How to check if a port is open on a remote or local…

To check an open port on a remote or local host, you can use the following methods:

netcat (nc): A versatile tool for working with network connections.
- Local host:
```
nc -zv localhost <port>
```
- Remote host:
```
nc -zv <remote_host> <port>
```
The options -z (zero-I/O mode) and -v (verbose) are used for scanning without sending data and for detailed output.
telnet: A protocol for remote management, which can be used to check connection on a specified port.
- Local host:
```
telnet localhost <port>
```
- Remote host:
```
telnet <remote_host> <port>
```
If the connection is established (usually a prompt or empty line appears), the port is open. You can close it by pressing Ctrl+], then quit and Enter.
nmap: A powerful port scanner. Requires installation.
- Local host:
```
nmap -p <port> localhost
```
- Remote host:
```
nmap -p <port> <remote_host>
```
The output will show the port status: open, closed, or filtered.

< /dev/tcp or < /dev/udp (in Bash): In Bash, you can use file descriptors to create TCP or UDP connections.

Local host (TCP):

timeout 1 bash -c "cat < /dev/tcp/localhost/<port>" 2>/dev/null && echo "Port <port> is open" || echo "Port <port> is closed"

Remote host (TCP):

timeout 1 bash -c "cat < /dev/tcp/<remote_host>/<port>" 2>/dev/null && echo "Port <port> is open" || echo "Port <port> is closed"

timeout is used to prevent hanging if the port is closed. 2>/dev/null hides error output. This method does not work for UDP.

Python: Using sockets for checking.

import socket

def check_port(host, port):
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.settimeout(1)  # Set timeout
    try:
        s.connect((host, port))
        print(f"Port {port} on {host} is open")
    except (socket.timeout, ConnectionRefusedError):
        print(f"Port {port} on {host} is closed or filtered")
    finally:
        s.close()

# Example usage
check_port("localhost", 80)       # Check local host
check_port("google.com", 443)  # Check remote host

The choice of method depends on available tools, operating system, and the specific task. netcat and nmap are among the most flexible and informative tools.

How to check if a port is open on a remote or local host?