Can curl make a connection to any TCP ports, not just HTTP/HTTPS?

Learn can curl make a connection to any tcp ports, not just http/https? with practical examples, diagrams, and best practices. Covers php, curl development techniques with visual explanations.

Beyond HTTP: Using cURL for Raw TCP Connections

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Discover how cURL, often associated with HTTP/HTTPS, can establish and interact with raw TCP connections to any port, enabling versatile network testing and communication.

While cURL is predominantly known for its robust capabilities in handling HTTP and HTTPS requests, its underlying power extends far beyond web protocols. At its core, cURL is a command-line tool and library for transferring data with URLs, supporting a wide range of protocols. This article explores how to leverage cURL to establish raw TCP connections to arbitrary ports, allowing you to interact with services that don't speak HTTP, such as database servers, mail servers, or custom network applications. Understanding this functionality can be incredibly useful for network diagnostics, testing, and even simple data exchange.

The --raw and --tcp-nodelay Flags

The key to making cURL behave like a raw TCP client lies in specific command-line flags. The --raw flag (or -r) tells cURL to send the data exactly as provided, without any protocol-specific processing like adding HTTP headers. This is crucial for interacting with non-HTTP services. Additionally, the --tcp-nodelay flag can be useful. It disables the Nagle algorithm, which can sometimes introduce small delays by buffering small packets. For interactive or time-sensitive raw TCP communication, disabling Nagle's algorithm can improve responsiveness, though it might slightly increase network overhead.

curl --raw telnet://localhost:23
curl --raw telnet://example.com:8080
curl --raw --tcp-nodelay telnet://192.168.1.1:1234

Basic cURL commands for raw TCP connections

Sending Data Over Raw TCP

To send data over a raw TCP connection, you can combine the --raw flag with the -d (data) or --data-binary flags. The -d flag sends data as a POST request by default, but with --raw, it simply sends the specified string. For binary data or to prevent cURL from interpreting special characters, --data-binary is preferred. You can also pipe input directly into cURL using standard input (-T - or < file). This allows for sending multi-line commands or file contents to a TCP service.

sequenceDiagram
    participant User
    participant cURL
    participant TCP_Service

    User->>cURL: curl --raw telnet://host:port -d "Hello"
    cURL->>TCP_Service: Establish TCP connection
    cURL->>TCP_Service: Send "Hello" (raw bytes)
    TCP_Service-->>cURL: Response (raw bytes)
    cURL-->>User: Display response

Sequence diagram of cURL sending raw data over TCP

# Sending a simple string
curl --raw telnet://localhost:12345 -d "Hello, TCP server!\n"

# Sending multi-line commands from a file
cat commands.txt | curl --raw telnet://localhost:12345 -T -

# Sending binary data (e.g., a file)
curl --raw telnet://localhost:12345 --data-binary @binary_file.bin

Examples of sending data using cURL over raw TCP

Practical Use Cases and Considerations

Using cURL for raw TCP connections opens up several practical applications:

  • Network Service Testing: Quickly check if a non-HTTP service (e.g., a custom daemon, a game server, or a database listener) is responding on a specific port and exchange basic commands.
  • Debugging: Send specific byte sequences to a service to debug its behavior without needing a full client application.
  • Simple Data Exchange: For very basic, ad-hoc data transfer where setting up a full client is overkill.
  • Firewall Testing: Verify if specific ports are open and reachable from a given machine.

Remember that cURL will simply send and receive raw bytes. It won't interpret or format the response in any way. You'll see the raw output from the server, which might be human-readable text, binary data, or a mix. For more complex interactions or protocol parsing, dedicated client libraries or tools are usually more appropriate.