Network Security & Segmentation

Network security remains fundamental to any security architecture. This lesson covers segmentation strategies, firewall concepts, and common interview scenarios.

Network Segmentation Strategies

Traditional vs Micro-segmentation

Traditional Segmentation:
┌─────────────────────────────────────────────────────────────┐
│                         Corporate Network                    │
│  ┌──────────────┐  ┌──────────────┐  ┌──────────────┐      │
│  │   VLAN 10    │  │   VLAN 20    │  │   VLAN 30    │      │
│  │   (Users)    │  │   (Servers)  │  │   (Database) │      │
│  └──────────────┘  └──────────────┘  └──────────────┘      │
│         └──────────────┴──────────────┘                     │
│                    Firewall Rules                           │
└─────────────────────────────────────────────────────────────┘

Micro-segmentation:
┌─────────────────────────────────────────────────────────────┐
│                         Environment                          │
│  ┌───────┐  ┌───────┐  ┌───────┐  ┌───────┐  ┌───────┐    │
│  │ App A │  │ App B │  │ App C │  │ DB A  │  │ DB B  │    │
│  └───┬───┘  └───┬───┘  └───┬───┘  └───┬───┘  └───┬───┘    │
│      │          │          │          │          │          │
│  Individual policies per workload with explicit allow rules │
└─────────────────────────────────────────────────────────────┘

Segmentation Models

Model	Description	Use Case
Perimeter	Single firewall at network edge	Legacy environments
VLAN-based	VLANs separate traffic types	Traditional enterprise
Zone-based	Trust zones with inter-zone rules	Multi-tier applications
Micro-segmentation	Per-workload policies	Cloud-native, Zero Trust

Firewall Concepts

Firewall Types

Type	OSI Layer	Capabilities
Packet Filter	3-4	IP, port filtering
Stateful	3-4	Connection tracking
Application (WAF)	7	HTTP/HTTPS inspection
Next-Gen (NGFW)	3-7	DPI, IPS, application awareness

Security Group Rules (AWS Example)

{
  "SecurityGroupRules": [
    {
      "Description": "Allow HTTPS from load balancer",
      "IpProtocol": "tcp",
      "FromPort": 443,
      "ToPort": 443,
      "SourceSecurityGroupId": "sg-loadbalancer"
    },
    {
      "Description": "Allow SSH from bastion only",
      "IpProtocol": "tcp",
      "FromPort": 22,
      "ToPort": 22,
      "SourceSecurityGroupId": "sg-bastion"
    }
  ]
}

Interview Question

Q: "Explain the difference between Security Groups and NACLs in AWS."

Answer:

Aspect	Security Groups	NACLs
Level	Instance/ENI	Subnet
State	Stateful (return traffic allowed)	Stateless (explicit rules both ways)
Rules	Allow only	Allow and Deny
Evaluation	All rules evaluated	Rules evaluated in order
Default	Deny all inbound	Allow all
Use Case	Primary access control	Additional layer, subnet-wide blocks

Network Attack Scenarios

Man-in-the-Middle (MITM)

Normal Flow:
Client ────────────────────────────────▶ Server

MITM Attack:
Client ────────▶ Attacker ────────▶ Server
        Intercept     Forward
        & Modify      (or drop)

Prevention:

• TLS everywhere (HSTS)
• Certificate pinning for mobile apps
• Mutual TLS (mTLS) for service-to-service
• Network segmentation

ARP Spoofing

# Detection: Monitor for duplicate IP-MAC mappings
def detect_arp_spoofing(arp_cache):
    ip_to_macs = {}
    for entry in arp_cache:
        ip = entry['ip']
        mac = entry['mac']
        if ip in ip_to_macs and ip_to_macs[ip] != mac:
            alert(f"Possible ARP spoofing: {ip} has multiple MACs")
        ip_to_macs[ip] = mac

Prevention:

• Dynamic ARP inspection (DAI)
• Static ARP entries for critical systems
• Port security on switches
• VLAN segmentation

DNS Spoofing

Prevention:

• DNSSEC validation
• DNS over HTTPS (DoH) / DNS over TLS (DoT)
• Internal DNS servers
• DNS query logging and monitoring

DDoS Protection

Attack Types

Type	Target	Mitigation
Volumetric	Bandwidth	CDN, scrubbing centers
Protocol	Connection state	SYN cookies, rate limiting
Application	L7 resources	WAF, request validation

Defense in Depth

                    ┌─────────────────────┐
                    │   CDN / Edge        │ ← Absorb volumetric
                    │   (CloudFlare, etc) │
                    └─────────┬───────────┘
                              │
                    ┌─────────┴───────────┐
                    │   Load Balancer     │ ← Rate limiting
                    │   + WAF             │ ← L7 filtering
                    └─────────┬───────────┘
                              │
                    ┌─────────┴───────────┐
                    │   Application       │ ← Request validation
                    │   Servers           │ ← Auto-scaling
                    └─────────────────────┘

Network Monitoring

What to Log

Source	Critical Events
Firewall	Denied connections, rule changes
DNS	Queries to suspicious domains
Proxy	Blocked requests, unusual User-Agents
IDS/IPS	Signature matches, anomalies
Flow Data	Unusual traffic patterns, data exfiltration

Network Security Interview Scenario

Q: "A user reports they can't access an internal application. How do you troubleshoot?"

Structured Response:

1. VERIFY THE PROBLEM
   └── Can other users access it?
   └── What error do they see?

2. CHECK NETWORK PATH
   └── Can user ping the server?
   └── Is DNS resolving correctly?
   └── What does traceroute show?

3. CHECK SECURITY CONTROLS
   └── Security group rules
   └── NACLs on subnet
   └── Host-based firewall
   └── Application-level auth

4. CHECK APPLICATION
   └── Is the service running?
   └── Are there error logs?
   └── Is there a certificate issue?

5. DOCUMENT AND RESOLVE
   └── Root cause analysis
   └── Fix and verify
   └── Update runbooks if needed

Cloud Network Security

VPC Design Best Practices

┌────────────────────────────────────────────────────────────────┐
│                          VPC (10.0.0.0/16)                      │
│                                                                 │
│  ┌─────────────────────────┐  ┌─────────────────────────────┐  │
│  │  Public Subnet (AZ-1)   │  │  Public Subnet (AZ-2)       │  │
│  │  10.0.1.0/24            │  │  10.0.2.0/24                │  │
│  │  ├── ALB                │  │  ├── ALB                    │  │
│  │  └── NAT Gateway        │  │  └── NAT Gateway            │  │
│  └─────────────────────────┘  └─────────────────────────────┘  │
│                                                                 │
│  ┌─────────────────────────┐  ┌─────────────────────────────┐  │
│  │  Private Subnet (AZ-1)  │  │  Private Subnet (AZ-2)      │  │
│  │  10.0.10.0/24           │  │  10.0.20.0/24               │  │
│  │  └── Application tier   │  │  └── Application tier       │  │
│  └─────────────────────────┘  └─────────────────────────────┘  │
│                                                                 │
│  ┌─────────────────────────┐  ┌─────────────────────────────┐  │
│  │  Data Subnet (AZ-1)     │  │  Data Subnet (AZ-2)         │  │
│  │  10.0.100.0/24          │  │  10.0.200.0/24              │  │
│  │  └── RDS (no internet)  │  │  └── RDS (no internet)      │  │
│  └─────────────────────────┘  └─────────────────────────────┘  │
└────────────────────────────────────────────────────────────────┘

Interview Tip: When discussing network security, always mention defense in depth - multiple layers of protection ensure that if one fails, others still provide protection.

In the next module, we'll cover detection, response, and GRC. :::