var (
	// ErrMissingAddr is the error returned when building a DNS resolver when
	// the provided target name is empty.
	ErrMissingAddr = errors.New("dns resolver: missing address")

	// ErrEndsWithColon is the error returned when building a DNS resolver when
	// the provided target name ends with a colon that is supposed to be the
	// separator between host and port.  E.g. "::" is a valid address as it is
	// an IPv6 address (host only) and "[::]:" is invalid as it ends with a
	// colon as the host and port separator
	ErrEndsWithColon = errors.New("dns resolver: missing port after port-separator colon")
)

var (
	// TimeAfterFunc is used by the DNS resolver to wait for the given duration
	// to elapse. In non-test code, this is implemented by time.After. In test
	// code, this can be used to control the amount of time the resolver is
	// blocked waiting for the duration to elapse.
	TimeAfterFunc func(time.Duration) <-chan time.Time

	// TimeNowFunc is used by the DNS resolver to get the current time.
	// In non-test code, this is implemented by time.Now. In test code,
	// this can be used to control the current time for the resolver.
	TimeNowFunc func() time.Time

	// TimeUntilFunc is used by the DNS resolver to calculate the remaining
	// wait time for re-resolution. In non-test code, this is implemented by
	// time.Until. In test code, this can be used to control the remaining
	// time for resolver to wait for re-resolution.
	TimeUntilFunc func(time.Time) time.Duration

	// NewNetResolver returns the net.Resolver instance for the given target.
	NewNetResolver func(string) (NetResolver, error)

	// AddressDialer is the dialer used to dial the DNS server. It accepts the
	// Host portion of the URL corresponding to the user's dial target and
	// returns a dial function.
	AddressDialer func(address string) func(context.Context, string, string) (net.Conn, error)
)

type NetResolver interface {
	LookupHost(ctx context.Context, host string) (addrs []string, err error)
	LookupSRV(ctx context.Context, service, proto, name string) (cname string, addrs []*net.SRV, err error)
	LookupTXT(ctx context.Context, name string) (txts []string, err error)
}