Manual Pages for UNIX Darwin command on man libcurl-tutorial

libcurl-tutorial(3) libcurl programming libcurl-tutorial(3)

NAME

libcurl-tutorial - libcurl programming tutorial

OObbjjeeccttiivvee This document attempts to describe the general principles and some basic approaches to consider when programming with libcurl. The text will focus mainly on the C interface but might apply fairly well on other interfaces as well as they usually follow the C one pretty closely. This document will refer to 'the user' as the person writing the source code that uses libcurl. That would probably be you or someone in your position. What will be generally referred to as 'the program' will be the collected source code that you write that is using libcurl for transfers. The program is outside libcurl and libcurl is outside of the program. To get the more details on all options and functions described herein, please refer to their respective man pages. BBuuiillddiinngg There are many different ways to build C programs. This chapter will

assume a unix-style build process. If you use a different build system,

you can still read this to get general information that may apply to your environment as well. Compiling the Program Your compiler needs to know where the libcurl headers are located. Therefore you must set your compiler's include path to

point to the directory where you installed them. The 'curl-con-

fig'[3] tool can be used to get this information:

$ curl-config -cflags

Linking the Program with libcurl When having compiled the program, you need to link your object files to create a single executable. For that to succeed, you need to link with libcurl and possibly also with other libraries that libcurl itself depends on. Like the OpenSSL libraries, but even some standard OS libraries may be needed on the command

line. To figure out which flags to use, once again the 'curl-

config' tool comes to the rescue:

$ curl-config -libs

SSL or Not libcurl can be built and customized in many ways. One of the things that varies from different libraries and builds is the

support for SSL-based transfers, like HTTPS and FTPS. If OpenSSL

was detected properly at build-time, libcurl will be built with

SSL support. To figure out if an installed libcurl has been

built with SSL support enabled, use 'curl-config' like this:

$ curl-config -feature

And if SSL is supported, the keyword 'SSL' will be written to stdout, possibly together with a few other features that can be on and off on different libcurls. See also the "Features libcurl Provides" further down. autoconf macro When you write your configure script to detect libcurl and setup variables accordingly, we offer a prewritten macro that probably does everything you need in this area. See

docs/libcurl/libcurl.m4 file - it includes docs on how to use

it. PPoorrttaabbllee CCooddee iinn aa PPoorrttaabbllee WWoorrlldd The people behind libcurl have put a considerable effort to make

libcurl work on a large amount of different operating systems and envi-

ronments. You program libcurl the same way on all platforms that libcurl runs on. There are only very few minor considerations that differs. If you just make sure to write your code portable enough, you may very well create yourself a very portable program. libcurl shouldn't stop you from that. GGlloobbaall PPrreeppaarraattiioonn The program must initialize some of the libcurl functionality globally. That means it should be done exactly once, no matter how many times you intend to use the library. Once for your program's entire life time. This is done using curlglobalinit() and it takes one parameter which is a bit pattern that tells libcurl what to initialize. Using CURLGLOBALALL will make it initialize all known internal sub modules, and might be a good default option. The current two bits that are specified are: CURLGLOBALWIN32 which only does anything on Windows machines. When used on a Windows machine, it'll make libcurl initialize the

win32 socket stuff. Without having that initialized prop-

erly, your program cannot use sockets properly. You should only do this once for each application, so if your program already does this or of another library in use does it, you should not tell libcurl to do this as well. CURLGLOBALSSL which only does anything on libcurls compiled and built

SSL-enabled. On these systems, this will make libcurl

initialize OpenSSL properly for this application. This is only needed to do once for each application so if your program or another library already does this, this bit should not be needed. libcurl has a default protection mechanism that detects if curlglobalinit(3) hasn't been called by the time curleasyperform(3) is called and if that is the case, libcurl runs the function itself with a guessed bit pattern. Please note that depending solely on this is not considered nice nor very good. When the program no longer uses libcurl, it should call curlglobalcleanup(3), which is the opposite of the init call. It will then do the reversed operations to cleanup the resources the curlglobalinit(3) call initialized. Repeated calls to curlglobalinit(3) and curlglobalcleanup(3) should be avoided. They should only be called once each. FFeeaattuurreess lliibbccuurrll PPrroovviiddeess

It is considered best-practice to determine libcurl features run-time

rather than build-time (if possible of course). By calling curlver-

sioninfo() and checking tout he details of the returned struct, your

program can figure out exactly what the currently running libcurl sup-

ports. HHaannddllee tthhee EEaassyy lliibbccuurrll libcurl first introduced the so called easy interface. All operations in the easy interface are prefixed with 'curleasy'. Recent libcurl versions also offer the multi interface. More about that interface, what it is targeted for and how to use it is detailed in a separate chapter further down. You still need to understand the easy interface first, so please continue reading for better understanding.

To use the easy interface, you must first create yourself an easy han-

dle. You need one handle for each easy session you want to perform. Basically, you should use one handle for every thread you plan to use for transferring. You must never share the same handle in multiple threads. Get an easy handle with easyhandle = curleasyinit(); It returns an easy handle. Using that you proceed to the next step: setting up your preferred actions. A handle is just a logic entity for the upcoming transfer or series of transfers. You set properties and options for this handle using

curleasysetopt(3). They control how the subsequent transfer or trans-

fers will be made. Options remain set in the handle until set again to something different. Alas, multiple requests using the same handle will use the same options. Many of the options you set in libcurl are "strings", pointers to data terminated with a zero byte. Keep in mind that when you set strings with curleasysetopt(3), libcurl will not copy the data. It will merely point to the data. You MUST make sure that the data remains available for libcurl to use until finished or until you use the same option again to point to something else. One of the most basic properties to set in the handle is the URL. You set your preferred URL to transfer with CURLOPTURL in a manner similar to: curleasysetopt(handle, CURLOPTURL, "http://domain.com/");

Let's assume for a while that you want to receive data as the URL iden-

tifies a remote resource you want to get here. Since you write a sort of application that needs this transfer, I assume that you would like to get the data passed to you directly instead of simply getting it passed to stdout. So, you write your own function that matches this prototype: sizet writedata(void *buffer, sizet size, sizet nmemb, void *userp);

You tell libcurl to pass all data to this function by issuing a func-

tion similar to this: curleasysetopt(easyhandle, CURLOPTWRITEFUNCTION, writedata); You can control what data your function get in the forth argument by setting another property: curleasysetopt(easyhandle, CURLOPTWRITEDATA, &internalstruct);

Using that property, you can easily pass local data between your appli-

cation and the function that gets invoked by libcurl. libcurl itself won't touch the data you pass with CURLOPTWRITEDATA. libcurl offers its own default internal callback that'll take care of the data if you don't set the callback with CURLOPTWRITEFUNCTION. It will then simply output the received data to stdout. You can have the default callback write the data to a different file handle by passing a 'FILE *' to a file opened for writing with the CURLOPTWRITEDATA option. Now, we need to take a step back and have a deep breath. Here's one of

those rare platform-dependent nitpicks. Did you spot it? On some plat-

forms[2], libcurl won't be able to operate on files opened by the pro-

gram. Thus, if you use the default callback and pass in a an open file with CURLOPTWRITEDATA, it will crash. You should therefore avoid this to make your program run fine virtually everywhere. (CURLOPTWRITEDATA was formerly known as CURLOPTFILE. Both names still work and do the same thing).

If you're using libcurl as a win32 DLL, you MUST use the CURLOPTWRITE-

FUNCTION if you set CURLOPTWRITEDATA - or you will experience crashes.

There are of course many more options you can set, and we'll get back to a few of them later. Let's instead continue to the actual transfer: success = curleasyperform(easyhandle); curleasyperform(3) will connect to the remote site, do the necessary commands and receive the transfer. Whenever it receives data, it calls the callback function we previously set. The function may get one byte at a time, or it may get many kilobytes at once. libcurl delivers as much as possible as often as possible. Your callback function should return the number of bytes it "took care of". If that is not the exact same amount of bytes that was passed to it, libcurl will abort the operation and return with an error code. When the transfer is complete, the function returns a return code that informs you if it succeeded in its mission or not. If a return code

isn't enough for you, you can use the CURLOPTERRORBUFFER to point

libcurl to a buffer of yours where it'll store a human readable error message as well. If you then want to transfer another file, the handle is ready to be

used again. Mind you, it is even preferred that you re-use an existing

handle if you intend to make another transfer. libcurl will then

attempt to re-use the previous

MMuullttii-tthhrreeaaddiinngg IIssssuueess

libcurl is completely thread safe, except for two issues: signals and alarm handlers. Signals are needed for a SIGPIPE handler, and the alarm() call is used to deal with timeouts (during DNS lookup).

If you are accessing HTTPS or FTPS URLs in a multi-threaded manner, you

are then of course using OpenSSL multi-threaded and it has itself a few

requirements on this. Basically, you need to provide one or two func-

tions to allow it to function properly. For all details, see this:

http://www.openssl.org/docs/crypto/threads.html#DESCRIPTION

When using multiple threads you should set the CURLOPTNOSIGNAL option to TRUE for all handles. Everything will work fine except that timeouts

are not honored during the DNS lookup - which you can work around by

building libcurl with c-ares support. c-ares is a library that provides

asynchronous name resolves. Unfortunately, c-ares does not yet support

IPv6.

Also, note that CURLOPTDNSUSEGLOBALCACHE is not thread-safe.

WWhheenn IItt DDooeessnn''tt WWoorrkk There will always be times when the transfer fails for some reason. You might have set the wrong libcurl option or misunderstood what the

libcurl option actually does, or the remote server might return non-

standard replies that confuse the library which then confuses your pro-

gram.

There's one golden rule when these things occur: set the CURLOPTVER-

BOSE option to TRUE. It'll cause the library to spew out the entire

protocol details it sends, some internal info and some received proto-

col data as well (especially when using FTP). If you're using HTTP, adding the headers in the received output to study is also a clever way to get a better understanding why the server behaves the way it does. Include headers in the normal body output with CURLOPTHEADER set TRUE. Of course there are bugs left. We need to get to know about them to be able to fix them, so we're quite dependent on your bug reports! When you do report suspected bugs in libcurl, please include as much details you possibly can: a protocol dump that CURLOPTVERBOSE produces, library version, as much as possible of your code that uses libcurl, operating system name and version, compiler name and version etc. If CURLOPTVERBOSE is not enough, you increase the level of debug data your application receive by using the CURLOPTDEBUGFUNCTION.

Getting some in-depth knowledge about the protocols involved is never

wrong, and if you're trying to do funny things, you might very well

understand libcurl and how to use it better if you study the appropri-

ate RFC documents at least briefly. UUppllooaadd DDaattaa ttoo aa RReemmoottee SSiittee

libcurl tries to keep a protocol independent approach to most trans-

fers, thus uploading to a remote FTP site is very similar to uploading data to a HTTP server with a PUT request.

Of course, first you either create an easy handle or you re-use one

existing one. Then you set the URL to operate on just like before. This is the remote URL, that we now will upload. Since we write an application, we most likely want libcurl to get the upload data by asking us for it. To make it do that, we set the read callback and the custom pointer libcurl will pass to our read callback. The read callback should have a prototype similar to: sizet function(char *bufptr, sizet size, sizet nitems, void *userp); Where bufptr is the pointer to a buffer we fill in with data to upload

and size*nitems is the size of the buffer and therefore also the maxi-

mum amount of data we can return to libcurl in this call. The 'userp' pointer is the custom pointer we set to point to a struct of ours to pass private data between the application and the callback. curleasysetopt(easyhandle, CURLOPTREADFUNCTION, readfunction); curleasysetopt(easyhandle, CURLOPTINFILE, &filedata); Tell libcurl that we want to upload: curleasysetopt(easyhandle, CURLOPTUPLOAD, TRUE); A few protocols won't behave properly when uploads are done without any prior knowledge of the expected file size. So, set the upload file size using the CURLOPTINFILESIZELARGE for all known file sizes like this[1]: /* in this example, filesize must be an offt variable */ curleasysetopt(easyhandle, CURLOPTINFILESIZELARGE, filesize); When you call curleasyperform(3) this time, it'll perform all the necessary operations and when it has invoked the upload it'll call your supplied callback to get the data to upload. The program should return as much data as possible in every invoke, as that is likely to make the

upload perform as fast as possible. The callback should return the num-

ber of bytes it wrote in the buffer. Returning 0 will signal the end of the upload. PPaasssswwoorrddss

Many protocols use or even require that user name and password are pro-

vided to be able to download or upload the data of your choice. libcurl offers several ways to specify them. Most protocols support that you specify the name and password in the URL itself. libcurl will detect this and use them accordingly. This is written like this: protocol://user:password@example.com/path/ If you need any odd letters in your user name or password, you should

enter them URL encoded, as %XX where XX is a two-digit hexadecimal num-

ber. libcurl also provides options to set various passwords. The user name and password as shown embedded in the URL can instead get set with the CURLOPTUSERPWD option. The argument passed to libcurl should be a char * to a string in the format "user:password:". In a manner like this: curleasysetopt(easyhandle, CURLOPTUSERPWD, "myname:thesecret"); Another case where name and password might be needed at times, is for those users who need to authenticate themselves to a proxy they use. libcurl offers another option for this, the CURLOPTPROXYUSERPWD. It is used quite similar to the CURLOPTUSERPWD option like this:

curleasysetopt(easyhandle, CURLOPTPROXYUSERPWD, "myname:these-

cret"); There's a long time unix "standard" way of storing ftp user names and

passwords, namely in the $HOME/.netrc file. The file should be made

private so that only the user may read it (see also the "Security Con-

siderations" chapter), as it might contain the password in plain text. libcurl has the ability to use this file to figure out what set of user name and password to use for a particular host. As an extension to the

normal functionality, libcurl also supports this file for non-FTP pro-

tocols such as HTTP. To make curl use this file, use the CURLOPTNETRC option: curleasysetopt(easyhandle, CURLOPTNETRC, TRUE); And a very basic example of how such a .netrc file may look like: machine myhost.mydomain.com login userlogin password secretword All these examples have been cases where the password has been optional, or at least you could leave it out and have libcurl attempt to do its job without it. There are times when the password isn't

optional, like when you're using an SSL private key for secure trans-

fers. To pass the known private key password to libcurl: curleasysetopt(easyhandle, CURLOPTSSLKEYPASSWD, "keypassword"); HHTTTTPP AAuutthheennttiiccaattiioonn

The previous chapter showed how to set user name and password for get-

ting URLs that require authentication. When using the HTTP protocol, there are many different ways a client can provide those credentials to the server and you can control what way libcurl will (attempt to) use. The default HTTP authentication method is called 'Basic', which is

sending the name and password in clear-text in the HTTP request,

base64-encoded. This is insecure.

At the time of this writing libcurl can be built to use: Basic, Digest,

NTLM, Negotiate, GSS-Negotiate and SPNEGO. You can tell libcurl which

one to use with CURLOPTHTTPAUTH as in: curleasysetopt(easyhandle, CURLOPTHTTPAUTH, CURLAUTHDIGEST);

And when you send authentication to a proxy, you can also set authenti-

cation type the same way but instead with CURLOPTPROXYAUTH: curleasysetopt(easyhandle, CURLOPTPROXYAUTH, CURLAUTHNTLM); Both these options allow you to set multiple types (by ORing them together), to make libcurl pick the most secure one out of the types the server/proxy claims to support. This method does however add a

round-trip since libcurl must first ask the server what it supports:

curleasysetopt(easyhandle, CURLOPTHTTPAUTH, CURLAUTHDIGEST|CURLAUTHBASIC); For convenience, you can use the 'CURLAUTHANY' define (instead of a list with specific types) which allows libcurl to use whatever method it wants. When asking for multiple types, libcurl will pick the available one it considers "best" in its own internal order of preference. HHTTTTPP PPOOSSTTiinngg We get many questions regarding how to issue HTTP POSTs with libcurl

the proper way. This chapter will thus include examples using both dif-

ferent versions of HTTP POST that libcurl supports. The first version is the simple POST, the most common version, that most HTML pages using the

tag uses. We provide a pointer to the data and tell libcurl to post it all to the remote site: char *data="name=daniel&project=curl"; curleasysetopt(easyhandle, CURLOPTPOSTFIELDS, data); curleasysetopt(easyhandle, CURLOPTURL, "http://posthere.com/"); curleasyperform(easyhandle); /* post away! */

Simple enough, huh? Since you set the POST options with the CUR-

LOPTPOSTFIELDS, this automatically switches the handle to use POST in the upcoming request. Ok, so what if you want to post binary data that also requires you to

set the Content-Type: header of the post? Well, binary posts prevents

libcurl from being able to do strlen() on the data to figure out the

size, so therefore we must tell libcurl the size of the post data. Set-

ting headers in libcurl requests are done in a generic way, by building a list of our own headers and then passing that list to libcurl. struct curlslist *headers=NULL;

headers = curlslistappend(headers, "Content-Type: text/xml");

/* post binary data */ curleasysetopt(easyhandle, CURLOPTPOSTFIELDS, binaryptr); /* set the size of the postfields data */ curleasysetopt(easyhandle, CURLOPTPOSTFIELDSIZE, 23); /* pass our list of custom made headers */ curleasysetopt(easyhandle, CURLOPTHTTPHEADER, headers); curleasyperform(easyhandle); /* post away! */ curlslistfreeall(headers); /* free the header list */ While the simple examples above cover the majority of all cases where

HTTP POST operations are required, they don't do multi-part formposts.

Multi-part formposts were introduced as a better way to post (possibly

large) binary data and was first documented in the RFC1867. They're

called multi-part because they're built by a chain of parts, each being

a single unit. Each part has its own name and contents. You can in fact

create and post a multi-part formpost with the regular libcurl POST

support described above, but that would require that you build a form-

post yourself and provide to libcurl. To make that easier, libcurl pro-

vides curlformadd(3). Using this function, you add parts to the form. When you're done adding parts, you post the whole form. The following example sets two simple text parts with plain textual contents, and then a file with binary contents and upload the whole thing. struct curlhttppost *post=NULL; struct curlhttppost *last=NULL; curlformadd(&post, &last,

CURLFORMCOPYNAME, "name",

CURLFORMCOPYCONTENTS, "daniel", CURLFORMEND); curlformadd(&post, &last,

CURLFORMCOPYNAME, "project",

CURLFORMCOPYCONTENTS, "curl", CURLFORMEND); curlformadd(&post, &last,

CURLFORMCOPYNAME, "logotype-image",

CURLFORMFILECONTENT, "curl.png", CURLFORMEND); /* Set the form info */ curleasysetopt(easyhandle, CURLOPTHTTPPOST, post); curleasyperform(easyhandle); /* post away! */ /* free the post data again */ curlformfree(post);

Multipart formposts are chains of parts using MIME-style separators and

headers. It means that each one of these separate parts get a few head-

ers set that describe the individual content-type, size etc. To enable

your application to handicraft this formpost even more, libcurl allows you to supply your own set of custom headers to such an individual form part. You can of course supply headers to as many parts you like, but this little example will show how you set headers to one specific part when you add that to the post handle: struct curlslist *headers=NULL;

headers = curlslistappend(headers, "Content-Type: text/xml");

curlformadd(&post, &last,

CURLFORMCOPYNAME, "logotype-image",

CURLFORMFILECONTENT, "curl.xml", CURLFORMCONTENTHEADER, headers, CURLFORMEND); curleasyperform(easyhandle); /* post away! */ curlformfree(post); /* free post */ curlslistfreeall(post); /* free custom header list */ Since all options on an easyhandle are "sticky", they remain the same until changed even if you do call curleasyperform(3), you may need to tell curl to go back to a plain GET request if you intend to do such a one as your next request. You force an easyhandle to back to GET by using the CURLOPTHTTPGET option: curleasysetopt(easyhandle, CURLOPTHTTPGET, TRUE); Just setting CURLOPTPOSTFIELDS to "" or NULL will *not* stop libcurl from doing a POST. It will just make it POST without any data to send! SShhoowwiinngg PPrrooggrreessss

For historical and traditional reasons, libcurl has a built-in progress

meter that can be switched on and then makes it presents a progress meter in your terminal. Switch on the progress meter by, oddly enough, set CURLOPTNOPROGRESS to FALSE. This option is set to TRUE by default.

For most applications however, the built-in progress meter is useless

and what instead is interesting is the ability to specify a progress callback. The function pointer you pass to libcurl will then be called on irregular intervals with information about the current transfer. Set the progress callback by using CURLOPTPROGRESSFUNCTION. And pass a pointer to a function that matches this prototype: int progresscallback(void *clientp, double dltotal, double dlnow, double ultotal, double ulnow); If any of the input arguments is unknown, a 0 will be passed. The first

argument, the 'clientp' is the pointer you pass to libcurl with CUR-

LOPTPROGRESSDATA. libcurl won't touch it. lliibbccuurrll wwiitthh CC++++ There's basically only one thing to keep in mind when using C++ instead of C when interfacing libcurl:

The callbacks CANNOT be non-static class member functions

Example C++ code: class AClass { static sizet writedata(void *ptr, sizet size, sizet nmemb, void *ourpointer) { /* do what you want with the data */ } } PPrrooxxiieess

What "proxy" means according to Merriam-Webster: "a person authorized

to act for another" but also "the agency, function, or office of a deputy who acts as a substitute for another". Proxies are exceedingly common these days. Companies often only offer Internet access to employees through their HTTP proxies. Network

clients or user-agents ask the proxy for documents, the proxy does the

actual request and then it returns them. libcurl has full support for HTTP proxies, so when a given URL is wanted, libcurl will ask the proxy for it instead of trying to connect to the actual host identified in the URL. The fact that the proxy is a HTTP proxy puts certain restrictions on what can actually happen. A requested URL that might not be a HTTP URL will be still be passed to the HTTP proxy to deliver back to libcurl. This happens transparently, and an application may not need to know. I say "may", because at times it is very important to understand that all operations over a HTTP proxy is using the HTTP protocol. For example,

you can't invoke your own custom FTP commands or even proper FTP direc-

tory listings. Proxy Options To tell libcurl to use a proxy at a given port number:

curleasysetopt(easyhandle, CURLOPTPROXY, "proxy-

host.com:8080"); Some proxies require user authentication before allowing a request, and you pass that information similar to this:

curleasysetopt(easyhandle, CURLOPTPROXYUSERPWD, "user:pass-

word");

If you want to, you can specify the host name only in the CUR-

LOPTPROXY option, and set the port number separately with CUR-

LOPTPROXYPORT. Environment Variables

libcurl automatically checks and uses a set of environment vari-

ables to know what proxies to use for certain protocols. The

names of the variables are following an ancient de facto stan-

dard and are built up as "[protocol]proxy" (note the lower cas-

ing). Which makes the variable HTTP. Following the same rule, the variable named 'ftpproxy' is checked for FTP URLs. Again, the proxies are always HTTP proxies, the different names of the variables simply allows different HTTP proxies to be used. The proxy environment variable contents should be in the format

"[protocol://][user:password@]machine[:port]". Where the proto-

col:// part is simply ignored if present (so http://proxy and bluerk://proxy will do the same) and the optional port number specifies on which port the proxy operates on the host. If not specified, the internal default port number will be used and that is most likely *not* the one you would like it to be. There are two special environment variables. 'allproxy' is what sets proxy for any URL in case the protocol specific variable wasn't set, and 'noproxy' defines a list of hosts that should not use a proxy even though a variable may say so. If 'noproxy' is a plain asterisk ("*") it matches all hosts. SSL and Proxies

SSL is for secure point-to-point connections. This involves

strong encryption and similar things, which effectively makes it impossible for a proxy to operate as a "man in between" which the proxy's task is, as previously discussed. Instead, the only way to have SSL work over a HTTP proxy is to ask the proxy to tunnel trough everything without being able to check or fiddle with the traffic. Opening an SSL connection over a HTTP proxy is therefor a matter of asking the proxy for a straight connection to the target host on a specified port. This is made with the HTTP request CONNECT. ("please mr proxy, connect me to that remote host"). Because of the nature of this operation, where the proxy has no idea what kind of data that is passed in and out through this tunnel, this breaks some of the very few advantages that come from using a proxy, such as caching. Many organizations prevent this kind of tunneling to other destination port numbers than 443 (which is the default HTTPS port number). Tunneling Through Proxy As explained above, tunneling is required for SSL to work and often even restricted to the operation intended for SSL; HTTPS.

This is however not the only time proxy-tunneling might offer

benefits to you or your application. As tunneling opens a direct connection from your application to

the remote machine, it suddenly also re-introduces the ability

to do non-HTTP operations over a HTTP proxy. You can in fact use

things such as FTP upload or FTP custom commands this way. Again, this is often prevented by the administrators of proxies and is rarely allowed. Tell libcurl to use proxy tunneling like this: curleasysetopt(easyhandle, CURLOPTHTTPPROXYTUNNEL, TRUE); In fact, there might even be times when you want to do plain HTTP operations using a tunnel like this, as it then enables you to operate on the remote server instead of asking the proxy to do so. libcurl will not stand in the way for such innovative actions either!

Proxy Auto-Config

Netscape first came up with this. It is basically a web page (usually using a .pac extension) with a javascript that when executed by the browser with the requested URL as input, returns information to the browser on how to connect to the URL. The returned information might be "DIRECT" (which means no proxy should be used), "PROXY host:port" (to tell the browser where the proxy for this particular URL is) or "SOCKS host:port" (to direct the browser to a SOCKS proxy). libcurl has no means to interpret or evaluate javascript and thus it doesn't support this. If you get yourself in a position where you face this nasty invention, the following advice have been mentioned and used in the past:

- 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 sim-

ilar. Persistence Is The Way to Happiness

Re-cycling the same easy handle several times when doing multiple

requests is the way to go. After each single curleasyperform(3) operation, libcurl will keep the connection alive and open. A subsequent request using the same easy

handle to the same host might just be able to use the already open con-

nection! This reduces network impact a lot. Even if the connection is dropped, all connections involving SSL to the same host again, will benefit from libcurl's session ID cache that

drastically reduces re-connection time.

FTP connections that are kept alive saves a lot of time, as the com-

mand- response round-trips are skipped, and also you don't risk getting

blocked without permission to login again like on many FTP servers only allowing N persons to be logged in at the same time.

libcurl caches DNS name resolving results, to make lookups of a previ-

ously looked up name a lot faster. Other interesting details that improve performance for subsequent requests may also be added in the future. Each easy handle will attempt to keep the last few connections alive for a while in case they are to be used again. You can set the size of this "cache" with the CURLOPTMAXCONNECTS option. Default is 5. It is very seldom any point in changing this value, and if you think of changing this it is often just a matter of thinking again. When the connection cache gets filled, libcurl must close an existing

connection in order to get room for the new one. To know which connec-

tion to close, libcurl uses a "close policy" that you can affect with the CURLOPTCLOSEPOLICY option. There's only two polices implemented as of this writing (libcurl 7.9.4) and they are: CURLCLOSEPOLICYLEASTRECENTLYUSED

simply close the one that hasn't been used for the long-

est time. This is the default behavior. CURLCLOSEPOLICYOLDEST closes the oldest connection, the one that was created the longest time ago. There are, or at least were, plans to support a close policy that would

call a user-specified callback to let the user be able to decide which

connection to dump when this is necessary and therefor is the CUR-

LOPTCLOSEFUNCTION an existing option still today. Nothing ever uses this though and this will not be used within the foreseeable future either.

To force your upcoming request to not use an already existing connec-

tion (it will even close one first if there happens to be one alive to the same host you're about to operate on), you can do that by setting CURLOPTFRESHCONNECT to TRUE. In a similar spirit, you can also forbid

the upcoming request to be "lying" around and possibly get re-used

after the request by setting CURLOPTFORBIDREUSE to TRUE. HHTTTTPP HHeeaaddeerrss UUsseedd bbyy lliibbccuurrll When you use libcurl to do HTTP requests, it'll pass along a series of headers automatically. It might be good for you to know and understand these ones. Host This header is required by HTTP 1.1 and even many 1.0 servers and should be the name of the server we want to talk to. This includes the port number if anything but default.

Pragma "no-cache". Tells a possible proxy to not grab a copy from the

cache but to fetch a fresh one. Accept "*/*". Expect:

When doing multi-part formposts, libcurl will set this header to

"100-continue" to ask the server for an "OK" message before it

proceeds with sending the data part of the post. CCuussttoommiizziinngg OOppeerraattiioonnss There is an ongoing development today where more and more protocols are built upon HTTP for transport. This has obvious benefits as HTTP is a tested and reliable protocol that is widely deployed and have excellent

proxy-support.

When you use one of these protocols, and even when doing other kinds of programming you may need to change the traditional HTTP (or FTP or...) manners. You may need to change words, headers or various data. libcurl is your friend here too. CUSTOMREQUEST If just changing the actual HTTP request keyword is what you want, like when GET, HEAD or POST is not good enough for you, CURLOPTCUSTOMREQUEST is there for you. It is very simple to use:

curleasysetopt(easyhandle, CURLOPTCUSTOMREQUEST, "MYOWN-

RUQUEST"); When using the custom request, you change the request keyword of the actual request you are performing. Thus, by default you make GET request but you can also make a POST operation (as described before) and then replace the POST keyword if you want to. You're the boss. Modify Headers

HTTP-like protocols pass a series of headers to the server when

doing the request, and you're free to pass any amount of extra headers that you think fit. Adding headers are this easy: struct curlslist *headers=NULL; /* init to NULL is important */

headers = curlslistappend(headers, "Hey-server-hey: how are you?");

headers = curlslistappend(headers, "X-silly-content: yes");

/* pass our list of custom made headers */ curleasysetopt(easyhandle, CURLOPTHTTPHEADER, headers); curleasyperform(easyhandle); /* transfer http */ curlslistfreeall(headers); /* free the header list */ ... and if you think some of the internally generated headers, such as Accept: or Host: don't contain the data you want them to contain, you can replace them by simply setting them too:

headers = curlslistappend(headers, "Accept: Agent-007");

headers = curlslistappend(headers, "Host: munged.host.line"); Delete Headers If you replace an existing header with one with no contents, you will prevent the header from being sent. Like if you want to

completely prevent the "Accept:" header to be sent, you can dis-

able it with code similar to this: headers = curlslistappend(headers, "Accept:"); Both replacing and canceling internal headers should be done with careful consideration and you should be aware that you may violate the HTTP protocol when doing so.

Enforcing chunked transfer-encoding

By making sure a request uses the custom header "Transfer-Encod-

ing: chunked" when doing a non-GET HTTP operation, libcurl will

switch over to "chunked" upload, even though the size of the data to upload might be known. By default, libcurl usually switches over to chunked upload automatically if the upload data size is unknown. HTTP Version There's only one aspect left in the HTTP requests that we haven't yet mentioned how to modify: the version field. All HTTP requests includes the version number to tell the server which version we support. libcurl speak HTTP 1.1 by default. Some very

old servers don't like getting 1.1-requests and when dealing

with stubborn old things like that, you can tell libcurl to use 1.0 instead by doing something like this:

curleasysetopt(easyhandle, CURLOPTHTTPVERSION, CURL-

HTTPVERSION10); FTP Custom Commands

Not all protocols are HTTP-like, and thus the above may not help

you when you want to make for example your FTP transfers to behave differently. Sending custom commands to a FTP server means that you need to send the commands exactly as the FTP server expects them (RFC959 is a good guide here), and you can only use commands that work

on the control-connection alone. All kinds of commands that

requires data interchange and thus needs a data-connection must

be left to libcurl's own judgment. Also be aware that libcurl

will do its very best to change directory to the target direc-

tory before doing any transfer, so if you change directory (with CWD or similar) you might confuse libcurl and then it might not attempt to transfer the file in the correct remote directory. A little example that deletes a given file before an operation:

headers = curlslistappend(headers, "DELE file-to-remove");

/* pass the list of custom commands to the handle */ curleasysetopt(easyhandle, CURLOPTQUOTE, headers); curleasyperform(easyhandle); /* transfer ftp data! */ curlslistfreeall(headers); /* free the header list */

If you would instead want this operation (or chain of opera-

tions) to happen after the data transfer took place the option to curleasysetopt(3) would instead be called CURLOPTPOSTQUOTE and used the exact same way. The custom FTP command will be issued to the server in the same order they are added to the list, and if a command gets an error code returned back from the server, no more commands will be issued and libcurl will bail out with an error code

(CURLEFTPQUOTEERROR). Note that if you use CURLOPTQUOTE to

send commands before a transfer, no transfer will actually take place when a quote command has failed. If you set the CURLOPTHEADER to true, you will tell libcurl to get information about the target file and output "headers" about

it. The headers will be in "HTTP-style", looking like they do in

HTTP. The option to enable headers or to run custom FTP commands may be useful to combine with CURLOPTNOBODY. If this option is set, no actual file content transfer will be performed. FTP Custom CUSTOMREQUEST If you do what list the contents of a FTP directory using your own defined FTP command, CURLOPTCUSTOMREQUEST will do just that. "NLST" is the default one for listing directories but you're free to pass in your idea of a good alternative. CCooookkiieess WWiitthhoouutt CChhooccoollaattee CChhiippss In the HTTP sense, a cookie is a name with an associated value. A server sends the name and value to the client, and expects it to get sent back on every subsequent request to the server that matches the particular conditions set. The conditions include that the domain name and path match and that the cookie hasn't become too old.

In real-world cases, servers send new cookies to replace existing one

to update them. Server use cookies to "track" users and to keep "ses-

sions".

Cookies are sent from server to clients with the header Set-Cookie: and

they're sent from clients to servers with the Cookie: header.

To just send whatever cookie you want to a server, you can use CUR-

LOPTCOOKIE to set a cookie string like this: curleasysetopt(easyhandle, CURLOPTCOOKIE, "name1=var1; name2=var2;"); In many cases, that is not enough. You might want to dynamically save whatever cookies the remote server passes to you, and make sure those cookies are then use accordingly on later requests. One way to do this, is to save all headers you receive in a plain file and when you make a request, you tell libcurl to read the previous headers to figure out which cookies to use. Set header file to read cookies from with CURLOPTCOOKIEFILE. The CURLOPTCOOKIEFILE option also automatically enables the cookie parser in libcurl. Until the cookie parser is enabled, libcurl will not parse or understand incoming cookies and they will just be ignored. However, when the parser is enabled the cookies will be understood and the cookies will be kept in memory and used properly in subsequent requests when the same handle is used. Many times this is enough, and you may not have to save the cookies to disk at all. Note that the file you specify to CURLOPTCOOKIEFILE doesn't have to exist to enable the parser, so a common way to just enable the parser and not read able might be to use a file name you know doesn't exist. If you rather use existing cookies that you've previously received with your Netscape or Mozilla browsers, you can make libcurl use that cookie file as input. The CURLOPTCOOKIEFILE is used for that too, as libcurl

will automatically find out what kind of file it is and act accord-

ingly. The perhaps most advanced cookie operation libcurl offers, is saving the entire internal cookie state back into a Netscape/Mozilla formatted

cookie file. We call that the cookie-jar. When you set a file name with

CURLOPTCOOKIEJAR, that file name will be created and all received cookies will be stored in it when curleasycleanup(3) is called. This enabled cookies to get passed on properly between multiple handles without any information getting lost. FFTTPP PPeeccuulliiaarriittiieess WWee NNeeeedd FTP transfers use a second TCP/IP connection for the data transfer. This is usually a fact you can forget and ignore but at times this fact will come back to haunt you. libcurl offers several different ways to custom how the second connection is being made. libcurl can either connect to the server a second time or tell the server to connect back to it. The first option is the default and it is

also what works best for all the people behind firewalls, NATs or IP-

masquerading setups. libcurl then tells the server to open up a new port and wait for a second connection. This is by default attempted with EPSV first, and if that doesn't work it tries PASV instead. (EPSV is an extension to the original FTP spec and does not exist nor work on all FTP servers.) You can prevent libcurl from first trying the EPSV command by setting CURLOPTFTPUSEEPSV to FALSE. In some cases, you will prefer to have the server connect back to you for the second connection. This might be when the server is perhaps behind a firewall or something and only allows connections on a single port. libcurl then informs the remote server which IP address and port number to connect to. This is made with the CURLOPTFTPPORT option. If

you set it to "-", libcurl will use your system's "default IP address".

If you want to use a particular IP, you can set the full IP address, a host name to resolve to an IP address or even a local network interface name that libcurl will get the IP address from. When doing the "PORT" approach, libcurl will attempt to use the EPRT and the LPRT before trying PORT, as they work with more protocols. You can disable this behavior by setting CURLOPTFTPUSEEPRT to FALSE. HHeeaaddeerrss EEqquuaall FFuunn

Some protocols provide "headers", meta-data separated from the normal

data. These headers are by default not included in the normal data

stream, but you can make them appear in the data stream by setting CUR-

LOPTHEADER to TRUE. What might be even more useful, is libcurl's ability to separate the headers from the data and thus make the callbacks differ. You can for example set a different pointer to pass to the ordinary write callback by setting CURLOPTWRITEHEADER. Or, you can set an entirely separate function to receive the headers, by using CURLOPTHEADERFUNCTION. The headers are passed to the callback function one by one, and you can depend on that fact. It makes it easier for you to add custom header parsers etc. "Headers" for FTP transfers equal all the FTP server responses. They

aren't actually true headers, but in this case we pretend they are! ;-)

PPoosstt TTrraannssffeerr IInnffoorrmmaattiioonn [ curleasygetinfo ] SSeeccuurriittyy CCoonnssiiddeerraattiioonnss libcurl is in itself not insecure. If used the right way, you can use libcurl to transfer data pretty safely. There are of course many things to consider that may loosen up this situation: Command Lines If you use a command line tool (such as curl) that uses libcurl, and you give option to the tool on the command line those options can very likely get read by other users of your system

when they use 'ps' or other tools to list currently running pro-

cesses. To avoid this problem, never feed sensitive things to programs using command line options. .netrc .netrc is a pretty handy file/feature that allows you to login quickly and automatically to frequently visited sites. The file contains passwords in clear text and is a real security risk. In some cases, your .netrc is also stored in a home directory that is NFS mounted or used on another network based file system, so the clear text password will fly through your network every time anyone reads that file! To avoid this problem, don't use .netrc files and never store passwords in plain text anywhere. Clear Text Passwords Many of the protocols libcurl supports send name and password

unencrypted as clear text (HTTP Basic authentication, FTP, TEL-

NET etc). It is very easy for anyone on your network or a net-

work nearby yours, to just fire up a network analyzer tool and eavesdrop on your passwords. Don't let the fact that HTTP uses base64 encoded passwords fool you. They may not look readable at a first glance, but they very easily "deciphered" by anyone within seconds. To avoid this problem, use protocols that don't let snoopers see

your password: HTTPS, FTPS and FTP-kerberos are a few examples.

HTTP Digest authentication allows this too, but isn't supported by libcurl as of this writing. Showing What You Do On a related issue, be aware that even in situations like when

you have problems with libcurl and ask someone for help, every-

thing you reveal in order to get best possible help might also impose certain security related risks. Host names, user names, paths, operating system specifics etc (not to mention passwords of course) may in fact be used by intruders to gain additional information of a potential target. To avoid this problem, you must of course use your common sense. Often, you can just edit out the sensitive data or just search/replace your true information with faked data. MMuullttiippllee TTrraannssffeerrss UUssiinngg tthhee mmuullttii IInntteerrffaaccee

The easy interface as described in detail in this document is a syn-

chronous interface that transfers one file at a time and doesn't return until its done. The multi interface on the other hand, allows your program to transfer multiple files in both directions at the same time, without forcing you to use multiple threads. To use this interface, you are better off if you first understand the basics of how to use the easy interface. The multi interface is simply

a way to make multiple transfers at the same time, by adding up multi-

ple easy handles in to a "multi stack". You create the easy handles you want and you set all the options just like you have been told above, and then you create a multi handle with curlmultiinit(3) and add all those easy handles to that multi handle with curlmultiaddhandle(3). When you've added the handles you have for the moment (you can still add new ones at any time), you start the transfers by call curlmultiperform(3). curlmultiperform(3) is asynchronous. It will only execute as little as possible and then return back control to your program. It is designed to never block. If it returns CURLMCALLMULTIPERFORM you better call it again soon, as that is a signal that it still has local data to send or remote data to receive.

The best usage of this interface is when you do a select() on all pos-

sible file descriptors or sockets to know when to call libcurl again. This also makes it easy for you to wait and respond to actions on your own application's sockets/handles. You figure out what to select() for by using curlmultifdset(3), that fills in a set of fdset variables for you with the particular file descriptors libcurl uses for the moment. When you then call select(), it'll return when one of the file handles signal action and you then call curlmultiperform(3) to allow libcurl to do what it wants to do. Take note that libcurl does also feature

some time-out code so we advice you to never use very long timeouts on

select() before you call curlmultiperform(3), which thus should be called unconditionally every now and then even if none of its file descriptors have signaled ready. Another precaution you should use: always call curlmultifdset(3) immediately before the select() call since the current set of file descriptors may change when calling a curl function. If you want to stop the transfer of one of the easy handles in the stack, you can use curlmultiremovehandle(3) to remove individual easy handles. Remember that easy handles should be curleasycleanup(3)ed. When a transfer within the multi stack has finished, the counter of running transfers (as filled in by curlmultiperform(3)) will decrease. When the number reaches zero, all transfers are done. curlmultiinforead(3) can be used to get information about completed transfers. It then returns the CURLcode for each easy transfer, to allow you to figure out success on each individual transfer. SSSSLL,, CCeerrttiiffiiccaatteess aanndd OOtthheerr TTrriicckkss [ seeding, passwords, keys, certificates, ENGINE, ca certs ] SShhaarriinngg DDaattaa BBeettwweeeenn EEaassyy HHaannddlleess [ fill in ] FFoooottnnootteess [1] libcurl 7.10.3 and later have the ability to switch over to

chunked Transfer-Encoding in cases were HTTP uploads are done

with data of an unknown size. [2] This happens on Windows machines when libcurl is built and used as a DLL. However, you can still do this on Windows if you link with a static library.

[3] The curl-config tool is generated at build-time (on unix-like

systems) and should be installed with the 'make install' or sim-

ilar instruction that installs the library, header files, man pages etc.