Core Concepts

1. Curl Essentials

# Basic requests
curl https://api.example.com            # GET
curl -X POST https://api.example.com    # POST
curl -o file.zip https://example.com/f  # Download

# Headers and data
curl -H "Authorization: Bearer $TOKEN" \
     -H "Content-Type: application/json" \
     -d '{"key":"value"}' \
     https://api.example.com

# Common options
curl -v url        # Verbose
curl -s url        # Silent
curl -L url        # Follow redirects
curl -k url        # Skip SSL verify
curl -w "%{http_code}" -o /dev/null -s url

2. SSH Operations

# Connect
ssh user@host
ssh -p 2222 user@host
ssh -i ~/.ssh/key.pem user@host

# File transfer
scp file.txt user@host:/path/
scp -r dir/ user@host:/path/
scp user@host:/path/file.txt ./

# Tunnels
ssh -L 8080:localhost:80 user@host
ssh -D 1080 user@host     # SOCKS proxy

3. Port Checking

# List listening ports
ss -tlnp                  # TCP
ss -ulnp                  # UDP
netstat -tlnp             # Alternative

# Check specific port
nc -zv host 80            # Port check
lsof -i :8080             # What's using port

# Scan ports
nmap -sT host             # TCP scan
nmap -p 80,443 host       # Specific ports

4. DNS Operations

# DNS lookup
dig example.com
dig +short example.com    # IP only
dig example.com MX        # MX records
dig @8.8.8.8 example.com  # Specific DNS

# Alternatives
host example.com
nslookup example.com

Common Patterns

API Request with Error Handling

response=$(curl -s -w "\n%{http_code}" \
    -H "Authorization: Bearer $TOKEN" \
    "https://api.example.com/data")
http_code=$(echo "$response" | tail -1)
body=$(echo "$response" | sed '$d')

if [[ "$http_code" != "200" ]]; then
    echo "Error: HTTP $http_code"
    exit 1
fi

Wait for Port

wait_for_port() {
    local host="$1" port="$2" timeout="${3:-30}"
    for ((i=0; i<timeout; i++)); do
        if nc -z "$host" "$port" 2>/dev/null; then
            return 0
        fi
        sleep 1
    done
    return 1
}

SSH Config

# ~/.ssh/config
Host myserver
    HostName 192.168.1.100
    User admin
    Port 2222
    IdentityFile ~/.ssh/mykey

Anti-Patterns

Don't	Do	Why
`curl \| bash`	Download, inspect, run	Security risk
Store passwords	Use SSH keys	More secure
Skip SSL verify	Fix certificates	Security

Practice Exercises

API Client: Script to interact with REST API
Health Checker: Check if services are up
SSH Automation: Run commands on multiple hosts
Port Scanner: Simple port availability checker

Troubleshooting

Common Errors

Error	Cause	Fix
`Connection refused`	Service down	Check if running
`Connection timed out`	Firewall/routing	Check network
`Name not resolved`	DNS issue	Check DNS
`Permission denied (publickey)`	SSH key	Check authorized_keys

Debug Techniques

# Test connectivity
ping -c 2 host
traceroute host

# Debug curl
curl -v https://example.com

# Debug SSH
ssh -vvv user@host

Security Guidelines

Use SSH keys instead of passwords
Verify SSL certificates in production
Don't store secrets in scripts
Use environment variables for credentials
Audit SSH access regularly

Core Concepts

1. Curl Essentials

2. SSH Operations

3. Port Checking

4. DNS Operations

Common Patterns

API Request with Error Handling

Wait for Port

SSH Config

Anti-Patterns

Practice Exercises

Troubleshooting

Common Errors

Debug Techniques

Security Guidelines

Resources

shell-networking

Shell Networking Skill

Learning Objectives

Prerequisites