docs(networking): IMDEEPMIND-29: updated notes on http, dns, smtp, and ssh

docs/networking/protocols/layer-7/smtp.md

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 ---
 
 # SMTP
+
+:::tip[Status]
+
+This note is complete, reviewed, and considered stable.
+
+:::
+
+**SMTP (Simple Mail Transfer Protocol)** is an **application-layer protocol** used for **sending and relaying emails across networks**.
+
+* Works on top of **TCP**
+* Default port: **25** (server-to-server), **587** (submission)
+* It is a **push protocol** (used to send emails, not retrieve)
+
+Important distinction:
+
+* **SMTP → Sending emails**
+* **IMAP / POP3 → Retrieving emails**
+
+SMTP defines:
+
+* Message transfer rules
+* Command/response format
+* Mail routing between servers
+
+## High-Level Architecture
+
+SMTP operates using a **client-server model**
+
+### Key Components
+
+| Component                     | Description                   |
+| ----------------------------- | ----------------------------- |
+| **MUA (Mail User Agent)**     | Email client (Gmail, Outlook) |
+| **MTA (Mail Transfer Agent)** | SMTP server handling transfer |
+| **MDA (Mail Delivery Agent)** | Final storage (mailbox)       |
+| **DNS (MX records)**          | Finds recipient mail server   |
+
+### Architecture Flow
+
+<div style={{textAlign: 'center'}}>
+
+```mermaid
+flowchart LR
+    A[User - MUA] --> B[SMTP Client]
+    B --> C["Sender SMTP Server (MTA)"]
+    C --> D["DNS Lookup (MX Record)"]
+    D --> E["Recipient SMTP Server (MTA)"]
+    E --> F["Mailbox (MDA)"]
+    F --> G["Recipient MUA via IMAP/POP3"]
+```
+
+</div>
+
+## How SMTP Works (Step-by-Step)
+
+SMTP follows a **store-and-forward model**
+
+### Step-by-step flow
+
+1. User composes email in client (MUA)
+2. Client connects to SMTP server via TCP
+3. Email is submitted to sender’s SMTP server
+4. Server queries DNS for recipient’s mail server
+5. Email is relayed through one or more MTAs
+6. Final server stores email in mailbox
+7. Recipient retrieves via IMAP/POP3
+
+### End-to-End Flow
+
+<div style={{textAlign: 'center'}}>
+
+```mermaid
+sequenceDiagram
+    participant User
+    participant SMTP_Client
+    participant Sender_SMTP
+    participant DNS
+    participant Receiver_SMTP
+    participant Mailbox
+
+    User->>SMTP_Client: Compose & Send Email
+    SMTP_Client->>Sender_SMTP: Open TCP Connection
+    Sender_SMTP->>DNS: Query MX Record
+    DNS-->>Sender_SMTP: Return Mail Server IP
+    Sender_SMTP->>Receiver_SMTP: Transfer Email
+    Receiver_SMTP->>Mailbox: Store Email
+```
+
+</div>
+
+## SMTP Session Phases
+
+SMTP communication consists of **three main phases**
+
+### Connection Establishment
+
+* TCP connection opened
+* Client sends `HELO` / `EHLO`
+
+### Mail Transfer
+
+* Sender & recipient defined
+* Message body transmitted
+
+### Connection Termination
+
+* Session ends with `QUIT`
+
+### Lifecycle Diagram
+
+<div style={{textAlign: 'center'}}>
+
+```mermaid
+flowchart TD
+    A[Connect to SMTP Server] --> B[HELO / EHLO]
+    B --> C[MAIL FROM]
+    C --> D[RCPT TO]
+    D --> E[DATA]
+    E --> F[Message Body]
+    F --> G[.]
+    G --> H[QUIT]
+```
+
+</div>
+
+## SMTP Commands & Responses
+
+SMTP is a **text-based protocol** using commands and replies
+
+### Common Commands
+
+| Command         | Description        |
+| --------------- | ------------------ |
+| `HELO` / `EHLO` | Identify client    |
+| `MAIL FROM`     | Sender address     |
+| `RCPT TO`       | Recipient address  |
+| `DATA`          | Start message body |
+| `QUIT`          | End session        |
+
+### Example SMTP Conversation
+
+```text
+Client: HELO example.com
+Server: 250 Hello
+
+Client: MAIL FROM:<alice@example.com>
+Server: 250 OK
+
+Client: RCPT TO:<bob@example.com>
+Server: 250 OK
+
+Client: DATA
+Server: 354 Start mail input
+
+Client: Hello Bob!
+Client: .
+Server: 250 Message accepted
+
+Client: QUIT
+Server: 221 Bye
+```
+
+### Response Codes
+
+| Code | Meaning                 |
+| ---- | ----------------------- |
+| 2xx  | Success                 |
+| 3xx  | Intermediate (continue) |
+| 4xx  | Temporary failure       |
+| 5xx  | Permanent failure       |
+
+## Message Routing & DNS (Critical Part)
+
+SMTP does **not directly know where to send email**.
+
+It relies on **DNS MX (Mail Exchange) records**:
+
+<div style={{textAlign: 'center'}}>
+
+```mermaid
+flowchart LR
+    A[Sender SMTP Server] --> B[DNS Query: MX Record]
+    B --> C[MX Record Returned]
+    C --> D[Recipient SMTP Server]
+```
+
+</div>
+
+* If same domain → local delivery
+* If different domain → DNS lookup required
+
+## SMTP Modes of Delivery
+
+### End-to-End Delivery
+
+* Direct sender → receiver
+* Used across organizations
+
+### Store-and-Forward
+
+* Email passes through multiple servers
+* Most common on the internet
+
+## SMTP Ports
+
+| Port | Usage                              |
+| ---- | ---------------------------------- |
+| 25   | Server-to-server communication     |
+| 587  | Email submission (modern standard) |
+| 465  | SMTPS (deprecated but used)        |
+
+## Extended SMTP (ESMTP)
+
+Enhancement over SMTP:
+
+* Introduced via `EHLO`
+* Supports:
+
+  * Authentication (AUTH)
+  * TLS encryption (STARTTLS)
+  * Attachments (MIME)
+  * Pipelining
+
+## Security in SMTP
+
+### Problems (Original SMTP)
+
+* No encryption
+* No authentication
+* Vulnerable to spoofing
+
+### Modern Fixes
+
+* **STARTTLS** → Encrypt connection
+* **SMTP AUTH** → Authenticate users
+* **SPF, DKIM, DMARC** → Prevent spoofing
+
+## Limitations of SMTP
+
+* Cannot retrieve emails
+* Originally plaintext (security issues)
+* No built-in spam protection
+* Depends on other protocols (IMAP/POP3)
+
+## SMTP vs IMAP vs POP3
+
+| Feature   | SMTP                 | IMAP            | POP3                |
+| --------- | -------------------- | --------------- | ------------------- |
+| Purpose   | Send email           | Retrieve & sync | Retrieve (download) |
+| Direction | Push                 | Pull            | Pull                |
+| Storage   | Server-side transfer | Server-side     | Local               |

docs/networking/protocols/layer-7/ssh.md

@@ -4,6 +4,12 @@ sidebar_position: 4
 
 # SSH
 
+:::tip[Status]
+
+This note is complete, reviewed, and considered stable.
+
+:::
+
 SSH (Secure Shell) is a cryptographic network protocol used to securely access remote systems over an unsecured network. It ensures **confidentiality**, **integrity**, and **authentication** for data communication between a client and a server. SSH is widely used for remote login, file transfers, and secure command execution.
 
 ## Key Features of SSH