Configuring Firewalls

Purpose

Guide engineers through configuring firewalls across host-based (iptables, nftables, UFW), cloud-based (AWS Security Groups, NACLs), and container-based (Kubernetes NetworkPolicies) environments with practical rule examples and safety patterns to prevent lockouts and security misconfigurations.

When to Use This Skill

Trigger Phrases:

• "Configure firewall for [server/service]"
• "Set up security groups for [AWS resource]"
• "Allow port [X] through firewall"
• "Block IP address [X.X.X.X]"
• "Set up UFW on Ubuntu server"
• "Create iptables/nftables rules"
• "Configure bastion host firewall"
• "Implement egress filtering"

Common Scenarios:

• Initial server setup and hardening
• Exposing a new service (web server, API, database)
• Implementing network segmentation
• Creating bastion host or jump box
• Migrating from iptables to nftables
• Configuring cloud security groups
• Troubleshooting connectivity issues

Decision Framework: Which Firewall Tool?

Cloud Environments

AWS:

• Instance-level control → Security Groups (stateful, allow-only rules)
• Subnet-level enforcement → Network ACLs (stateless, allow + deny rules)
• Use both for defense-in-depth

GCP:

• Use VPC Firewall Rules (stateful, priority-based)

Azure:

• Use Network Security Groups (NSGs) (stateful, priority-based)

Host-Based Linux Firewalls

Ubuntu/Debian + Simplicity:

• Use UFW (Uncomplicated Firewall) - recommended for most users
• Front-end for iptables/nftables with simplified syntax

RHEL/CentOS/Fedora:

• Use firewalld (default on Red Hat ecosystem)
• Zone-based configuration with dynamic updates

Modern Distro + Advanced Control:

• Use nftables (best performance, modern standard)
• O(log n) performance vs iptables O(n)
• Unified IPv4/IPv6/NAT syntax

Legacy Systems:

• Use iptables (migrate to nftables when feasible)
• Required for older kernels (< 4.14)

Kubernetes/Containers

• Use NetworkPolicies (requires CNI plugin: Calico, Cilium, Weave)
• See references/k8s-networkpolicies.md

Stateful vs Stateless

Stateful (recommended for most cases):

• Automatically allows return traffic
• Simpler configuration
• Examples: Security Groups, UFW, nftables default

Stateless (specialized use):

• Must explicitly allow both directions
• Fine-grained control, less state tracking
• Examples: Network ACLs, custom nftables rules

Quick Start Examples

UFW (Ubuntu/Debian)

# 1. Set defaults
sudo ufw default deny incoming
sudo ufw default allow outgoing

# 2. CRITICAL: Allow SSH before enabling (prevent lockout)
sudo ufw allow ssh
sudo ufw limit ssh  # Rate-limit to prevent brute force

# 3. Allow web traffic
sudo ufw allow http    # Port 80
sudo ufw allow https   # Port 443

# 4. Allow from specific IP (e.g., database access)
sudo ufw allow from 192.168.1.100 to any port 5432

# 5. Enable firewall
sudo ufw enable

# 6. Verify rules
sudo ufw status verbose

For complete UFW patterns, see references/ufw-patterns.md

nftables (Modern Linux)

#!/usr/sbin/nft -f
# /etc/nftables.conf

flush ruleset

table inet filter {
    chain input {
        type filter hook input priority 0; policy drop;

        # Accept loopback
        iif "lo" accept

        # Accept established connections (stateful)
        ct state established,related accept

        # Drop invalid packets
        ct state invalid drop

        # Allow SSH
        tcp dport 22 accept

        # Allow HTTP/HTTPS
        tcp dport { 80, 443 } accept

        # Log dropped packets
        log prefix "nftables-drop: " drop
    }

    chain forward {
        type filter hook forward priority 0; policy drop;
    }

    chain output {
        type filter hook output priority 0; policy accept;
    }
}

Apply: sudo nft -f /etc/nftables.conf Enable on boot: sudo systemctl enable nftables

For advanced patterns (sets, maps), see references/nftables-patterns.md

AWS Security Groups (Terraform)

# Web server security group
resource "aws_security_group" "web" {
  name        = "web-server-sg"
  description = "Security group for web servers"
  vpc_id      = aws_vpc.main.id

  # Allow HTTP/HTTPS from anywhere
  ingress {
    description = "HTTPS from anywhere"
    from_port   = 443
    to_port     = 443
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }

  # Allow SSH from bastion only
  ingress {
    description     = "SSH from bastion"
    from_port       = 22
    to_port         = 22
    protocol        = "tcp"
    security_groups = [aws_security_group.bastion.id]
  }

  # Allow all outbound
  egress {
    from_port   = 0
    to_port     = 0
    protocol    = "-1"
    cidr_blocks = ["0.0.0.0/0"]
  }

  tags = {
    Name = "web-server-sg"
  }
}

For Security Groups vs NACLs guide, see references/aws-security-groups.md

Safety Checklist

Before enabling any firewall:

• Always allow SSH before enabling (prevent lockout)
• Test rules before enabling (dry-run when possible)
• Enable logging for debugging
• Document rules in version control (Git)
• Verify externally with nmap: nmap -Pn <server-ip>
• Have console access (cloud) or physical access (on-prem)
• Start with default deny, explicitly allow required traffic
• Use rate limiting for SSH (ufw limit ssh)

Common Patterns

Pattern 1: Basic Web Server

Requirements:

• Allow HTTP (80) and HTTPS (443) from anywhere
• Allow SSH from specific IP or bastion only
• Default deny all other inbound traffic

UFW:

sudo ufw default deny incoming
sudo ufw allow from 203.0.113.0/24 to any port 22  # Office IP
sudo ufw allow http
sudo ufw allow https
sudo ufw enable

nftables: See references/nftables-patterns.md for complete example

AWS Security Group: See references/aws-security-groups.md for Terraform module

Pattern 2: Database Server (Private)

Requirements:

• Allow database port (5432, 3306, etc.) from app tier only
• No public internet access
• SSH from bastion only

See references/database-patterns.md for implementation

Pattern 3: Bastion Host (Jump Box)

Purpose: Single hardened entry point for SSH access

See references/bastion-pattern.md for complete implementation

Pattern 4: Egress Filtering

Purpose: Control outbound traffic to prevent data exfiltration

See references/egress-filtering.md for implementation

Key Concepts

Stateful Firewalls

Track connection state (established, related, new):

• Automatically allow return traffic
• Simpler rule configuration
• Used by: Security Groups, UFW, nftables (default)

Stateless Firewalls

No connection tracking:

• Must explicitly allow both directions
• Must allow ephemeral ports (1024-65535) for return traffic
• Used by: Network ACLs

Defense-in-Depth

Layer multiple firewall controls:

• Cloud: Security Groups + NACLs
• Host: UFW/nftables + fail2ban
• Container: NetworkPolicies

Rule Evaluation

Security Groups (AWS): All rules evaluated, most permissive wins Network ACLs (AWS): Sequential evaluation, first match wins nftables/iptables: Sequential, first match wins UFW: Sequential by rule number

Universal Best Practices

1. Default Deny: Start with deny-all, explicitly allow required traffic
2. Principle of Least Privilege: Only open necessary ports/IPs
3. No 0.0.0.0/0 on Sensitive Ports: Never allow SSH/RDP/database from anywhere
4. Version Control: Store firewall rules in Git
5. Logging: Enable and monitor firewall logs
6. Regular Audits: Review rules quarterly, remove unused
7. Don't Mix Tools: Avoid running iptables and nftables simultaneously
8. Test Before Production: Use staging environment first

Advanced Topics

Bastion Host Architecture: See references/bastion-pattern.md for single entry point patterns

DMZ (Demilitarized Zone): See references/dmz-pattern.md