docker/cli · error

bad format for add-host: %q

Error message

bad format for add-host: %q

What it means

Error "bad format for add-host: %q" thrown in docker/cli.

Source

Thrown at opts/hosts.go:174

//	my-hostname=::1
//	my-hostname:[::1]
//
// For compatibility with the API server, this function normalises the given
// argument to use the ':' separator and strip square brackets enclosing the
// address.
func ValidateExtraHost(val string) (string, error) {
	k, v, ok := strings.Cut(val, "=")
	if !ok {
		// allow for IPv6 addresses in extra hosts by only splitting on first ":"
		k, v, ok = strings.Cut(val, ":")
	}
	// Check that a hostname was given, and that it doesn't contain a ":". (Colon
	// isn't allowed in a hostname, along with many other characters. It's
	// special-cased here because the API server doesn't know about '=' separators in
	// '--add-host'. So, it'll split at the first colon and generate a strange error
	// message.)
	if !ok || k == "" || strings.Contains(k, ":") {
		return "", fmt.Errorf("bad format for add-host: %q", val)
	}
	// Skip IPaddr validation for "host-gateway" string
	if v != hostGatewayName {
		// If the address is enclosed in square brackets, extract it (for IPv6, but
		// permit it for IPv4 as well; we don't know the address family here, but it's
		// unambiguous).
		if len(v) > 2 && v[0] == '[' && v[len(v)-1] == ']' {
			v = v[1 : len(v)-1]
		}
		// ValidateIPAddress returns the address in canonical form (for example,
		// 0:0:0:0:0:0:0:1 -> ::1). But, stick with the original form, to avoid
		// surprising a user who's expecting to see the address they supplied in the
		// output of 'docker inspect' or '/etc/hosts'.
		if _, err := ValidateIPAddress(v); err != nil {
			return "", fmt.Errorf("invalid IP address in add-host: %q", v)
		}
	}
	// This result is passed directly to the API, the daemon doesn't accept the '='

View on GitHub (pinned to e9452d6e78)

When it happens

Trigger: Thrown at opts/hosts.go:174 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of docker/cli@e9452d6e78 (2026-08-01). Data as JSON: /data/errors/2a51253a20968b07.json. Report an issue: GitHub.