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