1562 lines
66 KiB
Plaintext
1562 lines
66 KiB
Plaintext
_ _ ____ _
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___| | | | _ \| |
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/ __| | | | |_) | |
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| (__| |_| | _ <| |___
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\___|\___/|_| \_\_____|
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FAQ
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1. Philosophy
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1.1 What is cURL?
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1.2 What is libcurl?
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1.3 What is curl not?
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1.4 When will you make curl do XXXX ?
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1.5 Who makes curl?
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1.6 What do you get for making curl?
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1.7 What about CURL from curl.com?
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1.8 I have a problem, who do I mail?
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1.9 Where do I buy commercial support for curl?
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1.10 How many are using curl?
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1.11 Why do you not update ca-bundle.crt
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1.12 I have a problem, who can I chat with?
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1.13 curl's ECCN number?
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1.14 How do I submit my patch?
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1.15 How do I port libcurl to my OS?
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2. Install Related Problems
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2.1 configure fails when using static libraries
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2.2 Does curl work/build with other SSL libraries?
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2.3 How do I upgrade curl.exe in Windows?
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2.4 Does curl support SOCKS (RFC 1928) ?
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3. Usage Problems
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3.1 curl: (1) SSL is disabled, https: not supported
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3.2 How do I tell curl to resume a transfer?
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3.3 Why does my posting using -F not work?
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3.4 How do I tell curl to run custom FTP commands?
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3.5 How can I disable the Accept: */* header?
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3.6 Does curl support ASP, XML, XHTML or HTML version Y?
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3.7 Can I use curl to delete/rename a file through FTP?
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3.8 How do I tell curl to follow HTTP redirects?
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3.9 How do I use curl in my favorite programming language?
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3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
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3.11 How do I POST with a different Content-Type?
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3.12 Why do FTP-specific features over HTTP proxy fail?
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3.13 Why do my single/double quotes fail?
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3.14 Does curl support JavaScript or PAC (automated proxy config)?
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3.15 Can I do recursive fetches with curl?
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3.16 What certificates do I need when I use SSL?
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3.17 How do I list the root directory of an FTP server?
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3.18 Can I use curl to send a POST/PUT and not wait for a response?
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3.19 How do I get HTTP from a host using a specific IP address?
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3.20 How to SFTP from my user's home directory?
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3.21 Protocol xxx not supported or disabled in libcurl
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3.22 curl -X gives me HTTP problems
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4. Running Problems
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4.2 Why do I get problems when I use & or % in the URL?
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4.3 How can I use {, }, [ or ] to specify multiple URLs?
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4.4 Why do I get downloaded data even though the web page does not exist?
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4.5 Why do I get return code XXX from an HTTP server?
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4.5.1 "400 Bad Request"
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4.5.2 "401 Unauthorized"
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4.5.3 "403 Forbidden"
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4.5.4 "404 Not Found"
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4.5.5 "405 Method Not Allowed"
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4.5.6 "301 Moved Permanently"
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4.6 Can you tell me what error code 142 means?
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4.7 How do I keep user names and passwords secret in curl command lines?
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4.8 I found a bug
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4.9 curl cannot authenticate to a server that requires NTLM?
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4.10 My HTTP request using HEAD, PUT or DELETE does not work
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4.11 Why do my HTTP range requests return the full document?
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4.12 Why do I get "certificate verify failed" ?
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4.13 Why is curl -R on Windows one hour off?
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4.14 Redirects work in browser but not with curl
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4.15 FTPS does not work
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4.16 My HTTP POST or PUT requests are slow
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4.17 Non-functional connect timeouts on Windows
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4.18 file:// URLs containing drive letters (Windows, NetWare)
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4.19 Why does not curl return an error when the network cable is unplugged?
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4.20 curl does not return error for HTTP non-200 responses
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5. libcurl Issues
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5.1 Is libcurl thread-safe?
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5.2 How can I receive all data into a large memory chunk?
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5.3 How do I fetch multiple files with libcurl?
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5.4 Does libcurl do Winsock initialization on win32 systems?
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5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
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5.6 What about Keep-Alive or persistent connections?
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5.7 Link errors when building libcurl on Windows
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5.8 libcurl.so.X: open failed: No such file or directory
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5.9 How does libcurl resolve host names?
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5.10 How do I prevent libcurl from writing the response to stdout?
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5.11 How do I make libcurl not receive the whole HTTP response?
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5.12 Can I make libcurl fake or hide my real IP address?
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5.13 How do I stop an ongoing transfer?
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5.14 Using C++ non-static functions for callbacks?
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5.15 How do I get an FTP directory listing?
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5.16 I want a different time-out
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5.17 Can I write a server with libcurl?
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5.18 Does libcurl use threads?
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6. License Issues
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6.1 I have a GPL program, can I use the libcurl library?
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6.2 I have a closed-source program, can I use the libcurl library?
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6.3 I have a BSD licensed program, can I use the libcurl library?
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6.4 I have a program that uses LGPL libraries, can I use libcurl?
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6.5 Can I modify curl/libcurl for my program and keep the changes secret?
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6.6 Can you please change the curl/libcurl license to XXXX?
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6.7 What are my obligations when using libcurl in my commercial apps?
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7. PHP/CURL Issues
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7.1 What is PHP/CURL?
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7.2 Who wrote PHP/CURL?
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7.3 Can I perform multiple requests using the same handle?
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7.4 Does PHP/CURL have dependencies?
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8. Development
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8.1 Why does curl use C89?
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8.2 Will curl be rewritten?
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==============================================================================
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1. Philosophy
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1.1 What is cURL?
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cURL is the name of the project. The name is a play on 'Client for URLs',
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originally with URL spelled in uppercase to make it obvious it deals with
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URLs. The fact it can also be read as 'see URL' also helped, it works as
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an abbreviation for "Client URL Request Library" or why not the recursive
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version: "curl URL Request Library".
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The cURL project produces two products:
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libcurl
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A client-side URL transfer library, supporting DICT, FILE, FTP, FTPS,
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GOPHER, GOPHERS, HTTP, HTTPS, IMAP, IMAPS, LDAP, LDAPS, MQTT, POP3, POP3S,
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RTMP, RTMPS, RTSP, SCP, SFTP, SMB, SMBS, SMTP, SMTPS, TELNET, TFTP, WS
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and WSS.
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libcurl supports HTTPS certificates, HTTP POST, HTTP PUT, FTP uploading,
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Kerberos, SPNEGO, HTTP form based upload, proxies, cookies, user+password
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authentication, file transfer resume, http proxy tunneling and more.
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libcurl is highly portable, it builds and works identically on numerous
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platforms, including Solaris, NetBSD, FreeBSD, OpenBSD, Darwin, HP-UX,
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IRIX, AIX, Tru64, Linux, UnixWare, HURD, Windows, Amiga, OS/2, macOS,
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Ultrix, QNX, OpenVMS, RISC OS, Novell NetWare, DOS, Symbian, OSF, Android,
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Minix, IBM TPF and more...
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libcurl is free, thread-safe, IPv6 compatible, feature rich, well
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supported and fast.
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curl
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A command line tool for getting or sending data using URL syntax.
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Since curl uses libcurl, curl supports the same wide range of common
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Internet protocols that libcurl does.
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We pronounce curl with an initial k sound. It rhymes with words like girl
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and earl. This is a short WAV file to help you:
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https://media.merriam-webster.com/soundc11/c/curl0001.wav
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There are numerous sub-projects and related projects that also use the word
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curl in the project names in various combinations, but you should take
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notice that this FAQ is directed at the command-line tool named curl (and
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libcurl the library), and may therefore not be valid for other curl-related
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projects. (There is however a small section for the PHP/CURL in this FAQ.)
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1.2 What is libcurl?
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libcurl is a reliable and portable library for doing Internet data transfers
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using one or more of its supported Internet protocols.
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You can use libcurl freely in your application, be it open source,
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commercial or closed-source.
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libcurl is most probably the most portable, most powerful and most often
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used C-based multi-platform file transfer library on this planet - be it
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open source or commercial.
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1.3 What is curl not?
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curl is not a wget clone. That is a common misconception. Never, during
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curl's development, have we intended curl to replace wget or compete on its
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market. curl is targeted at single-shot file transfers.
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curl is not a website mirroring program. If you want to use curl to mirror
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something: fine, go ahead and write a script that wraps around curl or use
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libcurl to make it reality.
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curl is not an FTP site mirroring program. Sure, get and send FTP with curl
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but if you want systematic and sequential behavior you should write a
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script (or write a new program that interfaces libcurl) and do it.
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curl is not a PHP tool, even though it works perfectly well when used from
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or with PHP (when using the PHP/CURL module).
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curl is not a program for a single operating system. curl exists, compiles,
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builds and runs under a wide range of operating systems, including all
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modern Unixes (and a bunch of older ones too), Windows, Amiga, OS/2, macOS,
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QNX etc.
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1.4 When will you make curl do XXXX ?
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We love suggestions of what to change in order to make curl and libcurl
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better. We do however believe in a few rules when it comes to the future of
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curl:
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curl -- the command line tool -- is to remain a non-graphical command line
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tool. If you want GUIs or fancy scripting capabilities, you should look for
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another tool that uses libcurl.
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We do not add things to curl that other small and available tools already do
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well at the side. curl's output can be piped into another program or
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redirected to another file for the next program to interpret.
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We focus on protocol related issues and improvements. If you want to do more
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magic with the supported protocols than curl currently does, chances are
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good we will agree. If you want to add more protocols, we may agree.
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If you want someone else to do all the work while you wait for us to
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implement it for you, that is not a friendly attitude. We spend a
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considerable time already on maintaining and developing curl. In order to
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get more out of us, you should consider trading in some of your time and
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effort in return. Simply go to the GitHub repository which resides at
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https://github.com/curl/curl, fork the project, and create pull requests
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with your proposed changes.
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If you write the code, chances are better that it will get into curl faster.
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1.5 Who makes curl?
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curl and libcurl are not made by any single individual. Daniel Stenberg is
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project leader and main developer, but other persons' submissions are
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important and crucial. Anyone can contribute and post their changes and
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improvements and have them inserted in the main sources (of course on the
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condition that developers agree that the fixes are good).
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The full list of all contributors is found in the docs/THANKS file.
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curl is developed by a community, with Daniel at the wheel.
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1.6 What do you get for making curl?
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Project cURL is entirely free and open. We do this voluntarily, mostly in
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our spare time. Companies may pay individual developers to work on curl.
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This is not controlled by nor supervised in any way by the curl project.
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We get help from companies. Haxx provides website, bandwidth, mailing lists
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etc, GitHub hosts the primary git repository and other services like the bug
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tracker at https://github.com/curl/curl. Also again, some companies have
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sponsored certain parts of the development in the past and I hope some will
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continue to do so in the future.
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If you want to support our project, consider a donation or a banner-program
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or even better: by helping us with coding, documenting or testing etc.
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See also: https://curl.se/sponsors.html
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1.7 What about CURL from curl.com?
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During the summer of 2001, curl.com was busy advertising their client-side
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programming language for the web, named CURL.
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We are in no way associated with curl.com or their CURL programming
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language.
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Our project name curl has been in effective use since 1998. We were not the
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first computer related project to use the name "curl" and do not claim any
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rights to the name.
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We recognize that we will be living in parallel with curl.com and wish them
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every success.
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1.8 I have a problem, who do I mail?
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Please do not mail any single individual unless you really need to. Keep
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curl-related questions on a suitable mailing list. All available mailing
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lists are listed in the MANUAL document and online at
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https://curl.se/mail/
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Keeping curl-related questions and discussions on mailing lists allows
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others to join in and help, to share their ideas, to contribute their
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suggestions and to spread their wisdom. Keeping discussions on public mailing
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lists also allows for others to learn from this (both current and future
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users thanks to the web based archives of the mailing lists), thus saving us
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from having to repeat ourselves even more. Thanks for respecting this.
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If you have found or simply suspect a security problem in curl or libcurl,
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submit all the details at https://hackerone.one/curl. On there we keep the
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issue private while we investigate, confirm it, work and validate a fix and
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agree on a time schedule for publication etc. That way we produce a fix in a
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timely manner before the flaw is announced to the world, reducing the impact
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the problem risks having on existing users.
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Security issues can also be taking to the curl security team by emailing
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security at curl.se (closed list of receivers, mails are not disclosed).
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1.9 Where do I buy commercial support for curl?
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curl is fully open source. It means you can hire any skilled engineer to fix
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your curl-related problems.
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We list available alternatives on the curl website:
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https://curl.se/support.html
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1.10 How many are using curl?
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It is impossible to tell.
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We do not know how many users that knowingly have installed and use curl.
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We do not know how many users that use curl without knowing that they are in
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fact using it.
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We do not know how many users that downloaded or installed curl and then
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never use it.
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In 2020, we estimate that curl runs in roughly ten billion installations
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world wide.
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1.11 Why do you not update ca-bundle.crt
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In the cURL project we have decided not to attempt to keep this file updated
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(or even present) since deciding what to add to a ca cert bundle is an
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undertaking we have not been ready to accept, and the one we can get from
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Mozilla is perfectly fine so there is no need to duplicate that work.
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Today, with many services performed over HTTPS, every operating system
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should come with a default ca cert bundle that can be deemed somewhat
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trustworthy and that collection (if reasonably updated) should be deemed to
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be a lot better than a private curl version.
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If you want the most recent collection of ca certs that Mozilla Firefox
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uses, we recommend that you extract the collection yourself from Mozilla
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Firefox (by running 'make ca-bundle), or by using our online service setup
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for this purpose: https://curl.se/docs/caextract.html
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1.12 I have a problem who, can I chat with?
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There is a bunch of friendly people hanging out in the #curl channel on the
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IRC network libera.chat. If you are polite and nice, chances are good that
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you can get -- or provide -- help instantly.
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1.13 curl's ECCN number?
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The US government restricts exports of software that contains or uses
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cryptography. When doing so, the Export Control Classification Number (ECCN)
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is used to identify the level of export control etc.
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Apache Software Foundation gives a good explanation of ECCNs at
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https://www.apache.org/dev/crypto.html
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We believe curl's number might be ECCN 5D002, another possibility is
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5D992. It seems necessary to write them (the authority that administers ECCN
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numbers), asking to confirm.
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Comprehensible explanations of the meaning of such numbers and how to obtain
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them (resp.) are here
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https://www.bis.doc.gov/licensing/exportingbasics.htm
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https://www.bis.doc.gov/licensing/do_i_needaneccn.html
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An incomprehensible description of the two numbers above is here
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https://www.bis.doc.gov/index.php/documents/new-encryption/1653-ccl5-pt2-3
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1.14 How do I submit my patch?
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We strongly encourage you to submit changes and improvements directly as
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"pull requests" on GitHub: https://github.com/curl/curl/pulls
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If you for any reason cannot or will not deal with GitHub, send your patch to
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the curl-library mailing list. We are many subscribers there and there are
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lots of people who can review patches, comment on them and "receive" them
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properly.
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Lots of more details are found in the CONTRIBUTE.md and INTERNALS.md
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documents.
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1.15 How do I port libcurl to my OS?
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Here's a rough step-by-step:
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1. copy a suitable lib/config-*.h file as a start to lib/config-[youros].h
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2. edit lib/config-[youros].h to match your OS and setup
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3. edit lib/curl_setup.h to include config-[youros].h when your OS is
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detected by the preprocessor, in the style others already exist
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4. compile lib/*.c and make them into a library
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2. Install Related Problems
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2.1 configure fails when using static libraries
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You may find that configure fails to properly detect the entire dependency
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chain of libraries when you provide static versions of the libraries that
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configure checks for.
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The reason why static libraries is much harder to deal with is that for them
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we do not get any help but the script itself must know or check what more
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libraries that are needed (with shared libraries, that dependency "chain" is
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handled automatically). This is a error-prone process and one that also
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tends to vary over time depending on the release versions of the involved
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components and may also differ between operating systems.
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For that reason, configure does few attempts to actually figure this out and
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you are instead encouraged to set LIBS and LDFLAGS accordingly when you
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invoke configure, and point out the needed libraries and set the necessary
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flags yourself.
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2.2 Does curl work with other SSL libraries?
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curl has been written to use a generic SSL function layer internally, and
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that SSL functionality can then be provided by one out of many different SSL
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backends.
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curl can be built to use one of the following SSL alternatives: OpenSSL,
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libressl, BoringSSL, AWS-LC, GnuTLS, wolfSSL, mbedTLS, Secure Transport
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(native iOS/OS X), Schannel (native Windows), BearSSL or Rustls. They all
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have their pros and cons, and we try to maintain a comparison of them here:
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https://curl.se/docs/ssl-compared.html
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2.3 How do I upgrade curl.exe in Windows?
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The curl tool that is shipped as an integrated component of Windows 10 and
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Windows 11 is managed by Microsoft. If you were to delete the file or
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replace it with a newer version downloaded from https://curl.se/windows,
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then Windows Update will cease to work on your system.
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There is no way to independently force an upgrade of the curl.exe that is
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part of Windows other than through the regular Windows update process. There
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is also nothing the curl project itself can do about this, since this is
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managed and controlled entirely by Microsoft as owners of the operating
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system.
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You can always download and install the latest version of curl for Windows
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from https://curl.se/windows into a separate location.
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2.4 Does curl support SOCKS (RFC 1928) ?
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Yes, SOCKS 4 and 5 are supported.
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3. Usage problems
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3.1 curl: (1) SSL is disabled, https: not supported
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If you get this output when trying to get anything from an HTTPS server, it
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means that the instance of curl/libcurl that you are using was built without
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support for this protocol.
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This could have happened if the configure script that was run at build time
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could not find all libs and include files curl requires for SSL to work. If
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the configure script fails to find them, curl is simply built without SSL
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support.
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To get HTTPS support into a curl that was previously built but that reports
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that HTTPS is not supported, you should dig through the document and logs
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|
and check out why the configure script does not find the SSL libs and/or
|
|
include files.
|
|
|
|
Also, check out the other paragraph in this FAQ labeled "configure does not
|
|
find OpenSSL even when it is installed".
|
|
|
|
3.2 How do I tell curl to resume a transfer?
|
|
|
|
curl supports resumed transfers both ways on both FTP and HTTP.
|
|
Try the -C option.
|
|
|
|
3.3 Why does my posting using -F not work?
|
|
|
|
You cannot arbitrarily use -F or -d, the choice between -F or -d depends on
|
|
the HTTP operation you need curl to do and what the web server that will
|
|
receive your post expects.
|
|
|
|
If the form you are trying to submit uses the type '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 do not 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.
|
|
|
|
3.4 How do I tell curl to run custom FTP commands?
|
|
|
|
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 do not normally use curl to
|
|
perform FTP commands without transferring anything. Therefore you must
|
|
always specify a URL to transfer to/from even when doing custom FTP
|
|
commands, or use -I which implies the "no body" option sent to libcurl.
|
|
|
|
3.5 How can I disable the Accept: */* header?
|
|
|
|
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 "Accept:" to disable that specific header.
|
|
|
|
3.6 Does curl support ASP, XML, XHTML or HTML version Y?
|
|
|
|
To curl, all contents are alike. It does not matter how the page was
|
|
generated. It may be ASP, PHP, Perl, shell-script, SSI or plain HTML
|
|
files. There is no difference to curl and it does not even know what kind of
|
|
language that generated the page.
|
|
|
|
See also item 3.14 regarding JavaScript.
|
|
|
|
3.7 Can I use curl to delete/rename a file through FTP?
|
|
|
|
Yes. You specify custom FTP commands with -Q/--quote.
|
|
|
|
One example would be to delete a file after you have downloaded it:
|
|
|
|
curl -O ftp://example.com/coolfile -Q '-DELE coolfile'
|
|
|
|
or rename a file after upload:
|
|
|
|
curl -T infile ftp://example.com/dir/ -Q "-RNFR infile" -Q "-RNTO newname"
|
|
|
|
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 are using the
|
|
-L/--location option. As in:
|
|
|
|
curl -L http://example.com
|
|
|
|
Not all redirects are HTTP ones, see 4.14
|
|
|
|
3.9 How do I use curl in my favorite programming language?
|
|
|
|
Many programming languages have interfaces/bindings that allow you to use
|
|
curl without having to use the command line tool. If you are fluent in such
|
|
a language, you may prefer to use one of these interfaces instead.
|
|
|
|
Find out more about which languages that support curl directly, and how to
|
|
install and use them, in the libcurl section of the curl website:
|
|
https://curl.se/libcurl/
|
|
|
|
All the various bindings to libcurl are made by other projects and people,
|
|
outside of the cURL project. The cURL project itself only produces libcurl
|
|
with its plain C API. If you do not find anywhere else to ask you can ask
|
|
about bindings on the curl-library list too, but be prepared that people on
|
|
that list may not know anything about bindings.
|
|
|
|
In December 2021, there were interfaces available for the following
|
|
languages: Ada95, Basic, C, C++, Ch, Cocoa, D, Delphi, Dylan, Eiffel,
|
|
Euphoria, Falcon, Ferite, Gambas, glib/GTK+, Go, Guile, Harbour, Haskell,
|
|
Java, Julia, Lisp, Lua, Mono, .NET, node.js, Object-Pascal, OCaml, Pascal,
|
|
Perl, PHP, PostgreSQL, Python, R, Rexx, Ring, RPG, Ruby, Rust, Scheme,
|
|
Scilab, S-Lang, Smalltalk, SP-Forth, SPL, Tcl, Visual Basic, Visual FoxPro,
|
|
Q, wxwidgets, XBLite and Xoho. 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 is of course just as good and you would 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 an HTTP proxy, the protocol spoken on the network will
|
|
be HTTP, even if you specify an FTP URL. This effectively means that you
|
|
normally cannot 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
|
|
ports other than 443 (which is used for HTTPS access through proxies).
|
|
|
|
3.13 Why do 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 " example.com
|
|
|
|
or perhaps
|
|
|
|
curl -d ' with spaces ' example.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 command prompts you must use double (") quotes, and if the
|
|
option string contains inner double quotes you can escape them with a
|
|
backslash.
|
|
|
|
For Windows powershell the arguments are not always passed on as expected
|
|
because curl is not a powershell script. You may or may not be able to use
|
|
single quotes. To escape inner double quotes seems to require a
|
|
backslash-backtick escape sequence and the outer quotes as double quotes.
|
|
|
|
Please study the documentation for your particular environment. Examples in
|
|
the curl docs will use a mix of both of these 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 does not support JavaScript,
|
|
it cannot support .pac proxy configuration either.
|
|
|
|
Some workarounds 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 interpreter, people have successfully used the
|
|
Mozilla JavaScript engine in the past.
|
|
|
|
Ask your admins to stop this, for a static proxy setup or similar.
|
|
|
|
3.15 Can I do recursive fetches with curl?
|
|
|
|
No. curl itself has no code that performs recursive operations, such as
|
|
those performed by wget and similar tools.
|
|
|
|
There exists wrapper scripts with that functionality (for example the
|
|
curlmirror perl script), and you can write programs based on libcurl to do
|
|
it, but the command line tool curl itself cannot.
|
|
|
|
3.16 What certificates do I need when I use SSL?
|
|
|
|
There are three different kinds of "certificates" to keep track of when we
|
|
talk about using SSL-based protocols (HTTPS or FTPS) using curl or libcurl.
|
|
|
|
CLIENT CERTIFICATE
|
|
|
|
The server you communicate with may require that you can provide this in
|
|
order to prove that you actually are who you claim to be. If the server
|
|
does not require this, you do not need a client certificate.
|
|
|
|
A client certificate is always used together with a private key, and the
|
|
private key has a pass phrase that protects it.
|
|
|
|
SERVER CERTIFICATE
|
|
|
|
The server you communicate with has a server certificate. You can and should
|
|
verify this certificate to make sure that you are truly talking to the real
|
|
server and not a server impersonating it.
|
|
|
|
CERTIFICATE AUTHORITY CERTIFICATE ("CA cert")
|
|
|
|
You often have several CA certs in a CA cert bundle that can be used to
|
|
verify a server certificate that was signed by one of the authorities in the
|
|
bundle. curl does not come with a CA cert bundle but most curl installs
|
|
provide one. You can also override the default.
|
|
|
|
The server certificate verification process is made by using a Certificate
|
|
Authority certificate ("CA cert") that was used to sign the server
|
|
certificate. Server certificate verification is enabled by default in curl
|
|
and libcurl and is often the reason for problems as explained in FAQ entry
|
|
4.12 and the SSLCERTS document
|
|
(https://curl.se/docs/sslcerts.html). Server certificates that are
|
|
"self-signed" or otherwise signed by a CA that you do not have a CA cert
|
|
for, cannot be verified. If the verification during a connect fails, you are
|
|
refused access. You then need to explicitly disable the verification to
|
|
connect to the server.
|
|
|
|
3.17 How do I list the root directory of an FTP server?
|
|
|
|
There are two ways. The way defined in the RFC is to use an encoded slash
|
|
in the first path part. List the "/tmp" directory like this:
|
|
|
|
curl ftp://ftp.example.com/%2ftmp/
|
|
|
|
or the not-quite-kosher-but-more-readable way, by simply starting the path
|
|
section of the URL with a slash:
|
|
|
|
curl ftp://ftp.example.com//tmp/
|
|
|
|
3.18 Can I use curl to send a POST/PUT and not wait for a response?
|
|
|
|
No.
|
|
|
|
You can easily write your own program using libcurl to do such stunts.
|
|
|
|
3.19 How do I get HTTP from a host using a specific IP address?
|
|
|
|
For example, you may be trying out a website installation that is not yet in
|
|
the DNS. Or you have a site using multiple IP addresses for a given host
|
|
name and you want to address a specific one out of the set.
|
|
|
|
Set a custom Host: header that identifies the server name you want to reach
|
|
but use the target IP address in the URL:
|
|
|
|
curl --header "Host: www.example.com" http://127.0.0.1/
|
|
|
|
You can also opt to add faked host name entries to curl with the --resolve
|
|
option. That has the added benefit that things like redirects will also work
|
|
properly. The above operation would instead be done as:
|
|
|
|
curl --resolve www.example.com:80:127.0.0.1 http://www.example.com/
|
|
|
|
3.20 How to SFTP from my user's home directory?
|
|
|
|
Contrary to how FTP works, SFTP and SCP URLs specify the exact directory to
|
|
work with. It means that if you do not specify that you want the user's home
|
|
directory, you get the actual root directory.
|
|
|
|
To specify a file in your user's home directory, you need to use the correct
|
|
URL syntax which for SFTP might look similar to:
|
|
|
|
curl -O -u user:password sftp://example.com/~/file.txt
|
|
|
|
and for SCP it is just a different protocol prefix:
|
|
|
|
curl -O -u user:password scp://example.com/~/file.txt
|
|
|
|
3.21 Protocol xxx not supported or disabled in libcurl
|
|
|
|
When passing on a URL to curl to use, it may respond that the particular
|
|
protocol is not supported or disabled. The particular way this error message
|
|
is phrased is because curl does not make a distinction internally of whether
|
|
a particular protocol is not supported (i.e. never got any code added that
|
|
knows how to speak that protocol) or if it was explicitly disabled. curl can
|
|
be built to only support a given set of protocols, and the rest would then
|
|
be disabled or not supported.
|
|
|
|
Note that this error will also occur if you pass a wrongly spelled protocol
|
|
part as in "htpt://example.com" or as in the less evident case if you prefix
|
|
the protocol part with a space as in " http://example.com/".
|
|
|
|
3.22 curl -X gives me HTTP problems
|
|
|
|
In normal circumstances, -X should hardly ever be used.
|
|
|
|
By default you use curl without explicitly saying which request method to
|
|
use when the URL identifies an HTTP transfer. If you just pass in a URL like
|
|
"curl http://example.com" it will use GET. If you use -d or -F curl will use
|
|
POST, -I will cause a HEAD and -T will make it a PUT.
|
|
|
|
If for whatever reason you are not happy with these default choices that curl
|
|
does for you, you can override those request methods by specifying -X
|
|
[WHATEVER]. This way you can for example send a DELETE by doing "curl -X
|
|
DELETE [URL]".
|
|
|
|
It is thus pointless to do "curl -XGET [URL]" as GET would be used
|
|
anyway. In the same vein it is pointless to do "curl -X POST -d data
|
|
[URL]"... But you can make a fun and somewhat rare request that sends a
|
|
request-body in a GET request with something like "curl -X GET -d data
|
|
[URL]"
|
|
|
|
Note that -X does not actually change curl's behavior as it only modifies the
|
|
actual string sent in the request, but that may of course trigger a
|
|
different set of events.
|
|
|
|
Accordingly, by using -XPOST on a command line that for example would follow
|
|
a 303 redirect, you will effectively prevent curl from behaving
|
|
correctly. Be aware.
|
|
|
|
|
|
4. Running Problems
|
|
|
|
4.2 Why do I get problems when I use & or % in the URL?
|
|
|
|
In general Unix shells, the & symbol is treated specially 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 (")
|
|
quotes around it. Similar problems can also occur on some shells with other
|
|
characters, including ?*!$~(){}<>\|;`. When in doubt, quote the URL.
|
|
|
|
An example that would invoke a remote CGI that uses &-symbols could be:
|
|
|
|
curl 'http://www.example.com/cgi-bin/query?text=yes&q=curl'
|
|
|
|
In Windows, the standard DOS shell treats the percent sign specially and you
|
|
need to use TWO percent signs for each single one you want to use in the
|
|
URL.
|
|
|
|
If you want a literal percent sign 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
|
|
percent sign doubled on Windows machines).
|
|
|
|
4.3 How can I use {, }, [ or ] to specify multiple URLs?
|
|
|
|
Because those letters have a special meaning to the shell, to be used in
|
|
a URL specified to curl you must quote them.
|
|
|
|
An example that downloads two URLs (sequentially) would be:
|
|
|
|
curl '{curl,www}.haxx.se'
|
|
|
|
To be able to use those characters as actual parts of the URL (without using
|
|
them for the curl URL "globbing" system), use the -g/--globoff option:
|
|
|
|
curl -g 'www.example.com/weirdname[].html'
|
|
|
|
4.4 Why do I get downloaded data even though the web page does not exist?
|
|
|
|
curl asks remote servers for the page you specify. If the page does not 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 is simply how
|
|
HTTP works.
|
|
|
|
By using the --fail option you can tell curl explicitly to not get any data
|
|
if the HTTP return code does not say success.
|
|
|
|
4.5 Why do I get return code XXX from an HTTP server?
|
|
|
|
RFC 2616 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"
|
|
|
|
The request requires user authentication.
|
|
|
|
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 as to 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 requested 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.
|
|
|
|
4.6 Can you tell me what error code 142 means?
|
|
|
|
All curl error codes are described at the end of the man page, in the
|
|
section called "EXIT CODES".
|
|
|
|
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 do not 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. curl itself will also
|
|
attempt to "hide" the given password by blanking out the option - this
|
|
does not work on all platforms.
|
|
|
|
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 (using Basic authentication) and FTP passwords
|
|
are sent as cleartext across the network. All it takes for anyone to fetch
|
|
them is to listen on the network. Eavesdropping is easy. Use more secure
|
|
authentication methods (like Digest, Negotiate or even NTLM) or consider the
|
|
SSL-based alternatives HTTPS and FTPS.
|
|
|
|
4.8 I found a bug
|
|
|
|
It is not a bug if the behavior is documented. Read the docs first.
|
|
Especially check out the KNOWN_BUGS file, it may be a documented bug.
|
|
|
|
If it is a problem with a binary you have 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 cannot authenticate to a server that requires NTLM?
|
|
|
|
NTLM support requires OpenSSL, GnuTLS, mbedTLS, Secure Transport, or
|
|
Microsoft Windows libraries at build-time to provide this functionality.
|
|
|
|
4.10 My HTTP request using HEAD, PUT or DELETE does not 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 are trying to interact with. This is not anything curl can do
|
|
anything about.
|
|
|
|
4.11 Why do my HTTP range requests return the full document?
|
|
|
|
Because the range may not be supported by the server, or the server may
|
|
choose to ignore it and return the full document anyway.
|
|
|
|
4.12 Why do I get "certificate verify failed" ?
|
|
|
|
When you invoke curl and get an error 60 error back it means that curl
|
|
could not verify that the server's certificate was good. curl verifies the
|
|
certificate using the CA cert bundle and verifying for which names the
|
|
certificate has been granted.
|
|
|
|
To completely disable the certificate verification, use -k. This does
|
|
however enable man-in-the-middle attacks and makes the transfer INSECURE.
|
|
We strongly advise against doing this for more than experiments.
|
|
|
|
If you get this failure with a CA cert bundle installed and used, the
|
|
server's certificate might not be signed by one of the CA's in your CA
|
|
store. It might for example be self-signed. You then correct this problem by
|
|
obtaining a valid CA cert for the server. Or again, decrease the security by
|
|
disabling this check.
|
|
|
|
At times, you find that the verification works in your favorite browser but
|
|
fails in curl. When this happens, the reason is usually that the server
|
|
sends an incomplete cert chain. The server is mandated to send all
|
|
"intermediate certificates" but does not. This typically works with browsers
|
|
anyway since they A) cache such certs and B) supports AIA which downloads
|
|
such missing certificates on demand. This is a server misconfiguration. A
|
|
good way to figure out if this is the case it to use the SSL Labs server
|
|
test and check the certificate chain: https://www.ssllabs.com/ssltest/
|
|
|
|
Details are also in the SSLCERTS.md document, found online here:
|
|
https://curl.se/docs/sslcerts.html
|
|
|
|
4.13 Why is curl -R on Windows one hour off?
|
|
|
|
Since curl 7.53.0 this issue should be fixed as long as curl was built with
|
|
any modern compiler that allows for a 64-bit curl_off_t type. For older
|
|
compilers or prior curl versions it may set a time that appears one hour off.
|
|
This happens due to a flaw in how Windows stores and uses file modification
|
|
times and it is not easily worked around. For more details read this:
|
|
https://www.codeproject.com/Articles/1144/Beating-the-Daylight-Savings-Time-bug-and-getting
|
|
|
|
4.14 Redirects work in browser but not with curl
|
|
|
|
curl supports HTTP redirects well (see item 3.8). Browsers generally support
|
|
at least two other ways to perform redirects that curl does not:
|
|
|
|
Meta tags. You can write an HTML tag that will cause the browser to redirect
|
|
to another given URL after a certain time.
|
|
|
|
JavaScript. You can write a JavaScript program embedded in an HTML page that
|
|
redirects the browser to another given URL.
|
|
|
|
There is no way to make curl follow these redirects. You must either
|
|
manually figure out what the page is set to do, or write a script that parses
|
|
the results and fetches the new URL.
|
|
|
|
4.15 FTPS does not work
|
|
|
|
curl supports FTPS (sometimes known as FTP-SSL) both implicit and explicit
|
|
mode.
|
|
|
|
When a URL is used that starts with FTPS://, curl assumes implicit SSL on
|
|
the control connection and will therefore immediately connect and try to
|
|
speak SSL. FTPS:// connections default to port 990.
|
|
|
|
To use explicit FTPS, you use an FTP:// URL and the --ftp-ssl option (or one
|
|
of its related flavors). This is the most common method, and the one
|
|
mandated by RFC 4217. This kind of connection will then of course use the
|
|
standard FTP port 21 by default.
|
|
|
|
4.16 My HTTP POST or PUT requests are slow
|
|
|
|
libcurl makes all POST and PUT requests (except for requests with a small
|
|
request body) use the "Expect: 100-continue" header. This header allows the
|
|
server to deny the operation early so that libcurl can bail out before having
|
|
to send any data. This is useful in authentication cases and others.
|
|
|
|
However, many servers do not implement the Expect: stuff properly and if the
|
|
server does not respond (positively) within 1 second libcurl will continue
|
|
and send off the data anyway.
|
|
|
|
You can disable libcurl's use of the Expect: header the same way you disable
|
|
any header, using -H / CURLOPT_HTTPHEADER, or by forcing it to use HTTP 1.0.
|
|
|
|
4.17 Non-functional connect timeouts
|
|
|
|
In most Windows setups having a timeout longer than 21 seconds make no
|
|
difference, as it will only send 3 TCP SYN packets and no more. The second
|
|
packet sent three seconds after the first and the third six seconds after
|
|
the second. No more than three packets are sent, no matter how long the
|
|
timeout is set.
|
|
|
|
See option TcpMaxConnectRetransmissions on this page:
|
|
https://support.microsoft.com/en-us/kb/175523/en-us
|
|
|
|
Also, even on non-Windows systems there may run a firewall or anti-virus
|
|
software or similar that accepts the connection but does not actually do
|
|
anything else. This will make (lib)curl to consider the connection connected
|
|
and thus the connect timeout will not trigger.
|
|
|
|
4.18 file:// URLs containing drive letters (Windows, NetWare)
|
|
|
|
When using curl to try to download a local file, one might use a URL
|
|
in this format:
|
|
|
|
file://D:/blah.txt
|
|
|
|
you will find that even if D:\blah.txt does exist, curl returns a 'file
|
|
not found' error.
|
|
|
|
According to RFC 1738 (https://www.ietf.org/rfc/rfc1738.txt),
|
|
file:// URLs must contain a host component, but it is ignored by
|
|
most implementations. In the above example, 'D:' is treated as the
|
|
host component, and is taken away. Thus, curl tries to open '/blah.txt'.
|
|
If your system is installed to drive C:, that will resolve to 'C:\blah.txt',
|
|
and if that does not exist you will get the not found error.
|
|
|
|
To fix this problem, use file:// URLs with *three* leading slashes:
|
|
|
|
file:///D:/blah.txt
|
|
|
|
Alternatively, if it makes more sense, specify 'localhost' as the host
|
|
component:
|
|
|
|
file://localhost/D:/blah.txt
|
|
|
|
In either case, curl should now be looking for the correct file.
|
|
|
|
4.19 Why does not curl return an error when the network cable is unplugged?
|
|
|
|
Unplugging a cable is not an error situation. The TCP/IP protocol stack
|
|
was designed to be fault tolerant, so even though there may be a physical
|
|
break somewhere the connection should not be affected, just possibly
|
|
delayed. Eventually, the physical break will be fixed or the data will be
|
|
re-routed around the physical problem through another path.
|
|
|
|
In such cases, the TCP/IP stack is responsible for detecting when the
|
|
network connection is irrevocably lost. Since with some protocols it is
|
|
perfectly legal for the client to wait indefinitely for data, the stack may
|
|
never report a problem, and even when it does, it can take up to 20 minutes
|
|
for it to detect an issue. The curl option --keepalive-time enables
|
|
keep-alive support in the TCP/IP stack which makes it periodically probe the
|
|
connection to make sure it is still available to send data. That should
|
|
reliably detect any TCP/IP network failure.
|
|
|
|
TCP keep alive will not detect the network going down before the TCP/IP
|
|
connection is established (e.g. during a DNS lookup) or using protocols that
|
|
do not use TCP. To handle those situations, curl offers a number of timeouts
|
|
on its own. --speed-limit/--speed-time will abort if the data transfer rate
|
|
falls too low, and --connect-timeout and --max-time can be used to put an
|
|
overall timeout on the connection phase or the entire transfer.
|
|
|
|
A libcurl-using application running in a known physical environment (e.g.
|
|
an embedded device with only a single network connection) may want to act
|
|
immediately if its lone network connection goes down. That can be achieved
|
|
by having the application monitor the network connection on its own using an
|
|
OS-specific mechanism, then signaling libcurl to abort (see also item 5.13).
|
|
|
|
4.20 curl does not return error for HTTP non-200 responses
|
|
|
|
Correct. Unless you use -f (--fail).
|
|
|
|
When doing HTTP transfers, curl will perform exactly what you are asking it
|
|
to do and if successful it will not return an error. You can use curl to
|
|
test your web server's "file not found" page (that gets 404 back), you can
|
|
use it to check your authentication protected web pages (that gets a 401
|
|
back) and so on.
|
|
|
|
The specific HTTP response code does not constitute a problem or error for
|
|
curl. It simply sends and delivers HTTP as you asked and if that worked,
|
|
everything is fine and dandy. The response code is generally providing more
|
|
higher level error information that curl does not care about. The error was
|
|
not in the HTTP transfer.
|
|
|
|
If you want your command line to treat error codes in the 400 and up range
|
|
as errors and thus return a non-zero value and possibly show an error
|
|
message, curl has a dedicated option for that: -f (CURLOPT_FAILONERROR in
|
|
libcurl speak).
|
|
|
|
You can also use the -w option and the variable %{response_code} to extract
|
|
the exact response code that was returned in the response.
|
|
|
|
5. libcurl Issues
|
|
|
|
5.1 Is libcurl thread-safe?
|
|
|
|
Yes.
|
|
|
|
We have written the libcurl code specifically adjusted for multi-threaded
|
|
programs. libcurl will use thread-safe functions instead of non-safe ones if
|
|
your system has such. Note that you must never share the same handle in
|
|
multiple threads.
|
|
|
|
There may be some exceptions to thread safety depending on how libcurl was
|
|
built. Please review the guidelines for thread safety to learn more:
|
|
https://curl.se/libcurl/c/threadsafe.html
|
|
|
|
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.
|
|
|
|
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
|
|
CURLOPT_WRITEDATA option. 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)
|
|
{
|
|
size_t 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;
|
|
}
|
|
|
|
5.3 How do I fetch multiple files with libcurl?
|
|
|
|
libcurl has 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, but you are even encouraged to reuse it if you can, as that
|
|
will enable libcurl to use persistent connections.
|
|
|
|
5.4 Does libcurl do Winsock initialization on win32 systems?
|
|
|
|
Yes, if told to in the curl_global_init() call.
|
|
|
|
5.5 Does CURLOPT_WRITEDATA and CURLOPT_READDATA work on win32 ?
|
|
|
|
Yes, but you cannot open a FILE * and pass the pointer to a DLL and have
|
|
that DLL use the FILE * (as the DLL and the client application cannot access
|
|
each others' variable memory areas). If you set CURLOPT_WRITEDATA 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_READDATA you must also specify
|
|
CURLOPT_READFUNCTION.
|
|
|
|
5.6 What about Keep-Alive or persistent connections?
|
|
|
|
curl and libcurl 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.
|
|
|
|
When you use the easy interface the connection cache is kept within the easy
|
|
handle. If you instead use the multi interface, the connection cache will be
|
|
kept within the multi handle and will be shared among all the easy handles
|
|
that are used within the same multi handle.
|
|
|
|
5.7 Link errors when building libcurl on Windows
|
|
|
|
You need to make sure that your project, and all the libraries (both static
|
|
and dynamic) that it links against, are compiled/linked against the same run
|
|
time library.
|
|
|
|
This is determined by the /MD, /ML, /MT (and their corresponding /M?d)
|
|
options to the command line compiler. /MD (linking against MSVCRT dll) seems
|
|
to be the most commonly used option.
|
|
|
|
When building an application that uses the static libcurl library, you must
|
|
add -DCURL_STATICLIB to your CFLAGS. Otherwise the linker will look for
|
|
dynamic import symbols. If you are using Visual Studio, you need to instead
|
|
add CURL_STATICLIB in the "Preprocessor Definitions" section.
|
|
|
|
If you get a linker error like "unknown symbol __imp__curl_easy_init ..." you
|
|
have linked against the wrong (static) library. If you want to use the
|
|
libcurl.dll and import lib, you do not need any extra CFLAGS, but use one of
|
|
the import libraries below. These are the libraries produced by the various
|
|
lib/Makefile.* files:
|
|
|
|
Target: static lib. import lib for libcurl*.dll.
|
|
-----------------------------------------------------------
|
|
MinGW: libcurl.a libcurldll.a
|
|
MSVC (release): libcurl.lib libcurl_imp.lib
|
|
MSVC (debug): libcurld.lib libcurld_imp.lib
|
|
Borland: libcurl.lib libcurl_imp.lib
|
|
|
|
5.8 libcurl.so.X: open failed: No such file or directory
|
|
|
|
This is an error message you might get when you try to run a program linked
|
|
with a shared version of libcurl and your runtime linker (ld.so) could not
|
|
find the shared library named libcurl.so.X. (Where X is the number of the
|
|
current libcurl ABI, typically 3 or 4).
|
|
|
|
You need to make sure that ld.so finds libcurl.so.X. You can do that
|
|
multiple ways, and it differs somewhat between different operating systems.
|
|
They are usually:
|
|
|
|
* Add an option to the linker command line that specify the hard-coded path
|
|
the runtime linker should check for the lib (usually -R)
|
|
|
|
* Set an environment variable (LD_LIBRARY_PATH for example) where ld.so
|
|
should check for libs
|
|
|
|
* Adjust the system's config to check for libs in the directory where you have
|
|
put the library (like Linux's /etc/ld.so.conf)
|
|
|
|
'man ld.so' and 'man ld' will tell you more details
|
|
|
|
5.9 How does libcurl resolve host names?
|
|
|
|
libcurl supports a large number of name resolve functions. One of them is
|
|
picked at build-time and will be used unconditionally. Thus, if you want to
|
|
change name resolver function you must rebuild libcurl and tell it to use a
|
|
different function.
|
|
|
|
- The non-IPv6 resolver that can use one of four different host name resolve
|
|
calls (depending on what your system supports):
|
|
|
|
A - gethostbyname()
|
|
B - gethostbyname_r() with 3 arguments
|
|
C - gethostbyname_r() with 5 arguments
|
|
D - gethostbyname_r() with 6 arguments
|
|
|
|
- The IPv6-resolver that uses getaddrinfo()
|
|
|
|
- The c-ares based name resolver that uses the c-ares library for resolves.
|
|
Using this offers asynchronous name resolves.
|
|
|
|
- The threaded resolver (default option on Windows). It uses:
|
|
|
|
A - gethostbyname() on plain IPv4 hosts
|
|
B - getaddrinfo() on IPv6 enabled hosts
|
|
|
|
Also note that libcurl never resolves or reverse-lookups addresses given as
|
|
pure numbers, such as 127.0.0.1 or ::1.
|
|
|
|
5.10 How do I prevent libcurl from writing the response to stdout?
|
|
|
|
libcurl provides a default built-in write function that writes received data
|
|
to stdout. Set the CURLOPT_WRITEFUNCTION to receive the data, or possibly
|
|
set CURLOPT_WRITEDATA to a different FILE * handle.
|
|
|
|
5.11 How do I make libcurl not receive the whole HTTP response?
|
|
|
|
You make the write callback (or progress callback) return an error and
|
|
libcurl will then abort the transfer.
|
|
|
|
5.12 Can I make libcurl fake or hide my real IP address?
|
|
|
|
No. libcurl operates on a higher level. Besides, faking IP address would
|
|
imply sending IP packets with a made-up source address, and then you normally
|
|
get a problem with receiving the packet sent back as they would then not be
|
|
routed to you.
|
|
|
|
If you use a proxy to access remote sites, the sites will not see your local
|
|
IP address but instead the address of the proxy.
|
|
|
|
Also note that on many networks NATs or other IP-munging techniques are used
|
|
that makes you see and use a different IP address locally than what the
|
|
remote server will see you coming from. You may also consider using
|
|
https://www.torproject.org/ .
|
|
|
|
5.13 How do I stop an ongoing transfer?
|
|
|
|
With the easy interface you make sure to return the correct error code from
|
|
one of the callbacks, but none of them are instant. There is no function you
|
|
can call from another thread or similar that will stop it immediately.
|
|
Instead, you need to make sure that one of the callbacks you use returns an
|
|
appropriate value that will stop the transfer. Suitable callbacks that you
|
|
can do this with include the progress callback, the read callback and the
|
|
write callback.
|
|
|
|
If you are using the multi interface, you can also stop a transfer by
|
|
removing the particular easy handle from the multi stack at any moment you
|
|
think the transfer is done or when you wish to abort the transfer.
|
|
|
|
5.14 Using C++ non-static functions for callbacks?
|
|
|
|
libcurl is a C library, it does not know anything about C++ member functions.
|
|
|
|
You can overcome this "limitation" with relative ease using a static
|
|
member function that is passed a pointer to the class:
|
|
|
|
// f is the pointer to your object.
|
|
static size_t YourClass::func(void *buffer, size_t sz, size_t n, void *f)
|
|
{
|
|
// Call non-static member function.
|
|
static_cast<YourClass*>(f)->nonStaticFunction();
|
|
}
|
|
|
|
// This is how you pass pointer to the static function:
|
|
curl_easy_setopt(hcurl, CURLOPT_WRITEFUNCTION, YourClass::func);
|
|
curl_easy_setopt(hcurl, CURLOPT_WRITEDATA, this);
|
|
|
|
5.15 How do I get an FTP directory listing?
|
|
|
|
If you end the FTP URL you request with a slash, libcurl will provide you
|
|
with a directory listing of that given directory. You can also set
|
|
CURLOPT_CUSTOMREQUEST to alter what exact listing command libcurl would use
|
|
to list the files.
|
|
|
|
The follow-up question tends to be how is a program supposed to parse the
|
|
directory listing. How does it know what's a file and what's a directory and
|
|
what's a symlink etc. If the FTP server supports the MLSD command then it
|
|
will return data in a machine-readable format that can be parsed for type.
|
|
The types are specified by RFC 3659 section 7.5.1. If MLSD is not supported
|
|
then you have to work with what you are given. The LIST output format is
|
|
entirely at the server's own liking and the NLST output does not reveal any
|
|
types and in many cases does not even include all the directory entries.
|
|
Also, both LIST and NLST tend to hide unix-style hidden files (those that
|
|
start with a dot) by default so you need to do "LIST -a" or similar to see
|
|
them.
|
|
|
|
Example - List only directories.
|
|
ftp.funet.fi supports MLSD and ftp.kernel.org does not:
|
|
|
|
curl -s ftp.funet.fi/pub/ -X MLSD | \
|
|
perl -lne 'print if s/(?:^|;)type=dir;[^ ]+ (.+)$/$1/'
|
|
|
|
curl -s ftp.kernel.org/pub/linux/kernel/ | \
|
|
perl -lne 'print if s/^d[-rwx]{9}(?: +[^ ]+){7} (.+)$/$1/'
|
|
|
|
If you need to parse LIST output in libcurl one such existing
|
|
list parser is available at https://cr.yp.to/ftpparse.html Versions of
|
|
libcurl since 7.21.0 also provide the ability to specify a wildcard to
|
|
download multiple files from one FTP directory.
|
|
|
|
5.16 I want a different time-out
|
|
|
|
Sometimes users realize that CURLOPT_TIMEOUT and CURLOPT_CONNECTIMEOUT are
|
|
not sufficiently advanced or flexible to cover all the various use cases and
|
|
scenarios applications end up with.
|
|
|
|
libcurl offers many more ways to time-out operations. A common alternative
|
|
is to use the CURLOPT_LOW_SPEED_LIMIT and CURLOPT_LOW_SPEED_TIME options to
|
|
specify the lowest possible speed to accept before to consider the transfer
|
|
timed out.
|
|
|
|
The most flexible way is by writing your own time-out logic and using
|
|
CURLOPT_XFERINFOFUNCTION (perhaps in combination with other callbacks) and
|
|
use that to figure out exactly when the right condition is met when the
|
|
transfer should get stopped.
|
|
|
|
5.17 Can I write a server with libcurl?
|
|
|
|
No. libcurl offers no functions or building blocks to build any kind of
|
|
Internet protocol server. libcurl is only a client-side library. For server
|
|
libraries, you need to continue your search elsewhere but there exist many
|
|
good open source ones out there for most protocols you could want a server
|
|
for. There are also really good stand-alone servers that have been tested
|
|
and proven for many years. There is no need for you to reinvent them.
|
|
|
|
5.18 Does libcurl use threads?
|
|
|
|
Put simply: no, libcurl will execute in the same thread you call it in. All
|
|
callbacks will be called in the same thread as the one you call libcurl in.
|
|
|
|
If you want to avoid your thread to be blocked by the libcurl call, you make
|
|
sure you use the non-blocking multi API which will do transfers
|
|
asynchronously - still in the same single thread.
|
|
|
|
libcurl will potentially internally use threads for name resolving, if it
|
|
was built to work like that, but in those cases it will create the child
|
|
threads by itself and they will only be used and then killed internally by
|
|
libcurl and never exposed to the outside.
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|
|
|
6. License Issues
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|
|
|
curl and libcurl are released under a MIT/X derivative license. The license
|
|
is 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.)
|
|
|
|
We are not lawyers and this is not legal advice. You should probably consult
|
|
one if you want true and accurate legal insights without our prejudice. Note
|
|
especially that this section concerns the libcurl license only; compiling in
|
|
features of libcurl that depend on other libraries (e.g. OpenSSL) may affect
|
|
the licensing obligations of your application.
|
|
|
|
6.1 I have a GPL program, can I use the libcurl library?
|
|
|
|
Yes
|
|
|
|
Since libcurl may be distributed under the MIT/X derivative 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?
|
|
|
|
Yes
|
|
|
|
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?
|
|
|
|
Yes
|
|
|
|
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?
|
|
|
|
Yes
|
|
|
|
The LGPL license does not clash with other licenses.
|
|
|
|
6.5 Can I modify curl/libcurl for my program and keep the changes secret?
|
|
|
|
Yes
|
|
|
|
The MIT/X derivative 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?
|
|
|
|
No.
|
|
|
|
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. It should be possible for everyone to use libcurl or
|
|
curl in their projects, no matter what license they already have in use.
|
|
|
|
6.7 What are my obligations when using libcurl in my commercial apps?
|
|
|
|
Next to none. All you need to adhere to is the MIT-style license (stated in
|
|
the COPYING file) which basically says you have to include the copyright
|
|
notice in "all copies" and that you may not use the copyright holder's name
|
|
when promoting your software.
|
|
|
|
You do not have to release any of your source code.
|
|
|
|
You do not have to reveal or make public any changes to the libcurl source
|
|
code.
|
|
|
|
You do not have to broadcast to the world that you are using libcurl within
|
|
your app.
|
|
|
|
All we ask is that you disclose "the copyright notice and this permission
|
|
notice" somewhere. Most probably like in the documentation or in the section
|
|
where other third party dependencies already are mentioned and acknowledged.
|
|
|
|
As can be seen here: https://curl.se/docs/companies.html and elsewhere,
|
|
more and more companies are discovering the power of libcurl and take
|
|
advantage of it even in commercial environments.
|
|
|
|
|
|
7. PHP/CURL Issues
|
|
|
|
7.1 What is PHP/CURL?
|
|
|
|
The module for PHP that makes it possible for PHP programs to access curl-
|
|
functions from within PHP.
|
|
|
|
In the cURL project we call this module PHP/CURL to differentiate it from
|
|
curl the command line tool and libcurl the library. The PHP team however
|
|
does not refer to it like this (for unknown reasons). They call it plain
|
|
CURL (often using all caps) or sometimes ext/curl, but both cause much
|
|
confusion to users which in turn gives us a higher question load.
|
|
|
|
7.2 Who wrote PHP/CURL?
|
|
|
|
PHP/CURL was initially written by Sterling Hughes.
|
|
|
|
7.3 Can I perform multiple requests using the same handle?
|
|
|
|
Yes - at least in PHP version 4.3.8 and later (this has been known to not
|
|
work in earlier versions, but the exact version when it started to work is
|
|
unknown to me).
|
|
|
|
After a transfer, you just set new options in the handle and make another
|
|
transfer. This will make libcurl reuse the same connection if it can.
|
|
|
|
7.4 Does PHP/CURL have dependencies?
|
|
|
|
PHP/CURL is a module that comes with the regular PHP package. It depends on
|
|
and uses libcurl, so you need to have libcurl installed properly before
|
|
PHP/CURL can be used.
|
|
|
|
8. Development
|
|
|
|
8.1 Why does curl use C89?
|
|
|
|
As with everything in curl, there is a history and we keep using what we have
|
|
used before until someone brings up the subject and argues for and works on
|
|
changing it.
|
|
|
|
We started out using C89 in the 1990s because that was the only way to write
|
|
a truly portable C program and have it run as widely as possible. C89 was for
|
|
a long time even necessary to make things work on otherwise considered modern
|
|
platforms such as Windows. Today, we do not really know how many users that
|
|
still require the use of a C89 compiler.
|
|
|
|
We will continue to use C89 for as long as nobody brings up a strong enough
|
|
reason for us to change our minds. The core developers of the project do not
|
|
feel restricted by this and we are not convinced that going C99 will offer us
|
|
enough of a benefit to warrant the risk of cutting off a share of users.
|
|
|
|
8.2 Will curl be rewritten?
|
|
|
|
In one go: no. Little by little over time? Maybe.
|
|
|
|
Over the years, new languages and clever operating environments come and go.
|
|
Every now and then the urge apparently arises to request that we rewrite curl
|
|
in another language.
|
|
|
|
Some the most important properties in curl are maintaining the API and ABI
|
|
for libcurl and keeping the behavior for the command line tool. As long as we
|
|
can do that, everything else is up for discussion. To maintain the ABI, we
|
|
probably have to maintain a certain amount of code in C, and to remain rock
|
|
stable, we will never risk anything by rewriting a lot of things in one go.
|
|
That said, we can certainly offer more and more optional backends written in
|
|
other languages, as long as those backends can be plugged in at build-time.
|
|
Backends can be written in any language, but should probably provide APIs
|
|
usable from C to ease integration and transition.
|