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extended the proxy chapter mucho

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Daniel Stenberg 2002-01-30 10:04:40 +00:00
parent 5b58e61f28
commit 14e9420d2c
1 changed files with 144 additions and 23 deletions
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docs/libcurl-the-guide
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@ -137,9 +137,22 @@ Handle the Easy libcurl
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. One of the most basic properties to set in
the handle is the URL. You set your preferred URL to transfer with
CURLOPT_URL in a manner similar to:
transfer or series of transfers.
You set properties and options for this handle using curl_easy_setopt(). They
control how the subsequent transfer or transfers 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 informationals you set in libcurl are "strings", pointers to data
terminated with a zero byte. Keep in mind that when you set strings with
curl_easy_setopt(), 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 CURLOPT_URL in a manner similar to:
curl_easy_setopt(easyhandle, CURLOPT_URL, "http://curl.haxx.se/");
@ -358,12 +371,16 @@ HTTP POSTing
curl_easy_perform(easyhandle); /* post away! */
Simple enough, huh? 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. Setting
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.
Simple enough, huh? Since you set the POST options with the
CURLOPT_POSTFIELDS, this automaticly 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. Setting 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 curl_slist *headers=NULL;
headers = curl_slist_append(headers, "Content-Type: text/xml");
@ -416,14 +433,14 @@ HTTP POSTing
/* free the post data again */
curl_formfree(post);
The multipart formposts are a chain of parts using MIME-style separators and
headers. That means that each of these separate parts get a few headers set
that describes its individual content-type, size etc. Now, to enable your
Multipart formposts are chains of parts using MIME-style separators and
headers. It means that each one of these separate parts get a few headers 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 custom headers to an individual form part. You can of course
supply headers to as many parts you like, but this little example will show
how you have set headers to one specific part when you add that to post
handle:
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 curl_slist *headers=NULL;
headers = curl_slist_append(headers, "Content-Type: text/xml");
@ -439,9 +456,22 @@ HTTP POSTing
curl_formfree(post); /* free post */
curl_slist_free_all(post); /* free custom header list */
Since all options on an easyhandle are "sticky", they remain the same until
changed even if you do call curl_easy_perform(), 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 CURLOPT_HTTPGET
option:
curl_easy_setopt(easyhandle, CURLOPT_HTTPGET, TRUE);
Just setting CURLOPT_POSTFIELDS to "" or NULL will *not* stop libcurl from
doing a POST. It will just make it POST without any data to send!
Showing Progress
[ built-in progress meter, progress callback ]
libcurl with C++
@ -488,16 +518,107 @@ Proxies
proxy is using the HTTP protocol. For example, you can't invoke your own
custom FTP commands or even proper FTP directory listings.
To tell libcurl to use a proxy at a given port number:
Proxy Options
curl_easy_setopt(easyhandle, CURLOPT_PROXY, "proxy-host.com:8080");
To tell libcurl to use a proxy at a given port number:
Some proxies require user authentication before allowing a request, and you
pass that information similar to this:
curl_easy_setopt(easyhandle, CURLOPT_PROXY, "proxy-host.com:8080");
curl_easy_setopt(easyhandle, CURLOPT_PROXYUSERPWD, "user:password");
Some proxies require user authentication before allowing a request, and
you pass that information similar to this:
[ environment variables, SSL, tunneling, automatic proxy config (.pac) ]
curl_easy_setopt(easyhandle, CURLOPT_PROXYUSERPWD, "user:password");
If you want to, you can specify the host name only in the CURLOPT_PROXY
option, and set the port number separately with CURLOPT_PROXYPORT.
Environment Variables
libcurl automaticly checks and uses a set of environment variables to know
what proxies to use for certain protocols. The names of the variables are
following an ancient de facto standard and are built up as
"[protocol]_proxy" (note the lower casing). Which makes the variable
'http_proxy' checked for a name of a proxy to use when the input URL is
HTTP. Following the same rule, the variable named 'ftp_proxy' 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://]machine[:port]". Where the protocol:// 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. 'all_proxy' is what sets
proxy for any URL in case the protocol specific variable wasn't set, and
'no_proxy' defines a list of hosts that should not use a proxy even though
a variable may say so. If 'no_proxy' is a plain asterisk ("*") it matches
all hosts.
SSL and Proxies
SSL is for secure point-to-point connections. This envolves strong
encryption and similar things, which effectivly 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 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.
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
effectively breaks some of the pros a proxy might offer, 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 adminstrators of proxies and is
rarely allowed.
Tell libcurl to use proxy tunneling like this:
curl_easy_setopt(easyhandle, CURLOPT_HTTPPROXYTUNNEL, TRUE);
Proxy Auto-Config
Netscape first came up with this. It is basicly 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 brower 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 similar.
Security Considerations
@ -505,7 +626,7 @@ Security Considerations
[ ps output, netrc plain text, plain text protocols / base64 ]
Certificates and Other SSL Tricks
SSL, Certificates and Other Tricks
Future