Server Roles, OS Types, User Management & Security Fundamentals

Core Concepts

What is a Server?

A server is a computer or system that provides resources, data, or services to other computers (clients) over a network. Unlike personal computers, servers are designed for 24/7 uptime, high performance, and scalability.

Key Characteristics of Servers:

• Dedicated Hardware: High-end CPUs, RAM, storage, and redundancy (RAID, backup power).
• Specialized OS: Server-grade operating systems (Windows Server, Linux distributions like Ubuntu Server, CentOS).
• Network-Centric: Optimized for network services (TCP/IP, DNS, HTTP, FTP).
• Automation: Scripts, scheduled tasks, and remote management tools.

![server rack in data center](/media/567d4351153a5a182ff3.jpg "A typical enterprise server rack with labeled components (power supply, network interfaces, cooling fans). (Image: Derrick Coetzee from Berkeley, CA, USA, CC0, via Wikimedia Commons)")

Server Roles and Their Functions

Servers are classified based on the services they provide. Below is a comparison table of common server roles (focus on these for exams):

Worked Example: File Server in a University Setting

Scenario: Tribhuvan University’s Computer Science department needs a file server to share course materials, student submissions, and software installations across 50+ computers in the lab.

1. Hardware Requirements:

   • RAID 10 for redundancy (mirroring + striping).
   • 10Gbps NIC for fast file transfers.
   • 16+ TB HDD to store course files, backups, and student projects.

2. Software Configuration:

   • OS: Ubuntu Server 22.04 LTS (open-source, cost-effective).
   • File Sharing Protocol: Samba (for Windows clients) + NFS (for Linux clients).
   • Permissions:
     • drwxr-xr-x for course folders (readable by all students).
     • drwx------ for faculty-only folders (restricted access).

3. Security:

   • Firewall: Allow only ports 139, 445 (SMB), and 2049 (NFS).
   • User Quotas: Limit student storage to 5GB to prevent abuse.
   • Backup: Automated daily backups to an offsite server.

Operating Systems for Servers

Comparison: Windows Server vs. Linux

Why Linux Dominates Servers (Real-World Example)

Example: Google’s Data Centers

• Google uses Linux (custom kernel) for nearly all its servers because:
  • Cost: No licensing fees for millions of servers.
  • Performance: Lightweight, optimized for scalability.
  • Control: Google modifies the kernel for its needs (e.g., Borg, its container system).
• WhatsApp also runs on Erlang/OTP (Linux-based) to handle billions of messages daily.

User and Group Management

Users, Groups, and Permissions

Servers manage access via users, groups, and permissions. This is critical for security and resource allocation.

Key Commands (Linux/Windows):

Worked Example: Managing Users in a Bank’s ATM Network

Scenario: Nepal Bank Limited (NBL) has a file server storing transaction logs. Only auditors and branch managers should access sensitive files.

1. Create Groups:

   • sudo groupadd auditors
   • sudo groupadd branch_managers

2. Add Users to Groups:

   • sudo usermod -aG auditors alice
   • sudo usermod -aG branch_managers bob

3. Set File Permissions:

   • chmod 750 /var/log/transactions (owner: read/write/execute; group: read/execute; others: no access).
   • chown root:auditors /var/log/transactions (owner = root, group = auditors).

4. Verify:

   • Alice (auditor) can read files: ls -l /var/log/transactions → -rwxr-x---.
   • A regular employee (not in the group) gets "Permission denied."

Security Fundamentals

Firewalls and Ports

Firewalls control inbound/outbound traffic based on ports and rules. Common ports:

• 21: FTP (file transfer).
• 22: SSH (secure remote access).
• 80: HTTP (web traffic).
• 443: HTTPS (secure web).
• 3389: RDP (Windows remote desktop).

Example Firewall Rule (Linux iptables):

# Allow SSH (port 22) from any IP, but block all other incoming traffic.
iptables -A INPUT -p tcp --dport 22 -j ACCEPT
iptables -P INPUT DROP

Worked Example: Securing a Web Server for an E-Commerce Site (Daraz-like)

Scenario: A Nepalese e-commerce site (e.g., Hamrobazaar) hosts its web server on a VPS.

1. Install and Configure Firewall (UFW on Ubuntu):

   sudo ufw allow 80/tcp    # HTTP
   sudo ufw allow 443/tcp   # HTTPS
   sudo ufw allow 22/tcp    # SSH (restrict to your IP)
   sudo ufw enable
   

2. Block Brute Force Attacks:

   • Install fail2ban to ban IPs after 3 failed SSH login attempts.

   sudo apt install fail2ban
   sudo systemctl enable fail2ban
   

3. Disable Unused Services:

   • Turn off FTP (port 21) if not needed: sudo systemctl stop vsftpd.

4. Use HTTPS:

   • Install Let’s Encrypt for free SSL certificates:

   sudo apt install certbot
   sudo certbot --nginx -d hamrobazaar.com
   

Exam Tip

What to Expect in TU/PU Exams:

1. Definitions: Be ready to define terms like server, RAID, firewall, user permissions.
2. Comparisons: Expect questions comparing Windows Server vs. Linux, SMB vs. NFS, or FTP vs. SFTP.
3. Configuration Scenarios:
   • You may be asked to write commands (e.g., create a user, set permissions).
   • Trace a process: e.g., "Explain how a file server handles a request from a client."
4. Real-World Applications:
   • Questions may link concepts to banks (ATM networks), universities (file sharing), or e-commerce (web servers).
   • Example: "How would you secure a mail server for a company like Ncell?"
5. Diagrams:
   • Draw layered models (e.g., OSI vs. TCP/IP) or network topologies (star, mesh).
   • Label packet formats (e.g., IP header fields) or protocol handshakes (e.g., TCP 3-way handshake).

Common Pitfalls:

• Mixing up ports: Remember 22 is SSH, not 21 (FTP).
• Permission errors: Know 755 vs. 700 (world-readable vs. private).
• OS-specific commands: Don’t confuse useradd (Linux) with New-LocalUser (Windows).

In the Real World

1. eSewa (Nepal):

   • Role: Web Server + Database Server.
   • How it uses this unit:
     • Runs on Linux servers (cost-effective, scalable).
     • Uses Apache/Nginx for web traffic and MySQL for transaction data.
     • Implements firewall rules to block DDoS attacks during peak hours (e.g., during festival seasons like Dashain).
     • Manages user roles (e.g., admin, customer, merchant) with strict permissions.

2. Khalti (Digital Payments):

   • Role: Mail Server + Authentication Server.
   • How it uses this unit:
     • Uses Postfix (Linux mail server) to send OTP emails/SMS for transactions.
     • Implements LDAP for secure user authentication (e.g., when you log in via email).
     • Firewall rules restrict access to only HTTPS (443) and SMTP (25) ports.
     • RAID 1 for critical databases to prevent data loss during power outages.

3. NTC (Nepal Telecommunications):

   • Role: DNS Server + Proxy Server.
   • How it uses this unit:
     • Runs BIND (Linux DNS server) to resolve domain names (e.g., ntc.net.np to IP).
     • Uses Squid proxy to cache web requests and reduce bandwidth costs.
     • User management: Technicians have different access levels (e.g., read-only for junior staff, full-admin for network admins).
     • Security: Firewall blocks all ports except 53 (DNS) and 80/443 (web).

Visual: Server OS Architecture (Layered Model)

Visual: TCP 3-Way Handshake (Protocol Exchange)

sequenceDiagram
    participant Client
    participant Server
    Client->>Server: SYN (Port 80)
    Server->>Client: SYN-ACK
    Client->>Server: ACK
    Note right of Client: Client sends SYN
    Note right of Server: Server acknowledges
    Note right of Client: Connection established
    Note over Client,Server: TCP 3-Way Handshake

Visual: File Server Permission Flow