netbirdio/netbird · warning

%s is not a valid input for %s. it should be formatted as "S

Error message

%s is not a valid input for %s. it should be formatted as "String" or "String/String"

What it means

validateNATExternalIPs splits each --external-ip-map element on '/' and requires at most two sub-elements (an IP and optionally a mask-or-interface). More than one slash means three or more parts, which matches no accepted form, so it is rejected with this message stating the "String" or "String/String" shape.

Source

Thrown at client/cmd/up.go:745

		loginRequest.BlockInbound = &blockInbound
	}

	if cmd.Flag(disableIPv6Flag).Changed {
		loginRequest.DisableIpv6 = &disableIPv6
	}

	return &loginRequest, nil
}

func validateNATExternalIPs(list []string) error {
	for _, element := range list {
		if element == "" {
			return fmt.Errorf("empty string is not a valid input for %s", externalIPMapFlag)
		}

		subElements := strings.Split(element, "/")
		if len(subElements) > 2 {
			return fmt.Errorf("%s is not a valid input for %s. it should be formatted as \"String\" or \"String/String\"", element, externalIPMapFlag)
		}

		if len(subElements) == 1 && !isValidIP(subElements[0]) {
			return fmt.Errorf("%s is not a valid input for %s. it should be formatted as \"IP\" or \"IP/IP\", or \"IP/Interface Name\"", element, externalIPMapFlag)
		}

		last := 0
		for _, singleElement := range subElements {
			inputType, err := validateElement(singleElement)
			if err != nil {
				return fmt.Errorf("%s is not a valid input for %s. it should be an IP string or a network name", singleElement, externalIPMapFlag)
			}
			if last == interfaceInputType && inputType == interfaceInputType {
				return fmt.Errorf("%s is not a valid input for %s. it should not contain two interface names", element, externalIPMapFlag)
			}
			last = inputType
		}
	}

View on GitHub (pinned to 93e97f4bf1)

Solutions

1. Use at most one '/': "IP" or "IP/secondary-IP" or "IP/interface-name"
2. Split a doubled value into two comma-separated elements if two mappings were intended

Example fix

# before
netbird up --external-ip-map "1.2.3.4/5.6.7.8/9"
# after
netbird up --external-ip-map "1.2.3.4/5.6.7.8"

for _, e := range list {
	if strings.Count(e, "/") > 1 {
		return fmt.Errorf("%q has more than one '/'", e)
	}
}

When it happens

Trigger: Values like "1.2.3.4/24/eth0" or "1.2.3.4/5.6.7.8/9" — anything with two or more '/' characters in a single element.

Common situations: Attempting to pass CIDR notation plus an interface, or copy-paste artifacts gluing two entries together with a slash.

Related errors

• empty string is not a valid input for %s
• %s is not a valid input for %s. it should be formatted as "I
• %s is not a valid input for %s. it should be an IP string or
• %s is not a valid input for %s. it should not contain two in
• invalid IP or network interface name not found

AI-assisted analysis of netbirdio/netbird@93e97f4bf1 (2026-08-16). Data as JSON: /api/errors/f61d9152f675349a. Report an issue: GitHub.