What causes "invalid start port %s: %s" in bettercap/bettercap?

Pass a plain decimal integer for the start port argument, e.g. `syn.parse_ports 192.168.1.10 1 65535`.

invalid start port %s: %s

Error message

invalid start port %s: %s

What it means

syn_scan's `syn.parse_ports` (parsePorts) accepts an optional start port as its second argument and parses it with strconv.Atoi. If the trimmed argument is not a valid integer, the module refuses to configure the port range and returns this error. It is a pure argument-validation error raised before any packets are sent.

Source

Thrown at modules/syn_scan/syn_scan_parsers.go:41

			return fmt.Errorf("error while parsing IP range '%s': %s", arg, err)
		} else {
			mod.addresses = list.Expand()
		}
	}

	return nil
}

func (mod *SynScanner) parsePorts(args []string) (err error) {
	argc := len(args)
	mod.stats.totProbes = 0
	mod.stats.doneProbes = 0
	mod.startPort = 1
	mod.endPort = 65535

	if argc > 1 && str.Trim(args[1]) != "" {
		if mod.startPort, err = strconv.Atoi(str.Trim(args[1])); err != nil {
			return fmt.Errorf("invalid start port %s: %s", args[1], err)
		} else if mod.startPort > 65535 {
			mod.startPort = 65535
		}
		mod.endPort = mod.startPort
	}

	if argc > 2 && str.Trim(args[2]) != "" {
		if mod.endPort, err = strconv.Atoi(str.Trim(args[2])); err != nil {
			return fmt.Errorf("invalid end port %s: %s", args[2], err)
		}
	}

	if mod.endPort < mod.startPort {
		return fmt.Errorf("end port %d is greater than start port %d", mod.endPort, mod.startPort)
	}

	return
}

View on GitHub (pinned to 8eca2820f3)

Solutions

Pass a plain decimal integer for the start port argument, e.g. `syn.parse_ports 192.168.1.10 1 65535`.
If passing variables from a script, verify they are non-empty numeric strings before invoking the module.
Remember ports above 65535 are silently clamped, so only non-numeric values cause this error.

Example fix

// before
syn.parse_ports 10.0.0.1 $START-
// after
syn.parse_ports 10.0.0.1 80 443

Defensive patterns

Strategy: validation

Validate before calling

const start = args[1]?.trim() ?? ''
if (start !== '' && !/^\d+$/.test(start)) {
  throw new Error(`start port must be an integer, got: ${start}`)
}

Type guard

function isValidPort(s) { return /^\d+$/.test(s.trim()) && +s.trim() <= 65535 }

Try / catch

try {
  session.Run('syn.parse_ports ' + target + ' ' + start)
} catch (e) {
  if (String(e).includes('invalid start port')) {
    console.error('Fix the start port argument:', start)
  } else { throw e }
}

Prevention

Always pass plain decimal integers for port arguments
Validate scripted variables are numeric before interpolation
Remember only values >65535 are clamped; all other bad input errors out

When it happens

Trigger: Running `syn.parse_ports <target> <start>` where the start port argument is empty-looking-but-non-numeric text (e.g. a placeholder, a stray letter, or a value with units like `80/tcp`). The trimmed string must parse as an int.

Common situations: Typing a port range in a non-numeric notation (`80-90` as the start argument instead of two args), copy-pasting commands with hidden characters, or scripting with an unset shell variable that renders as literal text.

Related errors

AI-assisted analysis of bettercap/bettercap@8eca2820f3 (2026-09-02). Data as JSON: /api/errors/1288396b7245f75f. Report an issue: GitHub.