curl / libcurl / API / curl_easy_setopt / CURLOPT_URL

CURLOPT_URL explained

Name

CURLOPT_URL - URL for this transfer

Synopsis

#include <curl/curl.h>
 
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_URL, char *URL);

Description

Pass in a pointer to the URL to work with. The parameter should be a char * to a null-terminated string which must be URL-encoded in the following format:

scheme://host:port/path

For a greater explanation of the format please see RFC 3986.

libcurl does not validate the syntax or use the URL until the transfer is started. Even if you set a crazy value here, curl_easy_setopt might still return CURLE_OK.

If the given URL is missing a scheme name (such as "http://" or "ftp://" etc) then libcurl guesses based on the host. If the outermost subdomain name matches DICT, FTP, IMAP, LDAP, POP3 or SMTP then that protocol gets used, otherwise HTTP is used. Since 7.45.0 guessing can be disabled by setting a default protocol, see CURLOPT_DEFAULT_PROTOCOL for details.

Should the protocol, either as specified by the URL scheme or deduced by libcurl from the hostname, not be supported by libcurl then CURLE_UNSUPPORTED_PROTOCOL is returned from either the curl_easy_perform or curl_multi_perform functions when you call them. Use curl_version_info for detailed information of which protocols are supported by the build of libcurl you are using.

CURLOPT_PROTOCOLS_STR can be used to limit what protocols libcurl may use for this transfer, independent of what libcurl has been compiled to support. That may be useful if you accept the URL from an external source and want to limit the accessibility.

The CURLOPT_URL string is ignored if CURLOPT_CURLU is set.

Either CURLOPT_URL or CURLOPT_CURLU must be set before a transfer is started.

The application does not have to keep the string around after setting this option.

Using this option multiple times makes the last set string override the previous ones. Set it to NULL to disable its use again. Note however that libcurl needs a URL set to be able to performed a transfer.

The parser used for handling the URL set with CURLOPT_URL is the same that curl_url_set uses.

Encoding

The string pointed to in the CURLOPT_URL argument is generally expected to be a sequence of characters using an ASCII compatible encoding.

If libcurl is built with IDN support, the server name part of the URL can use an "international name" by using the current encoding (according to locale) or UTF-8 (when WinIDN is used; or a Windows Unicode build using libidn2).

If libcurl is built without IDN support, the server name is used exactly as specified when passed to the name resolver functions.

Default

NULL. If this option is not set, no transfer can be performed.

Security concerns

Applications may at times find it convenient to allow users to specify URLs for various purposes and that string would then end up fed to this option.

Getting a URL from an external untrusted party brings several security concerns:

If you have an application that runs as or in a server application, getting an unfiltered URL can easily trick your application to access a local resource instead of a remote. Protecting yourself against localhost accesses is hard when accepting user provided URLs.

Such custom URLs can also access other ports than you planned as port numbers are part of the regular URL format. The combination of a local host and a custom port number can allow external users to play tricks with your local services.

Accepting external URLs may also use other protocols than http:// or other common ones. Restrict what accept with CURLOPT_PROTOCOLS_STR.

User provided URLs can also be made to point to sites that redirect further on (possibly to other protocols too). Consider your CURLOPT_FOLLOWLOCATION and CURLOPT_REDIR_PROTOCOLS_STR settings.

Protocols

This functionality affects all supported protocols

Example

int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
 
    curl_easy_perform(curl);
  }
}

Availability

Added in curl 7.1

Return value

Returns CURLE_OK on success or CURLE_OUT_OF_MEMORY if there was insufficient heap space.

Note that curl_easy_setopt does not parse the given string so given a bad URL, it is not detected until curl_easy_perform or similar is called.