Network Diagnostics

Network Diagnostics

Diagnose and troubleshoot network issues including connectivity, DNS, ports, and routing.

Quick Start

Connectivity Check

# 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

# 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

# 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

# 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

# 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

# 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)

# 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

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

# 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"

1. Ping the host: ping -c 3 <host> — Is the machine up?
2. Check the port: nc -zv <host> <port> — Is the service listening?
3. Check locally: SSH in, run ss -tlnp | grep <port> — Is it bound?
4. Check firewall: Is the port allowed through?
5. Check the service: systemctl status <service> — Is it running?
6. Check logs: journalctl -u <service> -n 20 — Any errors?

"Internet is Slow"

1. Check DNS: time dig google.com — Slow DNS resolution?
2. Check latency: ping -c 10 1.1.1.1 — High latency?
3. Check bandwidth: curl -o /dev/null -w "%{speed_download}" https://speed.cloudflare.com/__down?bytes=10000000
4. Check for packet loss: mtr --report -c 20 1.1.1.1
5. Check local network: iperf3 -c <local-host> — LAN speed OK?

"Pi-hole Stopped Blocking Ads"

1. Check status: curl -s "http://pi.hole/admin/api.php?status" | jq '.status'
2. Check if disabled: Someone might have paused it
3. Check blocklists: curl -s "http://pi.hole/admin/api.php?summary" | jq '.gravity_last_updated'
4. Update gravity: pihole -g (requires SSH to Pi-hole host)
5. Check client DNS: Is the client actually using Pi-hole? cat /etc/resolv.conf

Useful One-Liners

# 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