{
  "id": "linux-dns-http-diagnosis",
  "title": "DNS Resolves, But the Connection Still Fails — Why?",
  "category": "Networking",
  "isolation_type": "shared_namespace",
  "difficulty": "Medium",
  "type": "task",
  "weight": 6,
  "description": "## DNS Resolves, But the Connection Still Fails — Why?\n\nAn application is supposed to be reachable at `http://broken-app.test:9091/`. A teammate insists \"DNS must be broken.\" Prove or disprove that, then actually fix the problem.\n\n**Your task:**\n\n1. Confirm whether `broken-app.test` actually resolves (`getent hosts broken-app.test`) — record your finding\n2. Try `curl http://broken-app.test:9091/` and observe what actually happens\n3. Find out what's REALLY running and on which port (hint: something already answers on a different port)\n4. Fix it so `curl http://broken-app.test:9091/` succeeds with a 200 response\n\nDo not just change the app's advertised port in documentation — make the SERVICE actually listen on 9091.",
  "hints": [
    {
      "title": "Separate DNS from connectivity",
      "body": "`getent hosts broken-app.test` only tells you whether the NAME resolves — it says nothing about whether anything is listening on the port you're trying to reach. Test them as two separate questions.",
      "command": "getent hosts broken-app.test"
    },
    {
      "title": "Finding what's actually running",
      "body": "`ss -lnt` lists every port something is actually listening on right now — compare that against 9091.",
      "command": "ss -lnt"
    },
    {
      "title": "The fix",
      "body": "Stop whatever's running on the wrong port, then start the real service correctly on 9091.",
      "command": "kill $(cat /srv/dns-http-lab/wrong.pid)\npython3 -m http.server 9091 --directory /srv/dns-http-lab &\necho $! > /srv/dns-http-lab/fixed.pid"
    }
  ],
  "setup_commands": [
    { "command": "mkdir -p /srv/dns-http-lab && echo 'app running' > /srv/dns-http-lab/index.html" },
    { "command": "grep -q broken-app.test /etc/hosts || echo '127.0.0.1 broken-app.test' >> /etc/hosts" },
    { "command": "python3 -m http.server 9095 --directory /srv/dns-http-lab >/srv/dns-http-lab/wrong.log 2>&1 & echo $! > /srv/dns-http-lab/wrong.pid" }
  ],
  "validation": {
    "commands": [
      { "description": "broken-app.test genuinely resolves (DNS was never the actual problem)", "command": "getent hosts broken-app.test | awk '{print $1}'", "expected_output": "127.0.0.1", "match": "exact" },
      { "description": "the service now actually answers on port 9091", "command": "curl -sf http://broken-app.test:9091/ -o /dev/null -w '%{http_code}'", "expected_output": "200", "match": "exact" }
    ]
  },
  "default_namespace": "default",
  "teardown_commands": [
    { "command": "[ -f /srv/dns-http-lab/wrong.pid ] && kill $(cat /srv/dns-http-lab/wrong.pid) 2>/dev/null; [ -f /srv/dns-http-lab/fixed.pid ] && kill $(cat /srv/dns-http-lab/fixed.pid) 2>/dev/null; rm -rf /srv/dns-http-lab" }
  ]
}