Networking Commands

🎯 Learning Objectives

• Diagnose network connectivity with ping, traceroute
• Transfer data with curl and wget
• Connect securely with SSH
• Copy files with SCP and rsync
• Inspect network connections with netstat, ss, lsof

1. Network Diagnostics

bash
# Ping — test connectivity
ping google.com                 # Continuous ping
ping -c 4 google.com            # 4 pings only
ping -c 1 -W 2 192.168.1.1      # 1 ping, 2 second timeout
# Traceroute — show path to host
traceroute google.com           # Show each hop
traceroute -n google.com        # Skip DNS lookup (faster)
# DNS lookup
nslookup google.com             # Query DNS records
dig google.com                  # Detailed DNS information
host google.com                 # Simple DNS lookup
# IP configuration
ip addr                         # Show all IP addresses (replaces ifconfig)
ip route                        # Show routing table
ip link                         # Show network interfaces

2. Data Transfer — curl

bash
# Basic HTTP GET
curl https://api.github.com/users/octocat
curl -O https://example.com/file.zip     # Download file (keeps filename)
curl -o myfile.zip https://example.com/file.zip  # Download with custom name
# HTTP headers
curl -I https://google.com              # Show response headers only
curl -v https://example.com             # Verbose (request + response headers)
# POST with data
curl -X POST https://api.example.com/login \
  -H "Content-Type: application/json" \
  -d '{"username":"alice","password":"secret"}'
# Authentication
curl -u username:password https://api.example.com/secure
curl -H "Authorization: Bearer TOKEN" https://api.example.com
# Follow redirects
curl -L https://bit.ly/example          # Follow redirects

3. Downloading — wget

bash
# Basic download
wget https://example.com/file.zip
# Resume interrupted download
wget -c https://example.com/largefile.zip
# Download to specific directory
wget -P /downloads/ https://example.com/file.zip
# Recursive download (website mirroring)
wget -r -l 2 -np https://example.com/docs/
# -r: recursive, -l 2: 2 levels deep, -np: no parent directories

4. Secure Shell — ssh

bash
# Basic connection
ssh user@hostname                      # Connect to remote host
ssh user@192.168.1.100                 # Connect by IP
ssh -p 2222 user@hostname              # Custom port (default: 22)
# Key-based authentication
ssh-keygen -t ed25519                  # Generate SSH key pair
ssh-copy-id user@hostname              # Copy public key to server
# Now: ssh user@hostname (no password!)
# Execute remote command
ssh user@hostname "ls -la /var/log"
ssh user@hostname "df -h"
# Tunnel (port forwarding)
ssh -L 8080:localhost:80 user@hostname  # Forward local 8080 → remote 80
# SCP — secure copy over SSH
scp file.txt user@hostname:/remote/path/    # Copy file TO remote
scp user@hostname:/remote/file.txt ./       # Copy file FROM remote
scp -r dir/ user@hostname:/remote/path/     # Copy directory recursively
# rsync — efficient file sync (only transfers differences)
rsync -avz source/ user@hostname:/dest/     # Sync TO remote
rsync -avz user@hostname:/source/ ./dest/   # Sync FROM remote
# -a: archive (preserve permissions), -v: verbose, -z: compress

5. Network Connections — netstat and ss

bash
# Show active connections
netstat -tuln                      # Listening ports (-t: TCP, -u: UDP, -l: listen, -n: numeric)
ss -tuln                           # Modern replacement for netstat (faster)
# Show all connections
netstat -an                        # All connections
ss -an                             # Modern version
# Show process using port
lsof -i :80                        # What's running on port 80?
lsof -i :3000                      # What's on port 3000?
fuser 80/tcp                       # Process using port 80
# Check if port is open
nc -zv hostname 80                 # Check if port 80 is open
nc -zv hostname 22 80 443          # Check multiple ports

6. Firewall

bash
# ufw (Uncomplicated Firewall) — Ubuntu
sudo ufw status                    # Show firewall rules
sudo ufw allow 80/tcp              # Allow HTTP
sudo ufw allow 22                  # Allow SSH
sudo ufw enable                    # Enable firewall

7. Practice Questions

Q1: How do you test if a remote server is reachable?

Answer: ping -c 4 server.com. Sends 4 ICMP packets and shows response times. If the server is unreachable, you'll see "Destination Host Unreachable" or 100% packet loss. Q2: What does curl -I do?

Answer: Sends an HTTP HEAD request, which retrieves only the response headers (not the body). Useful for checking content type, file size, status code, or last modified date without downloading the entire file. Q3: How do you securely copy a file to a remote server?

Answer: scp localfile.txt user@server:/remote/path/. For directories: scp -r localdir/ user@server:/remote/path/. For large/syncing: rsync -avz source/ user@server:/dest/. Q4: How do you find what process is using port 3000?

Answer: lsof -i :3000 or fuser 3000/tcp. Shows the PID and process name. If nothing is listening, there's no output. Q5: What does ssh-keygen do?

Answer: Generates a public/private key pair for SSH authentication. By default creates ~/.ssh/id_rsa (private key) and ~/.ssh/id_rsa.pub (public key). After copying the public key to a server with ssh-copy-id, you can log in without a password. Q6: What is the difference between curl and wget?

Answer: curl is more versatile: supports more protocols, more options for sending data (POST, PUT), better for API interaction. wget is simpler for downloading files: supports recursive download, resuming, and works better for large file downloads. Both can download files; use curl for API testing, wget for bulk downloads. Q7: How do you check which ports are open on your machine?

Answer: ss -tuln or netstat -tuln. Shows listening TCP and UDP ports with their numeric addresses. -t=TCP, -u=UDP, -l=listening, -n=numeric (no DNS). Q8: What does ssh -L 8080:localhost:3000 user@server do?

Answer: Creates a local port forward: connections to localhost:8080 are tunneled through SSH to server, which forwards to localhost:3000 (from server's perspective). This allows accessing a service running on the server's port 3000 via your machine's port 8080.

📐 Key Concepts

🔗 Cross-References

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