# Troubleshooting network applications with curl

> Use curl from the command line to test web and network connections and diagnose problems with your website.

Source: https://helpcenter.ruachost.com/developers/networking/troubleshooting-network-applications-with-curl/

Troubleshooting Network Applications with Curl

The **curl** program is a powerful tool for network troubleshooting. This guide explains how to use curl to test connectivity and diagnose network issues.

**About Curl**\
Curl is a command-line utility used to transfer data to or from a server. It supports a wide range of protocols, including HTTP, HTTPS, FTP, and SMTP.

You can use curl in:

- **Linux**: available by default in most distributions.
- **macOS**: accessible via the Terminal.
- **Windows 10**: included in newer versions and available through the Windows Subsystem for Linux (WSL).
- Alternatively, curl can be downloaded from [https://curl.haxx.se](https://curl.haxx.se/).

**Tip:**\
If you’d rather not install new software, you can also use **Telnet** or **PowerShell (tnc)** for network troubleshooting—most computers already have one of these tools installed.

**Using Curl to Troubleshoot**\
To test basic network connectivity with curl, you’ll need:

- The server’s **domain name or IP address**.
- The **protocol** used by the service (e.g., HTTP, FTP, SMTP).
- The **port number** of the application you’re testing.

To connect to a remote server, open a terminal and type:

```
curl protocol://IP_or_host:port
```

Here:

- **protocol** is the communication method (HTTP, FTP, etc.).
- **IP_or_host** is the hostname or IP address.
- **port** is optional if the standard port is used.

Example:

```
curl http://example.com
```

**Tip:**\
For a full list of standard TCP and UDP port numbers, visit [Wikipedia’s port list](http://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers).

When you try to connect:

- If the **server accepts** the connection, curl displays the server’s response or data.
- If the **server rejects** it, you’ll see an error such as *Connection refused* or *Connect failed*.

Below are examples of how to troubleshoot with curl.

**Troubleshooting Web Servers**\
Web server testing is one of curl’s most common uses. You can use it to check if a website or web server is responding.\
Example:

```
$ curl http://example.com -I
HTTP/1.1 200 OK
Content-Type: text/html
Last-Modified: Mon, 13 Jan 2025 20:11:20 GMT
Cache-Control: max-age=2112
Date: Fri, 13 Jun 2025 12:57:49 GMT
Connection: keep-alive
```

By default, curl downloads the full web page. The **-I** option requests only the HTTP headers, confirming that the web server is online and responding correctly.

**Troubleshooting Mail (SMTP) Servers**\
Curl can also test mail servers using the SMTP protocol.\
Example:

```
$ curl smtp://example.com
214-Commands supported:
214 AUTH STARTTLS HELO EHLO MAIL RCPT DATA BDAT NOOP QUIT RSET HELP
```

These responses confirm that the mail server is active and ready to process requests.

**Note:**\
If you’re testing connectivity to a **ruachost.com** mail server, you can also use ports **2525** or **587**, since some internet providers block the default SMTP port (25) to reduce spam.

Example for port 2525:

```
$ curl smtp://example.com:2525
214-Commands supported:
214 AUTH STARTTLS HELO EHLO MAIL RCPT DATA BDAT NOOP QUIT RSET HELP
The response again confirms the SMTP service is available.
```

```
Troubleshooting FTP Servers\
To check an FTP server, use the ftp protocol or connect directly to port 21.\
Example:
```

```
$ curl ftp://example.com
curl: (67) Access denied: 530
```

This message indicates that the FTP server is active but requires authentication.

For more detailed information, you can connect to port 21 directly:

```
$ curl example.com:21
220----------
---- Welcome to Pure-FTPd [privsep] [TLS] ----------
220-You are user number 18 of 50 allowed.
220-Local time is now 12:46. Server port: 21.
220-This is a private system - No anonymous login
220-IPv6 connections are also welcome on this server.
220 You will be disconnected after 15 minutes of inactivity.
```

Either response confirms that the FTP service is online and functional.

**More Information**\
Curl is highly versatile. Beyond simple connectivity checks, it can also:

- Send emails.
- Upload and download files.
- Post or retrieve information from web servers.

To explore curl’s full capabilities:

- Visit the [official curl website](https://curl.haxx.se/).
- Read the **curl book** online for advanced usage examples.