Consider Using the nc Command for Connectivity Verification

kawasaki031677 — Sun, 01 Nov 2026 02:33:17 +0000

Introduction

When verifying connectivity to external servers or databases, which command do you typically use? I'd like to recommend the nc (Netcat) command for this purpose.

Why I Recommend the nc Command

The nc (Netcat) command is convenient for connectivity verification. Here's how to use it:

$ nc -zv <hostname or IP address> <port number>
$ nc -zv google.com 80

If you see Connection to ... succeeded! as output, the connectivity is successful.

user@localhost:~$ nc -zv google.com 80
Connection to google.com (142.250.207.46) 80 port [tcp/http] succeeded!
user@localhost:~$

What About Using ping?

It's common to just try ping first. However, ping uses the ICMP protocol, so if the other side doesn't allow ICMP, it will fail (security groups and firewalls often block it).

Why Not Just Use curl or psql?

Ultimately, it's best to verify that you can properly access the API with curl or the database with psql. However, before that, you want to confirm: "Can we connect at the network level in the first place?"

If you only verify at the application layer, troubleshooting becomes tedious when a connection fails. It's better to use the nc command to verify layer by layer.

What About the Traditional telnet?

While telnet is a classic option, it's often considered a security risk and is not recommended. Personally, I prefer not to use it.

Why nc is Underrated but Practical

It straightforwardly tells you whether a TCP port is open
It also supports UDP (surprisingly powerful)

However, nc is not installed by default on systems like Amazon Linux, so install it beforehand:

$ dnf install nmap-ncat

Getting Started with Keycloak - Building a Simple Login Page with Docker

kawasaki031677 — Sun, 01 Nov 2026 01:06:42 +0000

Introduction

Recently, it seems Keycloak is being used quite frequently. (According to my colleague)
I hadn't tried it yet myself, so I decided to give it a simple try to see what it's all about.

Getting Started with Keycloak

1. Pull the Docker image and start Keycloak

docker run --rm -p 8080:8080 \
  -e KEYCLOAK_ADMIN=admin \
  -e KEYCLOAK_ADMIN_PASSWORD=admin \
  quay.io/keycloak/keycloak:24.0.3 \
  start-dev

2. Access the admin console

Once Keycloak is running, access the admin console at the following URL:

http://localhost:8080

You'll see the Keycloak login page.

3. Log in to the admin console

Enter the username (admin) and password (admin) to log in to the admin console. The login was successful!

4. Create a test user

Navigate to the left menu and go to Users → Add user to create a new user. In this example, I created a user named "test".

5. Create a test client

Navigate to Clients → Create client in the left menu and enter the following information:

Client ID: my-html-client
Home URL / Valid redirect URIs / Web origins: http://localhost:8000

(We'll place the HTML file we create later at this URL)

6. Create a simple login page

Create a simple HTML file (index.html) to test the login functionality:

<!DOCTYPE html>
<html>
<head>
  <title>Keycloak Demo</title>
  <script src="https://cdn.jsdelivr.net/npm/keycloak-js@24.0.3/dist/keycloak.min.js"></script>
</head>
<body>
  <h1>Keycloak Test Page</h1>
  <button onclick="login()">Login</button>
  <pre id="output"></pre>

  <script>
    const keycloak = new Keycloak({
      url: 'http://localhost:8080/',
      realm: 'master',
      clientId: 'my-html-client',
    });

    function login() {
      keycloak.init({ onLoad: 'login-required' }).then(authenticated => {
        document.getElementById('output').textContent =
          authenticated ? 'Authenticated\n' + JSON.stringify(keycloak.tokenParsed, null, 2) : 'Not Authenticated';
      });
    }
  </script>
</body>
</html>

7. Start a simple web server

Navigate to the folder where you placed the index.html file and run the following command to start a simple Python web server:

python -m http.server 8000

8. Access the login page

Open your browser and navigate to the following URL to see the test page:

http://localhost:8000

The page displays successfully with the login button visible.

9. Test the login functionality

Click the Login button and you'll be redirected to the Keycloak authentication screen. After entering your credentials (username: test, password: password), you'll be successfully authenticated!

Summary

Since configuration can be done through a GUI, it left a beginner-friendly impression on me. I was able to verify the authentication flow in a short time, so it seems very useful for future development. Next time, I plan to explore the more detailed configuration settings and advanced features.

DEV Community: kawasaki031677

Consider Using the nc Command for Connectivity Verification

Introduction

Why I Recommend the nc Command

What About Using ping?

Why Not Just Use curl or psql?

What About the Traditional telnet?

Why nc is Underrated but Practical

Getting Started with Keycloak - Building a Simple Login Page with Docker

Introduction

Getting Started with Keycloak

Summary