network

Network Diagnostics

Before running commands, detect the OS and load the matching reference for platform-specific syntax:

• Linux — references/linux.md (ss, ip, iptables/nftables, iproute2)
• macOS — references/macos.md (lsof, ifconfig, pfctl, networkQuality)
• Windows — references/windows.md (PowerShell: Test-NetConnection, Get-NetTCPConnection)

OS detection:

uname -s 2>/dev/null || echo Windows

Quick Reference

Task	Command
Check host reachability	ping -c 4 host
Trace route to host	traceroute host
DNS lookup	dig +short host
Reverse DNS	dig -x IP
Check specific port	nc -zv host port
HTTP status	curl -sI -o /dev/null -w "%{http_code}" URL
SSL certificate	openssl s_client -connect host:443 </dev/null

Diagnostic Workflow

Is a service accessible?

1. DNS resolves — dig +short host
2. Host reachable — ping -c 2 host
3. Port open — nc -zv -w 3 host 443
4. HTTP responds — curl -sI -o /dev/null -w "%{http_code}" https://host
5. SSL valid — openssl s_client -connect host:443 </dev/null 2>/dev/null | openssl x509 -noout -checkend 0

Diagnose slow connections

1. DNS time — dig +stats host | grep "Query time"
2. Latency — ping -c 10 host
3. Route bottleneck — mtr -r -c 10 host (or traceroute host)
4. HTTP timing — use curl timing breakdown (see below)

HTTP Debugging (curl, cross-platform)

# Response headers
curl -sI https://host

# Status code only
curl -s -o /dev/null -w "%{http_code}" https://host

# Full verbose debug
curl -v https://host 2>&1

# Follow redirects
curl -sIL https://host

# Timing breakdown
curl -s -o /dev/null -w "\
  DNS:     %{time_namelookup}s\n\
  Connect: %{time_connect}s\n\
  TLS:     %{time_appconnect}s\n\
  TTFB:    %{time_starttransfer}s\n\
  Total:   %{time_total}s\n\
  Status:  %{http_code}\n\
  Size:    %{size_download} bytes\n" https://host

# Resolve to specific IP (bypass DNS)
curl --resolve host:443:93.184.216.34 https://host

# Test specific Host header
curl -s -H "Host: host" http://10.0.0.1/