can I find a copy of LIBEAY32.DLL?
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That is an OpenSSL binary built for Windows.
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Curl uses OpenSSL to do the SSL stuff. The LIBEAY32.DLL is what curl needs
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on a windows machine to do https://. Check out the curl web page to find
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accurate and up-to-date pointers to recent OpenSSL DDLs and other binary
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packages.
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2.4. Does cURL support Socks (RFC 1928) ?
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No. Nobody has wanted it that badly yet. I would appriciate patches that
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brings this functionality.
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3. Usage problems
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3.1. curl: (1) SSL is disabled, https: not supported
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If you get this output when trying to get anything from a https:// server,
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it means that the configure script couldn't find all libs and include files
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it requires for SSL to work. If the configure script fails to find them,
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curl is simply built without SSL support.
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To get the https:// support into a curl that was previously built but that
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reports that https:// is not supported, you should dig through the document
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and logs and check out why the configure script doesn't find the SSL libs
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and/or include files.
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Also, check out the other paragraph in this FAQ labeled "configure doesn't
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find OpenSSL even when it is installed".
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3.2. How do I tell curl to resume a transfer?
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Curl supports resume both ways on FTP, download ways on HTTP.
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Try the -c and -C options.
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3.3. Why doesn't my posting using -F work?
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You can't simply use -F or -d at your choice. The web server that will
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receive your post assumes one of the formats. If the form you're trying to
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"fake" sets the type to 'multipart/form-data', than and only then you must
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use the -F type. In all the most common cases, you should use -d which then
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causes a posting with the type 'application/x-www-form-urlencoded'.
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I have described this in some detail in the README.curl file, and if you
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don't understand it the first time, read it again before you post questions
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about this to the mailing list. I would also suggest that you read through
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the mailing list archives for old postings and questions regarding this.
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3.4. How do I tell curl to run custom FTP commands?
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You can tell curl to perform optional commands both before and/or after a
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file transfer. Study the -Q/--quote option.
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Since curl is used for file transfers, you don't use curl to just perform
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ftp commands without transfering anything. Therefore you must always specify
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a URL to transfer to/from even when doing custom FTP commands.
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3.5. How can I disable the Pragma: nocache header?
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You can change all internally generated headers by adding a replacement with
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the -H/--header option. By adding a header with empty contents you safely
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disable that one. Use -H "Pragma:" to disable that specific header.
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3.6. Does curl support javascript, ASP, XML, XHTML or HTML version Y?
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To curl, all contents are alike. It doesn't matter how the page was
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generated. It may be ASP, PHP, perl, shell-script, SSI or plain
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HTML-files. There's no difference to curl and it doesn't even know what kind
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of language that generated the page.
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Javascript is slightly different since that is code embedded in the HTML
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that is sent for the client to interpret and curl has no javascript
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interpreter.
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3.7. Can I use curl to delete/rename a file through FTP?
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Yes. You specify custom ftp commands with -Q/--quote.
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One example would be to delete a file after you have downloaded it:
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curl -O ftp://download.com/coolfile -Q '-DELE coolfile'
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3.8 How do I tell curl to follow HTTP redirects?
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Curl does not follow so-called redirects by default. The Location: header
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that informs the client about this is only interpreted if you're using the
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-L/--location option. As in:
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curl -L http://redirector.com
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4. Running Problems
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4.1. Problems connecting to SSL servers.
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It took a very long time before I could sort out why curl had problems
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to connect to certain SSL servers when using SSLeay or OpenSSL v0.9+.
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The error sometimes showed up similar to:
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16570:error:1407D071:SSL routines:SSL2_READ:bad mac decode:s2_pkt.c:233:
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It turned out to be because many older SSL servers don't deal with SSLv3
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requests properly. To correct this problem, tell curl to select SSLv2 from
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the command line (-2/--sslv2).
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I have also seen examples where the remote server didn't like the SSLv2
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request and instead you had to force curl to use SSLv3 with -3/--sslv3.
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4.2. Why do I get problems when I use & in the URL?
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In general unix shells, the & letter is treated special and when used it
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runs the specified command in the background. To safely send the & as a part
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of a URL, you should qoute the entire URL by using single (') or double (")
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quotes around it.
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An example that would invoke a remote CGI that uses &-letters could be:
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curl 'http://www.altavista.com/cgi-bin/query?text=yes&q=curl'
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4.3. How can I use {, }, [ or ] to specify multiple URLs?
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Because those letters have a special meaning to the shell, and to be used in
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a URL specified to curl you must quote them.
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An example that downloads two URLs (sequentially) would do:
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curl '{curl,www}.haxx.se'
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4.4. Why do I get downloaded data even though the web page doesn't exist?
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Curl asks remote servers for the page you specify. If the page doesn't exist
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at the server, the HTTP protocol defines how the server should respond and
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that means that headers and a "page" will be returned. That's simply how
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HTTP works.
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By using the --fail option you can tell curl explicitly to not get any data
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if the HTTP return code doesn't say success.
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4.5 Why do I get return code XXX from a HTTP server?
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RFC2616 clearly explains the return codes. I'll make a short transcript
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here. Go read the RFC for exact details:
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4.5.1 "400 Bad Request"
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The request could not be understood by the server due to malformed
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syntax. The client SHOULD NOT repeat the request without modifications.
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4.5.2 "401 Unauthorized"
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The request requires user authentication.
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4.5.3 "403 Forbidden"
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The server understood the request, but is refusing to fulfill it.
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Authorization will not help and the request SHOULD NOT be repeated.
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4.5.4 "404 Not Found"
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The server has not found anything matching the Request-URI. No indication
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is given of whether the condition is temporary or permanent.
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4.5.5 "405 Method Not Allowed"
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The method specified in the Request-Line is not allowed for the resource
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identified by the Request-URI. The response MUST include an Allow header
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containing a list of valid methods for the requested resource.
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4.6. Can you tell me what error code 142 means?
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All error codes that are larger than the highest documented error code means
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that curl has existed due to a timeout. There is currentl no nice way for
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curl to abort from such a condition and that's why it gets this undocumented
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error. This is planned to change in a future release.
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4.7. How do I keep usernames and passwords secret in Curl command lines?
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I see this problem as two parts:
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The first part is to avoid having clear-text passwords in the command line
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so that they don't appear in 'ps' outputs and similar. That is easily
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avoided by using the "-K" option tho tell curl to read parameters from a
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file or stdin to which you can pass the secret info.
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To keep the passwords in your account secret from the rest of the world is
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not a task that curl addresses. You could of course encrypt them somehow to
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at least hide them from being read by human eyes, but that is not what
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anyone would call security.
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5. libcurl Issues
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5.1. Is libcurl thread safe?
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As version seven is slowly marching in as the libcurl version to use, we
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have made a serious attempt to address all places in the code where we could
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forsee problems for multi-threaded programs. If your system has them, curl
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will attempt to use threadsafe functions instead of non-safe ones.
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I am very interested in once and for all getting some kind of report or
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README file from those who have used libcurl in a threaded environment,
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since I haven't and I get this question more and more frequently!
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6. License Issues
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Curl and libcurl are released under the MPL, the Mozilla Public License. To
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get a really good answer to this or other licensing questions, you should
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study the MPL license and the license you are about to use and check for
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clashes yourself. This is a brief summary for the cases we get the most
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questions. (Parts of this section was enhanced by Bjorn Reese.)
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6.1. I have a GPL program, can I use the libcurl library?
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No, unfortunately you cannot distribute the (lib)curl code with your code.
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According to both the Free Software Foundation and the Mozilla
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Organization, MPL and GPL are not compatible, because GPL requires of
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other licenses that they do "not impose any further restrictions on the
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recipients' exercise of the rights granted herein" [GPL paragraph 6] (One
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could argue about which license that actually imposes restrictions, but
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that'll be left for you to ponder about.)
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However, you have two alternative options:
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1) Nothing prevents you from distributing your program separately from
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curl and libcurl, and request that the user receives it separately.
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The incompability issues between MPL and GPL only applies to the
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distribution.
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2) Add the following exception to your distribution (you must have the
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consent of all the copyright holders of the source code in your
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project in order to do this)
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As a special exception, you have permission to link this program
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with the curl library and distribute executables, as long as you
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follow the requirements of the GNU GPL in regard to all of the
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software in the executable aside from curl.
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This exception must be added to your GPL covered source code files
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immediately after the notification mentioned in the appendix of GPL.
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The wording was originally suggested by the Free Software Foundation
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in relationship with the Qt library. We have changed the wording to
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apply to curl.
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6.2. I have a closed-source program, can I use the libcurl library?
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Yes, libcurl does not put any restrictions on the program that uses the
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library. If you end up doing changes to the library, only those changes
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must be made available, not the ones to your program.
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6.3. I have a BSD licensed program, can I use the libcurl library?
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Yes, libcurl does not put any restrictions on the program that uses the
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library. If you end up doing changes to the library, only those changes
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must be made available, not the ones to your program.
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6.4. I have a program that uses LGPL libraries, can I use libcurl?
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Yes you can. LGPL libraries don't spread to other libraries the same way
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GPL ones do.
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However, when you read paragraph (3) of the LGPL license, you'll see that
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anyone - at will - may at any time convert that LGPL program into GPL. And
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GPL programs can't be distributed together with MPL programs, neither with
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(lib)curl source code and not as a binary.
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6.5. Can I modify curl/libcurl for my program and keep the changes secret?
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No, you're not allowed to do that.
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6.6. Can you please change the curl/libcurl license to XXXX?
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No. We carefully picked this license years ago and a large amount of
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people have contributed with source code knowing that this is the license
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we use. This license puts the restrictions we want on curl/libcurl and it
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does not spread to other programs or libraries that use it.
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