Enum FallbackMode 

#[non_exhaustive]
pub enum FallbackMode {
    Never,
    Eager,
    IfSystemEmpty,
    Deferred,
}

How the resolver uses its fallback nameservers relative to the primary ones.

The primary nameservers come from the system DNS configuration and the nameservers added on the Builder. The fallback nameservers default to a set of public resolvers, which Builder::fallback_nameserver_configs can override. Select the mode with Builder::with_fallback_mode.

Variants (Non-exhaustive)

Non-exhaustive enums could have additional variants added in future. Therefore, when matching against variants of non-exhaustive enums, an extra wildcard arm must be added to account for any future variants.

Never

Never queries the fallback nameservers.

Eager

Races the fallback nameservers alongside the primary ones from the start.

IfSystemEmpty

Uses the fallback nameservers only when the system configuration is empty.

A system configuration counts as empty when it yields no nameservers, whether because it could not be read or because it listed none. One that did yield nameservers is never supplemented: if those fail at query time the lookup fails rather than escalating. Without Builder::with_system_defaults there is no configuration at all, which also counts as empty, so this behaves like Self::Eager then.

Deferred

Keeps the fallback nameservers as a lower-priority tier.

They are queried only once every primary nameserver has failed or timed out. This is the default.

Trait Implementations

impl Clone for FallbackMode

fn clone(&self) -> FallbackMode

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for FallbackMode

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for FallbackMode

fn default() -> FallbackMode

Returns the “default value” for a type.

impl PartialEq for FallbackMode

fn eq(&self, other: &FallbackMode) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl Copy for FallbackMode

impl Eq for FallbackMode

impl StructuralPartialEq for FallbackMode