I’m trying to build an nmap-like CLI port scanning tool in Rust. I want to ensure that if the user doesn’t pass the optional –ports flag in their command, the tool scans a range of commonly used ports and outputs their state, service and other relevant information. However, when no –ports flag is passed, I’m scanning a very large number of ports instead of the commonly used ports. This is what the output looks like for the command cargo run -- --host scanme.nmap.org:

...
[2024-06-12T06:19:42Z INFO  ferric] Port 264 is filtered! Service: Unknown
[2024-06-12T06:19:42Z INFO  ferric] Port 994 is filtered! Service: Unknown
[2024-06-12T06:19:42Z INFO  ferric] Port 995 is filtered! Service: Unknown
[2024-06-12T06:19:42Z INFO  ferric] Port 997 is filtered! Service: Unknown
[2024-06-12T06:19:42Z INFO  ferric] Port 988 is filtered! Service: Unknown
[2024-06-12T06:19:42Z INFO  ferric] Port 976 is filtered! Service: Unknown
[2024-06-12T06:19:42Z INFO  ferric] Port 984 is filtered! Service: Unknown
...

I tried modifying my parse_ports function to check if there is no –ports flag in the command, but this doesn’t seem to work. Should I use a match statement instead? Here is the parse_ports function:

    const COMMON_PORTS: &[u16] = &[80, 443, 22, 21, 25, 110, 143, 3306, 5432, 27017, 6379, 8080];

    // Function to parse ports
`
    fn parse_ports(ports: Option<&str>) -> Result<Vec<u16>, String> {

        if let Some(port_str) = ports {
            let mut ports_vec = Vec::new();
            for part in port_str.split(',') {
                if let Some(range) = part.split_once('-') {
                    let start: u16 = range.0.parse().map_err(|e| format!("Invalid port range: {}", e))?;
                    let end: u16 = range.1.parse().map_err(|e| format!("Invalid port range: {}", e))?;
                    if start > end {
                        return Err(format!("Invalid port range: {} - {}", start, end));
                    }
                    ports_vec.extend(start..=end);
                } else {
                    let port: u16 = part.parse().map_err(|_| format!("Invalid port: {}", part))?; 
                    ports_vec.push(port);
                }
            }
            Ok(ports_vec)
        } else {
            Ok(COMMON_PORTS.to_vec())
        }
    }