Linux: ネットワーク基礎 — ip / curl / sshとネットワーク診断

curl http://localhost:3000
# ✅ Returns HTML normally

# Check if the port is listening
ss -tlnp | grep 3000
# LISTEN 0 128 127.0.0.1:3000 0.0.0.0:*  ← Only listening on localhost!

# Node.js bound to 127.0.0.1 instead of 0.0.0.0

# View IP addresses
ip addr show
# Or shorthand
ip a

# View routing table
ip route
# Or shorthand
ip r

# View network interface status
ip link show

# Enable/disable interface
sudo ip link set eth0 up
sudo ip link set eth0 down

# ping (test network connectivity)
ping google.com              # Continuous ping, Ctrl+C to stop
ping -c 4 google.com         # Ping only 4 times
ping -c 4 -i 2 google.com    # 2-second interval
ping -c 10 -q google.com     # Summary only

# mtr (combines traceroute and ping)
mtr google.com               # Continuously trace route
mtr -r -c 10 google.com      # Report mode (output after 10 rounds)

# curl (HTTP requests)
curl https://example.com              # GET request
curl -I https://example.com           # Headers only
curl -o page.html https://example.com # Save to file

> 💡 ヒント：`curl -v`（verboseモード）はHTTP問題のデバッグに強力なツール — 完全なリクエストヘッダー、レスポンスヘッダー、TLSハンドシェイク過程を表示し、DNS問題、接続タイムアウト、証明書エラー、リダイレクト問題の特定に役立ちます。`curl`リクエストが失敗した時、まず`-v`を追加して詳細を確認してください。
curl -L http://httpbin.org/redirect/3 # Follow redirects
curl -k https://self-signed.com       # Skip SSL verification
curl -X POST -d '{"key":"value"}' -H "Content-Type: application/json" https://api.example.com

# wget (file download)
wget https://example.com/file.zip     # Download file
wget -c https://example.com/big.zip   # Resume download
wget -r -np https://example.com/docs/ # Recursive download

# ss (recommended, faster)
ss -tlnp           # TCP listening ports
ss -ulnp           # UDP listening ports
ss -tlnp | grep 80 # Filter port 80
ss -tuna           # All TCP/UDP connections

# netstat (traditional, may need install)
netstat -tlnp
netstat -i         # Network interface statistics

# dig
dig example.com                    # Full DNS query
dig +short example.com             # IP only
dig example.com A                  # Query A record
dig example.com MX                 # Query mail record
dig @8.8.8.8 example.com          # Use specific DNS server

# Other DNS tools
nslookup example.com               # Simple query
host example.com                   # More concise

# View all IP addresses
ip -br addr
# lo               UNKNOWN        127.0.0.1/8
# eth0             UP             192.168.1.100/24

# View default gateway
ip route | grep default
# default via 192.168.1.1 dev eth0

# View DNS
cat /etc/resolv.conf
# nameserver 8.8.8.8
# nameserver 1.1.1.1

# 1. Ping the gateway first (confirm LAN works)
ping -c 2 192.168.1.1

# 2. Ping an external IP (confirm internet works)
ping -c 2 8.8.8.8

# 3. DNS resolution (confirm domain resolution works)
dig +short google.com

# 4. Port connectivity (confirm service port is open)
nc -zv google.com 80
# Connection to google.com (142.250.80.14) 80 port [tcp/http] succeeded!

# 5. HTTP request (confirm service is working)
curl -I https://example.com

# GET request
curl https://api.github.com/users/octocat

# View response headers
curl -I https://httpbin.org

# POST JSON data
curl -X POST https://httpbin.org/post \
    -H "Content-Type: application/json" \
    -d '{"name":"Alice","role":"developer"}'

# Authenticated request
curl -u "username:token" https://api.github.com/user

# Test HTTP status code
curl -s -o /dev/null -w "%{http_code}" https://example.com

# View all listening TCP ports
ss -tlnp

# Check if port 3306 (MySQL) is listening
ss -tlnp | grep 3306

# View ports for a specific process
ss -tlnp | grep nginx

# Test if a remote port is open
echo "Testing port 80..."
timeout 3 bash -c "echo >/dev/tcp/example.com/80" && echo "Open" || echo "Closed"

# View connection states
ss -tuna | grep ESTAB

# wget download
wget -c https://releases.ubuntu.com/22.04/ubuntu-22.04.3-live-server-amd64.iso

# curl download
curl -L -o ubuntu.iso https://releases.ubuntu.com/22.04/ubuntu-22.04.3-live-server-amd64.iso

# Download and extract
curl -L https://github.com/user/repo/archive/main.tar.gz | tar -xz

#!/bin/bash
# net-diag.sh - Network diagnostics tool

TARGET="${1:-google.com}"

echo "========================================"
echo "    Network Diagnostics Report"
echo "    Time: $(date)"
echo "========================================"

echo ""
echo "1. Local Network Configuration"
echo "----------------"
ip -br addr 2>/dev/null || ifconfig

echo ""
echo "2. Default Gateway"
echo "-----------"
ip route | grep default

echo ""
echo "3. DNS Servers"
echo "--------------"
cat /etc/resolv.conf | grep nameserver

echo ""
echo "4. DNS Resolution ($TARGET)"
echo "----------------------"
dig +short "$TARGET" 2>/dev/null || nslookup "$TARGET" 2>/dev/null || host "$TARGET"

echo ""
echo "5. Network Connectivity"
echo "--------------"
ping -c 4 -q "$TARGET" 2>&1 | tail -3

echo ""
echo "6. HTTP Response"
echo "-------------"
curl -s -o /dev/null -w "HTTP %{http_code}, time: %{time_total}s\n" "https://$TARGET" --connect-timeout 5

echo ""
echo "7. Route Tracing"
echo "------------"
mtr -r -c 3 "$TARGET" 2>/dev/null | tail -5 || echo "mtr not installed"

echo ""
echo "8. Listening Ports"
echo "-----------"
ss -tlnp 2>/dev/null | head -10 || netstat -tlnp 2>/dev/null | head -10

echo ""
echo "========================================"