What is Curl?
Curl is a command line utility for transferring data to or from a remote server using one of the supported protocols. Curl is widely used for testing APIs, sending requests to the server, and viewing the content and headers of the server's response. Curl supports over 25+ protocols, including HTTP, HTTPS, FTP, FTPS, and SFTP, and has built-in support for SSL certificates, web form submission, file upload, HTTP Cookies, user authentication, and more.
What is an SSL Certificate?
Secure Sockets Layer (SSL) is a network protocol for establishing safe, authenticated, and encrypted connections between two devices. An SSL certificate is a digital certificate that identifies a website and encrypts the information sent to/from the server using SSL. SSL protects user privacy when sending data over the Internet by encrypting all traffic between the user's computer and the web server, making it impossible for the data to be intercepted or read. An SSL certificate allows secure communication over the HTTPS protocol when installed on the server.
Examples of using the Curl -k flag
The following are examples with the -k command line option:
Sending a Curl request without the -k flag to a page with an expired certificate
Curl automatically checks the validity of the SSL certificate. Without the -k flag, if the certificate is invalid or cannot be verified, it will throw an error.
You will receive an invalid SSL certificate error message:
Sending a Curl request without SSL certificate verification
The following is an example of sending a request without validating the SSL certificate:
Curl file download without SSL certificate verification
The following is an example of downloading a file without SSL verification in Curl: