Network Tools and Diagnostics
Overview
Linux network diagnostics, port scanning, traffic analysis and other tool usage skills.
Network Configuration
View Configuration
bash# IP address ip addr ip a ifconfig # Legacy command # Routing table ip route route -n netstat -rn # DNS configuration cat /etc/resolv.conf systemd-resolve --status
Configure Network
bash# Temporary IP configuration ip addr add 192.168.1.100/24 dev eth0 ip addr del 192.168.1.100/24 dev eth0 # Enable/Disable interface ip link set eth0 up ip link set eth0 down # Add route ip route add 10.0.0.0/8 via 192.168.1.1 ip route del 10.0.0.0/8
Connectivity Testing
ping
bashping hostname ping -c 4 hostname # Send 4 packets ping -i 0.2 hostname # 0.2 second interval ping -s 1000 hostname # Specify packet size
traceroute
bashtraceroute hostname traceroute -n hostname # Don't resolve hostnames traceroute -T hostname # Use TCP mtr hostname # Real-time trace
DNS Query
bashnslookup hostname dig hostname dig +short hostname dig @8.8.8.8 hostname # Specify DNS server host hostname
Ports and Connections
ss Command (Recommended)
bash# Listening ports ss -tlnp # TCP listening ss -ulnp # UDP listening ss -tlnp | grep :80 # All connections ss -tanp # TCP connections ss -s # Statistics # Filter ss -t state established ss -t dst 192.168.1.1 ss -t sport = :80
netstat Command
bashnetstat -tlnp # TCP listening netstat -ulnp # UDP listening netstat -anp # All connections netstat -s # Statistics
lsof Network
bashlsof -i # All network connections lsof -i :80 # Specific port lsof -i tcp # TCP connections lsof -i @192.168.1.1 # Specific host
HTTP Tools
curl
bash# Basic request curl http://example.com curl -I http://example.com # Headers only curl -v http://example.com # Verbose output # POST request curl -X POST -d "data=value" http://example.com curl -X POST -H "Content-Type: application/json" -d '{"key":"value"}' http://example.com # Download curl -O http://example.com/file.zip curl -o output.zip http://example.com/file.zip # Authentication curl -u user:pass http://example.com curl -H "Authorization: Bearer token" http://example.com
wget
bashwget http://example.com/file.zip wget -c http://example.com/file.zip # Resume download wget -r http://example.com # Recursive download wget --mirror http://example.com # Mirror site
Packet Capture
tcpdump
bash# Basic capture tcpdump -i eth0 tcpdump -i any # Filter tcpdump -i eth0 port 80 tcpdump -i eth0 host 192.168.1.1 tcpdump -i eth0 'tcp port 80 and host 192.168.1.1' # Save/Read tcpdump -i eth0 -w capture.pcap tcpdump -r capture.pcap # Display content tcpdump -i eth0 -A port 80 # ASCII tcpdump -i eth0 -X port 80 # Hexadecimal
Traffic Monitoring
bash# Real-time traffic iftop iftop -i eth0 # By process nethogs nethogs eth0 # Bandwidth test iperf3 -s # Server iperf3 -c server_ip # Client
Common Scenarios
Scenario 1: Troubleshoot Port Usage
bash# Check port usage ss -tlnp | grep :8080 lsof -i :8080 # Find process and handle kill -9 PID # Or fuser -k 8080/tcp
Scenario 2: Test Service Connectivity
bash# TCP port test nc -zv hostname 80 telnet hostname 80 # HTTP service test curl -I http://hostname curl -w "HTTP Code: %{http_code}\nTime: %{time_total}s\n" -o /dev/null -s http://hostname
Scenario 3: Network Performance Diagnosis
bash# Latency test ping -c 100 hostname | tail -1 # Route analysis mtr --report hostname # Bandwidth test iperf3 -c server -t 30
Troubleshooting
| Problem | Solution |
|---|---|
| Network unreachable | ping, traceroute, check routing |
| DNS resolution failed | dig, nslookup, check resolv.conf |
| Port unreachable | ss -tlnp, check firewall |
| Connection timeout | mtr, tcpdump packet capture |
| Insufficient bandwidth | iftop, iperf3 test |
