Let’s go through the steps and data flow when using `curl` to make an HTTPS request and check for an HTTP status code. This process involves a number of stages and protocols, primarily DNS (Domain Name System), TCP (Transmission Control Protocol), TLS (Transport Layer Security), and HTTP (Hypertext Transfer Protocol).
1. DNS Lookup:
`curl` begins by performing a DNS lookup on the domain name you’ve specified in the URL (e.g., `www.cordero.me`). This resolves the domain name into an IP address that can be used to contact the server.
2. TCP Connection:
Next, `curl` initiates a TCP connection with the server at the IP address it obtained in the DNS lookup. This involves a three-way handshake:
• `curl` sends a SYN packet to the server.
• The server responds with a SYN-ACK packet.
• `curl` sends an ACK packet back to the server.
At this point, a TCP connection has been established.
3. TLS Handshake:
Now that `curl` has a TCP connection, it begins the TLS handshake to establish a secure, encrypted connection:
• `curl` sends a ClientHello message, which includes the highest version of TLS it supports and a list of supported cipher suites.
• The server responds with a ServerHello message, which includes the chosen TLS version and cipher suite, as well as the server’s digital certificate. The certificate includes the server’s public key and has been signed by a trusted Certificate Authority (CA).
• `curl` verifies the server’s certificate by checking that the signature matches one of the trusted CAs in its certificate store. If the certificate is valid, `curl` generates a pre-master secret, encrypts it with the server’s public key, and sends it back to the server.
• Both `curl` and the server use this pre-master secret to generate the same session key. This session key is used for symmetric encryption of all data sent during the session.
• `curl` and the server exchange Finished messages to confirm that the handshake was successful.
4. HTTP Request and Response:
With the secure TLS connection in place, `curl` can now send the HTTP request:
• `curl` sends an HTTP GET request over the TLS connection. This request includes the path to the resource (e.g., `/`) and the HTTP version (e.g., `HTTP/1.1`).
• The server processes the request and sends an HTTP response. This response includes a status code (e.g., `200 OK`), headers, and potentially a body with the requested resource.
Note that the entire HTTP request and response are encrypted with the session key from the TLS handshake.
5. Checking the Status Code:
`curl` reads the HTTP response and checks the status code. If you’re using `curl` in a script or automated tool, you can parse this status code to handle different types of responses (e.g., successful, client error, server error).
6. Closing the Connection:
Finally, unless the HTTP headers specified a keep-alive connection, `curl` and the server close the TCP connection.
In summary, `curl` uses a combination of DNS, TCP, TLS, and HTTP to securely request a resource over HTTPS and check the HTTP status code of the response. This process involves several layers of the networking stack and requires a valid certificate on the server side for the TLS handshake.