Connectivity Tests
# Basic ping test
ping -c 4 example.com
# Traceroute (show network path)
traceroute example.com
# Or using mtr (combines ping + traceroute)
mtr example.com
DNS Troubleshooting
# DNS lookup
dig example.com
dig example.com MX
# Trace DNS resolution path
dig +trace example.com
# Check specific DNS server
dig @8.8.8.8 example.com
# Reverse lookup
dig -x 198.51.100.10
Port and Connection Testing
# Check if a port is open
nc -zv example.com 443
# Or using nmap
nmap -p 80,443 example.com
# List all listening ports
ss -tlnp
# Active connections
ss -tnp
Network Traffic Analysis
# Capture packets on interface
sudo tcpdump -i eth0 -n port 80
# Capture to file for analysis
sudo tcpdump -i eth0 -w capture.pcap
# Real-time bandwidth per connection
sudo iftop -i eth0
# Bandwidth by process
sudo nethogs eth0
Interface and Routing
# Show IP addresses
ip addr show
# Show routing table
ip route show
# Show ARP table
ip neigh show
# Check MTU
ip link show eth0