02. Linux File Permissions (chmod, chown, SUID/SGID) & Sticky Bits Security • DevOps, Docker & Cloud VPS Deployment Mastery • Rishabh Ki Kaksha

DevOps, Docker & Cloud VPS Deployment Mastery › 02. Linux File Permissions (chmod, chown, SUID/SGID) & Sticky Bits Security

40 mins

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Course Syllabus

Module 1: Linux System Administration & Kernel Performance Tuning

01. Linux OS Architecture: Kernel vs User Space, System Calls & Process Management (ps, top, htop) 02. Linux File Permissions (chmod, chown, SUID/SGID) & Sticky Bits Security 03. Networking & Diagnostics: netstat, ss, lsof, tcpdump, curl, dig & DNS Resolution

Part 1: Linux & Docker Containerization

Module 2: Containerization with Docker & Multi-Stage Builds

04. Docker Architecture: Namespaces, Cgroups, UnionFS & Docker Engine Internals 05. Dockerfile Optimization: Layer Caching, Non-Root Users & Ultra-Light Alpine Multi-Stage Builds 06. Docker Compose: Multi-Container Networking, Persistent Volumes & Environment Config 07. Docker Production Security: Read-Only Root FS, Capabilities Drop & Container Scanning

Part 2: Nginx, SSL, PM2 & Automated CI/CD

Module 3: Nginx Reverse Proxy, SSL Automation & Security Hardening

08. Nginx Masterclass: Event-Driven Non-Blocking Architecture vs Apache Multi-Threaded 09. Nginx as Reverse Proxy, Load Balancer (upstream) & Gzip/Brotli Compression Config 10. SSL/TLS Automation with Certbot Let's Encrypt, HTTP/2 & Strict-Transport-Security (HSTS) 11. Nginx Rate Limiting (limit_req), DDoS Mitigation & Security Headers Injection

Module 4: Process Management with PM2 & Systemd Services

12. Systemd Services: Creating Custom System Daemons with Automatic Restart & Logging 13. PM2 Enterprise Process Manager: Cluster Mode, Ecosystem Config & Graceful Reloads 14. Log Management & Rotation: logrotate, journalctl & Centralized Syslog Streaming

Module 5: CI/CD Automation with GitHub Actions & Zero-Downtime Deployments

15. CI/CD Foundations: Building Automated Test & Build Pipelines with GitHub Actions 16. Automated SSH Deployments: Self-Hosted Runners, Deploy Keys & Zero-Downtime Atomic Symlinks 17. Blue-Green & Canary Deployment Strategies on Cloud VPS Infrastructure

Module 6: Cloud VPS Production Hardening, Firewall (UFW) & Backups

18. Cloud VPS Hardening: Non-Root Sudo Users, SSH Key-Only Auth, Fail2ban & UFW Firewall Rules 19. Automated Database & Media Backups to Cloud Storage (AWS S3 / Cloudflare R2 / MinIO) 20. Server Health Monitoring & Alerting: Uptime Kuma, Netdata, Prometheus & Telegram Bot Webhooks

Interactive Lesson Studio

# 02. Linux File Permissions (chmod, chown, SUID/SGID) & Sticky Bits Security

> DevOps & Cloud VPS Notes by Rishabh - Topic 02

---

### 1. The 9-Bit Permission Octal Model 🔒

```
- r w x  r - x  r - -
  └──┬──┘ └──┬──┘ └──┬──┘
   Owner   Group   Others
  (User)  (Group) (World)
```

*   `r` (Read) = **4**
*   `w` (Write) = **2**
*   `x` (Execute) = **1**
*   *Total Standard Webroot Permissions:*
    *   Directories: `chmod 755` (`rwxr-xr-x` = $4+2+1, 4+1, 4+1$).
    *   Files: `chmod 644` (`rw-r--r--` = $4+2, 4, 4$).
    *   SSH Private Keys: `chmod 600` (`rw-------` = $4+2, 0, 0$).

---

### 2. Special Permissions: SUID, SGID & Sticky Bit 🌟

1. **SUID (Set UID - 4000):** File execute karne par user temporary file owner ke permissions se run karta hai (e.g. `/usr/bin/passwd` runs as root).
2. **SGID (Set GID - 2000):** Directory me banayi gayi nayi files automatically parent directory ka group inherit karti hain.
3. **Sticky Bit (1000 - `chmod +t /tmp`):** Directory me koi bhi user file bana sakta hai, lekin **sirf file ka original creator hi use delete ya rename kar sakta hai!**

---

### 3. Web Server Ownership Hardening (`chown`)

```bash
# Nginx/Apache standard ownership on Ubuntu:
chown -R www-data:www-data /var/www/rishabh-kaksha
find /var/www/rishabh-kaksha -type d -exec chmod 755 {} \;
find /var/www/rishabh-kaksha -type f -exec chmod 644 {} \;
```

---

### 4. Master Comparison Matrix

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### 5. Interview Ke 3 Sawal Is Topic Se

1. **`umask` kya hota hai?** -> User Mask; nayi file ya directory create hone par default permissions me se kaunse bits mask (disable) karne hain (e.g. `umask 022` creates files with `644` and folders with `755`).
2. **Production web server par `chmod 777` lagana cyber suicide kyu hai?** -> Kyunki world-writable ho jata hai; hacker PHP web shell inject karke kisi bhi file ko overwrite ya execute kar sakta hai.
3. **Linux me file delete karne ke liye file par write permission chahiye ya directory par?** -> Directory par! Kyunki file ka name directory tree table me entry hota hai.

---
**Next Topic:** 03. Networking & Diagnostics: netstat, ss, lsof, tcpdump, curl, dig & DNS Resolution

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Rishabh Gehlot

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