1 Updated: Oct 22, 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
78 5.1 Is libcurl thread-safe?
79 5.2 How can I receive all data into a large memory chunk?
80 5.3 How do I fetch multiple files with libcurl?
81 5.4 Does libcurl do Winsock initing on win32 systems?
82 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
83 5.6 What about Keep-Alive or persistent connections?
84 5.7 Link errors when building libcurl on Windows!
85 5.8 libcurl.so.3: open failed: No such file or directory
86 5.9 How does libcurl resolve host names?
87 5.10 How do I prevent libcurl from writing the response to stdout?
88 5.11 How do I make libcurl not receive the whole HTTP response?
89 5.12 Can I make libcurl fake or hide my real IP address?
90 5.13 How do I stop an ongoing transfer?
91 5.14 Using C++ non-static functions for callbacks?
92 5.15 How do I get an FTP directory listing?
95 6.1 I have a GPL program, can I use the libcurl library?
96 6.2 I have a closed-source program, can I use the libcurl library?
97 6.3 I have a BSD licensed program, can I use the libcurl library?
98 6.4 I have a program that uses LGPL libraries, can I use libcurl?
99 6.5 Can I modify curl/libcurl for my program and keep the changes secret?
100 6.6 Can you please change the curl/libcurl license to XXXX?
101 6.7 What are my obligations when using libcurl in my commercial apps?
104 7.1 What is PHP/CURL?
105 7.2 Who write PHP/CURL?
106 7.3 Can I perform multiple requests using the same handle?
108 ==============================================================================
114 cURL is the name of the project. The name is a play on 'Client for URLs',
115 originally with URL spelled in uppercase to make it obvious it deals with
116 URLs. The fact it can also be pronounced 'see URL' also helped, it works as
117 an abbreviation for "Client URL Request Library" or why not the recursive
118 version: "Curl URL Request Library".
120 The cURL project produces two products:
124 A free and easy-to-use client-side URL transfer library, supporting FTP,
125 FTPS, HTTP, HTTPS, SCP, SFTP, TFTP, TELNET, DICT, FILE, LDAP and LDAPS.
126 libcurl supports HTTPS certificates, HTTP POST, HTTP PUT, FTP uploading,
127 kerberos, HTTP form based upload, proxies, cookies, user+password
128 authentication, file transfer resume, http proxy tunneling and more!
130 libcurl is highly portable, it builds and works identically on numerous
131 platforms, including Solaris, NetBSD, FreeBSD, OpenBSD, Darwin, HPUX,
132 IRIX, AIX, Tru64, Linux, UnixWare, HURD, Windows, Amiga, OS/2, BeOs, Mac
133 OS X, Ultrix, QNX, OpenVMS, RISC OS, Novell NetWare, DOS and more...
135 libcurl is free, thread-safe, IPv6 compatible, feature rich, well
140 A command line tool for getting or sending files using URL syntax.
142 Since curl uses libcurl, it supports a range of common Internet protocols,
143 currently including HTTP, HTTPS, FTP, FTPS, SCP, SFTP, TFTP, LDAP, LDAPS,
144 DICT, TELNET and FILE.
146 We pronounce curl and cURL with an initial k sound: [kurl].
148 There are numerous sub-projects and related projects that also use the word
149 curl in the project names in various combinations, but you should take
150 notice that this FAQ is directed at the command-line tool named curl (and
151 libcurl the library), and may therefore not be valid for other curl-related
152 projects. (There is however a small section for the PHP/CURL in this FAQ.)
156 libcurl is a reliable and portable library which provides you with an easy
157 interface to a range of common Internet protocols.
159 You can use libcurl for free in your application, be it open source,
160 commercial or closed-source.
162 libcurl is most probably the most portable, most powerful and most often
163 used C-based multi-platform file transfer library on this planet - be it
164 open source or commercial.
166 1.3 What is curl not?
168 Curl is not a wget clone. That is a common misconception. Never, during
169 curl's development, have we intended curl to replace wget or compete on its
170 market. Curl is targeted at single-shot file transfers.
172 Curl is not a web site mirroring program. If you want to use curl to mirror
173 something: fine, go ahead and write a script that wraps around curl to make
174 it reality (like curlmirror.pl does).
176 Curl is not an FTP site mirroring program. Sure, get and send FTP with curl
177 but if you want systematic and sequential behavior you should write a
178 script (or write a new program that interfaces libcurl) and do it.
180 Curl is not a PHP tool, even though it works perfectly well when used from
181 or with PHP (when using the PHP/CURL module).
183 Curl is not a program for a single operating system. Curl exists, compiles,
184 builds and runs under a wide range of operating systems, including all
185 modern Unixes (and a bunch of older ones too), Windows, Amiga, BeOS, OS/2,
188 1.4 When will you make curl do XXXX ?
190 We love suggestions of what to change in order to make curl and libcurl
191 better. We do however believe in a few rules when it comes to the future of
194 * Curl -- the command line tool -- is to remain a non-graphical command line
195 tool. If you want GUIs or fancy scripting capabilities, you should look
196 for another tool that uses libcurl.
198 * We do not add things to curl that other small and available tools already
199 do very fine at the side. Curl's output is fine to pipe into another
200 program or redirect to another file for the next program to interpret.
202 * We focus on protocol related issues and improvements. If you wanna do more
203 magic with the supported protocols than curl currently does, chances are
204 big we will agree. If you wanna add more protocols, we may very well
207 * If you want someone else to make all the work while you wait for us to
208 implement it for you, that is not a very friendly attitude. We spend a
209 considerable time already on maintaining and developing curl. In order to
210 get more out of us, you should consider trading in some of your time and
213 * If you write the code, chances are bigger that it will get into curl
218 curl and libcurl are not made by any single individual. Daniel Stenberg is
219 project leader and main developer, but other persons' submissions are
220 important and crucial. Anyone can contribute and post their changes and
221 improvements and have them inserted in the main sources (of course on the
222 condition that developers agree on that the fixes are good).
224 The full list of all contributors is found in the docs/THANKS file.
226 curl is developed by a community, with Daniel at the wheel.
228 1.6 What do you get for making curl?
230 Project cURL is entirely free and open. No person gets paid for developing
231 (lib)curl on full or even part time. We do this voluntarily on our spare
232 time. Occasionally companies pay individual developers to work on curl, but
233 that's up to each company and developer. It is not controlled by nor
234 supervised in any way by the project.
236 We still get help from companies. Haxx provides web site, bandwidth, mailing
237 lists, CVS server etc and sourceforge.net hosts project services we take
238 advantage from, like the bug tracker. Also again, some companies have
239 sponsored certain parts of the development in the past and I hope some will
240 continue to do so in the future.
242 If you want to support our project, consider a donation or a banner-program
243 or even better: by helping us coding, documenting, testing etc.
245 1.7 What about CURL from curl.com?
247 During the summer 2001, curl.com was busy advertising their client-side
248 programming language for the web, named CURL.
250 We are in no way associated with curl.com or their CURL programming
253 Our project name curl has been in effective use since 1998. We were not the
254 first computer related project to use the name "curl" and do not claim any
255 first-hand rights to the name.
257 We recognize that we will be living in parallel with curl.com and wish them
260 1.8 I have a problem who do I mail?
262 Please do not mail any single individual unless you really need to. Keep
263 curl-related questions on a suitable mailing list. All available mailing
264 lists are listed in the MANUAL document and online at
265 http://curl.haxx.se/mail/
267 Keeping curl-related questions and discussions on mailing lists allows
268 others to join in and help, to share their ideas, contribute their
269 suggestions and spread their wisdom. Keeping discussions on public mailing
270 lists also allows for others to learn from this (both current and future
271 users thanks to the web based archives of the mailing lists), thus saving us
272 from having to repeat ourselves even more. Thanks for respecting this.
274 If you have found or simply suspect a security problem in curl or libcurl,
275 mail curl-security at haxx.se (closed list of receivers, mails are not
276 disclosed) and tell. Then we can produce a fix in a timely manner before the
277 flaw is announced to the world, thus lessen the impact the problem will have
280 1.9 Where do I buy commercial support for curl?
282 curl is fully open source. It means you can hire any skilled engineer to fix
283 your curl-related problems.
285 We list available alternatives on the curl web site:
286 http://curl.haxx.se/support.html
288 1.10 How many are using curl?
290 It is impossible to tell.
292 We don't know how many users that knowingly have installed and use curl.
294 We don't know how many users that use curl without knowing that they are in
297 We don't know how many users that downloaded or installed curl and then
300 Some facts to use as input to the math:
302 curl packages are downloaded from the curl.haxx.se and mirrors over a
303 million times per year. curl is installed by default with most Linux
304 distributions. curl is installed by default with Mac OS X. curl and libcurl
305 as used by numerous applications that include libcurl binaries in their
306 distribution packages (like Adobe Acrobat Reader and Google Earth).
308 More than 90 known named companies use curl in commercial environments and
309 products. More than 100 known named open source projects depend on
312 In a poll on the curl web site mid-2005, more than 50% of the 300+ answers
313 estimated a user base of one million users or more.
315 In March 2005, the "Linux Counter project" estimated a total Linux user base
316 of some 29 millions, while Netcraft detected some 4 million "active" Linux
317 based web servers. A guess is that a fair amount of these Linux
318 installations have curl installed.
320 All this taken together, there is no doubt that there are millions of
323 http://curl.haxx.se/docs/companies.html
324 http://curl.haxx.se/docs/programs.html
325 http://curl.haxx.se/libcurl/using/apps.html
326 http://counter.li.org/estimates.php
327 http://news.netcraft.com/archives/2005/03/14/fedora_makes_rapid_progress.html
329 1.11 Why don't you update ca-bundle.crt
331 The ca-bundle.crt file that used to be bundled with curl was very outdated
332 (it being last modified year 2000 should tell) and must be replaced with a
333 much more modern and up-to-date version by anyone who wants to verify peers
334 anyway. It is no longer provided, the last curl release that shipped it was
337 In the cURL project we've decided not to attempt to keep this file updated
338 (or even present anymore) since deciding what to add to a ca cert bundle is
339 an undertaking we've not been ready to accept, and the one we can get from
340 Mozilla is perfectly fine so there's no need to duplicate that work.
342 Today, with many services performed over HTTPS, every operating system
343 should come with a default ca cert bundle that can be deemed somewhat
344 trustworthy and that collection (if reasonably updated) should be deemed to
345 be a lot better than a private curl version.
347 If you want the most recent collection of ca certs that Mozilla Firefox
348 uses, we recommend that you extract the collection yourself from Mozilla
349 Firefox (by running 'make ca-bundle), or by using our online service setup
350 for this purpose: http://curl.haxx.se/docs/caextract.html
352 1.12 I have a problem who can I chat with?
354 There's a bunch of friendly people hanging out in the #curl channel on the
355 IRC network irc.freenode.net. If you're polite and nice, chances are big
356 that you can get -- or provide -- help instantly.
359 2. Install Related Problems
361 2.1 configure doesn't find OpenSSL even when it is installed
363 This may be because of several reasons.
365 2.1.1 native linker doesn't find openssl
368 Solaris (native cc compiler)
369 HPUX (native cc compiler)
370 SGI IRIX (native cc compiler)
371 SCO UNIX (native cc compiler)
373 When configuring curl, I specify --with-ssl. OpenSSL is installed in
374 /usr/local/ssl Configure reports SSL in /usr/local/ssl, but fails to find
375 CRYPTO_lock in -lcrypto
377 Cause: The cc for this test places the -L/usr/local/ssl/lib AFTER
378 -lcrypto, so ld can't find the library. This is due to a bug in the GNU
381 Workaround: Specifying "LDFLAGS=-L/usr/local/ssl/lib" in front of
382 ./configure places the -L/usr/local/ssl/lib early enough in the command
383 line to make things work
385 2.1.2 only the libssl lib is missing
387 If all include files and the libcrypto lib is present, with only the
388 libssl being missing according to configure, this is mostly likely because
389 a few functions are left out from the libssl.
391 If the function names missing include RSA or RSAREF you can be certain
392 that this is because libssl requires the RSA and RSAREF libs to build.
394 See the INSTALL file section that explains how to add those libs to
395 configure. Make sure that you remove the config.cache file before you
396 rerun configure with the new flags.
398 2.2 Does curl work/build with other SSL libraries?
400 Curl has been written to use OpenSSL, GnuTLS, yassl or NSS, although there
401 should not be many problems using a different library. If anyone does "port"
402 curl to use a different SSL library, we are of course very interested in
405 2.3 Where can I find a copy of LIBEAY32.DLL?
407 That is an OpenSSL binary built for Windows.
409 Curl uses OpenSSL to do the SSL stuff. The LIBEAY32.DLL is what curl needs
410 on a windows machine to do https://. Check out the curl web site to find
411 accurate and up-to-date pointers to recent OpenSSL DLLs and other binary
414 2.4 Does curl support SOCKS (RFC 1928) ?
416 Yes, SOCKS 4 and 5 are supported.
421 3.1 curl: (1) SSL is disabled, https: not supported
423 If you get this output when trying to get anything from a https:// server,
424 it means that the configure script couldn't find all libs and include files
425 it requires for SSL to work. If the configure script fails to find them,
426 curl is simply built without SSL support.
428 To get the https:// support into a curl that was previously built but that
429 reports that https:// is not supported, you should dig through the document
430 and logs and check out why the configure script doesn't find the SSL libs
431 and/or include files.
433 Also, check out the other paragraph in this FAQ labeled "configure doesn't
434 find OpenSSL even when it is installed".
436 3.2 How do I tell curl to resume a transfer?
438 Curl supports resumed transfers both ways on both FTP and HTTP.
442 3.3 Why doesn't my posting using -F work?
444 You can't simply use -F or -d at your choice. The web server that will
445 receive your post assumes one of the formats. If the form you're trying to
446 "fake" sets the type to 'multipart/form-data', then and only then you must
447 use the -F type. In all the most common cases, you should use -d which then
448 causes a posting with the type 'application/x-www-form-urlencoded'.
450 This is described in some detail in the MANUAL and TheArtOfHttpScripting
451 documents, and if you don't understand it the first time, read it again
452 before you post questions about this to the mailing list. Also, try reading
453 through the mailing list archives for old postings and questions regarding
456 3.4 How do I tell curl to run custom FTP commands?
458 You can tell curl to perform optional commands both before and/or after a
459 file transfer. Study the -Q/--quote option.
461 Since curl is used for file transfers, you don't use curl to just perform
462 FTP commands without transferring anything. Therefore you must always specify
463 a URL to transfer to/from even when doing custom FTP commands.
465 3.5 How can I disable the Pragma: nocache header?
467 You can change all internally generated headers by adding a replacement with
468 the -H/--header option. By adding a header with empty contents you safely
469 disable that one. Use -H "Pragma:" to disable that specific header.
471 3.6 Does curl support ASP, XML, XHTML or HTML version Y?
473 To curl, all contents are alike. It doesn't matter how the page was
474 generated. It may be ASP, PHP, Perl, shell-script, SSI or plain
475 HTML-files. There's no difference to curl and it doesn't even know what kind
476 of language that generated the page.
478 See also item 3.14 regarding javascript.
480 3.7 Can I use curl to delete/rename a file through FTP?
482 Yes. You specify custom FTP commands with -Q/--quote.
484 One example would be to delete a file after you have downloaded it:
486 curl -O ftp://download.com/coolfile -Q '-DELE coolfile'
488 or rename a file after upload:
490 curl -T infile ftp://upload.com/dir/ -Q "-RNFR infile" -Q "-RNTO newname"
492 3.8 How do I tell curl to follow HTTP redirects?
494 Curl does not follow so-called redirects by default. The Location: header
495 that informs the client about this is only interpreted if you're using the
496 -L/--location option. As in:
498 curl -L http://redirector.com
500 Not all redirects are HTTP ones, see 4.14
502 3.9 How do I use curl in my favorite programming language?
504 There exist many language interfaces/bindings for curl that integrates it
505 better with various languages. If you are fluid in a script language, you
506 may very well opt to use such an interface instead of using the command line
509 Find out more about which languages that support curl directly, and how to
510 install and use them, in the libcurl section of the curl web site:
511 http://curl.haxx.se/libcurl/
513 In February 2007, there are interfaces available for the following
514 languages: Ada95, Basic, C, C++, Ch, Cocoa, D, Dylan, Euphoria, Ferite,
515 Gambas, glib/GTK+, Java, Lisp, Lua, Mono, .NET, Object-Pascal, O'Caml,
516 Pascal, Perl, PHP, PostgreSQL, Python, R, Rexx, Ruby, Scheme, S-Lang,
517 Smalltalk, SPL, Tcl, Visual Basic, Q, wxwidgets and XBLite. By the time you
518 read this, additional ones may have appeared!
520 3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
522 Curl adheres to the HTTP spec, which basically means you can play with *any*
523 protocol that is built on top of HTTP. Protocols such as SOAP, WEBDAV and
524 XML-RPC are all such ones. You can use -X to set custom requests and -H to
525 set custom headers (or replace internally generated ones).
527 Using libcurl is of course just as fine and you'd just use the proper
528 library options to do the same.
530 3.11 How do I POST with a different Content-Type?
532 You can always replace the internally generated headers with -H/--header.
533 To make a simple HTTP POST with text/xml as content-type, do something like:
535 curl -d "datatopost" -H "Content-Type: text/xml" [URL]
537 3.12 Why do FTP specific features over HTTP proxy fail?
539 Because when you use a HTTP proxy, the protocol spoken on the network will
540 be HTTP, even if you specify a FTP URL. This effectively means that you
541 normally can't use FTP specific features such as FTP upload and FTP quote
544 There is one exception to this rule, and that is if you can "tunnel through"
545 the given HTTP proxy. Proxy tunneling is enabled with a special option (-p)
546 and is generally not available as proxy admins usually disable tunneling to
547 other ports than 443 (which is used for HTTPS access through proxies).
549 3.13 Why does my single/double quotes fail?
551 To specify a command line option that includes spaces, you might need to
552 put the entire option within quotes. Like in:
554 curl -d " with spaces " url.com
558 curl -d ' with spaces ' url.com
560 Exactly what kind of quotes and how to do this is entirely up to the shell
561 or command line interpreter that you are using. For most unix shells, you
562 can more or less pick either single (') or double (") quotes. For
563 Windows/DOS prompts I believe you're forced to use double (") quotes.
565 Please study the documentation for your particular environment. Examples in
566 the curl docs will use a mix of both these ones as shown above. You must
567 adjust them to work in your environment.
569 Remember that curl works and runs on more operating systems than most single
570 individuals have ever tried.
572 3.14 Does curl support Javascript or PAC (automated proxy config)?
574 Many web pages do magic stuff using embedded Javascript. Curl and libcurl
575 have no built-in support for that, so it will be treated just like any other
578 .pac files are a netscape invention and are sometimes used by organizations
579 to allow them to differentiate which proxies to use. The .pac contents is
580 just a Javascript program that gets invoked by the browser and that returns
581 the name of the proxy to connect to. Since curl doesn't support Javascript,
582 it can't support .pac proxy configuration either.
584 Some work-arounds usually suggested to overcome this Javascript dependency:
586 - Depending on the Javascript complexity, write up a script that
587 translates it to another language and execute that.
589 - Read the Javascript code and rewrite the same logic in another language.
591 - Implement a Javascript interpreter, people have successfully used the
592 Mozilla Javascript engine in the past.
594 - Ask your admins to stop this, for a static proxy setup or similar.
596 3.15 Can I do recursive fetches with curl?
598 No. curl itself has no code that performs recursive operations, such as
599 those performed by wget and similar tools.
601 There exist wrapper scripts with that functionality (for example the
602 curlmirror perl script), and you can write programs based on libcurl to do
603 it, but the command line tool curl itself cannot.
605 3.16 What certificates do I need when I use SSL?
607 There are three different kinds of "certificates" to keep track of when we
608 talk about using SSL-based protocols (HTTPS or FTPS) using curl or libcurl.
610 - Client certificate. The server you communicate may require that you can
611 provide this in order to prove that you actually are who you claim to be.
612 If the server doesn't require this, you don't need a client certificate.
614 - Server certificate. The server you communicate with has a server
615 certificate. You can and should verify this certificate to make sure that
616 you are truly talking to the real server and not a server impersonating
619 - Certificate Authority certificate ("CA cert"). You often have several CA
620 certs in a CA cert bundle that can be used to verify a server certificate
621 that was signed by one of the authorities in the bundle. curl comes with a
622 default CA cert bundle. You can override the default.
624 The server certificate verification process is made by using a Certificate
625 Authority certificate ("CA cert") that was used to sign the server
626 certificate. Server certificate verification is enabled by default in curl
627 and libcurl and is often the reason for problems as explained in FAQ entry
628 4.12 and the SSLCERTS document
629 (http://curl.haxx.se/docs/sslcerts.html). Server certificates that are
630 "self-signed" or otherwise signed by a CA that you do not have a CA cert
631 for, cannot be verified. If the verification during a connect fails, you
632 are refused access. You then need to explicitly disable the verification
633 to connect to the server.
635 3.17 How do I list the root dir of an FTP server?
637 There are two ways. The way defined in the RFC is to use an encoded slash
638 in the first path part. List the "/tmp" dir like this:
640 curl ftp://ftp.sunet.se/%2ftmp/
642 or the not-quite-kosher-but-more-readable way, by simply starting the path
643 section of the URL with a slash:
645 curl ftp://ftp.sunet.se//tmp/
647 3.18 Can I use curl to send a POST/PUT and not wait for a response?
651 But you could easily write your own program using libcurl to do such stunts.
656 4.1 Problems connecting to SSL servers.
658 It took a very long time before we could sort out why curl had problems to
659 connect to certain SSL servers when using SSLeay or OpenSSL v0.9+. The
660 error sometimes showed up similar to:
662 16570:error:1407D071:SSL routines:SSL2_READ:bad mac decode:s2_pkt.c:233:
664 It turned out to be because many older SSL servers don't deal with SSLv3
665 requests properly. To correct this problem, tell curl to select SSLv2 from
666 the command line (-2/--sslv2).
668 There have also been examples where the remote server didn't like the SSLv2
669 request and instead you had to force curl to use SSLv3 with -3/--sslv3.
671 4.2 Why do I get problems when I use & or % in the URL?
673 In general unix shells, the & symbol is treated specially and when used, it
674 runs the specified command in the background. To safely send the & as a part
675 of a URL, you should quote the entire URL by using single (') or double (")
676 quotes around it. Similar problems can also occur on some shells with other
677 characters, including ?*!$~(){}<>\|;`. When in doubt, quote the URL.
679 An example that would invoke a remote CGI that uses &-symbols could be:
681 curl 'http://www.altavista.com/cgi-bin/query?text=yes&q=curl'
683 In Windows, the standard DOS shell treats the %-symbol specially and you
684 need to use TWO %-symbols for each single one you want to use in the URL.
686 Also note that if you want the literal %-symbol to be part of the data you
687 pass in a POST using -d/--data you must encode it as '%25' (which then also
688 needs the %-symbol doubled on Windows machines).
690 4.3 How can I use {, }, [ or ] to specify multiple URLs?
692 Because those letters have a special meaning to the shell, and to be used in
693 a URL specified to curl you must quote them.
695 An example that downloads two URLs (sequentially) would do:
697 curl '{curl,www}.haxx.se'
699 To be able to use those letters as actual parts of the URL (without using
700 them for the curl URL "globbing" system), use the -g/--globoff option:
702 curl -g 'www.site.com/weirdname[].html'
704 4.4 Why do I get downloaded data even though the web page doesn't exist?
706 Curl asks remote servers for the page you specify. If the page doesn't exist
707 at the server, the HTTP protocol defines how the server should respond and
708 that means that headers and a "page" will be returned. That's simply how
711 By using the --fail option you can tell curl explicitly to not get any data
712 if the HTTP return code doesn't say success.
714 4.5 Why do I get return code XXX from a HTTP server?
716 RFC2616 clearly explains the return codes. This is a short transcript. Go
717 read the RFC for exact details:
719 4.5.1 "400 Bad Request"
721 The request could not be understood by the server due to malformed
722 syntax. The client SHOULD NOT repeat the request without modifications.
724 4.5.2 "401 Unauthorized"
726 The request requires user authentication.
728 4.5.3 "403 Forbidden"
730 The server understood the request, but is refusing to fulfill it.
731 Authorization will not help and the request SHOULD NOT be repeated.
733 4.5.4 "404 Not Found"
735 The server has not found anything matching the Request-URI. No indication
736 is given of whether the condition is temporary or permanent.
738 4.5.5 "405 Method Not Allowed"
740 The method specified in the Request-Line is not allowed for the resource
741 identified by the Request-URI. The response MUST include an Allow header
742 containing a list of valid methods for the requested resource.
744 4.5.6 "301 Moved Permanently"
746 If you get this return code and an HTML output similar to this:
748 <H1>Moved Permanently</H1> The document has moved <A
749 HREF="http://same_url_now_with_a_trailing_slash/">here</A>.
751 it might be because you request a directory URL but without the trailing
752 slash. Try the same operation again _with_ the trailing URL, or use the
753 -L/--location option to follow the redirection.
755 4.6 Can you tell me what error code 142 means?
757 All curl error codes are described at the end of the man page, in the
758 section called "EXIT CODES".
760 Error codes that are larger than the highest documented error code means
761 that curl has exited due to a crash. This is a serious error, and we
762 appreciate a detailed bug report from you that describes how we could go
763 ahead and repeat this!
765 4.7 How do I keep user names and passwords secret in Curl command lines?
767 This problem has two sides:
769 The first part is to avoid having clear-text passwords in the command line
770 so that they don't appear in 'ps' outputs and similar. That is easily
771 avoided by using the "-K" option to tell curl to read parameters from a file
772 or stdin to which you can pass the secret info. curl itself will also
773 attempt to "hide" the given password by blanking out the option - this
774 doesn't work on all platforms.
776 To keep the passwords in your account secret from the rest of the world is
777 not a task that curl addresses. You could of course encrypt them somehow to
778 at least hide them from being read by human eyes, but that is not what
779 anyone would call security.
781 Also note that regular HTTP (using Basic authentication) and FTP passwords
782 are sent in clear across the network. All it takes for anyone to fetch them
783 is to listen on the network. Eavesdropping is very easy. Use more secure
784 authentication methods (like Digest, Negotiate or even NTLM) or consider the
785 SSL-based alternatives HTTPS and FTPS.
789 It is not a bug if the behavior is documented. Read the docs first.
790 Especially check out the KNOWN_BUGS file, it may be a documented bug!
792 If it is a problem with a binary you've downloaded or a package for your
793 particular platform, try contacting the person who built the package/archive
796 If there is a bug, read the BUGS document first. Then report it as described
799 4.9 Curl can't authenticate to the server that requires NTLM?
801 This is supported in curl 7.10.6 or later. No earlier curl version knows
802 of this magic. Later versions require the OpenSSL or Microsoft Windows
803 libraries to provide this functionality. Using the NSS library will
804 not provide NTLM authentication functionality in curl.
806 NTLM is a Microsoft proprietary protocol. Proprietary formats are evil. You
807 should not use such ones.
809 4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
811 Many web servers allow or demand that the administrator configures the
812 server properly for these requests to work on the web server.
814 Some servers seem to support HEAD only on certain kinds of URLs.
816 To fully grasp this, try the documentation for the particular server
817 software you're trying to interact with. This is not anything curl can do
820 4.11 Why does my HTTP range requests return the full document?
822 Because the range may not be supported by the server, or the server may
823 choose to ignore it and return the full document anyway.
825 4.12 Why do I get "certificate verify failed" ?
827 You invoke curl 7.10 or later to communicate on a https:// URL and get an
828 error back looking something similar to this:
830 curl: (35) SSL: error:14090086:SSL routines:
831 SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
833 Then it means that curl couldn't verify that the server's certificate was
834 good. Curl verifies the certificate using the CA cert bundle that comes with
835 the curl installation.
837 To disable the verification (which makes it act like curl did before 7.10),
838 use -k. This does however enable man-in-the-middle attacks.
840 If you get this failure but are having a CA cert bundle installed and used,
841 the server's certificate is not signed by one of the CA's in the bundle. It
842 might for example be self-signed. You then correct this problem by obtaining
843 a valid CA cert for the server. Or again, decrease the security by disabling
846 Details are also in the SSLCERTS file in the release archives, found online
847 here: http://curl.haxx.se/docs/sslcerts.html
849 4.13 Why is curl -R on Windows one hour off?
851 During daylight savings time, when -R is used, curl will set a time that
852 appears one hour off. This happens due to a flaw in how Windows stores and
853 uses file modification times and it is not easily worked around. For details
854 on this problem, read this: http://www.codeproject.com/datetime/dstbugs.asp
856 4.14 Redirects work in browser but not with curl!
858 curl supports HTTP redirects fine (see item 3.8). Browsers generally support
859 at least two other ways to perform directs that curl does not:
861 - Meta tags. You can write a HTML tag that will cause the browser to
862 redirect to another given URL after a certain time.
864 - Javascript. You can write a Javascript program embedded in a HTML page
865 that redirects the browser to another given URL.
867 There is no way to make curl follow these redirects. You must either
868 manually figure out what the page is set to do, or you write a script that
869 parses the results and fetches the new URL.
871 4.15 FTPS doesn't work
873 curl supports FTPS (sometimes known as FTP-SSL) both implicit and explicit
876 When a URL is used that starts with FTPS://, curl assumes implicit SSL on
877 the control connection and will therefore immediately connect and try to
878 speak SSL. FTPS:// connections default to port 990.
880 To use explicit FTPS, you use a FTP:// URL and the --ftp-ssl option (or one
881 of its related flavours). This is the most common method, and the one
882 mandated by RFC4217. This kind of connection then of course uses the
883 standard FTP port 21 by default.
885 4.16 My HTTP POST or PUT requests are slow!
887 libcurl makes all POST and PUT requests (except for POST requests with a
888 very tiny request body) use the "Expect: 100-continue" header. This header
889 allows the server to deny the operation early so that libcurl can bail out
890 already before having to send any data. This is useful in authentication
893 However, many servers don't implement the Expect: stuff properly and if the
894 server doesn't respond (positively) within 1 second libcurl will continue
895 and send off the data anyway.
897 You can disable libcurl's use of the Expect: header the same way you disable
898 any header, using -H / CURLOPT_HTTPHEADER, or by forcing it to use HTTP 1.0.
900 4.17 Non-functional connect timeouts
902 In most Windows setups having a timeout longer than 21 seconds make no
903 difference, as it will only send 3 TCP SYN packets and no more. The second
904 packet sent three seconds after the first and the third six seconds after
905 the second. No more than three packets are sent, no matter how long the
908 See option TcpMaxConnectRetransmissions on this page:
909 http://support.microsoft.com/?scid=kb%3Ben-us%3B175523&x=6&y=7
911 Also, even on non-Windows systems there may run a firewall or anti-virus
912 software or similar that accepts the connection but does not actually do
913 anything else. This will make (lib)curl to consider the connection connected
914 and thus the connect timeout won't trigger.
919 5.1 Is libcurl thread-safe?
923 We have written the libcurl code specifically adjusted for multi-threaded
924 programs. libcurl will use thread-safe functions instead of non-safe ones if
925 your system has such.
927 If you use a OpenSSL-powered libcurl in a multi-threaded environment, you
928 need to provide one or two locking functions:
930 http://www.openssl.org/docs/crypto/threads.html
932 If you use a GnuTLS-powered libcurl in a multi-threaded environment, you
933 need to provide locking function(s) for libgcrypt (which is used by GnuTLS
934 for the crypto functions).
936 http://www.gnu.org/software/gnutls/manual/html_node/Multi_002dthreaded-applications.html
938 No special locking is needed with a NSS-powered libcurl. NSS is thread-safe.
940 5.2 How can I receive all data into a large memory chunk?
942 [ See also the examples/getinmemory.c source ]
944 You are in full control of the callback function that gets called every time
945 there is data received from the remote server. You can make that callback do
946 whatever you want. You do not have to write the received data to a file.
948 One solution to this problem could be to have a pointer to a struct that you
949 pass to the callback function. You set the pointer using the
950 CURLOPT_WRITEDATA option. Then that pointer will be passed to the callback
951 instead of a FILE * to a file:
953 /* imaginary struct */
954 struct MemoryStruct {
959 /* imaginary callback function */
961 WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data)
963 size_t realsize = size * nmemb;
964 struct MemoryStruct *mem = (struct MemoryStruct *)data;
966 mem->memory = (char *)realloc(mem->memory, mem->size + realsize + 1);
968 memcpy(&(mem->memory[mem->size]), ptr, realsize);
969 mem->size += realsize;
970 mem->memory[mem->size] = 0;
975 5.3 How do I fetch multiple files with libcurl?
977 libcurl has excellent support for transferring multiple files. You should
978 just repeatedly set new URLs with curl_easy_setopt() and then transfer it
979 with curl_easy_perform(). The handle you get from curl_easy_init() is not
980 only reusable, but you're even encouraged to reuse it if you can, as that
981 will enable libcurl to use persistent connections.
983 5.4 Does libcurl do Winsock initialization on win32 systems?
985 Yes, if told to in the curl_global_init() call.
987 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
989 Yes, but you cannot open a FILE * and pass the pointer to a DLL and have
990 that DLL use the FILE * (as the DLL and the client application cannot access
991 each others' variable memory areas). If you set CURLOPT_WRITEDATA you must
992 also use CURLOPT_WRITEFUNCTION as well to set a function that writes the
993 file, even if that simply writes the data to the specified FILE *.
994 Similarly, if you use CURLOPT_READDATA you must also specify
995 CURLOPT_READFUNCTION.
997 5.6 What about Keep-Alive or persistent connections?
999 curl and libcurl have excellent support for persistent connections when
1000 transferring several files from the same server. Curl will attempt to reuse
1001 connections for all URLs specified on the same command line/config file, and
1002 libcurl will reuse connections for all transfers that are made using the
1003 same libcurl handle.
1005 5.7 Link errors when building libcurl on Windows!
1007 You need to make sure that your project, and all the libraries (both static
1008 and dynamic) that it links against, are compiled/linked against the same run
1011 This is determined by the /MD, /ML, /MT (and their corresponding /M?d)
1012 options to the command line compiler. /MD (linking against MSVCRT dll) seems
1013 to be the most commonly used option.
1015 When building an application that uses the static libcurl library, you must
1016 add -DCURL_STATICLIB to your CFLAGS. Otherwise the linker will look for
1017 dynamic import symbols. If you get linker error like "unknown symbol
1018 __imp__curl_easy_init ..." you have linked against the wrong (static)
1019 library. If you want to use the libcurl.dll and import lib, you don't need
1020 any extra CFLAGS, but use one of the import libraries below. These are the
1021 libraries produced by the various lib/Makefile.* files:
1023 Target: static lib. import lib for libcurl*.dll.
1024 -----------------------------------------------------------
1025 MingW: libcurl.a libcurldll.a
1026 MSVC (release): libcurl.lib libcurl_imp.lib
1027 MSVC (debug): libcurld.lib libcurld_imp.lib
1028 Borland: libcurl.lib libcurl_imp.lib
1031 5.8 libcurl.so.3: open failed: No such file or directory
1033 This is an error message you might get when you try to run a program linked
1034 with a shared version of libcurl and your run-time linker (ld.so) couldn't
1035 find the shared library named libcurl.so.3.
1037 You need to make sure that ld.so finds libcurl.so.3. You can do that
1038 multiple ways, and it differs somewhat between different operating systems,
1039 but they are usually:
1041 * Add an option to the linker command line that specify the hard-coded path
1042 the run-time linker should check for the lib (usually -R)
1044 * Set an environment variable (LD_LIBRARY_PATH for example) where ld.so
1045 should check for libs
1047 * Adjust the system's config to check for libs in the directory where you've
1048 put the dir (like Linux's /etc/ld.so.conf)
1050 'man ld.so' and 'man ld' will tell you more details
1052 5.9 How does libcurl resolve host names?
1054 libcurl supports a large a number of different name resolve functions. One
1055 of them is picked at build-time and will be used unconditionally. Thus, if
1056 you want to change name resolver function you must rebuild libcurl and tell
1057 it to use a different function.
1059 - The non-ipv6 resolver that can use one out of four host name resolve calls
1060 (depending on what your system supports):
1063 B - gethostbyname_r() with 3 arguments
1064 C - gethostbyname_r() with 5 arguments
1065 D - gethostbyname_r() with 6 arguments
1067 - The ipv6-resolver that uses getaddrinfo()
1069 - The c-ares based name resolver that uses the c-ares library for resolves.
1070 Using this offers asynchronous name resolves but it currently has no IPv6
1073 - The Windows threaded resolver. It use:
1075 A - gethostbyname() on plain ipv4 windows hosts
1076 B - getaddrinfo() on ipv6-enabled windows hosts
1078 Also note that libcurl never resolves or reverse-lookups addresses given as
1079 pure numbers, such as 127.0.0.1 or ::1.
1081 5.10 How do I prevent libcurl from writing the response to stdout?
1083 libcurl provides a default built-in write function that writes received data
1084 to stdout. Set the CURLOPT_WRITEFUNCTION to receive the data, or possibly
1085 set CURLOPT_WRITEDATA to a different FILE * handle.
1087 5.11 How do I make libcurl not receive the whole HTTP response?
1089 You make the write callback (or progress callback) return an error and
1090 libcurl will then abort the transfer.
1092 5.12 Can I make libcurl fake or hide my real IP address?
1094 No. libcurl operates on a higher level than so. Besides, faking IP address
1095 would imply sending IP packages with a made-up source address, and then you
1096 normally get a problem with intercepting the packages sent back as they
1097 would then not be routed to you!
1099 If you use a proxy to access remote sites, the sites will not see your local
1100 IP address but instead the address of the proxy.
1102 Also note that on many networks NATs or other IP-munging techniques are used
1103 that makes you see and use a different IP address locally than what the
1104 remote server will see you coming from.
1106 5.13 How do I stop an ongoing transfer?
1108 There are several ways, but none of them are instant. There is no function
1109 you can call from another thread or similar that will stop it immediately.
1110 Instead you need to make sure that one of the callbacks you use return an
1111 appropriate value that will stop the transfer.
1113 Suitable callbacks that you can do this with include the progress callback,
1114 the read callback and the write callback.
1116 If you're using the multi interface, you also stop a transfer by removing
1117 the particular easy handle from the multi stack.
1119 5.14 Using C++ non-static functions for callbacks?
1121 libcurl is a C library, it doesn't know anything about C++ member functions.
1123 You can overcome this "limitation" with a relative ease using a static
1124 member function that is passed a pointer to the class:
1126 // f is the pointer to your object.
1127 static YourClass::staticFunction(void *buffer, size_t sz, size_t n, void *f)
1129 // Call non-static member function.
1130 static_cast<YourClass*>(f)->nonStaticFunction();
1133 // This is how you pass pointer to the static function:
1134 curl_easy_setopt(hcurl, CURLOPT_WRITEFUNCTION, YourClass:staticFunction);
1135 curl_easy_setopt(hcurl, CURLOPT_WRITEDATA, this);
1137 5.15 How do I get an FTP directory listing?
1139 If you end the FTP URL you request with a slash, libcurl will provide you
1140 with a directory listing of that given directory. You can also set
1141 CURLOPT_CUSTOMREQUEST to alter what exact listing command libcurl would use
1144 The follow-up question that tend to follow the previous one, is how a
1145 program is supposed to parse the directory listing. How does it know what's
1146 a file and what's a dir and what's a symlink etc. The harsh reality is that
1147 FTP provides no such fine and easy-to-parse output. The output format FTP
1148 servers respond to LIST commands are entirely at the server's own liking and
1149 the NLST output doesn't reveal any types and in many cases don't even
1150 include all the directory entries. Also, both LIST and NLST tend to hide
1151 unix-style hidden files (those that start with a dot) by default so you need
1152 to do "LIST -a" or similar to see them.
1154 The application thus needs to parse the LIST output. One such existing
1155 list parser is available at http://cr.yp.to/ftpparse.html
1160 Curl and libcurl are released under a MIT/X derivate license. The license is
1161 very liberal and should not impose a problem for your project. This section
1162 is just a brief summary for the cases we get the most questions. (Parts of
1163 this section was much enhanced by Bjorn Reese.)
1165 We are not lawyers and this is not legal advice. You should probably consult
1166 one if you want true and accurate legal insights without our prejudice.
1168 6.1 I have a GPL program, can I use the libcurl library?
1172 Since libcurl may be distributed under the MIT/X derivate license, it can be
1173 used together with GPL in any software.
1175 6.2 I have a closed-source program, can I use the libcurl library?
1179 libcurl does not put any restrictions on the program that uses the library.
1181 6.3 I have a BSD licensed program, can I use the libcurl library?
1185 libcurl does not put any restrictions on the program that uses the library.
1187 6.4 I have a program that uses LGPL libraries, can I use libcurl?
1191 The LGPL license doesn't clash with other licenses.
1193 6.5 Can I modify curl/libcurl for my program and keep the changes secret?
1197 The MIT/X derivate license practically allows you to do almost anything with
1198 the sources, on the condition that the copyright texts in the sources are
1201 6.6 Can you please change the curl/libcurl license to XXXX?
1205 We have carefully picked this license after years of development and
1206 discussions and a large amount of people have contributed with source code
1207 knowing that this is the license we use. This license puts the restrictions
1208 we want on curl/libcurl and it does not spread to other programs or
1209 libraries that use it. It should be possible for everyone to use libcurl or
1210 curl in their projects, no matter what license they already have in use.
1212 6.7 What are my obligations when using libcurl in my commercial apps?
1214 Next to none. All you need to adhere to is the MIT-style license (stated in
1215 the COPYING file) which basically says you have to include the copyright
1216 notice in "all copies" and that you may not use the copyright holder's name
1217 when promoting your software.
1219 You do not have to release any of your source code.
1221 You do not have to reveal or make public any changes to the libcurl source
1224 You do not have to reveal or make public that you are using libcurl within
1227 As can be seen here: http://curl.haxx.se/docs/companies.html and
1228 elsewhere, more and more companies are discovering the power
1229 of libcurl and take advantage of it even in commercial environments.
1234 7.1 What is PHP/CURL?
1236 The module for PHP that makes it possible for PHP programs to access curl-
1237 functions from within PHP.
1239 In the cURL project we call this module PHP/CURL to differentiate it from
1240 curl the command line tool and libcurl the library. The PHP team however
1241 does not refer to it like this (for unknown reasons). They call it plain
1242 CURL (often using all caps) or sometimes ext/curl, but both cause much
1243 confusion to users which in turn gives us a higher question load.
1245 7.2 Who write PHP/CURL?
1247 PHP/CURL is a module that comes with the regular PHP package. It depends and
1248 uses libcurl, so you need to have libcurl installed properly first before
1249 PHP/CURL can be used. PHP/CURL was initially written by Sterling Hughes.
1251 7.3 Can I perform multiple requests using the same handle?
1253 Yes - at least in PHP version 4.3.8 and later (this has been known to not
1254 work in earlier versions, but the exact version when it started to work is
1257 After a transfer, you just set new options in the handle and make another
1258 transfer. This will make libcurl to re-use the same connection if it can.