5 \___|\___/|_| \_\_____|
13 1.4 When will you make curl do XXXX ?
15 1.6 What do you get for making curl?
16 1.7 What about CURL from curl.com?
17 1.8 I have a problem who do I mail?
18 1.9 Where do I buy commercial support for curl?
19 1.10 How many are using curl?
20 1.11 Why don't you update ca-bundle.crt
21 1.12 I have a problem who can I chat with?
22 1.13 curl's ECCN number?
23 1.14 How do I submit my patch?
24 1.15 How do I port libcurl to my OS?
26 2. Install Related Problems
27 2.1 configure doesn't find OpenSSL even when it is installed
28 2.1.1 native linker doesn't find OpenSSL
29 2.1.2 only the libssl lib is missing
30 2.2 Does curl work/build with other SSL libraries?
31 2.3 Where can I find a copy of LIBEAY32.DLL?
32 2.4 Does curl support SOCKS (RFC 1928) ?
35 3.1 curl: (1) SSL is disabled, https: not supported
36 3.2 How do I tell curl to resume a transfer?
37 3.3 Why doesn't my posting using -F work?
38 3.4 How do I tell curl to run custom FTP commands?
39 3.5 How can I disable the Accept: */* header?
40 3.6 Does curl support ASP, XML, XHTML or HTML version Y?
41 3.7 Can I use curl to delete/rename a file through FTP?
42 3.8 How do I tell curl to follow HTTP redirects?
43 3.9 How do I use curl in my favorite programming language?
44 3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
45 3.11 How do I POST with a different Content-Type?
46 3.12 Why do FTP specific features over HTTP proxy fail?
47 3.13 Why does my single/double quotes fail?
48 3.14 Does curl support Javascript or PAC (automated proxy config)?
49 3.15 Can I do recursive fetches with curl?
50 3.16 What certificates do I need when I use SSL?
51 3.17 How do I list the root dir of an FTP server?
52 3.18 Can I use curl to send a POST/PUT and not wait for a response?
53 3.19 How do I get HTTP from a host using a specific IP address?
54 3.20 How to SFTP from my user's home directory?
55 3.21 Protocol xxx not supported or disabled in libcurl
56 3.22 curl -X gives me HTTP problems
59 4.1 Problems connecting to SSL servers.
60 4.2 Why do I get problems when I use & or % in the URL?
61 4.3 How can I use {, }, [ or ] to specify multiple URLs?
62 4.4 Why do I get downloaded data even though the web page doesn't exist?
63 4.5 Why do I get return code XXX from a HTTP server?
64 4.5.1 "400 Bad Request"
65 4.5.2 "401 Unauthorized"
68 4.5.5 "405 Method Not Allowed"
69 4.5.6 "301 Moved Permanently"
70 4.6 Can you tell me what error code 142 means?
71 4.7 How do I keep user names and passwords secret in Curl command lines?
73 4.9 Curl can't authenticate to the server that requires NTLM?
74 4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
75 4.11 Why does my HTTP range requests return the full document?
76 4.12 Why do I get "certificate verify failed" ?
77 4.13 Why is curl -R on Windows one hour off?
78 4.14 Redirects work in browser but not with curl!
79 4.15 FTPS doesn't work
80 4.16 My HTTP POST or PUT requests are slow!
81 4.17 Non-functional connect timeouts on Windows
82 4.18 file:// URLs containing drive letters (Windows, NetWare)
83 4.19 Why doesn't cURL return an error when the network cable is unplugged?
84 4.20 curl doesn't return error for HTTP non-200 responses!
85 4.21 Why is there a HTTP/1.1 in my HTTP/2 request?
88 5.1 Is libcurl thread-safe?
89 5.2 How can I receive all data into a large memory chunk?
90 5.3 How do I fetch multiple files with libcurl?
91 5.4 Does libcurl do Winsock initing on win32 systems?
92 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
93 5.6 What about Keep-Alive or persistent connections?
94 5.7 Link errors when building libcurl on Windows!
95 5.8 libcurl.so.X: open failed: No such file or directory
96 5.9 How does libcurl resolve host names?
97 5.10 How do I prevent libcurl from writing the response to stdout?
98 5.11 How do I make libcurl not receive the whole HTTP response?
99 5.12 Can I make libcurl fake or hide my real IP address?
100 5.13 How do I stop an ongoing transfer?
101 5.14 Using C++ non-static functions for callbacks?
102 5.15 How do I get an FTP directory listing?
103 5.16 I want a different time-out!
104 5.17 Can I write a server with libcurl?
105 5.18 Does libcurl use threads?
108 6.1 I have a GPL program, can I use the libcurl library?
109 6.2 I have a closed-source program, can I use the libcurl library?
110 6.3 I have a BSD licensed program, can I use the libcurl library?
111 6.4 I have a program that uses LGPL libraries, can I use libcurl?
112 6.5 Can I modify curl/libcurl for my program and keep the changes secret?
113 6.6 Can you please change the curl/libcurl license to XXXX?
114 6.7 What are my obligations when using libcurl in my commercial apps?
117 7.1 What is PHP/CURL?
118 7.2 Who wrote PHP/CURL?
119 7.3 Can I perform multiple requests using the same handle?
121 ==============================================================================
127 cURL is the name of the project. The name is a play on 'Client for URLs',
128 originally with URL spelled in uppercase to make it obvious it deals with
129 URLs. The fact it can also be pronounced 'see URL' also helped, it works as
130 an abbreviation for "Client URL Request Library" or why not the recursive
131 version: "Curl URL Request Library".
133 The cURL project produces two products:
137 A free and easy-to-use client-side URL transfer library, supporting DICT,
138 FILE, FTP, FTPS, GOPHER, HTTP, HTTPS, IMAP, IMAPS, LDAP, LDAPS, POP3,
139 POP3S, RTMP, RTSP, SCP, SFTP, SMB, SMBS, SMTP, SMTPS, TELNET and TFTP.
141 libcurl supports HTTPS certificates, HTTP POST, HTTP PUT, FTP uploading,
142 Kerberos, SPNEGO, HTTP form based upload, proxies, cookies, user+password
143 authentication, file transfer resume, http proxy tunneling and more!
145 libcurl is highly portable, it builds and works identically on numerous
146 platforms, including Solaris, NetBSD, FreeBSD, OpenBSD, Darwin, HP-UX,
147 IRIX, AIX, Tru64, Linux, UnixWare, HURD, Windows, Amiga, OS/2, BeOS, Mac
148 OS X, Ultrix, QNX, OpenVMS, RISC OS, Novell NetWare, DOS, Symbian, OSF,
149 Android, Minix, IBM TPF and more...
151 libcurl is free, thread-safe, IPv6 compatible, feature rich, well
156 A command line tool for getting or sending files using URL syntax.
158 Since curl uses libcurl, curl supports the same wide range of common
159 Internet protocols that libcurl does.
161 We pronounce curl with an initial k sound. It rhymes with words like girl
162 and earl. This is a short WAV file to help you:
164 http://media.merriam-webster.com/soundc11/c/curl0001.wav
166 There are numerous sub-projects and related projects that also use the word
167 curl in the project names in various combinations, but you should take
168 notice that this FAQ is directed at the command-line tool named curl (and
169 libcurl the library), and may therefore not be valid for other curl-related
170 projects. (There is however a small section for the PHP/CURL in this FAQ.)
174 libcurl is a reliable and portable library which provides you with an easy
175 interface to a range of common Internet protocols.
177 You can use libcurl for free in your application, be it open source,
178 commercial or closed-source.
180 libcurl is most probably the most portable, most powerful and most often
181 used C-based multi-platform file transfer library on this planet - be it
182 open source or commercial.
184 1.3 What is curl not?
186 Curl is not a wget clone. That is a common misconception. Never, during
187 curl's development, have we intended curl to replace wget or compete on its
188 market. Curl is targeted at single-shot file transfers.
190 Curl is not a web site mirroring program. If you want to use curl to mirror
191 something: fine, go ahead and write a script that wraps around curl to make
192 it reality (like curlmirror.pl does).
194 Curl is not an FTP site mirroring program. Sure, get and send FTP with curl
195 but if you want systematic and sequential behavior you should write a
196 script (or write a new program that interfaces libcurl) and do it.
198 Curl is not a PHP tool, even though it works perfectly well when used from
199 or with PHP (when using the PHP/CURL module).
201 Curl is not a program for a single operating system. Curl exists, compiles,
202 builds and runs under a wide range of operating systems, including all
203 modern Unixes (and a bunch of older ones too), Windows, Amiga, BeOS, OS/2,
206 1.4 When will you make curl do XXXX ?
208 We love suggestions of what to change in order to make curl and libcurl
209 better. We do however believe in a few rules when it comes to the future of
212 Curl -- the command line tool -- is to remain a non-graphical command line
213 tool. If you want GUIs or fancy scripting capabilities, you should look for
214 another tool that uses libcurl.
216 We do not add things to curl that other small and available tools already do
217 very fine at the side. Curl's output is fine to pipe into another program or
218 redirect to another file for the next program to interpret.
220 We focus on protocol related issues and improvements. If you wanna do more
221 magic with the supported protocols than curl currently does, chances are big
222 we will agree. If you wanna add more protocols, we may very well agree.
224 If you want someone else to make all the work while you wait for us to
225 implement it for you, that is not a very friendly attitude. We spend a
226 considerable time already on maintaining and developing curl. In order to
227 get more out of us, you should consider trading in some of your time and
230 If you write the code, chances are bigger that it will get into curl faster.
234 curl and libcurl are not made by any single individual. Daniel Stenberg is
235 project leader and main developer, but other persons' submissions are
236 important and crucial. Anyone can contribute and post their changes and
237 improvements and have them inserted in the main sources (of course on the
238 condition that developers agree on that the fixes are good).
240 The full list of all contributors is found in the docs/THANKS file.
242 curl is developed by a community, with Daniel at the wheel.
244 1.6 What do you get for making curl?
246 Project cURL is entirely free and open. No person gets paid for developing
247 curl on full time. We do this voluntarily, mostly on spare time.
248 Occasionally companies pay individual developers to work on curl, but that's
249 up to each company and developer. It is not controlled by nor supervised in
250 any way by the project.
252 We still get help from companies. Haxx provides web site, bandwidth, mailing
253 lists etc, sourceforge.net hosts project services we take advantage from,
254 like the bug tracker and github hosts the primary git repository. Also
255 again, some companies have sponsored certain parts of the development in the
256 past and I hope some will continue to do so in the future.
258 If you want to support our project, consider a donation or a banner-program
259 or even better: by helping us coding, documenting, testing etc.
261 1.7 What about CURL from curl.com?
263 During the summer 2001, curl.com was busy advertising their client-side
264 programming language for the web, named CURL.
266 We are in no way associated with curl.com or their CURL programming
269 Our project name curl has been in effective use since 1998. We were not the
270 first computer related project to use the name "curl" and do not claim any
273 We recognize that we will be living in parallel with curl.com and wish them
276 1.8 I have a problem who do I mail?
278 Please do not mail any single individual unless you really need to. Keep
279 curl-related questions on a suitable mailing list. All available mailing
280 lists are listed in the MANUAL document and online at
281 http://curl.haxx.se/mail/
283 Keeping curl-related questions and discussions on mailing lists allows
284 others to join in and help, to share their ideas, contribute their
285 suggestions and spread their wisdom. Keeping discussions on public mailing
286 lists also allows for others to learn from this (both current and future
287 users thanks to the web based archives of the mailing lists), thus saving us
288 from having to repeat ourselves even more. Thanks for respecting this.
290 If you have found or simply suspect a security problem in curl or libcurl,
291 mail curl-security at haxx.se (closed list of receivers, mails are not
292 disclosed) and tell. Then we can produce a fix in a timely manner before the
293 flaw is announced to the world, thus lessen the impact the problem will have
296 1.9 Where do I buy commercial support for curl?
298 curl is fully open source. It means you can hire any skilled engineer to fix
299 your curl-related problems.
301 We list available alternatives on the curl web site:
302 http://curl.haxx.se/support.html
304 1.10 How many are using curl?
306 It is impossible to tell.
308 We don't know how many users that knowingly have installed and use curl.
310 We don't know how many users that use curl without knowing that they are in
313 We don't know how many users that downloaded or installed curl and then
316 In May 2012 Daniel did a counting game and came up with a number that may
317 be completely wrong or somewhat accurate. Over 500 million!
319 See http://daniel.haxx.se/blog/2012/05/16/300m-users/
321 1.11 Why don't you update ca-bundle.crt
323 The ca cert bundle that used to shipped with curl was very outdated and must
324 be replaced with an up-to-date version by anyone who wants to verify
325 peers. It is no longer provided by curl. The last curl release ever that
326 shipped a ca cert bundle was curl 7.18.0.
328 In the cURL project we've decided not to attempt to keep this file updated
329 (or even present anymore) since deciding what to add to a ca cert bundle is
330 an undertaking we've not been ready to accept, and the one we can get from
331 Mozilla is perfectly fine so there's no need to duplicate that work.
333 Today, with many services performed over HTTPS, every operating system
334 should come with a default ca cert bundle that can be deemed somewhat
335 trustworthy and that collection (if reasonably updated) should be deemed to
336 be a lot better than a private curl version.
338 If you want the most recent collection of ca certs that Mozilla Firefox
339 uses, we recommend that you extract the collection yourself from Mozilla
340 Firefox (by running 'make ca-bundle), or by using our online service setup
341 for this purpose: http://curl.haxx.se/docs/caextract.html
343 1.12 I have a problem who can I chat with?
345 There's a bunch of friendly people hanging out in the #curl channel on the
346 IRC network irc.freenode.net. If you're polite and nice, chances are big
347 that you can get -- or provide -- help instantly.
349 1.13 curl's ECCN number?
351 The US government restricts exports of software that contains or uses
352 cryptography. When doing so, the Export Control Classification Number (ECCN)
353 is used to identify the level of export control etc.
355 ASF gives a good explanation at https://www.apache.org/dev/crypto.html
357 We believe curl's number might be ECCN 5D002, another possibility is
358 5D992. It seems necessary to write them, asking to confirm.
360 Comprehensible explanations of the meaning of such numbers and how to
361 obtain them (resp.) are here
363 http://www.bis.doc.gov/licensing/exportingbasics.htm
364 http://www.bis.doc.gov/licensing/do_i_needaneccn.html
366 An incomprehensible description of the two numbers above is here
367 http://www.access.gpo.gov/bis/ear/pdf/ccl5-pt2.pdf
369 1.14 How do I submit my patch?
371 When you have made a patch or a change of whatever sort, and want to submit
372 that to the project, there are a few different ways we prefer:
374 o send a patch to the curl-library mailing list. We're many subscribers
375 there and there are lots of people who can review patches, comment on them
376 and "receive" them properly.
378 o if your patch changes or fixes a bug, you can also opt to submit a bug
379 report in the bug tracker and attach your patch there. There are less
380 people involved there.
382 Lots of more details are found in the CONTRIBUTE and INTERNALS docs.
384 1.15 How do I port libcurl to my OS?
386 Here's a rough step-by-step:
388 1. copy a suitable lib/config-*.h file as a start to lib/config-[youros].h
390 2. edit lib/config-[youros].h to match your OS and setup
392 3. edit lib/curl_setup.h to include config-[youros].h when your OS is
393 detected by the preprocessor, in the style others already exist
395 4. compile lib/*.c and make them into a library
398 2. Install Related Problems
400 2.1 configure doesn't find OpenSSL even when it is installed
402 This may be because of several reasons.
404 2.1.1 native linker doesn't find openssl
407 Solaris (native cc compiler)
408 HPUX (native cc compiler)
409 SGI IRIX (native cc compiler)
410 SCO UNIX (native cc compiler)
412 When configuring curl, I specify --with-ssl. OpenSSL is installed in
413 /usr/local/ssl Configure reports SSL in /usr/local/ssl, but fails to find
414 CRYPTO_lock in -lcrypto
416 Cause: The cc for this test places the -L/usr/local/ssl/lib AFTER
417 -lcrypto, so ld can't find the library. This is due to a bug in the GNU
420 Workaround: Specifying "LDFLAGS=-L/usr/local/ssl/lib" in front of
421 ./configure places the -L/usr/local/ssl/lib early enough in the command
422 line to make things work
424 2.1.2 only the libssl lib is missing
426 If all include files and the libcrypto lib is present, with only the
427 libssl being missing according to configure, this is mostly likely because
428 a few functions are left out from the libssl.
430 If the function names missing include RSA or RSAREF you can be certain
431 that this is because libssl requires the RSA and RSAREF libs to build.
433 See the INSTALL file section that explains how to add those libs to
434 configure. Make sure that you remove the config.cache file before you
435 rerun configure with the new flags.
437 2.2 Does curl work/build with other SSL libraries?
439 Curl has been written to use a generic SSL function layer internally, and
440 that SSL functionality can then be provided by one out of many different SSL
443 curl can be built to use one of the following SSL alternatives: OpenSSL,
444 GnuTLS, yassl, NSS, PolarSSL, axTLS, Secure Transport (native iOS/OS X),
445 WinSSL (native Windows) or GSKit (native IBM i). They all have their pros
446 and cons, and we try to maintain a comparison of them here:
447 http://curl.haxx.se/docs/ssl-compared.html
449 2.3 Where can I find a copy of LIBEAY32.DLL?
451 That is an OpenSSL binary built for Windows.
453 Curl can be built with OpenSSL to do the SSL stuff. The LIBEAY32.DLL is then
454 what curl needs on a windows machine to do https:// etc. Check out the curl
455 web site to find accurate and up-to-date pointers to recent OpenSSL DLLs and
456 other binary packages.
458 2.4 Does curl support SOCKS (RFC 1928) ?
460 Yes, SOCKS 4 and 5 are supported.
465 3.1 curl: (1) SSL is disabled, https: not supported
467 If you get this output when trying to get anything from a https:// server,
468 it means that the instance of curl/libcurl that you're using was built
469 without support for this protocol.
471 This could've happened if the configure script that was run at build time
472 couldn't find all libs and include files curl requires for SSL to work. If
473 the configure script fails to find them, curl is simply built without SSL
476 To get the https:// support into a curl that was previously built but that
477 reports that https:// is not supported, you should dig through the document
478 and logs and check out why the configure script doesn't find the SSL libs
479 and/or include files.
481 Also, check out the other paragraph in this FAQ labelled "configure doesn't
482 find OpenSSL even when it is installed".
484 3.2 How do I tell curl to resume a transfer?
486 Curl supports resumed transfers both ways on both FTP and HTTP.
489 3.3 Why doesn't my posting using -F work?
491 You can't simply use -F or -d at your choice. The web server that will
492 receive your post expects one of the formats. If the form you're trying to
493 submit uses the type 'multipart/form-data', then and only then you must use
494 the -F type. In all the most common cases, you should use -d which then
495 causes a posting with the type 'application/x-www-form-urlencoded'.
497 This is described in some detail in the MANUAL and TheArtOfHttpScripting
498 documents, and if you don't understand it the first time, read it again
499 before you post questions about this to the mailing list. Also, try reading
500 through the mailing list archives for old postings and questions regarding
503 3.4 How do I tell curl to run custom FTP commands?
505 You can tell curl to perform optional commands both before and/or after a
506 file transfer. Study the -Q/--quote option.
508 Since curl is used for file transfers, you don't normally use curl to
509 perform FTP commands without transferring anything. Therefore you must
510 always specify a URL to transfer to/from even when doing custom FTP
511 commands, or use -I which implies the "no body" option sent to libcurl.
513 3.5 How can I disable the Accept: */* header?
515 You can change all internally generated headers by adding a replacement with
516 the -H/--header option. By adding a header with empty contents you safely
517 disable that one. Use -H "Accept:" to disable that specific header.
519 3.6 Does curl support ASP, XML, XHTML or HTML version Y?
521 To curl, all contents are alike. It doesn't matter how the page was
522 generated. It may be ASP, PHP, Perl, shell-script, SSI or plain HTML
523 files. There's no difference to curl and it doesn't even know what kind of
524 language that generated the page.
526 See also item 3.14 regarding javascript.
528 3.7 Can I use curl to delete/rename a file through FTP?
530 Yes. You specify custom FTP commands with -Q/--quote.
532 One example would be to delete a file after you have downloaded it:
534 curl -O ftp://download.com/coolfile -Q '-DELE coolfile'
536 or rename a file after upload:
538 curl -T infile ftp://upload.com/dir/ -Q "-RNFR infile" -Q "-RNTO newname"
540 3.8 How do I tell curl to follow HTTP redirects?
542 Curl does not follow so-called redirects by default. The Location: header
543 that informs the client about this is only interpreted if you're using the
544 -L/--location option. As in:
546 curl -L http://redirector.com
548 Not all redirects are HTTP ones, see 4.14
550 3.9 How do I use curl in my favorite programming language?
552 There exist many language interfaces/bindings for curl that integrates it
553 better with various languages. If you are fluid in a script language, you
554 may very well opt to use such an interface instead of using the command line
557 Find out more about which languages that support curl directly, and how to
558 install and use them, in the libcurl section of the curl web site:
559 http://curl.haxx.se/libcurl/
561 All the various bindings to libcurl are made by other projects and people,
562 outside of the cURL project. The cURL project itself only produces libcurl
563 with its plain C API. If you don't find anywhere else to ask you can ask
564 about bindings on the curl-library list too, but be prepared that people on
565 that list may not know anything about bindings.
567 In October 2009, there were interfaces available for the following
568 languages: Ada95, Basic, C, C++, Ch, Cocoa, D, Dylan, Eiffel, Euphoria,
569 Ferite, Gambas, glib/GTK+, Haskell, ILE/RPG, Java, Lisp, Lua, Mono, .NET,
570 Object-Pascal, O'Caml, Pascal, Perl, PHP, PostgreSQL, Python, R, Rexx, Ruby,
571 Scheme, S-Lang, Smalltalk, SP-Forth, SPL, Tcl, Visual Basic, Visual FoxPro,
572 Q, wxwidgets and XBLite. By the time you read this, additional ones may have
575 3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
577 Curl adheres to the HTTP spec, which basically means you can play with *any*
578 protocol that is built on top of HTTP. Protocols such as SOAP, WEBDAV and
579 XML-RPC are all such ones. You can use -X to set custom requests and -H to
580 set custom headers (or replace internally generated ones).
582 Using libcurl is of course just as fine and you'd just use the proper
583 library options to do the same.
585 3.11 How do I POST with a different Content-Type?
587 You can always replace the internally generated headers with -H/--header.
588 To make a simple HTTP POST with text/xml as content-type, do something like:
590 curl -d "datatopost" -H "Content-Type: text/xml" [URL]
592 3.12 Why do FTP specific features over HTTP proxy fail?
594 Because when you use a HTTP proxy, the protocol spoken on the network will
595 be HTTP, even if you specify a FTP URL. This effectively means that you
596 normally can't use FTP specific features such as FTP upload and FTP quote
599 There is one exception to this rule, and that is if you can "tunnel through"
600 the given HTTP proxy. Proxy tunneling is enabled with a special option (-p)
601 and is generally not available as proxy admins usually disable tunneling to
602 other ports than 443 (which is used for HTTPS access through proxies).
604 3.13 Why does my single/double quotes fail?
606 To specify a command line option that includes spaces, you might need to
607 put the entire option within quotes. Like in:
609 curl -d " with spaces " url.com
613 curl -d ' with spaces ' url.com
615 Exactly what kind of quotes and how to do this is entirely up to the shell
616 or command line interpreter that you are using. For most unix shells, you
617 can more or less pick either single (') or double (") quotes. For
618 Windows/DOS prompts I believe you're forced to use double (") quotes.
620 Please study the documentation for your particular environment. Examples in
621 the curl docs will use a mix of both these ones as shown above. You must
622 adjust them to work in your environment.
624 Remember that curl works and runs on more operating systems than most single
625 individuals have ever tried.
627 3.14 Does curl support Javascript or PAC (automated proxy config)?
629 Many web pages do magic stuff using embedded Javascript. Curl and libcurl
630 have no built-in support for that, so it will be treated just like any other
633 .pac files are a netscape invention and are sometimes used by organizations
634 to allow them to differentiate which proxies to use. The .pac contents is
635 just a Javascript program that gets invoked by the browser and that returns
636 the name of the proxy to connect to. Since curl doesn't support Javascript,
637 it can't support .pac proxy configuration either.
639 Some workarounds usually suggested to overcome this Javascript dependency:
641 Depending on the Javascript complexity, write up a script that translates it
642 to another language and execute that.
644 Read the Javascript code and rewrite the same logic in another language.
646 Implement a Javascript interpreter, people have successfully used the
647 Mozilla Javascript engine in the past.
649 Ask your admins to stop this, for a static proxy setup or similar.
651 3.15 Can I do recursive fetches with curl?
653 No. curl itself has no code that performs recursive operations, such as
654 those performed by wget and similar tools.
656 There exist wrapper scripts with that functionality (for example the
657 curlmirror perl script), and you can write programs based on libcurl to do
658 it, but the command line tool curl itself cannot.
660 3.16 What certificates do I need when I use SSL?
662 There are three different kinds of "certificates" to keep track of when we
663 talk about using SSL-based protocols (HTTPS or FTPS) using curl or libcurl.
667 The server you communicate may require that you can provide this in order to
668 prove that you actually are who you claim to be. If the server doesn't
669 require this, you don't need a client certificate.
671 A client certificate is always used together with a private key, and the
672 private key has a pass phrase that protects it.
676 The server you communicate with has a server certificate. You can and should
677 verify this certificate to make sure that you are truly talking to the real
678 server and not a server impersonating it.
680 CERTIFICATE AUTHORITY CERTIFICATE ("CA cert")
682 You often have several CA certs in a CA cert bundle that can be used to
683 verify a server certificate that was signed by one of the authorities in the
684 bundle. curl does not come with a CA cert bundle but most curl installs
685 provide one. You can also override the default.
687 The server certificate verification process is made by using a Certificate
688 Authority certificate ("CA cert") that was used to sign the server
689 certificate. Server certificate verification is enabled by default in curl
690 and libcurl and is often the reason for problems as explained in FAQ entry
691 4.12 and the SSLCERTS document
692 (http://curl.haxx.se/docs/sslcerts.html). Server certificates that are
693 "self-signed" or otherwise signed by a CA that you do not have a CA cert
694 for, cannot be verified. If the verification during a connect fails, you are
695 refused access. You then need to explicitly disable the verification to
696 connect to the server.
698 3.17 How do I list the root dir of an FTP server?
700 There are two ways. The way defined in the RFC is to use an encoded slash
701 in the first path part. List the "/tmp" dir like this:
703 curl ftp://ftp.sunet.se/%2ftmp/
705 or the not-quite-kosher-but-more-readable way, by simply starting the path
706 section of the URL with a slash:
708 curl ftp://ftp.sunet.se//tmp/
710 3.18 Can I use curl to send a POST/PUT and not wait for a response?
714 But you could easily write your own program using libcurl to do such stunts.
716 3.19 How do I get HTTP from a host using a specific IP address?
718 For example, you may be trying out a web site installation that isn't yet in
719 the DNS. Or you have a site using multiple IP addresses for a given host
720 name and you want to address a specific one out of the set.
722 Set a custom Host: header that identifies the server name you want to reach
723 but use the target IP address in the URL:
725 curl --header "Host: www.example.com" http://127.0.0.1/
727 You can also opt to add faked host name entries to curl with the --resolve
728 option. That has the added benefit that things like redirects will also work
729 properly. The above operation would instead be done as:
731 curl --resolve www.example.com:80:127.0.0.1 http://www.example.com/
733 3.20 How to SFTP from my user's home directory?
735 Contrary to how FTP works, SFTP and SCP URLs specify the exact directory to
736 work with. It means that if you don't specify that you want the user's home
737 directory, you get the actual root directory.
739 To specify a file in your user's home directory, you need to use the correct
740 URL syntax which for sftp might look similar to:
742 curl -O -u user:password sftp://example.com/~/file.txt
744 and for SCP it is just a different protocol prefix:
746 curl -O -u user:password scp://example.com/~/file.txt
748 3.21 Protocol xxx not supported or disabled in libcurl
750 When passing on a URL to curl to use, it may respond that the particular
751 protocol is not supported or disabled. The particular way this error message
752 is phrased is because curl doesn't make a distinction internally of whether
753 a particular protocol is not supported (i.e. never got any code added that
754 knows how to speak that protocol) or if it was explicitly disabled. curl can
755 be built to only support a given set of protocols, and the rest would then
756 be disabled or not supported.
758 Note that this error will also occur if you pass a wrongly spelled protocol
759 part as in "htpt://example.com" or as in the less evident case if you prefix
760 the protocol part with a space as in " http://example.com/".
762 3.22 curl -X gives me HTTP problems
764 In normal circumstances, -X should hardly ever be used.
766 By default you use curl without explicitly saying which request method to
767 use when the URL identifies a HTTP transfer. If you just pass in a URL like
768 "curl http://example.com" it will use GET. If you use -d or -F curl will use
769 POST, -I will cause a HEAD and -T will make it a PUT.
771 If for whatever reason you're not happy with these default choices that curl
772 does for you, you can override those request methods by specifying -X
773 [WHATEVER]. This way you can for example send a DELETE by doing "curl -X
776 It is thus pointless to do "curl -XGET [URL]" as GET would be used
777 anyway. In the same vein it is pointless to do "curl -X POST -d data
778 [URL]"... But you can make a fun and somewhat rare request that sends a
779 request-body in a GET request with something like "curl -X GET -d data
782 Note that -X doesn't actually change curl's behavior as it only modifies the
783 actual string sent in the request, but that may of course trigger a
784 different set of events.
786 Accordingly, by using -XPOST on a command line that for example would follow
787 a 303 redirect, you will effectively prevent curl from behaving
793 4.1 Problems connecting to SSL servers.
795 It took a very long time before we could sort out why curl had problems to
796 connect to certain SSL servers when using SSLeay or OpenSSL v0.9+. The
797 error sometimes showed up similar to:
799 16570:error:1407D071:SSL routines:SSL2_READ:bad mac decode:s2_pkt.c:233:
801 It turned out to be because many older SSL servers don't deal with SSLv3
802 requests properly. To correct this problem, tell curl to select SSLv2 from
803 the command line (-2/--sslv2).
805 There have also been examples where the remote server didn't like the SSLv2
806 request and instead you had to force curl to use SSLv3 with -3/--sslv3.
808 4.2 Why do I get problems when I use & or % in the URL?
810 In general unix shells, the & symbol is treated specially and when used, it
811 runs the specified command in the background. To safely send the & as a part
812 of a URL, you should quote the entire URL by using single (') or double (")
813 quotes around it. Similar problems can also occur on some shells with other
814 characters, including ?*!$~(){}<>\|;`. When in doubt, quote the URL.
816 An example that would invoke a remote CGI that uses &-symbols could be:
818 curl 'http://www.altavista.com/cgi-bin/query?text=yes&q=curl'
820 In Windows, the standard DOS shell treats the percent sign specially and you
821 need to use TWO percent signs for each single one you want to use in the
824 If you want a literal percent sign to be part of the data you pass in a POST
825 using -d/--data you must encode it as '%25' (which then also needs the
826 percent sign doubled on Windows machines).
828 4.3 How can I use {, }, [ or ] to specify multiple URLs?
830 Because those letters have a special meaning to the shell, and to be used in
831 a URL specified to curl you must quote them.
833 An example that downloads two URLs (sequentially) would do:
835 curl '{curl,www}.haxx.se'
837 To be able to use those letters as actual parts of the URL (without using
838 them for the curl URL "globbing" system), use the -g/--globoff option:
840 curl -g 'www.site.com/weirdname[].html'
842 4.4 Why do I get downloaded data even though the web page doesn't exist?
844 Curl asks remote servers for the page you specify. If the page doesn't exist
845 at the server, the HTTP protocol defines how the server should respond and
846 that means that headers and a "page" will be returned. That's simply how
849 By using the --fail option you can tell curl explicitly to not get any data
850 if the HTTP return code doesn't say success.
852 4.5 Why do I get return code XXX from a HTTP server?
854 RFC2616 clearly explains the return codes. This is a short transcript. Go
855 read the RFC for exact details:
857 4.5.1 "400 Bad Request"
859 The request could not be understood by the server due to malformed
860 syntax. The client SHOULD NOT repeat the request without modifications.
862 4.5.2 "401 Unauthorized"
864 The request requires user authentication.
866 4.5.3 "403 Forbidden"
868 The server understood the request, but is refusing to fulfil it.
869 Authorization will not help and the request SHOULD NOT be repeated.
871 4.5.4 "404 Not Found"
873 The server has not found anything matching the Request-URI. No indication
874 is given of whether the condition is temporary or permanent.
876 4.5.5 "405 Method Not Allowed"
878 The method specified in the Request-Line is not allowed for the resource
879 identified by the Request-URI. The response MUST include an Allow header
880 containing a list of valid methods for the requested resource.
882 4.5.6 "301 Moved Permanently"
884 If you get this return code and an HTML output similar to this:
886 <H1>Moved Permanently</H1> The document has moved <A
887 HREF="http://same_url_now_with_a_trailing_slash/">here</A>.
889 it might be because you request a directory URL but without the trailing
890 slash. Try the same operation again _with_ the trailing URL, or use the
891 -L/--location option to follow the redirection.
893 4.6 Can you tell me what error code 142 means?
895 All curl error codes are described at the end of the man page, in the
896 section called "EXIT CODES".
898 Error codes that are larger than the highest documented error code means
899 that curl has exited due to a crash. This is a serious error, and we
900 appreciate a detailed bug report from you that describes how we could go
901 ahead and repeat this!
903 4.7 How do I keep user names and passwords secret in Curl command lines?
905 This problem has two sides:
907 The first part is to avoid having clear-text passwords in the command line
908 so that they don't appear in 'ps' outputs and similar. That is easily
909 avoided by using the "-K" option to tell curl to read parameters from a file
910 or stdin to which you can pass the secret info. curl itself will also
911 attempt to "hide" the given password by blanking out the option - this
912 doesn't work on all platforms.
914 To keep the passwords in your account secret from the rest of the world is
915 not a task that curl addresses. You could of course encrypt them somehow to
916 at least hide them from being read by human eyes, but that is not what
917 anyone would call security.
919 Also note that regular HTTP (using Basic authentication) and FTP passwords
920 are sent in clear across the network. All it takes for anyone to fetch them
921 is to listen on the network. Eavesdropping is very easy. Use more secure
922 authentication methods (like Digest, Negotiate or even NTLM) or consider the
923 SSL-based alternatives HTTPS and FTPS.
927 It is not a bug if the behavior is documented. Read the docs first.
928 Especially check out the KNOWN_BUGS file, it may be a documented bug!
930 If it is a problem with a binary you've downloaded or a package for your
931 particular platform, try contacting the person who built the package/archive
934 If there is a bug, read the BUGS document first. Then report it as described
937 4.9 Curl can't authenticate to the server that requires NTLM?
939 NTLM support requires OpenSSL, GnuTLS, NSS, Secure Transport, or Microsoft
940 Windows libraries at build-time to provide this functionality.
942 NTLM is a Microsoft proprietary protocol. Proprietary formats are evil. You
943 should not use such ones.
945 4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
947 Many web servers allow or demand that the administrator configures the
948 server properly for these requests to work on the web server.
950 Some servers seem to support HEAD only on certain kinds of URLs.
952 To fully grasp this, try the documentation for the particular server
953 software you're trying to interact with. This is not anything curl can do
956 4.11 Why does my HTTP range requests return the full document?
958 Because the range may not be supported by the server, or the server may
959 choose to ignore it and return the full document anyway.
961 4.12 Why do I get "certificate verify failed" ?
963 You invoke curl 7.10 or later to communicate on a https:// URL and get an
964 error back looking something similar to this:
966 curl: (35) SSL: error:14090086:SSL routines:
967 SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
969 Then it means that curl couldn't verify that the server's certificate was
970 good. Curl verifies the certificate using the CA cert bundle that comes with
971 the curl installation.
973 To disable the verification (which makes it act like curl did before 7.10),
974 use -k. This does however enable man-in-the-middle attacks.
976 If you get this failure but are having a CA cert bundle installed and used,
977 the server's certificate is not signed by one of the CA's in the bundle. It
978 might for example be self-signed. You then correct this problem by obtaining
979 a valid CA cert for the server. Or again, decrease the security by disabling
982 Details are also in the SSLCERTS file in the release archives, found online
983 here: http://curl.haxx.se/docs/sslcerts.html
985 4.13 Why is curl -R on Windows one hour off?
987 During daylight savings time, when -R is used, curl will set a time that
988 appears one hour off. This happens due to a flaw in how Windows stores and
989 uses file modification times and it is not easily worked around. For details
990 on this problem, read this: http://www.codeproject.com/datetime/dstbugs.asp
992 4.14 Redirects work in browser but not with curl!
994 curl supports HTTP redirects fine (see item 3.8). Browsers generally support
995 at least two other ways to perform redirects that curl does not:
997 Meta tags. You can write a HTML tag that will cause the browser to redirect
998 to another given URL after a certain time.
1000 Javascript. You can write a Javascript program embedded in a HTML page that
1001 redirects the browser to another given URL.
1003 There is no way to make curl follow these redirects. You must either
1004 manually figure out what the page is set to do, or you write a script that
1005 parses the results and fetches the new URL.
1007 4.15 FTPS doesn't work
1009 curl supports FTPS (sometimes known as FTP-SSL) both implicit and explicit
1012 When a URL is used that starts with FTPS://, curl assumes implicit SSL on
1013 the control connection and will therefore immediately connect and try to
1014 speak SSL. FTPS:// connections default to port 990.
1016 To use explicit FTPS, you use a FTP:// URL and the --ftp-ssl option (or one
1017 of its related flavours). This is the most common method, and the one
1018 mandated by RFC4217. This kind of connection then of course uses the
1019 standard FTP port 21 by default.
1021 4.16 My HTTP POST or PUT requests are slow!
1023 libcurl makes all POST and PUT requests (except for POST requests with a
1024 very tiny request body) use the "Expect: 100-continue" header. This header
1025 allows the server to deny the operation early so that libcurl can bail out
1026 already before having to send any data. This is useful in authentication
1029 However, many servers don't implement the Expect: stuff properly and if the
1030 server doesn't respond (positively) within 1 second libcurl will continue
1031 and send off the data anyway.
1033 You can disable libcurl's use of the Expect: header the same way you disable
1034 any header, using -H / CURLOPT_HTTPHEADER, or by forcing it to use HTTP 1.0.
1036 4.17 Non-functional connect timeouts
1038 In most Windows setups having a timeout longer than 21 seconds make no
1039 difference, as it will only send 3 TCP SYN packets and no more. The second
1040 packet sent three seconds after the first and the third six seconds after
1041 the second. No more than three packets are sent, no matter how long the
1044 See option TcpMaxConnectRetransmissions on this page:
1045 https://support.microsoft.com/en-us/kb/175523/en-us
1047 Also, even on non-Windows systems there may run a firewall or anti-virus
1048 software or similar that accepts the connection but does not actually do
1049 anything else. This will make (lib)curl to consider the connection connected
1050 and thus the connect timeout won't trigger.
1052 4.18 file:// URLs containing drive letters (Windows, NetWare)
1054 When using cURL to try to download a local file, one might use a URL
1059 You'll find that even if D:\blah.txt does exist, cURL returns a 'file
1062 According to RFC 1738 (https://www.ietf.org/rfc/rfc1738.txt),
1063 file:// URLs must contain a host component, but it is ignored by
1064 most implementations. In the above example, 'D:' is treated as the
1065 host component, and is taken away. Thus, cURL tries to open '/blah.txt'.
1066 If your system is installed to drive C:, that will resolve to 'C:\blah.txt',
1067 and if that doesn't exist you will get the not found error.
1069 To fix this problem, use file:// URLs with *three* leading slashes:
1073 Alternatively, if it makes more sense, specify 'localhost' as the host
1076 file://localhost/D:/blah.txt
1078 In either case, cURL should now be looking for the correct file.
1080 4.19 Why doesn't cURL return an error when the network cable is unplugged?
1082 Unplugging a cable is not an error situation. The TCP/IP protocol stack
1083 was designed to be fault tolerant, so even though there may be a physical
1084 break somewhere the connection shouldn't be affected, just possibly
1085 delayed. Eventually, the physical break will be fixed or the data will be
1086 re-routed around the physical problem through another path.
1088 In such cases, the TCP/IP stack is responsible for detecting when the
1089 network connection is irrevocably lost. Since with some protocols it is
1090 perfectly legal for the client wait indefinitely for data, the stack may
1091 never report a problem, and even when it does, it can take up to 20 minutes
1092 for it to detect an issue. The curl option --keepalive-time enables
1093 keep-alive support in the TCP/IP stack which makes it periodically probe the
1094 connection to make sure it is still available to send data. That should
1095 reliably detect any TCP/IP network failure.
1097 But even that won't detect the network going down before the TCP/IP
1098 connection is established (e.g. during a DNS lookup) or using protocols that
1099 don't use TCP. To handle those situations, curl offers a number of timeouts
1100 on its own. --speed-limit/--speed-time will abort if the data transfer rate
1101 falls too low, and --connect-timeout and --max-time can be used to put an
1102 overall timeout on the connection phase or the entire transfer.
1104 A libcurl-using application running in a known physical environment (e.g.
1105 an embedded device with only a single network connection) may want to act
1106 immediately if its lone network connection goes down. That can be achieved
1107 by having the application monitor the network connection on its own using an
1108 OS-specific mechanism, then signalling libcurl to abort (see also item 5.13).
1110 4.20 curl doesn't return error for HTTP non-200 responses!
1112 Correct. Unless you use -f (--fail).
1114 When doing HTTP transfers, curl will perform exactly what you're asking it
1115 to do and if successful it will not return an error. You can use curl to
1116 test your web server's "file not found" page (that gets 404 back), you can
1117 use it to check your authentication protected web pages (that get a 401
1120 The specific HTTP response code does not constitute a problem or error for
1121 curl. It simply sends and delivers HTTP as you asked and if that worked,
1122 everything is fine and dandy. The response code is generally providing more
1123 higher level error information that curl doesn't care about. The error was
1124 not in the HTTP transfer.
1126 If you want your command line to treat error codes in the 400 and up range
1127 as errors and thus return a non-zero value and possibly show an error
1128 message, curl has a dedicated option for that: -f (CURLOPT_FAILONERROR in
1131 You can also use the -w option and the variable %{response_code} to extract
1132 the exact response code that was return in the response.
1134 4.21 Why is there a HTTP/1.1 in my HTTP/2 request?
1136 If you use verbose to see the HTTP request when you send off a HTTP/2
1137 request, it will still say 1.1.
1139 The reason for this is that we first generate the request to send using the
1140 old 1.1 style and show that request in the verbose output, and then we
1141 convert it over to the binary header-compressed HTTP/2 style. The actual
1142 "1.1" part from that request is then not actually used in the transfer. The
1143 binary HTTP/2 headers are not human readable.
1147 5.1 Is libcurl thread-safe?
1151 We have written the libcurl code specifically adjusted for multi-threaded
1152 programs. libcurl will use thread-safe functions instead of non-safe ones if
1153 your system has such. Note that you must never share the same handle in
1156 libcurl's implementation of timeouts might use signals (depending on what it
1157 was built to use for name resolving), and signal handling is generally not
1158 thread-safe. Multi-threaded Applicationss that call libcurl from different
1159 threads (on different handles) might want to use CURLOPT_NOSIGNAL, e.g.:
1161 curl_easy_setopt(handle, CURLOPT_NOSIGNAL, true);
1163 If you use a OpenSSL-powered libcurl in a multi-threaded environment, you
1164 need to provide one or two locking functions:
1166 https://www.openssl.org/docs/crypto/threads.html
1168 If you use a GnuTLS-powered libcurl in a multi-threaded environment, you
1169 need to provide locking function(s) for libgcrypt (which is used by GnuTLS
1170 for the crypto functions).
1172 https://web.archive.org/web/20111103083330/http://www.gnu.org/software/gnutls/manual/html_node/Multi_002dthreaded-applications.html
1174 No special locking is needed with a NSS-powered libcurl. NSS is thread-safe.
1176 5.2 How can I receive all data into a large memory chunk?
1178 [ See also the examples/getinmemory.c source ]
1180 You are in full control of the callback function that gets called every time
1181 there is data received from the remote server. You can make that callback do
1182 whatever you want. You do not have to write the received data to a file.
1184 One solution to this problem could be to have a pointer to a struct that you
1185 pass to the callback function. You set the pointer using the
1186 CURLOPT_WRITEDATA option. Then that pointer will be passed to the callback
1187 instead of a FILE * to a file:
1189 /* imaginary struct */
1190 struct MemoryStruct {
1195 /* imaginary callback function */
1197 WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data)
1199 size_t realsize = size * nmemb;
1200 struct MemoryStruct *mem = (struct MemoryStruct *)data;
1202 mem->memory = (char *)realloc(mem->memory, mem->size + realsize + 1);
1204 memcpy(&(mem->memory[mem->size]), ptr, realsize);
1205 mem->size += realsize;
1206 mem->memory[mem->size] = 0;
1211 5.3 How do I fetch multiple files with libcurl?
1213 libcurl has excellent support for transferring multiple files. You should
1214 just repeatedly set new URLs with curl_easy_setopt() and then transfer it
1215 with curl_easy_perform(). The handle you get from curl_easy_init() is not
1216 only reusable, but you're even encouraged to reuse it if you can, as that
1217 will enable libcurl to use persistent connections.
1219 5.4 Does libcurl do Winsock initialization on win32 systems?
1221 Yes, if told to in the curl_global_init() call.
1223 5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
1225 Yes, but you cannot open a FILE * and pass the pointer to a DLL and have
1226 that DLL use the FILE * (as the DLL and the client application cannot access
1227 each others' variable memory areas). If you set CURLOPT_WRITEDATA you must
1228 also use CURLOPT_WRITEFUNCTION as well to set a function that writes the
1229 file, even if that simply writes the data to the specified FILE *.
1230 Similarly, if you use CURLOPT_READDATA you must also specify
1231 CURLOPT_READFUNCTION.
1233 5.6 What about Keep-Alive or persistent connections?
1235 curl and libcurl have excellent support for persistent connections when
1236 transferring several files from the same server. Curl will attempt to reuse
1237 connections for all URLs specified on the same command line/config file, and
1238 libcurl will reuse connections for all transfers that are made using the
1239 same libcurl handle.
1241 When you use the easy interface, the connection cache is kept within the
1242 easy handle. If you instead use the multi interface, the connection cache
1243 will be kept within the multi handle and will be shared among all the easy
1244 handles that are used within the same multi handle.
1246 5.7 Link errors when building libcurl on Windows!
1248 You need to make sure that your project, and all the libraries (both static
1249 and dynamic) that it links against, are compiled/linked against the same run
1252 This is determined by the /MD, /ML, /MT (and their corresponding /M?d)
1253 options to the command line compiler. /MD (linking against MSVCRT dll) seems
1254 to be the most commonly used option.
1256 When building an application that uses the static libcurl library, you must
1257 add -DCURL_STATICLIB to your CFLAGS. Otherwise the linker will look for
1258 dynamic import symbols. If you're using Visual Studio, you need to instead
1259 add CURL_STATICLIB in the "Preprocessor Definitions" section.
1261 If you get linker error like "unknown symbol __imp__curl_easy_init ..." you
1262 have linked against the wrong (static) library. If you want to use the
1263 libcurl.dll and import lib, you don't need any extra CFLAGS, but use one of
1264 the import libraries below. These are the libraries produced by the various
1265 lib/Makefile.* files:
1267 Target: static lib. import lib for libcurl*.dll.
1268 -----------------------------------------------------------
1269 MingW: libcurl.a libcurldll.a
1270 MSVC (release): libcurl.lib libcurl_imp.lib
1271 MSVC (debug): libcurld.lib libcurld_imp.lib
1272 Borland: libcurl.lib libcurl_imp.lib
1274 5.8 libcurl.so.X: open failed: No such file or directory
1276 This is an error message you might get when you try to run a program linked
1277 with a shared version of libcurl and your run-time linker (ld.so) couldn't
1278 find the shared library named libcurl.so.X. (Where X is the number of the
1279 current libcurl ABI, typically 3 or 4).
1281 You need to make sure that ld.so finds libcurl.so.X. You can do that
1282 multiple ways, and it differs somewhat between different operating systems,
1283 but they are usually:
1285 * Add an option to the linker command line that specify the hard-coded path
1286 the run-time linker should check for the lib (usually -R)
1288 * Set an environment variable (LD_LIBRARY_PATH for example) where ld.so
1289 should check for libs
1291 * Adjust the system's config to check for libs in the directory where you've
1292 put the dir (like Linux's /etc/ld.so.conf)
1294 'man ld.so' and 'man ld' will tell you more details
1296 5.9 How does libcurl resolve host names?
1298 libcurl supports a large a number of different name resolve functions. One
1299 of them is picked at build-time and will be used unconditionally. Thus, if
1300 you want to change name resolver function you must rebuild libcurl and tell
1301 it to use a different function.
1303 - The non-IPv6 resolver that can use one out of four host name resolve calls
1304 (depending on what your system supports):
1307 B - gethostbyname_r() with 3 arguments
1308 C - gethostbyname_r() with 5 arguments
1309 D - gethostbyname_r() with 6 arguments
1311 - The IPv6-resolver that uses getaddrinfo()
1313 - The c-ares based name resolver that uses the c-ares library for resolves.
1314 Using this offers asynchronous name resolves.
1316 - The threaded resolver (default option on Windows). It uses:
1318 A - gethostbyname() on plain IPv4 hosts
1319 B - getaddrinfo() on IPv6 enabled hosts
1321 Also note that libcurl never resolves or reverse-lookups addresses given as
1322 pure numbers, such as 127.0.0.1 or ::1.
1324 5.10 How do I prevent libcurl from writing the response to stdout?
1326 libcurl provides a default built-in write function that writes received data
1327 to stdout. Set the CURLOPT_WRITEFUNCTION to receive the data, or possibly
1328 set CURLOPT_WRITEDATA to a different FILE * handle.
1330 5.11 How do I make libcurl not receive the whole HTTP response?
1332 You make the write callback (or progress callback) return an error and
1333 libcurl will then abort the transfer.
1335 5.12 Can I make libcurl fake or hide my real IP address?
1337 No. libcurl operates on a higher level. Besides, faking IP address would
1338 imply sending IP packet with a made-up source address, and then you normally
1339 get a problem with receiving the packet sent back as they would then not be
1342 If you use a proxy to access remote sites, the sites will not see your local
1343 IP address but instead the address of the proxy.
1345 Also note that on many networks NATs or other IP-munging techniques are used
1346 that makes you see and use a different IP address locally than what the
1347 remote server will see you coming from. You may also consider using
1348 https://www.torproject.org/ .
1350 5.13 How do I stop an ongoing transfer?
1352 With the easy interface you make sure to return the correct error code from
1353 one of the callbacks, but none of them are instant. There is no function you
1354 can call from another thread or similar that will stop it immediately.
1355 Instead, you need to make sure that one of the callbacks you use returns an
1356 appropriate value that will stop the transfer. Suitable callbacks that you
1357 can do this with include the progress callback, the read callback and the
1360 If you're using the multi interface, you can also stop a transfer by
1361 removing the particular easy handle from the multi stack at any moment you
1362 think the transfer is done or when you wish to abort the transfer.
1364 5.14 Using C++ non-static functions for callbacks?
1366 libcurl is a C library, it doesn't know anything about C++ member functions.
1368 You can overcome this "limitation" with a relative ease using a static
1369 member function that is passed a pointer to the class:
1371 // f is the pointer to your object.
1372 static YourClass::func(void *buffer, size_t sz, size_t n, void *f)
1374 // Call non-static member function.
1375 static_cast<YourClass*>(f)->nonStaticFunction();
1378 // This is how you pass pointer to the static function:
1379 curl_easy_setopt(hcurl, CURLOPT_WRITEFUNCTION, YourClass:func);
1380 curl_easy_setopt(hcurl, CURLOPT_WRITEDATA, this);
1382 5.15 How do I get an FTP directory listing?
1384 If you end the FTP URL you request with a slash, libcurl will provide you
1385 with a directory listing of that given directory. You can also set
1386 CURLOPT_CUSTOMREQUEST to alter what exact listing command libcurl would use
1389 The follow-up question that tend to follow the previous one, is how a
1390 program is supposed to parse the directory listing. How does it know what's
1391 a file and what's a dir and what's a symlink etc. The harsh reality is that
1392 FTP provides no such fine and easy-to-parse output. The output format FTP
1393 servers respond to LIST commands are entirely at the server's own liking and
1394 the NLST output doesn't reveal any types and in many cases don't even
1395 include all the directory entries. Also, both LIST and NLST tend to hide
1396 unix-style hidden files (those that start with a dot) by default so you need
1397 to do "LIST -a" or similar to see them.
1399 The application thus needs to parse the LIST output. One such existing
1400 list parser is available at http://cr.yp.to/ftpparse.html Versions of
1401 libcurl since 7.21.0 also provide the ability to specify a wildcard to
1402 download multiple files from one FTP directory.
1404 5.16 I want a different time-out!
1406 Time and time again users realize that CURLOPT_TIMEOUT and
1407 CURLOPT_CONNECTIMEOUT are not sufficiently advanced or flexible to cover all
1408 the various use cases and scenarios applications end up with.
1410 libcurl offers many more ways to time-out operations. A common alternative
1411 is to use the CURLOPT_LOW_SPEED_LIMIT and CURLOPT_LOW_SPEED_TIME options to
1412 specify the lowest possible speed to accept before to consider the transfer
1415 The most flexible way is by writing your own time-out logic and using
1416 CURLOPT_PROGRESSFUNCTION (perhaps in combination with other callbacks) and
1417 use that to figure out exactly when the right condition is met when the
1418 transfer should get stopped.
1420 5.17 Can I write a server with libcurl?
1422 No. libcurl offers no functions or building blocks to build any kind of
1423 internet protocol server. libcurl is only a client-side library. For server
1424 libraries, you need to continue your search elsewhere but there exist many
1425 good open source ones out there for most protocols you could possibly want a
1426 server for. And there are really good stand-alone ones that have been tested
1427 and proven for many years. There's no need for you to reinvent them!
1429 5.18 Does libcurl use threads?
1431 Put simply: no, libcurl will execute in the same thread you call it in. All
1432 callbacks will be called in the same thread as the one you call libcurl in.
1434 If you want to avoid your thread to be blocked by the libcurl call, you make
1435 sure you use the non-blocking API which will do transfers asynchronously -
1436 but still in the same single thread.
1438 libcurl will potentially internally use threads for name resolving, if it
1439 was built to work like that, but in those cases it'll create the child
1440 threads by itself and they will only be used and then killed internally by
1441 libcurl and never exposed to the outside.
1445 Curl and libcurl are released under a MIT/X derivate license. The license is
1446 very liberal and should not impose a problem for your project. This section
1447 is just a brief summary for the cases we get the most questions. (Parts of
1448 this section was much enhanced by Bjorn Reese.)
1450 We are not lawyers and this is not legal advice. You should probably consult
1451 one if you want true and accurate legal insights without our prejudice. Note
1452 especially that this section concerns the libcurl license only; compiling in
1453 features of libcurl that depend on other libraries (e.g. OpenSSL) may affect
1454 the licensing obligations of your application.
1456 6.1 I have a GPL program, can I use the libcurl library?
1460 Since libcurl may be distributed under the MIT/X derivate license, it can be
1461 used together with GPL in any software.
1463 6.2 I have a closed-source program, can I use the libcurl library?
1467 libcurl does not put any restrictions on the program that uses the library.
1469 6.3 I have a BSD licensed program, can I use the libcurl library?
1473 libcurl does not put any restrictions on the program that uses the library.
1475 6.4 I have a program that uses LGPL libraries, can I use libcurl?
1479 The LGPL license doesn't clash with other licenses.
1481 6.5 Can I modify curl/libcurl for my program and keep the changes secret?
1485 The MIT/X derivate license practically allows you to do almost anything with
1486 the sources, on the condition that the copyright texts in the sources are
1489 6.6 Can you please change the curl/libcurl license to XXXX?
1493 We have carefully picked this license after years of development and
1494 discussions and a large amount of people have contributed with source code
1495 knowing that this is the license we use. This license puts the restrictions
1496 we want on curl/libcurl and it does not spread to other programs or
1497 libraries that use it. It should be possible for everyone to use libcurl or
1498 curl in their projects, no matter what license they already have in use.
1500 6.7 What are my obligations when using libcurl in my commercial apps?
1502 Next to none. All you need to adhere to is the MIT-style license (stated in
1503 the COPYING file) which basically says you have to include the copyright
1504 notice in "all copies" and that you may not use the copyright holder's name
1505 when promoting your software.
1507 You do not have to release any of your source code.
1509 You do not have to reveal or make public any changes to the libcurl source
1512 You do not have to broadcast to the world that you are using libcurl within
1515 All we ask is that you disclose "the copyright notice and this permission
1516 notice" somewhere. Most probably like in the documentation or in the section
1517 where other third party dependencies already are mentioned and acknowledged.
1519 As can be seen here: http://curl.haxx.se/docs/companies.html and elsewhere,
1520 more and more companies are discovering the power of libcurl and take
1521 advantage of it even in commercial environments.
1526 7.1 What is PHP/CURL?
1528 The module for PHP that makes it possible for PHP programs to access curl-
1529 functions from within PHP.
1531 In the cURL project we call this module PHP/CURL to differentiate it from
1532 curl the command line tool and libcurl the library. The PHP team however
1533 does not refer to it like this (for unknown reasons). They call it plain
1534 CURL (often using all caps) or sometimes ext/curl, but both cause much
1535 confusion to users which in turn gives us a higher question load.
1537 7.2 Who wrote PHP/CURL?
1539 PHP/CURL is a module that comes with the regular PHP package. It depends and
1540 uses libcurl, so you need to have libcurl installed properly first before
1541 PHP/CURL can be used. PHP/CURL was initially written by Sterling Hughes.
1543 7.3 Can I perform multiple requests using the same handle?
1545 Yes - at least in PHP version 4.3.8 and later (this has been known to not
1546 work in earlier versions, but the exact version when it started to work is
1549 After a transfer, you just set new options in the handle and make another
1550 transfer. This will make libcurl to re-use the same connection if it can.