0x11. What happens when you type google.com in your browser and press Enter

Have you ever wondered what goes on behind the scenes when you type a URL into your browser and press Enter? It's a fascinating journey through various layers of technology that work together seamlessly to deliver the webpage you requested. In this blog post, we'll explore the key components involved in this process: DNS request, TCP/IP, firewall, HTTPS/SSL, load balancer, web server, application server, and database. Understanding these concepts will enhance your technical knowledge and prepare you for software engineering interviews.

DNS Request

The first step in this journey is the Domain Name System (DNS) request. When you type "https://www.google.com" into your browser, it needs to translate this human-readable domain name into an IP address, which computers use to identify each other on the network.

Browser Cache: The browser checks its cache to see if it has recently resolved the domain name.

OS Cache: If not found, it queries the operating system's DNS cache.

Router: The request then goes to your router, which might have its cache.

ISP DNS Server: Finally, the request reaches your ISP's DNS server. If the ISP's server doesn't have the IP address cached, it performs a recursive search starting from the root DNS servers down to the authoritative DNS server for "google.com".

The result of this DNS resolution is the IP address associated with "www.google.com".

TCP/IP

With the IP address in hand, the browser needs to establish a connection to the web server. This is where the TCP/IP protocol suite comes into play.

TCP Handshake: The browser initiates a TCP (Transmission Control Protocol) handshake with the server. This involves three steps:

SYN: The browser sends a SYN (synchronize) packet to the server.

SYN-ACK: The server responds with a SYN-ACK (synchronize-acknowledge) packet.

ACK: The browser sends an ACK (acknowledge) packet back to the server, establishing a stable connection.

IP: The TCP packets are encapsulated in IP (Internet Protocol) packets, which are routed through the internet to the server's IP address.

Firewall

Firewalls play a crucial role in network security. Both your computer and the server likely have firewalls that inspect incoming and outgoing traffic.

Client-side Firewall: Ensures that the outgoing request is allowed to leave your network.

Server-side Firewall: The server's firewall checks if the incoming request is permitted. If all is well, the request proceeds to the next stage.

HTTPS/SSL

Given that the URL starts with "https", the communication is encrypted using HTTPS (HyperText Transfer Protocol Secure). This involves SSL/TLS (Secure Sockets Layer/Transport Layer Security) protocols.

SSL Handshake: The browser and server perform an SSL handshake to establish a secure connection.

Certificate Exchange: The server sends its SSL certificate to the browser.

Public Key Encryption: The browser uses the public key in the certificate to encrypt a session key.

Secure Session: This session key is used for symmetric encryption of the subsequent data transfer, ensuring confidentiality and integrity.

Load Balancer

Once the secure connection is established, the request may pass through a load balancer, especially for large-scale websites like Google.

Distribution: The load balancer distributes incoming requests across multiple web servers to ensure no single server is overwhelmed.

Health Checks: It also performs health checks to route traffic away from any servers that are down or performing poorly.

Web Server

The request now reaches the web server, which is responsible for handling HTTP(S) requests. Common web servers include Apache, Nginx, and Microsoft's IIS.

Request Handling: The web server processes the request, checking the URL path to determine what resource is being requested.

Application Server

For dynamic content, the web server forwards the request to an application server. The application server runs the backend application logic, which might involve various programming languages and frameworks such as Node.js, Java, Ruby on Rails, or Django.

Business Logic: The application server executes the necessary business logic to generate a response. This might involve querying a database, processing data, or interacting with other services.

Database

Often, the application server needs to retrieve or store data in a database.

Query Execution: The application server sends a query to the database (e.g., MySQL, PostgreSQL, MongoDB).

Data Retrieval: The database executes the query and sends the results back to the application server.

Response Back to the Browser

After processing the request, the application server generates the appropriate response (e.g., an HTML page) and sends it back to the web server. The web server then sends this response through the load balancer (if present) and back to your browser via the secure TCP connection.

Rendering the Page

Finally, the browser receives the response and begins rendering the page.

HTML Parsing: The browser parses the HTML document.

DOM Construction: A Document Object Model (DOM) is constructed.

CSS and JavaScript: The browser fetches and applies CSS for styling and executes JavaScript for interactivity.

Rendering: The browser renders the page, displaying the content to you.