1 Updated: Nov 7, 2009 (http://curl.haxx.se/docs/faq.html)
6 \___|\___/|_| \_\_____|
14 1.4 When will you make curl do XXXX ?
16 1.6 What do you get for making curl?
17 1.7 What about CURL from curl.com?
18 1.8 I have a problem who do I mail?
19 1.9 Where do I buy commercial support for curl?
20 1.10 How many are using curl?
21 1.11 Why don't you update ca-bundle.crt
22 1.12 I have a problem who can I chat with?
24 2. Install Related Problems
25 2.1 configure doesn't find OpenSSL even when it is installed
26 2.1.1 native linker doesn't find OpenSSL
27 2.1.2 only the libssl lib is missing
28 2.2 Does curl work/build with other SSL libraries?
29 2.3 Where can I find a copy of LIBEAY32.DLL?
30 2.4 Does curl support SOCKS (RFC 1928) ?
33 3.1 curl: (1) SSL is disabled, https: not supported
34 3.2 How do I tell curl to resume a transfer?
35 3.3 Why doesn't my posting using -F work?
36 3.4 How do I tell curl to run custom FTP commands?
37 3.5 How can I disable the Pragma: nocache header?
38 3.6 Does curl support ASP, XML, XHTML or HTML version Y?
39 3.7 Can I use curl to delete/rename a file through FTP?
40 3.8 How do I tell curl to follow HTTP redirects?
41 3.9 How do I use curl in my favorite programming language?
42 3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
43 3.11 How do I POST with a different Content-Type?
44 3.12 Why do FTP specific features over HTTP proxy fail?
45 3.13 Why does my single/double quotes fail?
46 3.14 Does curl support Javascript or PAC (automated proxy config)?
47 3.15 Can I do recursive fetches with curl?
48 3.16 What certificates do I need when I use SSL?
49 3.17 How do I list the root dir of an FTP server?
50 3.18 Can I use curl to send a POST/PUT and not wait for a response?
53 4.1 Problems connecting to SSL servers.
54 4.2 Why do I get problems when I use & or % in the URL?
55 4.3 How can I use {, }, [ or ] to specify multiple URLs?
56 4.4 Why do I get downloaded data even though the web page doesn't exist?
57 4.5 Why do I get return code XXX from a HTTP server?
58 4.5.1 "400 Bad Request"
59 4.5.2 "401 Unauthorized"
62 4.5.5 "405 Method Not Allowed"
63 4.5.6 "301 Moved Permanently"
64 4.6 Can you tell me what error code 142 means?
65 4.7 How do I keep user names and passwords secret in Curl command lines?
67 4.9 Curl can't authenticate to the server that requires NTLM?
68 4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
69 4.11 Why does my HTTP range requests return the full document?
70 4.12 Why do I get "certificate verify failed" ?
71 4.13 Why is curl -R on Windows one hour off?
72 4.14 Redirects work in browser but not with curl!
73 4.15 FTPS doesn't work
74 4.16 My HTTP POST or PUT requests are slow!
75 4.17 Non-functional connect timeouts on Windows
76 4.18 file:// URLs containing drive letters (Windows, NetWare)
79 5.1 Is libcurl thread-safe?
80 5.2 How can I receive all data into a large memory chunk?
81 5.3 How do I fetch multiple files with libcurl?
82 5.4 Does libcurl do Winsock initing on win32 systems?
83 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
84 5.6 What about Keep-Alive or persistent connections?
85 5.7 Link errors when building libcurl on Windows!
86 5.8 libcurl.so.3: open failed: No such file or directory
87 5.9 How does libcurl resolve host names?
88 5.10 How do I prevent libcurl from writing the response to stdout?
89 5.11 How do I make libcurl not receive the whole HTTP response?
90 5.12 Can I make libcurl fake or hide my real IP address?
91 5.13 How do I stop an ongoing transfer?
92 5.14 Using C++ non-static functions for callbacks?
93 5.15 How do I get an FTP directory listing?
96 6.1 I have a GPL program, can I use the libcurl library?
97 6.2 I have a closed-source program, can I use the libcurl library?
98 6.3 I have a BSD licensed program, can I use the libcurl library?
99 6.4 I have a program that uses LGPL libraries, can I use libcurl?
100 6.5 Can I modify curl/libcurl for my program and keep the changes secret?
101 6.6 Can you please change the curl/libcurl license to XXXX?
102 6.7 What are my obligations when using libcurl in my commercial apps?
105 7.1 What is PHP/CURL?
106 7.2 Who write PHP/CURL?
107 7.3 Can I perform multiple requests using the same handle?
109 ==============================================================================
115 cURL is the name of the project. The name is a play on 'Client for URLs',
116 originally with URL spelled in uppercase to make it obvious it deals with
117 URLs. The fact it can also be pronounced 'see URL' also helped, it works as
118 an abbreviation for "Client URL Request Library" or why not the recursive
119 version: "Curl URL Request Library".
121 The cURL project produces two products:
125 A free and easy-to-use client-side URL transfer library, supporting FTP,
126 FTPS, HTTP, HTTPS, SCP, SFTP, TFTP, TELNET, DICT, FILE, LDAP and LDAPS.
127 libcurl supports HTTPS certificates, HTTP POST, HTTP PUT, FTP uploading,
128 kerberos, HTTP form based upload, proxies, cookies, user+password
129 authentication, file transfer resume, http proxy tunneling and more!
131 libcurl is highly portable, it builds and works identically on numerous
132 platforms, including Solaris, NetBSD, FreeBSD, OpenBSD, Darwin, HPUX,
133 IRIX, AIX, Tru64, Linux, UnixWare, HURD, Windows, Amiga, OS/2, BeOs, Mac
134 OS X, Ultrix, QNX, OpenVMS, RISC OS, Novell NetWare, DOS, Symbian, OSF,
135 Android, Minix, IBM TPF and more...
137 libcurl is free, thread-safe, IPv6 compatible, feature rich, well
142 A command line tool for getting or sending files using URL syntax.
144 Since curl uses libcurl, it supports a range of common Internet protocols,
145 currently including HTTP, HTTPS, FTP, FTPS, SCP, SFTP, TFTP, LDAP, LDAPS,
146 DICT, TELNET and FILE.
148 We pronounce curl and cURL with an initial k sound: [kurl].
150 There are numerous sub-projects and related projects that also use the word
151 curl in the project names in various combinations, but you should take
152 notice that this FAQ is directed at the command-line tool named curl (and
153 libcurl the library), and may therefore not be valid for other curl-related
154 projects. (There is however a small section for the PHP/CURL in this FAQ.)
158 libcurl is a reliable and portable library which provides you with an easy
159 interface to a range of common Internet protocols.
161 You can use libcurl for free in your application, be it open source,
162 commercial or closed-source.
164 libcurl is most probably the most portable, most powerful and most often
165 used C-based multi-platform file transfer library on this planet - be it
166 open source or commercial.
168 1.3 What is curl not?
170 Curl is not a wget clone. That is a common misconception. Never, during
171 curl's development, have we intended curl to replace wget or compete on its
172 market. Curl is targeted at single-shot file transfers.
174 Curl is not a web site mirroring program. If you want to use curl to mirror
175 something: fine, go ahead and write a script that wraps around curl to make
176 it reality (like curlmirror.pl does).
178 Curl is not an FTP site mirroring program. Sure, get and send FTP with curl
179 but if you want systematic and sequential behavior you should write a
180 script (or write a new program that interfaces libcurl) and do it.
182 Curl is not a PHP tool, even though it works perfectly well when used from
183 or with PHP (when using the PHP/CURL module).
185 Curl is not a program for a single operating system. Curl exists, compiles,
186 builds and runs under a wide range of operating systems, including all
187 modern Unixes (and a bunch of older ones too), Windows, Amiga, BeOS, OS/2,
190 1.4 When will you make curl do XXXX ?
192 We love suggestions of what to change in order to make curl and libcurl
193 better. We do however believe in a few rules when it comes to the future of
196 * Curl -- the command line tool -- is to remain a non-graphical command line
197 tool. If you want GUIs or fancy scripting capabilities, you should look
198 for another tool that uses libcurl.
200 * We do not add things to curl that other small and available tools already
201 do very fine at the side. Curl's output is fine to pipe into another
202 program or redirect to another file for the next program to interpret.
204 * We focus on protocol related issues and improvements. If you wanna do more
205 magic with the supported protocols than curl currently does, chances are
206 big we will agree. If you wanna add more protocols, we may very well
209 * If you want someone else to make all the work while you wait for us to
210 implement it for you, that is not a very friendly attitude. We spend a
211 considerable time already on maintaining and developing curl. In order to
212 get more out of us, you should consider trading in some of your time and
215 * If you write the code, chances are bigger that it will get into curl
220 curl and libcurl are not made by any single individual. Daniel Stenberg is
221 project leader and main developer, but other persons' submissions are
222 important and crucial. Anyone can contribute and post their changes and
223 improvements and have them inserted in the main sources (of course on the
224 condition that developers agree on that the fixes are good).
226 The full list of all contributors is found in the docs/THANKS file.
228 curl is developed by a community, with Daniel at the wheel.
230 1.6 What do you get for making curl?
232 Project cURL is entirely free and open. No person gets paid for developing
233 (lib)curl on full or even part time. We do this voluntarily on our spare
234 time. Occasionally companies pay individual developers to work on curl, but
235 that's up to each company and developer. It is not controlled by nor
236 supervised in any way by the project.
238 We still get help from companies. Haxx provides web site, bandwidth, mailing
239 lists, CVS server etc and sourceforge.net hosts project services we take
240 advantage from, like the bug tracker. Also again, some companies have
241 sponsored certain parts of the development in the past and I hope some will
242 continue to do so in the future.
244 If you want to support our project, consider a donation or a banner-program
245 or even better: by helping us coding, documenting, testing etc.
247 1.7 What about CURL from curl.com?
249 During the summer 2001, curl.com was busy advertising their client-side
250 programming language for the web, named CURL.
252 We are in no way associated with curl.com or their CURL programming
255 Our project name curl has been in effective use since 1998. We were not the
256 first computer related project to use the name "curl" and do not claim any
257 first-hand rights to the name.
259 We recognize that we will be living in parallel with curl.com and wish them
262 1.8 I have a problem who do I mail?
264 Please do not mail any single individual unless you really need to. Keep
265 curl-related questions on a suitable mailing list. All available mailing
266 lists are listed in the MANUAL document and online at
267 http://curl.haxx.se/mail/
269 Keeping curl-related questions and discussions on mailing lists allows
270 others to join in and help, to share their ideas, contribute their
271 suggestions and spread their wisdom. Keeping discussions on public mailing
272 lists also allows for others to learn from this (both current and future
273 users thanks to the web based archives of the mailing lists), thus saving us
274 from having to repeat ourselves even more. Thanks for respecting this.
276 If you have found or simply suspect a security problem in curl or libcurl,
277 mail curl-security at haxx.se (closed list of receivers, mails are not
278 disclosed) and tell. Then we can produce a fix in a timely manner before the
279 flaw is announced to the world, thus lessen the impact the problem will have
282 1.9 Where do I buy commercial support for curl?
284 curl is fully open source. It means you can hire any skilled engineer to fix
285 your curl-related problems.
287 We list available alternatives on the curl web site:
288 http://curl.haxx.se/support.html
290 1.10 How many are using curl?
292 It is impossible to tell.
294 We don't know how many users that knowingly have installed and use curl.
296 We don't know how many users that use curl without knowing that they are in
299 We don't know how many users that downloaded or installed curl and then
302 Some facts to use as input to the math:
304 curl packages are downloaded from the curl.haxx.se and mirrors over a
305 million times per year. curl is installed by default with most Linux
306 distributions. curl is installed by default with Mac OS X. curl and libcurl
307 as used by numerous applications that include libcurl binaries in their
308 distribution packages (like Adobe Acrobat Reader and Google Earth).
310 More than 90 known named companies use curl in commercial environments and
311 products. More than 100 known named open source projects depend on
314 In a poll on the curl web site mid-2005, more than 50% of the 300+ answers
315 estimated a user base of one million users or more.
317 In March 2005, the "Linux Counter project" estimated a total Linux user base
318 of some 29 millions, while Netcraft detected some 4 million "active" Linux
319 based web servers. A guess is that a fair amount of these Linux
320 installations have curl installed.
322 All this taken together, there is no doubt that there are millions of
325 http://curl.haxx.se/docs/companies.html
326 http://curl.haxx.se/docs/programs.html
327 http://curl.haxx.se/libcurl/using/apps.html
328 http://counter.li.org/estimates.php
329 http://news.netcraft.com/archives/2005/03/14/fedora_makes_rapid_progress.html
331 1.11 Why don't you update ca-bundle.crt
333 The ca-bundle.crt file that used to be bundled with curl was very outdated
334 (it being last modified year 2000 should tell) and must be replaced with a
335 much more modern and up-to-date version by anyone who wants to verify peers
336 anyway. It is no longer provided, the last curl release that shipped it was
339 In the cURL project we've decided not to attempt to keep this file updated
340 (or even present anymore) since deciding what to add to a ca cert bundle is
341 an undertaking we've not been ready to accept, and the one we can get from
342 Mozilla is perfectly fine so there's no need to duplicate that work.
344 Today, with many services performed over HTTPS, every operating system
345 should come with a default ca cert bundle that can be deemed somewhat
346 trustworthy and that collection (if reasonably updated) should be deemed to
347 be a lot better than a private curl version.
349 If you want the most recent collection of ca certs that Mozilla Firefox
350 uses, we recommend that you extract the collection yourself from Mozilla
351 Firefox (by running 'make ca-bundle), or by using our online service setup
352 for this purpose: http://curl.haxx.se/docs/caextract.html
354 1.12 I have a problem who can I chat with?
356 There's a bunch of friendly people hanging out in the #curl channel on the
357 IRC network irc.freenode.net. If you're polite and nice, chances are big
358 that you can get -- or provide -- help instantly.
361 2. Install Related Problems
363 2.1 configure doesn't find OpenSSL even when it is installed
365 This may be because of several reasons.
367 2.1.1 native linker doesn't find openssl
370 Solaris (native cc compiler)
371 HPUX (native cc compiler)
372 SGI IRIX (native cc compiler)
373 SCO UNIX (native cc compiler)
375 When configuring curl, I specify --with-ssl. OpenSSL is installed in
376 /usr/local/ssl Configure reports SSL in /usr/local/ssl, but fails to find
377 CRYPTO_lock in -lcrypto
379 Cause: The cc for this test places the -L/usr/local/ssl/lib AFTER
380 -lcrypto, so ld can't find the library. This is due to a bug in the GNU
383 Workaround: Specifying "LDFLAGS=-L/usr/local/ssl/lib" in front of
384 ./configure places the -L/usr/local/ssl/lib early enough in the command
385 line to make things work
387 2.1.2 only the libssl lib is missing
389 If all include files and the libcrypto lib is present, with only the
390 libssl being missing according to configure, this is mostly likely because
391 a few functions are left out from the libssl.
393 If the function names missing include RSA or RSAREF you can be certain
394 that this is because libssl requires the RSA and RSAREF libs to build.
396 See the INSTALL file section that explains how to add those libs to
397 configure. Make sure that you remove the config.cache file before you
398 rerun configure with the new flags.
400 2.2 Does curl work/build with other SSL libraries?
402 Curl has been written to use OpenSSL, GnuTLS, yassl or NSS, although there
403 should not be many problems using a different library. If anyone does "port"
404 curl to use a different SSL library, we are of course very interested in
407 2.3 Where can I find a copy of LIBEAY32.DLL?
409 That is an OpenSSL binary built for Windows.
411 Curl uses OpenSSL to do the SSL stuff. The LIBEAY32.DLL is what curl needs
412 on a windows machine to do https://. Check out the curl web site to find
413 accurate and up-to-date pointers to recent OpenSSL DLLs and other binary
416 2.4 Does curl support SOCKS (RFC 1928) ?
418 Yes, SOCKS 4 and 5 are supported.
423 3.1 curl: (1) SSL is disabled, https: not supported
425 If you get this output when trying to get anything from a https:// server,
426 it means that the configure script couldn't find all libs and include files
427 it requires for SSL to work. If the configure script fails to find them,
428 curl is simply built without SSL support.
430 To get the https:// support into a curl that was previously built but that
431 reports that https:// is not supported, you should dig through the document
432 and logs and check out why the configure script doesn't find the SSL libs
433 and/or include files.
435 Also, check out the other paragraph in this FAQ labelled "configure doesn't
436 find OpenSSL even when it is installed".
438 3.2 How do I tell curl to resume a transfer?
440 Curl supports resumed transfers both ways on both FTP and HTTP.
444 3.3 Why doesn't my posting using -F work?
446 You can't simply use -F or -d at your choice. The web server that will
447 receive your post assumes one of the formats. If the form you're trying to
448 "fake" sets the type to 'multipart/form-data', then and only then you must
449 use the -F type. In all the most common cases, you should use -d which then
450 causes a posting with the type 'application/x-www-form-urlencoded'.
452 This is described in some detail in the MANUAL and TheArtOfHttpScripting
453 documents, and if you don't understand it the first time, read it again
454 before you post questions about this to the mailing list. Also, try reading
455 through the mailing list archives for old postings and questions regarding
458 3.4 How do I tell curl to run custom FTP commands?
460 You can tell curl to perform optional commands both before and/or after a
461 file transfer. Study the -Q/--quote option.
463 Since curl is used for file transfers, you don't use curl to just perform
464 FTP commands without transferring anything. Therefore you must always specify
465 a URL to transfer to/from even when doing custom FTP commands.
467 3.5 How can I disable the Pragma: nocache header?
469 You can change all internally generated headers by adding a replacement with
470 the -H/--header option. By adding a header with empty contents you safely
471 disable that one. Use -H "Pragma:" to disable that specific header.
473 3.6 Does curl support ASP, XML, XHTML or HTML version Y?
475 To curl, all contents are alike. It doesn't matter how the page was
476 generated. It may be ASP, PHP, Perl, shell-script, SSI or plain
477 HTML-files. There's no difference to curl and it doesn't even know what kind
478 of language that generated the page.
480 See also item 3.14 regarding javascript.
482 3.7 Can I use curl to delete/rename a file through FTP?
484 Yes. You specify custom FTP commands with -Q/--quote.
486 One example would be to delete a file after you have downloaded it:
488 curl -O ftp://download.com/coolfile -Q '-DELE coolfile'
490 or rename a file after upload:
492 curl -T infile ftp://upload.com/dir/ -Q "-RNFR infile" -Q "-RNTO newname"
494 3.8 How do I tell curl to follow HTTP redirects?
496 Curl does not follow so-called redirects by default. The Location: header
497 that informs the client about this is only interpreted if you're using the
498 -L/--location option. As in:
500 curl -L http://redirector.com
502 Not all redirects are HTTP ones, see 4.14
504 3.9 How do I use curl in my favorite programming language?
506 There exist many language interfaces/bindings for curl that integrates it
507 better with various languages. If you are fluid in a script language, you
508 may very well opt to use such an interface instead of using the command line
511 Find out more about which languages that support curl directly, and how to
512 install and use them, in the libcurl section of the curl web site:
513 http://curl.haxx.se/libcurl/
515 In October 2009, there were interfaces available for the following
516 languages: Ada95, Basic, C, C++, Ch, Cocoa, D, Dylan, Eiffel, Euphoria,
517 Ferite, Gambas, glib/GTK+, Haskell, ILE/RPG, Java, Lisp, Lua, Mono, .NET,
518 Object-Pascal, O'Caml, Pascal, Perl, PHP, PostgreSQL, Python, R, Rexx, Ruby,
519 Scheme, S-Lang, Smalltalk, SP-Forth, SPL, Tcl, Visual Basic, Visual FoxPro,
520 Q, wxwidgets and XBLite. By the time you read this, additional ones may have
523 3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
525 Curl adheres to the HTTP spec, which basically means you can play with *any*
526 protocol that is built on top of HTTP. Protocols such as SOAP, WEBDAV and
527 XML-RPC are all such ones. You can use -X to set custom requests and -H to
528 set custom headers (or replace internally generated ones).
530 Using libcurl is of course just as fine and you'd just use the proper
531 library options to do the same.
533 3.11 How do I POST with a different Content-Type?
535 You can always replace the internally generated headers with -H/--header.
536 To make a simple HTTP POST with text/xml as content-type, do something like:
538 curl -d "datatopost" -H "Content-Type: text/xml" [URL]
540 3.12 Why do FTP specific features over HTTP proxy fail?
542 Because when you use a HTTP proxy, the protocol spoken on the network will
543 be HTTP, even if you specify a FTP URL. This effectively means that you
544 normally can't use FTP specific features such as FTP upload and FTP quote
547 There is one exception to this rule, and that is if you can "tunnel through"
548 the given HTTP proxy. Proxy tunneling is enabled with a special option (-p)
549 and is generally not available as proxy admins usually disable tunneling to
550 other ports than 443 (which is used for HTTPS access through proxies).
552 3.13 Why does my single/double quotes fail?
554 To specify a command line option that includes spaces, you might need to
555 put the entire option within quotes. Like in:
557 curl -d " with spaces " url.com
561 curl -d ' with spaces ' url.com
563 Exactly what kind of quotes and how to do this is entirely up to the shell
564 or command line interpreter that you are using. For most unix shells, you
565 can more or less pick either single (') or double (") quotes. For
566 Windows/DOS prompts I believe you're forced to use double (") quotes.
568 Please study the documentation for your particular environment. Examples in
569 the curl docs will use a mix of both these ones as shown above. You must
570 adjust them to work in your environment.
572 Remember that curl works and runs on more operating systems than most single
573 individuals have ever tried.
575 3.14 Does curl support Javascript or PAC (automated proxy config)?
577 Many web pages do magic stuff using embedded Javascript. Curl and libcurl
578 have no built-in support for that, so it will be treated just like any other
581 .pac files are a netscape invention and are sometimes used by organizations
582 to allow them to differentiate which proxies to use. The .pac contents is
583 just a Javascript program that gets invoked by the browser and that returns
584 the name of the proxy to connect to. Since curl doesn't support Javascript,
585 it can't support .pac proxy configuration either.
587 Some workarounds usually suggested to overcome this Javascript dependency:
589 - Depending on the Javascript complexity, write up a script that
590 translates it to another language and execute that.
592 - Read the Javascript code and rewrite the same logic in another language.
594 - Implement a Javascript interpreter, people have successfully used the
595 Mozilla Javascript engine in the past.
597 - Ask your admins to stop this, for a static proxy setup or similar.
599 3.15 Can I do recursive fetches with curl?
601 No. curl itself has no code that performs recursive operations, such as
602 those performed by wget and similar tools.
604 There exist wrapper scripts with that functionality (for example the
605 curlmirror perl script), and you can write programs based on libcurl to do
606 it, but the command line tool curl itself cannot.
608 3.16 What certificates do I need when I use SSL?
610 There are three different kinds of "certificates" to keep track of when we
611 talk about using SSL-based protocols (HTTPS or FTPS) using curl or libcurl.
613 - Client certificate. The server you communicate may require that you can
614 provide this in order to prove that you actually are who you claim to be.
615 If the server doesn't require this, you don't need a client certificate.
617 - Server certificate. The server you communicate with has a server
618 certificate. You can and should verify this certificate to make sure that
619 you are truly talking to the real server and not a server impersonating
622 - Certificate Authority certificate ("CA cert"). You often have several CA
623 certs in a CA cert bundle that can be used to verify a server certificate
624 that was signed by one of the authorities in the bundle. curl comes with a
625 default CA cert bundle. You can override the default.
627 The server certificate verification process is made by using a Certificate
628 Authority certificate ("CA cert") that was used to sign the server
629 certificate. Server certificate verification is enabled by default in curl
630 and libcurl and is often the reason for problems as explained in FAQ entry
631 4.12 and the SSLCERTS document
632 (http://curl.haxx.se/docs/sslcerts.html). Server certificates that are
633 "self-signed" or otherwise signed by a CA that you do not have a CA cert
634 for, cannot be verified. If the verification during a connect fails, you
635 are refused access. You then need to explicitly disable the verification
636 to connect to the server.
638 3.17 How do I list the root dir of an FTP server?
640 There are two ways. The way defined in the RFC is to use an encoded slash
641 in the first path part. List the "/tmp" dir like this:
643 curl ftp://ftp.sunet.se/%2ftmp/
645 or the not-quite-kosher-but-more-readable way, by simply starting the path
646 section of the URL with a slash:
648 curl ftp://ftp.sunet.se//tmp/
650 3.18 Can I use curl to send a POST/PUT and not wait for a response?
654 But you could easily write your own program using libcurl to do such stunts.
659 4.1 Problems connecting to SSL servers.
661 It took a very long time before we could sort out why curl had problems to
662 connect to certain SSL servers when using SSLeay or OpenSSL v0.9+. The
663 error sometimes showed up similar to:
665 16570:error:1407D071:SSL routines:SSL2_READ:bad mac decode:s2_pkt.c:233:
667 It turned out to be because many older SSL servers don't deal with SSLv3
668 requests properly. To correct this problem, tell curl to select SSLv2 from
669 the command line (-2/--sslv2).
671 There have also been examples where the remote server didn't like the SSLv2
672 request and instead you had to force curl to use SSLv3 with -3/--sslv3.
674 4.2 Why do I get problems when I use & or % in the URL?
676 In general unix shells, the & symbol is treated specially and when used, it
677 runs the specified command in the background. To safely send the & as a part
678 of a URL, you should quote the entire URL by using single (') or double (")
679 quotes around it. Similar problems can also occur on some shells with other
680 characters, including ?*!$~(){}<>\|;`. When in doubt, quote the URL.
682 An example that would invoke a remote CGI that uses &-symbols could be:
684 curl 'http://www.altavista.com/cgi-bin/query?text=yes&q=curl'
686 In Windows, the standard DOS shell treats the %-symbol specially and you
687 need to use TWO %-symbols for each single one you want to use in the URL.
689 Also note that if you want the literal %-symbol to be part of the data you
690 pass in a POST using -d/--data you must encode it as '%25' (which then also
691 needs the %-symbol doubled on Windows machines).
693 4.3 How can I use {, }, [ or ] to specify multiple URLs?
695 Because those letters have a special meaning to the shell, and to be used in
696 a URL specified to curl you must quote them.
698 An example that downloads two URLs (sequentially) would do:
700 curl '{curl,www}.haxx.se'
702 To be able to use those letters as actual parts of the URL (without using
703 them for the curl URL "globbing" system), use the -g/--globoff option:
705 curl -g 'www.site.com/weirdname[].html'
707 4.4 Why do I get downloaded data even though the web page doesn't exist?
709 Curl asks remote servers for the page you specify. If the page doesn't exist
710 at the server, the HTTP protocol defines how the server should respond and
711 that means that headers and a "page" will be returned. That's simply how
714 By using the --fail option you can tell curl explicitly to not get any data
715 if the HTTP return code doesn't say success.
717 4.5 Why do I get return code XXX from a HTTP server?
719 RFC2616 clearly explains the return codes. This is a short transcript. Go
720 read the RFC for exact details:
722 4.5.1 "400 Bad Request"
724 The request could not be understood by the server due to malformed
725 syntax. The client SHOULD NOT repeat the request without modifications.
727 4.5.2 "401 Unauthorized"
729 The request requires user authentication.
731 4.5.3 "403 Forbidden"
733 The server understood the request, but is refusing to fulfill it.
734 Authorization will not help and the request SHOULD NOT be repeated.
736 4.5.4 "404 Not Found"
738 The server has not found anything matching the Request-URI. No indication
739 is given of whether the condition is temporary or permanent.
741 4.5.5 "405 Method Not Allowed"
743 The method specified in the Request-Line is not allowed for the resource
744 identified by the Request-URI. The response MUST include an Allow header
745 containing a list of valid methods for the requested resource.
747 4.5.6 "301 Moved Permanently"
749 If you get this return code and an HTML output similar to this:
751 <H1>Moved Permanently</H1> The document has moved <A
752 HREF="http://same_url_now_with_a_trailing_slash/">here</A>.
754 it might be because you request a directory URL but without the trailing
755 slash. Try the same operation again _with_ the trailing URL, or use the
756 -L/--location option to follow the redirection.
758 4.6 Can you tell me what error code 142 means?
760 All curl error codes are described at the end of the man page, in the
761 section called "EXIT CODES".
763 Error codes that are larger than the highest documented error code means
764 that curl has exited due to a crash. This is a serious error, and we
765 appreciate a detailed bug report from you that describes how we could go
766 ahead and repeat this!
768 4.7 How do I keep user names and passwords secret in Curl command lines?
770 This problem has two sides:
772 The first part is to avoid having clear-text passwords in the command line
773 so that they don't appear in 'ps' outputs and similar. That is easily
774 avoided by using the "-K" option to tell curl to read parameters from a file
775 or stdin to which you can pass the secret info. curl itself will also
776 attempt to "hide" the given password by blanking out the option - this
777 doesn't work on all platforms.
779 To keep the passwords in your account secret from the rest of the world is
780 not a task that curl addresses. You could of course encrypt them somehow to
781 at least hide them from being read by human eyes, but that is not what
782 anyone would call security.
784 Also note that regular HTTP (using Basic authentication) and FTP passwords
785 are sent in clear across the network. All it takes for anyone to fetch them
786 is to listen on the network. Eavesdropping is very easy. Use more secure
787 authentication methods (like Digest, Negotiate or even NTLM) or consider the
788 SSL-based alternatives HTTPS and FTPS.
792 It is not a bug if the behavior is documented. Read the docs first.
793 Especially check out the KNOWN_BUGS file, it may be a documented bug!
795 If it is a problem with a binary you've downloaded or a package for your
796 particular platform, try contacting the person who built the package/archive
799 If there is a bug, read the BUGS document first. Then report it as described
802 4.9 Curl can't authenticate to the server that requires NTLM?
804 This is supported in curl 7.10.6 or later. No earlier curl version knows
805 of this magic. Later versions require the OpenSSL, GnuTLS or Microsoft
806 Windows libraries to provide this functionality. Using the NSS library
807 will not provide NTLM authentication functionality in curl.
809 NTLM is a Microsoft proprietary protocol. Proprietary formats are evil. You
810 should not use such ones.
812 4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
814 Many web servers allow or demand that the administrator configures the
815 server properly for these requests to work on the web server.
817 Some servers seem to support HEAD only on certain kinds of URLs.
819 To fully grasp this, try the documentation for the particular server
820 software you're trying to interact with. This is not anything curl can do
823 4.11 Why does my HTTP range requests return the full document?
825 Because the range may not be supported by the server, or the server may
826 choose to ignore it and return the full document anyway.
828 4.12 Why do I get "certificate verify failed" ?
830 You invoke curl 7.10 or later to communicate on a https:// URL and get an
831 error back looking something similar to this:
833 curl: (35) SSL: error:14090086:SSL routines:
834 SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
836 Then it means that curl couldn't verify that the server's certificate was
837 good. Curl verifies the certificate using the CA cert bundle that comes with
838 the curl installation.
840 To disable the verification (which makes it act like curl did before 7.10),
841 use -k. This does however enable man-in-the-middle attacks.
843 If you get this failure but are having a CA cert bundle installed and used,
844 the server's certificate is not signed by one of the CA's in the bundle. It
845 might for example be self-signed. You then correct this problem by obtaining
846 a valid CA cert for the server. Or again, decrease the security by disabling
849 Details are also in the SSLCERTS file in the release archives, found online
850 here: http://curl.haxx.se/docs/sslcerts.html
852 4.13 Why is curl -R on Windows one hour off?
854 During daylight savings time, when -R is used, curl will set a time that
855 appears one hour off. This happens due to a flaw in how Windows stores and
856 uses file modification times and it is not easily worked around. For details
857 on this problem, read this: http://www.codeproject.com/datetime/dstbugs.asp
859 4.14 Redirects work in browser but not with curl!
861 curl supports HTTP redirects fine (see item 3.8). Browsers generally support
862 at least two other ways to perform directs that curl does not:
864 - Meta tags. You can write a HTML tag that will cause the browser to
865 redirect to another given URL after a certain time.
867 - Javascript. You can write a Javascript program embedded in a HTML page
868 that redirects the browser to another given URL.
870 There is no way to make curl follow these redirects. You must either
871 manually figure out what the page is set to do, or you write a script that
872 parses the results and fetches the new URL.
874 4.15 FTPS doesn't work
876 curl supports FTPS (sometimes known as FTP-SSL) both implicit and explicit
879 When a URL is used that starts with FTPS://, curl assumes implicit SSL on
880 the control connection and will therefore immediately connect and try to
881 speak SSL. FTPS:// connections default to port 990.
883 To use explicit FTPS, you use a FTP:// URL and the --ftp-ssl option (or one
884 of its related flavours). This is the most common method, and the one
885 mandated by RFC4217. This kind of connection then of course uses the
886 standard FTP port 21 by default.
888 4.16 My HTTP POST or PUT requests are slow!
890 libcurl makes all POST and PUT requests (except for POST requests with a
891 very tiny request body) use the "Expect: 100-continue" header. This header
892 allows the server to deny the operation early so that libcurl can bail out
893 already before having to send any data. This is useful in authentication
896 However, many servers don't implement the Expect: stuff properly and if the
897 server doesn't respond (positively) within 1 second libcurl will continue
898 and send off the data anyway.
900 You can disable libcurl's use of the Expect: header the same way you disable
901 any header, using -H / CURLOPT_HTTPHEADER, or by forcing it to use HTTP 1.0.
903 4.17 Non-functional connect timeouts
905 In most Windows setups having a timeout longer than 21 seconds make no
906 difference, as it will only send 3 TCP SYN packets and no more. The second
907 packet sent three seconds after the first and the third six seconds after
908 the second. No more than three packets are sent, no matter how long the
911 See option TcpMaxConnectRetransmissions on this page:
912 http://support.microsoft.com/?scid=kb%3Ben-us%3B175523&x=6&y=7
914 Also, even on non-Windows systems there may run a firewall or anti-virus
915 software or similar that accepts the connection but does not actually do
916 anything else. This will make (lib)curl to consider the connection connected
917 and thus the connect timeout won't trigger.
919 4.18 file:// URLs containing drive letters (Windows, NetWare)
921 When using cURL to try to download a local file, one might use a URL
926 You'll find that even if D:\blah.txt does exist, cURL returns a 'file
929 According to RFC 1738 (http://www.faqs.org/rfcs/rfc1738.html),
930 file:// URLs must contain a host component, but it is ignored by
931 most implementations. In the above example, 'D:' is treated as the
932 host component, and is taken away. Thus, cURL tries to open '/blah.txt'.
933 If your system is installed to drive C:, that will resolve to 'C:\blah.txt',
934 and if that doesn't exist you will get the not found error.
936 To fix this problem, use file:// URLs with *three* leading slashes:
940 Alternatively, if it makes more sense, specify 'localhost' as the host
943 file://localhost/D:/blah.txt
945 In either case, cURL should now be looking for the correct file.
949 5.1 Is libcurl thread-safe?
953 We have written the libcurl code specifically adjusted for multi-threaded
954 programs. libcurl will use thread-safe functions instead of non-safe ones if
955 your system has such.
957 If you use a OpenSSL-powered libcurl in a multi-threaded environment, you
958 need to provide one or two locking functions:
960 http://www.openssl.org/docs/crypto/threads.html
962 If you use a GnuTLS-powered libcurl in a multi-threaded environment, you
963 need to provide locking function(s) for libgcrypt (which is used by GnuTLS
964 for the crypto functions).
966 http://www.gnu.org/software/gnutls/manual/html_node/Multi_002dthreaded-applications.html
968 No special locking is needed with a NSS-powered libcurl. NSS is thread-safe.
970 5.2 How can I receive all data into a large memory chunk?
972 [ See also the examples/getinmemory.c source ]
974 You are in full control of the callback function that gets called every time
975 there is data received from the remote server. You can make that callback do
976 whatever you want. You do not have to write the received data to a file.
978 One solution to this problem could be to have a pointer to a struct that you
979 pass to the callback function. You set the pointer using the
980 CURLOPT_WRITEDATA option. Then that pointer will be passed to the callback
981 instead of a FILE * to a file:
983 /* imaginary struct */
984 struct MemoryStruct {
989 /* imaginary callback function */
991 WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data)
993 size_t realsize = size * nmemb;
994 struct MemoryStruct *mem = (struct MemoryStruct *)data;
996 mem->memory = (char *)realloc(mem->memory, mem->size + realsize + 1);
998 memcpy(&(mem->memory[mem->size]), ptr, realsize);
999 mem->size += realsize;
1000 mem->memory[mem->size] = 0;
1005 5.3 How do I fetch multiple files with libcurl?
1007 libcurl has excellent support for transferring multiple files. You should
1008 just repeatedly set new URLs with curl_easy_setopt() and then transfer it
1009 with curl_easy_perform(). The handle you get from curl_easy_init() is not
1010 only reusable, but you're even encouraged to reuse it if you can, as that
1011 will enable libcurl to use persistent connections.
1013 5.4 Does libcurl do Winsock initialization on win32 systems?
1015 Yes, if told to in the curl_global_init() call.
1017 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
1019 Yes, but you cannot open a FILE * and pass the pointer to a DLL and have
1020 that DLL use the FILE * (as the DLL and the client application cannot access
1021 each others' variable memory areas). If you set CURLOPT_WRITEDATA you must
1022 also use CURLOPT_WRITEFUNCTION as well to set a function that writes the
1023 file, even if that simply writes the data to the specified FILE *.
1024 Similarly, if you use CURLOPT_READDATA you must also specify
1025 CURLOPT_READFUNCTION.
1027 5.6 What about Keep-Alive or persistent connections?
1029 curl and libcurl have excellent support for persistent connections when
1030 transferring several files from the same server. Curl will attempt to reuse
1031 connections for all URLs specified on the same command line/config file, and
1032 libcurl will reuse connections for all transfers that are made using the
1033 same libcurl handle.
1035 5.7 Link errors when building libcurl on Windows!
1037 You need to make sure that your project, and all the libraries (both static
1038 and dynamic) that it links against, are compiled/linked against the same run
1041 This is determined by the /MD, /ML, /MT (and their corresponding /M?d)
1042 options to the command line compiler. /MD (linking against MSVCRT dll) seems
1043 to be the most commonly used option.
1045 When building an application that uses the static libcurl library, you must
1046 add -DCURL_STATICLIB to your CFLAGS. Otherwise the linker will look for
1047 dynamic import symbols. If you get linker error like "unknown symbol
1048 __imp__curl_easy_init ..." you have linked against the wrong (static)
1049 library. If you want to use the libcurl.dll and import lib, you don't need
1050 any extra CFLAGS, but use one of the import libraries below. These are the
1051 libraries produced by the various lib/Makefile.* files:
1053 Target: static lib. import lib for libcurl*.dll.
1054 -----------------------------------------------------------
1055 MingW: libcurl.a libcurldll.a
1056 MSVC (release): libcurl.lib libcurl_imp.lib
1057 MSVC (debug): libcurld.lib libcurld_imp.lib
1058 Borland: libcurl.lib libcurl_imp.lib
1061 5.8 libcurl.so.3: open failed: No such file or directory
1063 This is an error message you might get when you try to run a program linked
1064 with a shared version of libcurl and your run-time linker (ld.so) couldn't
1065 find the shared library named libcurl.so.3.
1067 You need to make sure that ld.so finds libcurl.so.3. You can do that
1068 multiple ways, and it differs somewhat between different operating systems,
1069 but they are usually:
1071 * Add an option to the linker command line that specify the hard-coded path
1072 the run-time linker should check for the lib (usually -R)
1074 * Set an environment variable (LD_LIBRARY_PATH for example) where ld.so
1075 should check for libs
1077 * Adjust the system's config to check for libs in the directory where you've
1078 put the dir (like Linux's /etc/ld.so.conf)
1080 'man ld.so' and 'man ld' will tell you more details
1082 5.9 How does libcurl resolve host names?
1084 libcurl supports a large a number of different name resolve functions. One
1085 of them is picked at build-time and will be used unconditionally. Thus, if
1086 you want to change name resolver function you must rebuild libcurl and tell
1087 it to use a different function.
1089 - The non-ipv6 resolver that can use one out of four host name resolve calls
1090 (depending on what your system supports):
1093 B - gethostbyname_r() with 3 arguments
1094 C - gethostbyname_r() with 5 arguments
1095 D - gethostbyname_r() with 6 arguments
1097 - The ipv6-resolver that uses getaddrinfo()
1099 - The c-ares based name resolver that uses the c-ares library for resolves.
1100 Using this offers asynchronous name resolves but it currently has no IPv6
1103 - The Windows threaded resolver. It use:
1105 A - gethostbyname() on plain ipv4 windows hosts
1106 B - getaddrinfo() on ipv6-enabled windows hosts
1108 Also note that libcurl never resolves or reverse-lookups addresses given as
1109 pure numbers, such as 127.0.0.1 or ::1.
1111 5.10 How do I prevent libcurl from writing the response to stdout?
1113 libcurl provides a default built-in write function that writes received data
1114 to stdout. Set the CURLOPT_WRITEFUNCTION to receive the data, or possibly
1115 set CURLOPT_WRITEDATA to a different FILE * handle.
1117 5.11 How do I make libcurl not receive the whole HTTP response?
1119 You make the write callback (or progress callback) return an error and
1120 libcurl will then abort the transfer.
1122 5.12 Can I make libcurl fake or hide my real IP address?
1124 No. libcurl operates on a higher level than so. Besides, faking IP address
1125 would imply sending IP packages with a made-up source address, and then you
1126 normally get a problem with intercepting the packages sent back as they
1127 would then not be routed to you!
1129 If you use a proxy to access remote sites, the sites will not see your local
1130 IP address but instead the address of the proxy.
1132 Also note that on many networks NATs or other IP-munging techniques are used
1133 that makes you see and use a different IP address locally than what the
1134 remote server will see you coming from.
1136 5.13 How do I stop an ongoing transfer?
1138 There are several ways, but none of them are instant. There is no function
1139 you can call from another thread or similar that will stop it immediately.
1140 Instead you need to make sure that one of the callbacks you use return an
1141 appropriate value that will stop the transfer.
1143 Suitable callbacks that you can do this with include the progress callback,
1144 the read callback and the write callback.
1146 If you're using the multi interface, you also stop a transfer by removing
1147 the particular easy handle from the multi stack.
1149 5.14 Using C++ non-static functions for callbacks?
1151 libcurl is a C library, it doesn't know anything about C++ member functions.
1153 You can overcome this "limitation" with a relative ease using a static
1154 member function that is passed a pointer to the class:
1156 // f is the pointer to your object.
1157 static YourClass::staticFunction(void *buffer, size_t sz, size_t n, void *f)
1159 // Call non-static member function.
1160 static_cast<YourClass*>(f)->nonStaticFunction();
1163 // This is how you pass pointer to the static function:
1164 curl_easy_setopt(hcurl, CURLOPT_WRITEFUNCTION, YourClass:staticFunction);
1165 curl_easy_setopt(hcurl, CURLOPT_WRITEDATA, this);
1167 5.15 How do I get an FTP directory listing?
1169 If you end the FTP URL you request with a slash, libcurl will provide you
1170 with a directory listing of that given directory. You can also set
1171 CURLOPT_CUSTOMREQUEST to alter what exact listing command libcurl would use
1174 The follow-up question that tend to follow the previous one, is how a
1175 program is supposed to parse the directory listing. How does it know what's
1176 a file and what's a dir and what's a symlink etc. The harsh reality is that
1177 FTP provides no such fine and easy-to-parse output. The output format FTP
1178 servers respond to LIST commands are entirely at the server's own liking and
1179 the NLST output doesn't reveal any types and in many cases don't even
1180 include all the directory entries. Also, both LIST and NLST tend to hide
1181 unix-style hidden files (those that start with a dot) by default so you need
1182 to do "LIST -a" or similar to see them.
1184 The application thus needs to parse the LIST output. One such existing
1185 list parser is available at http://cr.yp.to/ftpparse.html
1190 Curl and libcurl are released under a MIT/X derivate license. The license is
1191 very liberal and should not impose a problem for your project. This section
1192 is just a brief summary for the cases we get the most questions. (Parts of
1193 this section was much enhanced by Bjorn Reese.)
1195 We are not lawyers and this is not legal advice. You should probably consult
1196 one if you want true and accurate legal insights without our prejudice.
1198 6.1 I have a GPL program, can I use the libcurl library?
1202 Since libcurl may be distributed under the MIT/X derivate license, it can be
1203 used together with GPL in any software.
1205 6.2 I have a closed-source program, can I use the libcurl library?
1209 libcurl does not put any restrictions on the program that uses the library.
1211 6.3 I have a BSD licensed program, can I use the libcurl library?
1215 libcurl does not put any restrictions on the program that uses the library.
1217 6.4 I have a program that uses LGPL libraries, can I use libcurl?
1221 The LGPL license doesn't clash with other licenses.
1223 6.5 Can I modify curl/libcurl for my program and keep the changes secret?
1227 The MIT/X derivate license practically allows you to do almost anything with
1228 the sources, on the condition that the copyright texts in the sources are
1231 6.6 Can you please change the curl/libcurl license to XXXX?
1235 We have carefully picked this license after years of development and
1236 discussions and a large amount of people have contributed with source code
1237 knowing that this is the license we use. This license puts the restrictions
1238 we want on curl/libcurl and it does not spread to other programs or
1239 libraries that use it. It should be possible for everyone to use libcurl or
1240 curl in their projects, no matter what license they already have in use.
1242 6.7 What are my obligations when using libcurl in my commercial apps?
1244 Next to none. All you need to adhere to is the MIT-style license (stated in
1245 the COPYING file) which basically says you have to include the copyright
1246 notice in "all copies" and that you may not use the copyright holder's name
1247 when promoting your software.
1249 You do not have to release any of your source code.
1251 You do not have to reveal or make public any changes to the libcurl source
1254 You do not have to reveal or make public that you are using libcurl within
1257 As can be seen here: http://curl.haxx.se/docs/companies.html and
1258 elsewhere, more and more companies are discovering the power
1259 of libcurl and take advantage of it even in commercial environments.
1264 7.1 What is PHP/CURL?
1266 The module for PHP that makes it possible for PHP programs to access curl-
1267 functions from within PHP.
1269 In the cURL project we call this module PHP/CURL to differentiate it from
1270 curl the command line tool and libcurl the library. The PHP team however
1271 does not refer to it like this (for unknown reasons). They call it plain
1272 CURL (often using all caps) or sometimes ext/curl, but both cause much
1273 confusion to users which in turn gives us a higher question load.
1275 7.2 Who write PHP/CURL?
1277 PHP/CURL is a module that comes with the regular PHP package. It depends and
1278 uses libcurl, so you need to have libcurl installed properly first before
1279 PHP/CURL can be used. PHP/CURL was initially written by Sterling Hughes.
1281 7.3 Can I perform multiple requests using the same handle?
1283 Yes - at least in PHP version 4.3.8 and later (this has been known to not
1284 work in earlier versions, but the exact version when it started to work is
1287 After a transfer, you just set new options in the handle and make another
1288 transfer. This will make libcurl to re-use the same connection if it can.