curl
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curl is a command-line utility for transferring data from or to a server designed to work without user interaction. With curl, you can download or upload data using one of the supported protocols including HTTP, HTTPS, , , and . curl provides a number of options allowing you to resume transfers, limit the bandwidth, proxy support, user authentication, and much more.
In this section, we will show you how to use the curl tool through practical examples and detailed explanations of the most common curl options.
The syntax for the curl command is as follows:
In its simplest form, when invoked without any option, curl displays the specified resource to the standard output (usually your Terminal Window). For example, to retrieve the example.com homepage you would run:
The command will print the source code of the example.com homepage in your terminal window:
curl is a very versatile tool, and as such it has an array of useful functionality. In this section, we will concentrate primarily on the download capabilities of curl, however, there is a section in the Penetration Testing section where some of the more advanced features of curl are explored.
To save the result of the curl command, use either the -o or -O option.
Lowercase -o saves the file with a predefined filename, which in the example below is vue-v2.6.10.js:
Uppercase -O saves the file with its original filename:
You can resume a download by using the -C - option. This is useful if your connection drops during the download of a large file, and instead of starting the download from scratch, you can continue the previous one.
For example, if you are downloading the Ubuntu 18.04 iso file using the following command:
and suddenly your connection drops you can resume the download with:
By default, curl doesn’t follow the HTTP Location headers. If you try to retrieve the non-www version of google.com, you will notice that instead of getting the source of the page you’ll be redirected to the www version:
The -L option instructs curl to follow any redirect until it reaches the final destination:
Sometimes when downloading a file, the remote server may be set to block the Curl User-Agent, or to return different contents depending on the visitor device and browser. In situations like this, to emulate a different browser you can use the -A option.
For example to emulates Firefox 60 you would use:
To access a protected FTP server with curl, use the -u option and specify the username and password as shown below:
Once logged in, the command lists all files and directories in the user’s home directory. You can then download a single file from the FTP server using the following syntax:
To upload a file to the FTP server, use the -T followed by the name of the file you want to upload:
curl supports different types of proxies, including HTTP, HTTPS and SOCKS. To transfer data through a proxy server, use the -x (--proxy) option, followed by the proxy URL.
The following command downloads the specified resource using a proxy on 192.168.44.1 port 8888:
If the proxy server requires authentication, use the -U (--proxy-user) option followed by the user name and password separated by a colon (user:password):
Sometimes you may need to make an HTTP request with specific cookies to access a remote resource or to debug an issue. By default, when requesting a resource with curl, no cookies are sent or stored.
To send cookies to the server, use the -b switch followed by a filename containing the cookies or a string.
For example, to download the Oracle Java JDK rpm file jdk-10.0.2_linux-x64_bin.rpm you’ll need to pass a cookie named oraclelicense with value a:
