Network Engineering

Comprehensive network engineering skill covering network design, troubleshooting, load balancing, DNS, and network security.

When to Use This Skill

• Designing network topologies
• Troubleshooting connectivity issues
• Configuring load balancers
• DNS configuration and troubleshooting
• SSL/TLS setup and debugging
• Network security implementation
• Performance optimization
• CDN configuration

Network Architecture

OSI Model Reference

Layer	Name	Protocols	Troubleshooting
7	Application	HTTP, DNS, SMTP	curl, browser tools
6	Presentation	SSL/TLS	openssl
5	Session	NetBIOS	-
4	Transport	TCP, UDP	netstat, ss
3	Network	IP, ICMP	ping, traceroute
2	Data Link	Ethernet	arp
1	Physical	-	cable tester

VPC/Network Design

Subnet Strategy:

VPC CIDR: 10.0.0.0/16 (65,536 IPs)

Public Subnets (internet-facing):
  - 10.0.1.0/24 (AZ-a) - Load balancers, bastion
  - 10.0.2.0/24 (AZ-b)
  - 10.0.3.0/24 (AZ-c)

Private Subnets (application tier):
  - 10.0.11.0/24 (AZ-a) - App servers
  - 10.0.12.0/24 (AZ-b)
  - 10.0.13.0/24 (AZ-c)

Database Subnets (isolated):
  - 10.0.21.0/24 (AZ-a) - Databases only
  - 10.0.22.0/24 (AZ-b)
  - 10.0.23.0/24 (AZ-c)

Traffic Flow:

• Internet → Load Balancer (public) → App (private) → DB (isolated)
• NAT Gateway for private subnet outbound
• VPC Endpoints for AWS services

Load Balancing

Load Balancer Types

Type	Layer	Use Case
Application (ALB)	7	HTTP/HTTPS, path routing
Network (NLB)	4	TCP/UDP, static IP, high performance
Classic	4/7	Legacy
Gateway	3	Third-party appliances

Health Checks

# ALB Health Check
health_check:
  path: /health
  protocol: HTTP
  port: 8080
  interval: 30
  timeout: 5
  healthy_threshold: 2
  unhealthy_threshold: 3
  matcher: "200-299"

Routing Strategies

• Round Robin: Equal distribution
• Least Connections: Route to least busy
• IP Hash: Sticky sessions by client IP
• Weighted: Percentage-based distribution
• Path-based: Route by URL path
• Host-based: Route by hostname

DNS

Record Types

Type	Purpose	Example
A	IPv4 address	example.com → 192.0.2.1
AAAA	IPv6 address	example.com → 2001:db8::1
CNAME	Alias	www → example.com
MX	Mail server	example.com → mail.example.com
TXT	Arbitrary text	SPF, DKIM, verification
NS	Name server	DNS delegation
SRV	Service location	_sip._tcp.example.com
CAA	Certificate authority	Restrict CA issuance

DNS Debugging

# Query specific record type
dig example.com A
dig example.com MX
dig example.com TXT

# Query specific DNS server
dig @8.8.8.8 example.com

# Trace DNS resolution
dig +trace example.com

# Check propagation
dig +short example.com @{dns-server}

TTL Strategy

Record Type	Recommended TTL
Static content	86400 (1 day)
Dynamic content	300 (5 min)
Failover records	60 (1 min)
Pre-migration	Lower to 60

SSL/TLS

Certificate Types

Type	Validation	Use Case
DV	Domain ownership	Basic sites
OV	Organization verified	Business sites
EV	Extended validation	High-trust sites
Wildcard	*.domain.com	Multiple subdomains
SAN	Multi-domain	Multiple specific domains

TLS Configuration

Recommended Settings:

• TLS 1.2 and 1.3 only
• Strong cipher suites (AEAD)
• HSTS enabled
• OCSP stapling
• Certificate transparency

Debugging SSL

# Check certificate
openssl s_client -connect example.com:443 -servername example.com

# Check certificate chain
openssl s_client -connect example.com:443 -showcerts

# Check expiration
echo | openssl s_client -connect example.com:443 2>/dev/null | openssl x509 -noout -dates

# Test TLS versions
openssl s_client -connect example.com:443 -tls1_2
openssl s_client -connect example.com:443 -tls1_3

Troubleshooting

Connectivity Checklist

1. Physical/Cloud layer: Is the instance running?
2. Security groups: Are ports open?
3. NACLs: Are subnets allowing traffic?
4. Route tables: Is routing correct?
5. DNS: Does name resolve?
6. Application: Is service listening?

Common Commands

# Check if port is listening
netstat -tlnp | grep :80
ss -tlnp | grep :80

# Test TCP connectivity
nc -zv hostname 443
telnet hostname 443

# Check routes
ip route
traceroute hostname
mtr hostname

# DNS resolution
nslookup hostname
dig hostname
host hostname

# Network interfaces
ip addr
ifconfig

# Active connections
netstat -an
ss -tuln

Performance Debugging

# Bandwidth test
iperf3 -c server-ip

# Latency analysis
ping -c 100 hostname | tail -1

# MTU issues
ping -M do -s 1472 hostname

# Packet capture
tcpdump -i eth0 port 443

Reference Files

• references/troubleshooting.md - Detailed troubleshooting workflows

Integration with Other Skills

• cloud-infrastructure - For cloud networking
• security-engineering - For network security
• performance - For network optimization