當您輸入 &#google.com&# 時會發生什麼？

您是否想過在瀏覽器中輸入「google.com」到看到熟悉的搜尋頁面出現之間的幾分之一秒內發生的一系列複雜事件？在這個詳細的探索中，我們將揭開網路技術、網路協議以及使我們的線上體驗成為可能的錯綜複雜的數據的迷人世界。

1. 旅程開始：您的瀏覽器和作業系統

1.1 瀏覽器的第一步

當您輸入「google.com」並按 Enter 鍵時，您的瀏覽器就會啟動：

1. URL 解析：瀏覽器首先分析您輸入的 URL。它標識協議（在本例中，隱含“http://”或“https://”）、網域名稱（“google.com”）以及任何其他路徑或查詢參數（在這個簡單示例中沒有） .

2. HSTS 檢查：對於 Google 等具有安全意識的網站，瀏覽器會檢查其 HTTP 嚴格傳輸安全 (HSTS) 清單。如果 google.com 在此清單中（確實如此），瀏覽器會自動將要求升級為 HTTPS。

3. 快取檢查：在連接到網路之前，瀏覽器會檢查其本機快取。此快取儲存先前存取的信息，包括：

   • DNS 快取：與 google.com 關聯的 IP 位址
   • 資源快取：HTML、CSS、JavaScript 檔案以及來自 Google 首頁的圖片

如果找到其中任何一個並且仍然有效（未過期），瀏覽器可以跳過以下一些步驟。

1.2 作業系統的作用

如果瀏覽器在快取中找不到必要的信息，則會向作業系統 (OS) 尋求協助：

1. Hosts 檔案檢查：作業系統先找本地「hosts」檔案。該檔案可以將網域名稱對應到 IP 位址，從而可能繞過 DNS 查找。但是，對於大多數用戶來說，google.com 不會在此文件中。

2. DNS 用戶端快取：作業系統維護自己的 DNS 緩存，與瀏覽器的快取分開。接下來檢查這裡。

3. 解析器設定：如果 IP 不在本機快取中，作業系統準備詢問 DNS 伺服器。它讀取其網路配置以找出要查詢的 DNS 伺服器（通常由您的網際網路服務供應商提供或手動設定）。

2. DNS解析：尋找Google的位址

如果 google.com 的 IP 位址未緩存，我們需要要求網域名稱系統 (DNS) 將人類可讀的「google.com」轉換為機器可用的 IP 位址。

2.1 DNS 層次結構

DNS 以層級結構組織：

1. 根伺服器：位於層次結構的頂端。他們知道在哪裡可以找到 .com、.org、.net 等頂級網域 (TLD) 的權威伺服器。

2. TLD 伺服器：這些伺服器了解在其 TLD 下註冊的所有網域。 .com TLD 伺服器了解 google.com。

3. 權威名稱伺服器：這些伺服器負責了解有關特定網域的所有信息，包括其 IP 位址。

2.2 DNS查詢過程

1. 遞歸解析器：您的 ISP 的 DNS 伺服器（或其他已設定的解析器）接收 google.com 的查詢。如果沒有快取答案，它將啟動遞歸過程：

• 它向根伺服器詢問 .com
• 根伺服器將其引用到 .com TLD 伺服器
• 它向 .com TLD 伺服器詢問 google.com
• .com 伺服器將其引用至 Google 的權威名稱伺服器
• 它向 Google 的名稱伺服器詢問 google.com 的 IP
• Google 的名稱伺服器使用 IP 位址進行回應

1. 快取：此過程中的每個步驟都可能涉及緩存，因此並不總是需要完整的旅程。解析器通常會在 Google 指定的時間內快取最終結果（生存時間或 TTL）。

2. 負載平衡：像 Google 這樣的大型服務通常會傳回多個 IP 位址。這可以實現負載平衡並提高可靠性。

2.3 DNS 查找範例

假設 DNS 尋找傳回以下（簡化的）結果：

google.com.     300    IN    A     172.217.167.78

這表示：

• 網域是 google.com
• 該記錄的 TTL 為 300 秒（5 分鐘）
• 這是網路（IN）紀錄
• 這是位址 (A) 記錄類型
• IP位址是172.217.167.78

3. 建立連線：TCP/IP

現在我們有了 Google 的 IP 位址，是時候建立連線了。

3.1 The TCP/IP Stack

1. Application Layer: Your browser operates here, using HTTP(S) to communicate.

2. Transport Layer: TCP is used here to ensure reliable, ordered delivery of data.

3. Internet Layer: IP is used to route packets between networks.

4. Link Layer: This handles the physical transmission of data, whether over Ethernet, Wi-Fi, cellular networks, etc.

3.2 The TCP Handshake

To establish a connection, a three-way handshake occurs:

1. SYN: Your computer sends a SYN (synchronize) packet to Google's server.
2. SYN-ACK: Google's server responds with a SYN-ACK packet.
3. ACK: Your computer sends an ACK (acknowledge) packet back.

This process establishes sequence numbers for the conversation, ensuring packets can be properly ordered and any lost packets can be detected and retransmitted.

3.3 TLS Handshake

For HTTPS connections (which Google uses), an additional TLS (Transport Layer Security) handshake occurs:

1. Client Hello: Your browser sends supported SSL/TLS versions, cipher suites, and a random number.
2. Server Hello: The server chooses the SSL/TLS version and cipher suite, sends its certificate, and another random number.
3. Authentication: Your browser verifies the server's certificate with a trusted Certificate Authority.
4. Key Exchange: A secure symmetric key is established for encrypting the session.

4. HTTP Request: Asking for the Page

With a secure connection established, your browser sends an HTTP GET request for the Google homepage.

4.1 Example HTTP Request

GET / HTTP/2
Host: www.google.com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:89.0) Gecko/20100101 Firefox/89.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate, br
Connection: keep-alive
Upgrade-Insecure-Requests: 1

This request includes:

• The method (GET) and path (/) we're requesting
• The HTTP version (HTTP/2)*
• Various headers providing information about the browser and its capabilities *Note: HTTP/2 refers to HTTPS only, not the http connection. This request is sent over an already-established HTTPS connection, even though the headers don't explicitly mention HTTPS.

5. Server Processing: Google Responds

Google's servers receive this request and process it. This might involve:

1. Load Balancing: Distributing the request among many servers.
2. Application Servers: Running code to generate a response.
3. Database Queries: Fetching personalized data or search suggestions.
4. Caching: Retrieving pre-generated content when possible.

6. HTTP Response: Sending the Page

Google's server sends back an HTTP response, which might look something like this:

HTTP/2 200 OK
Content-Type: text/html; charset=UTF-8
Date: Sat, 21 Sep 2024 12:00:00 GMT
Expires: Sat, 21 Sep 2024 12:00:00 GMT
Cache-Control: private, max-age=0
Server: gws
X-XSS-Protection: 0
X-Frame-Options: SAMEORIGIN
[... other headers ...]



  
    <meta charset="utf-8">
    <title>Google</title>
    [... rest of the HTML ...]
  
  
    [... body content ...]
  

This response includes:

• Status code (200 OK)
• Various headers providing metadata about the response
• The HTML content of the page

7. Rendering: Bringing the Page to Life

Your browser now has the HTML content and begins rendering the page:

1. Parsing HTML: The browser parses the HTML, creating the Document Object Model (DOM).

2. Requesting Additional Resources: As it encounters links to CSS, JavaScript, images, etc., it sends additional HTTP requests for these resources.

3. Parsing CSS: The browser parses CSS and applies styles to the DOM elements, creating the CSS Object Model (CSSOM).

4. Executing JavaScript: The browser executes JavaScript, which can modify the DOM and CSSOM.

5. Rendering: The browser uses the final DOM and CSSOM to render the page on your screen.

Conclusion

What seems like a simple action—typing "google.com" and pressing Enter—actually involves a complex series of steps, from DNS lookups and network protocols to server-side processing and client-side rendering. This intricate dance happens in mere milliseconds, showcasing the incredible engineering that powers our online experiences.

Understanding these processes not only satisfies our curiosity but also helps web developers and IT professionals optimize websites, troubleshoot issues, and build more efficient and secure web applications. The next time you navigate to a website, take a moment to appreciate the technological marvels working behind the scenes to bring the web to your screen!

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Images in this blog are AI generated.

Also read HTTP vs HTTPS what is difference between them

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