Network Diagnostics
Diagnose and troubleshoot network issues including connectivity, DNS, ports, and routing.
Quick Start
Connectivity Check
bash# Check internet connectivity ping -c 3 1.1.1.1 # Check DNS resolution dig google.com +short # Check a local service curl -s -o /dev/null -w "%{http_code}" http://pi.hole/admin/
DNS Diagnostics
Basic DNS Resolution
bash# Resolve via system DNS (Pi-hole) dig example.com +short # Resolve via specific DNS server dig @1.1.1.1 example.com +short # Cloudflare dig @8.8.8.8 example.com +short # Google dig @pi.hole example.com +short # Pi-hole # Reverse DNS lookup dig -x <ip-address> +short # Full DNS trace dig example.com +trace
Pi-hole DNS Analysis
bash# Check if Pi-hole is blocking correctly # Query a known ad domain — should return 0.0.0.0 or NXDOMAIN dig @pi.hole ads.google.com +short # Check Pi-hole query log (recent queries via API) curl -s "http://pi.hole/admin/api.php?getAllQueries=100" | jq '.data | length' # Check upstream DNS health curl -s "http://pi.hole/admin/api.php?getForwardDestinations" | jq '.' # Check if Pi-hole is enabled curl -s "http://pi.hole/admin/api.php?status" | jq '.status'
DNS Troubleshooting Flow
DNS not resolving? ├── Can you ping 1.1.1.1? │ ├── No → Network/routing issue (not DNS) │ └── Yes → DNS issue confirmed │ ├── Does dig @1.1.1.1 work? │ │ ├── No → Upstream DNS blocked (firewall?) │ │ └── Yes → Local DNS problem │ │ ├── Does dig @pi.hole work? │ │ │ ├── No → Pi-hole issue │ │ │ │ ├── Is pihole-FTL running? │ │ │ │ ├── Is port 53 listening? │ │ │ │ └── Check Pi-hole logs │ │ │ └── Yes → Client DNS config issue │ │ │ └── Check /etc/resolv.conf │ │ └── Is /etc/resolv.conf pointing to Pi-hole?
Connectivity Testing
Layer-by-Layer Diagnosis
bash# Layer 1-2: Physical/Link ip link show # Interface status ethtool <interface> # Link speed, duplex # Layer 3: Network ip addr show # IP addresses ip route show # Routing table ping -c 3 <gateway> # Gateway reachable? ping -c 3 1.1.1.1 # Internet reachable? # Layer 4: Transport ss -tlnp # Listening TCP ports ss -ulnp # Listening UDP ports # Layer 7: Application curl -v http://example.com # HTTP connectivity
Traceroute
bash# Standard traceroute traceroute <host> # TCP traceroute (bypasses ICMP blocks) traceroute -T -p 443 <host> # MTR for continuous monitoring mtr --report <host>
Port Reachability
Check if a Port is Open
bash# From this machine to a target nc -zv <host> <port> # Netcat timeout 3 bash -c "echo > /dev/tcp/<host>/<port>" && echo "open" || echo "closed" # Check multiple ports for port in 22 53 80 443 3000 8080 11434; do nc -zv <host> $port 2>&1 | grep -E "succeeded|refused" done
Port Scan (Local Network)
bash# Quick scan of a host nmap -F <host> # Scan specific ports nmap -p 22,53,80,443,3000,8080,11434 <host> # Scan entire local subnet nmap -sn <local-subnet-cidr> # Ping sweep (who's online?)
Known Service Ports
| Host | Port | Service | Protocol |
|---|---|---|---|
| pi.hole | 53 | DNS | UDP/TCP |
| pi.hole | 80 | Web UI | HTTP |
| 3000 | Gitea | HTTP | |
| NAS SSH | SSH | ||
| 22 | SSH server | SSH | |
| 11434 | Ollama | HTTP |
Network Topology
Local Network Map
Internet │ ▼ [Router/Gateway] │ ├── Pi-hole (DNS) ─── pi.hole:53, :80 │ ├── <gitea-or-nas-host> ─── Gitea :3000, NAS SSH :<ssh-port> │ ├── <ssh-host> ─── SSH server :22 │ ├── <ollama-host> ─── Ollama :11434 │ └── [This machine] ── Workstation
Discover Network Devices
bash# ARP table (devices recently seen) ip neigh show # Ping sweep nmap -sn <local-subnet-cidr> # Check DHCP leases (if accessible) # Often at router admin page
Troubleshooting Workflows
"Service X is Unreachable"
- Ping the host:
ping -c 3 <host>— Is the machine up? - Check the port:
nc -zv <host> <port>— Is the service listening? - Check locally: SSH in, run
ss -tlnp | grep <port>— Is it bound? - Check firewall: Is the port allowed through?
- Check the service:
systemctl status <service>— Is it running? - Check logs:
journalctl -u <service> -n 20— Any errors?
"Internet is Slow"
- Check DNS:
time dig google.com— Slow DNS resolution? - Check latency:
ping -c 10 1.1.1.1— High latency? - Check bandwidth:
curl -o /dev/null -w "%{speed_download}" https://speed.cloudflare.com/__down?bytes=10000000 - Check for packet loss:
mtr --report -c 20 1.1.1.1 - Check local network:
iperf3 -c <local-host>— LAN speed OK?
"Pi-hole Stopped Blocking Ads"
- Check status:
curl -s "http://pi.hole/admin/api.php?status" | jq '.status' - Check if disabled: Someone might have paused it
- Check blocklists:
curl -s "http://pi.hole/admin/api.php?summary" | jq '.gravity_last_updated' - Update gravity:
pihole -g(requires SSH to Pi-hole host) - Check client DNS: Is the client actually using Pi-hole?
cat /etc/resolv.conf
Useful One-Liners
bash# What's my public IP? curl -s ifconfig.me # What's my local IP? ip -4 addr show | grep -oP '(?<=inet\s)\d+(\.\d+){3}' | grep -v 127.0.0.1 # Check if a domain is blocked by Pi-hole dig @pi.hole <domain> +short # 0.0.0.0 = blocked # DNS response time time dig google.com @pi.hole > /dev/null # Check all network interfaces ip -br link show
Kai skill — Network diagnostics and troubleshooting
