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Updated: September 3, 2002 (http://curl.haxx.se/docs/faq.html)
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FAQ
1. Philosophy
1.1 What is cURL?
1.2 What is libcurl?
1.3 What is cURL not?
1.4 When will you make curl do XXXX ?
1.5 Who makes cURL?
1.6 What do you get for making cURL?
1.7 What about CURL from curl.com?
2. Install Related Problems
2.1 configure doesn't find OpenSSL even when it is installed
2.1.1. native linker doesn't find openssl
2.1.2. only the libssl lib is missing
2.2 Does curl work/build with other SSL libraries?
2.3 Where can I find a copy of LIBEAY32.DLL?
2.4 Does cURL support Socks (RFC 1928) ?
3. Usage Problems
3.1 curl: (1) SSL is disabled, https: not supported
3.2 How do I tell curl to resume a transfer?
3.3 Why doesn't my posting using -F work?
3.4 How do I tell curl to run custom FTP commands?
3.5 How can I disable the Pragma: nocache header?
3.6 Does curl support ASP, XML, XHTML or HTML version Y?
3.7 Can I use curl to delete/rename a file through FTP?
3.8 How do I tell curl to follow HTTP redirects?
3.9 How do I use curl in my favorite programming language?
3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
3.11 How do I POST with a different Content-Type?
3.13 Why does my single/double quotes fail?
3.14 Does curl support javascript or pac (automated proxy config)?
4. Running Problems
4.1 Problems connecting to SSL servers.
4.2 Why do I get problems when I use & or % in the URL?
4.3 How can I use {, }, [ or ] to specify multiple URLs?
4.4 Why do I get downloaded data even though the web page doesn't exist?
4.5 Why do I get return code XXX from a HTTP server?
4.5.1 "400 Bad Request"
4.5.2 "401 Unauthorized"
4.5.3 "403 Forbidden"
4.5.4 "404 Not Found"
4.5.5 "405 Method Not Allowed"
4.5.6 "301 Moved Permanently"
4.7 How do I keep user names and passwords secret in Curl command lines?
4.8 I found a bug!
4.9 Curl can't authenticate to the server that requires NTLM?
4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
5.1 Is libcurl thread-safe?
5.2 How can I receive all data into a large memory chunk?
5.4 Does libcurl do Winsock initing on win32 systems?
5.6 What about Keep-Alive or persistent connections?
6. License Issues
6.1 I have a GPL program, can I use the libcurl library?
6.2 I have a closed-source program, can I use the libcurl library?
6.3 I have a BSD licensed program, can I use the libcurl library?
6.4 I have a program that uses LGPL libraries, can I use libcurl?
6.5 Can I modify curl/libcurl for my program and keep the changes secret?
6.6 Can you please change the curl/libcurl license to XXXX?
==============================================================================
1. Philosophy
1.1 What is cURL?
cURL (or simply just 'curl') is a command line tool for getting or sending
files using URL syntax. The name is a play on 'Client for URLs', originally
with URL spelled in uppercase to make it obvious it deals with URLs. The
fact it can also be pronounced 'see URL' also helped, it works as an
abbrivation for "Client URL Request Library" or why not the recursive
version: "Curl is a URL Request Library".
Curl supports a range of common Internet protocols, currently including
HTTP, HTTPS, FTP, FTPS, GOPHER, LDAP, DICT, TELNET and FILE.
We spell it cURL or just curl. We pronounce it with an initial k sound:
[kurl].
libcurl is a reliable and portable library which provides you with an easy
interface to a range of common Internet protocols.
You can use libcurl for free in your application even if it is commercial
or closed-source.
Curl is *not* a wget clone even though that is a very common misconception.
Never, during curl's development, have we intended curl to replace wget or
compete on its market. Curl is targeted at single-shot file transfers.
Curl is not a web site mirroring program. If you wanna use curl to mirror
something: fine, go ahead and write a script that wraps around curl to make
it reality (like curlmirror.pl does).
Curl is not an FTP site mirroring program. Sure, get and send FTP with curl
but if you want systematic and sequential behavior you should write a
script (or write a new program that interfaces libcurl) and do it.
Curl is not a PHP tool, even though it works perfectly well when used from
or with PHP.
Curl is not a single-OS program. Curl exists, compiles, builds and runs
under a wide range of operating systems, including all modern Unixes (and a
bunch of older ones too), Windows, Amiga, BeOS, OS/2, OS X, QNX etc.
We love suggestions of what to change in order to make curl and libcurl
better. We do however believe in a few rules when it comes to the future of
* Curl is to remain a command line tool. If you want GUIs or fancy scripting
capabilities, you're free to write another tool that uses libcurl and that
offers this. There's no point in having a single tool that does every
imaginable thing. That's also one of the great advantages of having the
core of curl as a library.
* We do not add things to curl that other small and available tools already
do very fine at the side. Curl's output is fine to pipe into another
program or redirect to another file for the next program to interpret.
* We focus on protocol related issues and improvements. If you wanna do more
magic with the supported protocols than curl currently does, chances are
big we will agree. If you wanna add more protocols, we may very well
* If you want someone else to make all the work while you wait for us to
implement it for you, that is not a very friendly attitude. We spend a
considerable time already on maintaining and developing curl. In order to
get more out of us, you should consider trading in some of your time and
efforts in return.
* If you write the code, chances are bigger that it will get into curl
faster.
1.5 Who makes cURL?
cURL and libcurl are not made by any single individual. Sure, Daniel
Stenberg writes the major parts, but other persons' submissions are
important and crucial. Anyone can contribute and post their changes and
improvements and have them inserted in the main sources (of course on the
condition that developers agree on that the fixes are good).
The list of contributors in the docs/THANKS file is only a small part of all
the people that every day provide us with bug reports, suggestions, ideas
and source code.
curl is developed by a community, with Daniel at the wheel.
1.6 What do you get for making cURL?
Project cURL is entirely free and open. No person gets paid for developing
curl. We do this voluntarily on our spare time.
We get some help from companies. Contactor Data hosts the curl web site and
the main mailing list, Haxx owns the curl web site's domain and
sourceforge.net hosts several project tools we take advantage from like the
bug tracker, mailing lists and more.
If you want to support our project with a donation or similar, one way of
doing that would be to buy "gift certificates" at useful online shopping
sites, such as amazon.com or thinkgeek.com. Another way would be to sponsor
us through a banner-program or even better: by helping us coding,
documenting, testing etc.
1.7 What about CURL from curl.com?
During the summer 2001, curl.com has been busy advertising their client-side
programming language for the web, named CURL.
Daniel Stenberg
committed
We are in no way associated with curl.com or their CURL programming
language.
Daniel Stenberg
committed
Our project name curl has been in effective use since 1998. We were not the
first computer related project to use the name "curl" and do not claim any
Daniel Stenberg
committed
first-hand rights to the name.
Daniel Stenberg
committed
We recognize that we will be living in parallel with curl.com and wish them
every success.
1.8 I have a problem who do I mail?
Please do not attempt to mail any single individual unless you really need
to. Keep curl-related questions on a suitable mailing list. All available
mailing lists are listed in the MANUAL document and online at
http://curl.haxx.se/mail/
Keeping curl-related questions and discussions on mailing lists allows others
to join in and help, to share their ideas, contribute their suggestions and
spread their wisdom. Keeping discussions on public mailing lists also allows
for others to learn from this (both current and future users thanks to the
web based archives of the mailing lists), thus saving us from having to
repeat ourselves even more. Thanks for respecting this.
2. Install Related Problems
2.1. configure doesn't find OpenSSL even when it is installed
2.1.1. native linker doesn't find openssl
Affected platforms:
Solaris (native cc compiler)
HPUX (native cc compiler)
SGI IRIX (native cc compiler)
When configuring curl, I specify --with-ssl. OpenSSL is installed in
/usr/local/ssl Configure reports SSL in /usr/local/ssl, but fails to find
CRYPTO_lock in -lcrypto
Cause: The cc for this test places the -L/usr/local/ssl/lib AFTER
-lcrypto, so ld can't find the library. This is due to a bug in the GNU
autoconf tool.
Workaround: Specifying "LDFLAGS=-L/usr/local/ssl/lib" in front of
./configure places the -L/usr/local/ssl/lib early enough in the command
line to make things work
Solution submitted by: Bob Allison <allisonb@users.sourceforge.net>
2.1.2. only the libssl lib is missing
If all include files and the libcrypto lib is present, with only the
libssl being missing according to configure, this is mostly likely because
a few functions are left out from the libssl.
If the function names missing include RSA or RSAREF you can be certain
that this is because libssl requires the RSA and RSAREF libs to build.
See the INSTALL file section that explains how to add those libs to
configure. Make sure that you remove the config.cache file before you
rerun configure with the new flags.
Curl has been written to use OpenSSL, although there should not be much
problems using a different library. If anyone does "port" curl to use a
different SSL library, we are of course very interested in getting the
patch!
That is an OpenSSL binary built for Windows.
Curl uses OpenSSL to do the SSL stuff. The LIBEAY32.DLL is what curl needs
on a windows machine to do https://. Check out the curl web site to find
accurate and up-to-date pointers to recent OpenSSL DLLs and other binary
No. Nobody has wanted it that badly yet. We appreciate patches that bring
this functionality.
If you get this output when trying to get anything from a https:// server,
it means that the configure script couldn't find all libs and include files
it requires for SSL to work. If the configure script fails to find them,
curl is simply built without SSL support.
To get the https:// support into a curl that was previously built but that
reports that https:// is not supported, you should dig through the document
and logs and check out why the configure script doesn't find the SSL libs
and/or include files.
Also, check out the other paragraph in this FAQ labeled "configure doesn't
find OpenSSL even when it is installed".
Curl supports resumed transfers both ways on both FTP and HTTP.
You can't simply use -F or -d at your choice. The web server that will
receive your post assumes one of the formats. If the form you're trying to
"fake" sets the type to 'multipart/form-data', then and only then you must
use the -F type. In all the most common cases, you should use -d which then
causes a posting with the type 'application/x-www-form-urlencoded'.
This is described in some detail in the MANUAL and TheArtOfHttpScripting
documents, and if you don't understand it the first time, read it again
before you post questions about this to the mailing list. Also, try reading
through the mailing list archives for old postings and questions regarding
this.
You can tell curl to perform optional commands both before and/or after a
file transfer. Study the -Q/--quote option.
Since curl is used for file transfers, you don't use curl to just perform
FTP commands without transferring anything. Therefore you must always specify
a URL to transfer to/from even when doing custom FTP commands.
You can change all internally generated headers by adding a replacement with
the -H/--header option. By adding a header with empty contents you safely
disable that one. Use -H "Pragma:" to disable that specific header.
3.6. Does curl support ASP, XML, XHTML or HTML version Y?
To curl, all contents are alike. It doesn't matter how the page was
generated. It may be ASP, PHP, Perl, shell-script, SSI or plain
HTML-files. There's no difference to curl and it doesn't even know what kind
of language that generated the page.
Yes. You specify custom FTP commands with -Q/--quote.
One example would be to delete a file after you have downloaded it:
3.8 How do I tell curl to follow HTTP redirects?
Curl does not follow so-called redirects by default. The Location: header
that informs the client about this is only interpreted if you're using the
-L/--location option. As in:
3.9 How do I use curl in my favorite programming language?
There exist many language interfaces/bindings for curl that integrates it
better with various languages. If you are fluid in a script language, you
may very well opt to use such an interface instead of using the command line
tool.
Find out more about which languages that support curl directly, and how to
install and use them, in the libcurl section of the curl web site:
http://curl.haxx.se/libcurl/
In December 2001, there are interfaces available for the following
languages: C/C++, Cocoa, Dylan, Java, Perl, PHP, Python, Rexx, Ruby, Scheme
and Tcl. By the time you read this, additional ones may have appeared!
3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
Curl adheres to the HTTP spec, which basically means you can play with *any*
protocol that is built on top of HTTP. Protocols such as SOAP, WEBDAV and
XML-RPC are all such ones. You can use -X to set custom requests and -H to
set custom headers (or replace internally generated ones).
Using libcurl or PHP's curl modules is just as fine and you'd just use the
proper library options to do the same.
3.11 How do I POST with a different Content-Type?
You can always replace the internally generated headers with -H/--header.
To make a simple HTTP POST with text/xml as content-type, do something like:
curl -d "datatopost" -H "Content-Type: text/xml" [URL]
3.12 Why do FTP specific features over HTTP proxy fail?
Because when you use a HTTP proxy, the protocol spoken on the network will
be HTTP, even if you specify a FTP URL. This effectively means that you
normally can't use FTP specific features such as FTP upload and FTP quote
etc.
There is one exception to this rule, and that is if you can "tunnel through"
the given HTTP proxy. Proxy tunneling is enabled with a special option (-p)
and is generally not available as proxy admins usually disable tunneling to
other ports than 443 (which is used for HTTPS access through proxies).
3.13 Why does my single/double quotes fail?
To specify a command line option that includes spaces, you might need to
put the entire option within quotes. Like in:
curl -d " with spaces " url.com
or perhaps
curl -d ' with spaces ' url.com
Exactly what kind of quotes and how to do this is entirely up to the shell
or command line interpreter that you are using. For most unix shells, you
can more or less pick either single (') or double (") quotes. For
Windows/DOS prompts I believe you're forced to use double (") quotes.
Please study the documentation for your particular environment. Examples in
the curl docs will use a mix of both these ones as shown above. You must
adjust them to work in your environment.
Remember that curl works and runs on more operating systems than most single
individuals have ever tried.
3.14 Does curl support javascript or pac (automated proxy config)?
Many web pages do magic stuff using embedded javascript. Curl and libcurl
have no built-in support for that, so it will be treated just like any other
contents.
.pac files are a netscape invention and are sometimes used by organizations
to allow them to differentiate which proxies to use. The .pac contents is
just a javascript program that gets invoked by the browser and that returns
the name of the proxy to connect to. Since curl doesn't support javascript,
it can't support .pac proxy configuration either.
Some work-arounds usually suggested to overcome this javascript dependency:
- Depending on the javascript complexity, write up a script that
translates it to another language and execute that.
- Read the javascript code and rewrite the same logic in another language.
- Implement a javascript interpreted, people have successfully used the
Mozilla javascript engine in the past.
- Ask your admins to stop this, for a static proxy setup or similar.
4. Running Problems
4.1. Problems connecting to SSL servers.
It took a very long time before we could sort out why curl had problems to
connect to certain SSL servers when using SSLeay or OpenSSL v0.9+. The
error sometimes showed up similar to:
16570:error:1407D071:SSL routines:SSL2_READ:bad mac decode:s2_pkt.c:233:
It turned out to be because many older SSL servers don't deal with SSLv3
requests properly. To correct this problem, tell curl to select SSLv2 from
the command line (-2/--sslv2).
There has also been examples where the remote server didn't like the SSLv2
request and instead you had to force curl to use SSLv3 with -3/--sslv3.
4.2. Why do I get problems when I use & or % in the URL?
In general unix shells, the & letter is treated special and when used, it
runs the specified command in the background. To safely send the & as a part
of a URL, you should quote the entire URL by using single (') or double (")
An example that would invoke a remote CGI that uses &-letters could be:
curl 'http://www.altavista.com/cgi-bin/query?text=yes&q=curl'
In Windows, the standard DOS shell treats the %-letter specially and you
need to use TWO %-letters for each single one you want to use in the URL.
Daniel Stenberg
committed
Also note that if you want the literal %-letter to be part of the data you
pass in a POST using -d/--data you must encode it as '%25' (which then also
needs the %-letter doubled on Windows machines).
Daniel Stenberg
committed
4.3. How can I use {, }, [ or ] to specify multiple URLs?
Because those letters have a special meaning to the shell, and to be used in
a URL specified to curl you must quote them.
An example that downloads two URLs (sequentially) would do:
curl '{curl,www}.haxx.se'
To be able to use those letters as actual parts of the URL (without using
them for the curl URL "globbing" system), use the -g/--globoff option (curl
7.6 and later):
curl -g 'www.site.com/weirdname[].html'
4.4. Why do I get downloaded data even though the web page doesn't exist?
Curl asks remote servers for the page you specify. If the page doesn't exist
at the server, the HTTP protocol defines how the server should respond and
that means that headers and a "page" will be returned. That's simply how
HTTP works.
By using the --fail option you can tell curl explicitly to not get any data
if the HTTP return code doesn't say success.
RFC2616 clearly explains the return codes. This is a short transcript. Go
read the RFC for exact details:
4.5.1 "400 Bad Request"
The request could not be understood by the server due to malformed
syntax. The client SHOULD NOT repeat the request without modifications.
4.5.2 "401 Unauthorized"
4.5.3 "403 Forbidden"
The server understood the request, but is refusing to fulfill it.
Authorization will not help and the request SHOULD NOT be repeated.
4.5.4 "404 Not Found"
The server has not found anything matching the Request-URI. No indication
is given of whether the condition is temporary or permanent.
4.5.5 "405 Method Not Allowed"
The method specified in the Request-Line is not allowed for the resource
identified by the Request-URI. The response MUST include an Allow header
containing a list of valid methods for the requested resource.
4.5.6 "301 Moved Permanently"
If you get this return code and an HTML output similar to this:
<H1>Moved Permanently</H1> The document has moved <A
HREF="http://same_url_now_with_a_trailing_slash/">here</A>.
it might be because you request a directory URL but without the trailing
slash. Try the same operation again _with_ the trailing URL, or use the
-L/--location option to follow the redirection.
All error codes that are larger than the highest documented error code means
that curl has exited due to a crash. This is a serious error, and we
appreciate a detailed bug report from you that describes how we could go
ahead and repeat this!
4.7. How do I keep user names and passwords secret in Curl command lines?
This problem has two sides:
The first part is to avoid having clear-text passwords in the command line
so that they don't appear in 'ps' outputs and similar. That is easily
avoided by using the "-K" option to tell curl to read parameters from a
file or stdin to which you can pass the secret info.
To keep the passwords in your account secret from the rest of the world is
not a task that curl addresses. You could of course encrypt them somehow to
at least hide them from being read by human eyes, but that is not what
anyone would call security.
Also note that regular HTTP and FTP passwords are sent in clear across the
network. All it takes for anyone to fetch them is to listen on the network.
4.8 I found a bug!
It is not a bug if the behavior is documented. Read the docs first.
If it is a problem with a binary you've downloaded or a package for your
particular platform, try contacting the person who built the package/archive
you have.
If there is a bug, read the BUGS document first. Then report it as described
in there.
4.9. Curl can't authenticate to the server that requires NTLM?
NTLM is a Microsoft proprietary protocol. Unfortunately, curl does not
currently support that. Proprietary formats are evil. You should not use
such ones.
4.10 My HTTP request using HEAD, PUT or DELETE doesn't work!
Many web servers allow or demand that the administrator configures the
server properly for these requests to work on the web server.
Some servers seem to support HEAD only on certain kinds of URLs.
To fully grasp this, try the documentation for the particular server
software you're trying to interact with. This is not anything curl can do
anything about.
5.1. Is libcurl thread-safe?
Yes.
We have written the libcurl code specificly adjusted for multi-threaded
programs. libcurl will use thread-safe functions instead of non-safe ones if
your system has such.
We would appreciate some kind of report or README file from those who have
used libcurl in a threaded environment.
5.2 How can I receive all data into a large memory chunk?
[ See also the examples/getinmemory.c source ]
You are in full control of the callback function that gets called every time
there is data received from the remote server. You can make that callback do
whatever you want. You do not have to write the received data to a file.
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One solution to this problem could be to have a pointer to a struct that you
pass to the callback function. You set the pointer using the
curl_easy_setopt(CURLOPT_FILE) function. Then that pointer will be passed to
the callback instead of a FILE * to a file:
/* imaginary struct */
struct MemoryStruct {
char *memory;
size_t size;
};
/* imaginary callback function */
size_t
WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data)
{
register int realsize = size * nmemb;
struct MemoryStruct *mem = (struct MemoryStruct *)data;
mem->memory = (char *)realloc(mem->memory, mem->size + realsize + 1);
if (mem->memory) {
memcpy(&(mem->memory[mem->size]), ptr, realsize);
mem->size += realsize;
mem->memory[mem->size] = 0;
}
return realsize;
}
Starting with version 7.7, curl and libcurl will have excellent support for
transferring multiple files. You should just repeatedly set new URLs with
curl_easy_setopt() and then transfer it with curl_easy_perform(). The handle
you get from curl_easy_init() is not only reusable starting with libcurl
7.7, but also you're encouraged to reuse it if you can, as that will enable
For libcurl prior to 7.7, there was no multiple file support. The only
available way to do multiple requests was to init/perform/cleanup for each
transfer.
5.4 Does libcurl do Winsock initialization on win32 systems?
Yes (since 7.8.1) if told to in the curl_global_init() call.
Yes, but you cannot open a FILE * and pass the pointer to a DLL and have
that DLL use the FILE *. If you set CURLOPT_FILE you must also use
CURLOPT_WRITEFUNCTION as well to set a function that writes the file, even
if that simply writes the data to the specified FILE*. Similarly, if you use
CURLOPT_INFILE you must also specify CURLOPT_READFUNCTION.
5.6 What about Keep-Alive or persistent connections?
Starting with version 7.7, curl and libcurl will have excellent support for
persistent connections when transferring several files from the same server.
Curl will attempt to reuse connections for all URLs specified on the same
command line/config file, and libcurl will reuse connections for all
transfers that are made using the same libcurl handle.
Previous versions had no persistent connection support.
Curl and libcurl are released under a MIT/X derivate license. The license is
very liberal and should not impose a problem for your project. This section
is just a brief summary for the cases we get the most questions. (Parts of
this section was much enhanced by Bjorn Reese.)
6.1. I have a GPL program, can I use the libcurl library?
Since libcurl may be distributed under the MIT/X derivate license, it can be
used together with GPL in any software.
6.2. I have a closed-source program, can I use the libcurl library?
libcurl does not put any restrictions on the program that uses the library.
6.3. I have a BSD licensed program, can I use the libcurl library?
libcurl does not put any restrictions on the program that uses the library.
6.4. I have a program that uses LGPL libraries, can I use libcurl?
The LGPL license doesn't clash with other licenses.
6.5. Can I modify curl/libcurl for my program and keep the changes secret?
The MIT/X derivate license practically allows you to do almost anything with
the sources, on the condition that the copyright texts in the sources are
left intact.
6.6. Can you please change the curl/libcurl license to XXXX?
We have carefully picked this license after years of development and
discussions and a large amount of people have contributed with source code
knowing that this is the license we use. This license puts the restrictions
we want on curl/libcurl and it does not spread to other programs or
libraries that use it. The recent dual license modification should make it
possible for everyone to use libcurl or curl in their projects, no matter
what license they already have in use.