Network Engineering

Debug connectivity and configure network infrastructure.

When to use

• Connectivity issues
• Load balancer setup
• SSL/TLS problems
• DNS debugging
• Network performance

Diagnostic commands

Connectivity testing

# Basic connectivity
ping -c 4 host.example.com
traceroute host.example.com

# Port check
nc -zv host.example.com 443
telnet host.example.com 80

# DNS lookup
dig +short example.com
dig +trace example.com
nslookup -type=MX example.com

# HTTP testing
curl -v https://api.example.com/health
curl -w "@curl-format.txt" -o /dev/null -s https://example.com

curl-format.txt

     time_namelookup:  %{time_namelookup}s\n
        time_connect:  %{time_connect}s\n
     time_appconnect:  %{time_appconnect}s\n
    time_pretransfer:  %{time_pretransfer}s\n
       time_redirect:  %{time_redirect}s\n
  time_starttransfer:  %{time_starttransfer}s\n
                     ----------\n
          time_total:  %{time_total}s\n

SSL/TLS debugging

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

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

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

# Test specific TLS version
curl --tlsv1.2 --tls-max 1.2 https://example.com

Load balancer config

Nginx

upstream backend {
    least_conn;
    server backend1.example.com:8080 weight=5;
    server backend2.example.com:8080 weight=3;
    server backend3.example.com:8080 backup;

    keepalive 32;
}

server {
    listen 443 ssl http2;
    server_name api.example.com;

    ssl_certificate /etc/ssl/certs/api.crt;
    ssl_certificate_key /etc/ssl/private/api.key;

    location / {
        proxy_pass http://backend;
        proxy_http_version 1.1;
        proxy_set_header Connection "";
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_connect_timeout 5s;
        proxy_read_timeout 60s;
    }

    location /health {
        access_log off;
        return 200 "OK";
    }
}

Traffic analysis

# Capture packets
tcpdump -i eth0 -w capture.pcap port 443

# Read capture
tcpdump -r capture.pcap -n

# Filter by host
tcpdump -i any host 10.0.0.1 and port 80

# Show HTTP requests
tcpdump -i any -A -s 0 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'

Common issues

Symptom	Check	Fix
Connection refused	Port open? Service running?	Start service, open firewall
Connection timeout	Firewall? Route?	Check security groups, routing
SSL error	Cert valid? Chain complete?	Renew cert, fix chain
DNS failure	Resolver? Record exists?	Check DNS config, add record
Slow response	Latency? Bandwidth?	Optimize route, increase capacity

Examples

Input: "API calls timing out" Action: Test connectivity, check DNS, verify SSL, analyze latency

Input: "Set up load balancer" Action: Configure nginx/HAProxy, add health checks, test failover