ERR_CONNECTION_REFUSED 원인과 해결, 크롬에서 직접 해 보니

$ printf '#!/bin/sh\nexec "/Applications/Google Chrome.app/Contents/MacOS/Google Chrome" --headless=new --disable-gpu --no-first-run --no-default-browser-check --user-data-dir="$PWD/chrome-profile" "$@"\n' > chrome && chmod +x chrome && ./chrome --version && curl --version | head -1
Google Chrome 156.0.8078.48
curl 8.7.1 (x86_64-apple-darwin26.0) libcurl/8.7.1 (SecureTransport) LibreSSL/3.3.6 zlib/1.2.12 nghttp2/1.69.0

$ ./chrome --dump-dom http://localhost:8000/ 2>&1 | grep -o 'Page load failed: net::ERR_[A-Z_]*'
Page load failed: net::ERR_CONNECTION_REFUSED

$ curl -sS http://localhost:8000/
curl: (7) Failed to connect to localhost port 8000 after 0 ms: Couldn't connect to server

$ lsof -nP -iTCP:8000 -sTCP:LISTEN || echo '8000번을 듣는 프로그램 없음'
8000번을 듣는 프로그램 없음

$ mkdir -p site && printf '<meta charset="utf-8"><h1>열렸다</h1>\n' > site/index.html && (python3 -m http.server 8000 --directory site > server.log 2>&1 &) && sleep 1 && lsof -nP -iTCP:8000 -sTCP:LISTEN | awk '{print $1, $9}' && ./chrome --dump-dom http://localhost:8000/ 2>/dev/null
COMMAND NAME
Python *:8000
<html><head><meta charset="utf-8"></head><body><h1>열렸다</h1>
</body></html>

$ ./chrome --dump-dom http://localhost:8001/ 2>&1 | grep -o 'Page load failed: net::ERR_[A-Z_]*'
Page load failed: net::ERR_CONNECTION_REFUSED

$ (node -e "require('http').createServer((q, s) => { s.setHeader('content-type', 'text/plain; charset=utf-8'); s.end('127.0.0.1 서버\n'); }).listen(3000, '127.0.0.1')" > node.log 2>&1 &) && sleep 1 && lsof -nP -iTCP:3000 -sTCP:LISTEN | awk '{print $1, $9}' && curl -sS 'http://[::1]:3000/'
COMMAND NAME
node 127.0.0.1:3000
curl: (7) Failed to connect to ::1 port 3000 after 0 ms: Couldn't connect to server

$ curl -sS http://localhost:3000/ && ./chrome --dump-dom http://localhost:3000/ 2>/dev/null | head -1
127.0.0.1 서버
<html><head><meta name="color-scheme" content="light dark"></head><body><pre style="word-wrap: break-word; white-space: pre-wrap;">127.0.0.1 서버

$ printf "const http = require('http');\nconst port = process.env.PORT;\nif (!port) throw new Error('PORT 환경 변수가 없습니다');\nhttp.createServer((q, s) => s.end('ok')).listen(port);\n" > app.js && node app.js; sleep 0.5; curl -sS http://localhost:5050/
./app.js:3
if (!port) throw new Error('PORT 환경 변수가 없습니다');
           ^

Error: PORT 환경 변수가 없습니다
    at Object.<anonymous> (./app.js:3:18)
    at Module._compile (node:internal/modules/cjs/loader:1781:14)
    at Module._extensions..js (node:internal/modules/cjs/loader:1913:10)
    at Module.load (node:internal/modules/cjs/loader:1505:32)
    at Module._load (node:internal/modules/cjs/loader:1309:12)
    at wrapModuleLoad (node:internal/modules/cjs/loader:254:19)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:171:5)
    at node:internal/main/run_main_module:36:49

Node.js v22.23.1
curl: (7) Failed to connect to localhost port 5050 after 0 ms: Couldn't connect to server

$ claude --model opus -p "이 폴더의 app.js를 실행하면 브라우저에서 localhost:5050이 ERR_CONNECTION_REFUSED로 열리지 않는다. 파일을 읽고 원인과 고치는 실행 명령을 두 줄로만 답해. 파일은 고치지 마." --allowedTools "Read"
원인: `app.js`는 `PORT` 환경 변수가 없으면 바로 예외를 던지고 종료됩니다. 그래서 5050번 포트에서 대기하는 서버가 없어 `ERR_CONNECTION_REFUSED`가 납니다.
고치는 실행 명령: `PORT=5050 node app.js`

$ kill $(lsof -tiTCP:8000 -sTCP:LISTEN) $(lsof -tiTCP:3000 -sTCP:LISTEN) && sleep 0.5 && (lsof -nP -iTCP:8000 -iTCP:3000 -sTCP:LISTEN || echo '8000, 3000번 모두 꺼짐')
8000, 3000번 모두 꺼짐