# DNS over HTTPS (Trusted Recursive Resolver)

## Terminology

**DNS-over-HTTPS (DoH)** allows DNS to be resolved with enhanced
privacy, secure transfers and comparable performance. The protocol is
described in [RFC 8484](https://tools.ietf.org/html/rfc8484) .

**Trusted Recursive Resolver (TRR)** is the name of Firefox\'s
implementation of the protocol and the
[policy](https://wiki.mozilla.org/Security/DOH-resolver-policy) that
ensures only privacy-respecting DoH providers are recommended by
Firefox.

On this page we will use DoH when referring to the protocol, and TRR
when referring to the implementation.

**Unencrypted DNS (Do53)** is the regular way most programs resolve DNS
names. This is usually done by the operating system by sending an
unencrypted packet to the DNS server that normally listens on port 53.

## DoH Rollout

**DoH Rollout** refers to the frontend code that decides whether TRR
will be enabled automatically for users in the [rollout
population](https://support.mozilla.org/kb/firefox-dns-over-https#w_about-the-us-rollout-of-dns-over-https).

The functioning of this module is described
[here](https://wiki.mozilla.org/Security/DNS_Over_HTTPS).

The code lives in
[browser/components/doh](https://searchfox.org/firefox-main/source/browser/components/doh).

## Implementation

When enabled TRR may work in two modes, TRR-first (2) and TRR-only (3).
These are controlled by the **network.trr.mode** or **doh-rollout.mode**
prefs. The difference is that when a DoH request fails in TRR-first
mode, we then fallback to **Do53**.

For TRR-first mode, we have a strict-fallback setting which can be
enabled by setting network.trr.strict\_native\_fallback to true. With
this, while we will still completely skip TRR for certain requests (like
captive portal detection, bootstrapping the TRR provider, etc.) we will
only fall back after a TRR failure to **Do53** for three possible
reasons:
1. We detected, via Confirmation, that TRR is currently out of
service on the network. This could mean the provider is down or blocked.
2. The address successfully resolved via TRR could not be connected to.
3. TRR result is NXDOMAIN.

When a DNS resolution doesn't use TRR we will normally preserve that data in the form of a _TRRSkippedReason_. A detailed explanation of each one is available [here](trr-skip-reasons).

In other cases, instead of falling back, we will trigger a fresh
Confirmation (which will start us on a fresh connection to the provider)
and retry the lookup with TRR again. We only retry once.

DNS name resolutions are performed in _nsHostResolver::ResolveHost_. If a
cached response for the request could not be found,
_nsHostResolver::NameLookup_ will trigger either a DoH or a Do53 request.
First it checks the effective TRR mode of the request is as requests
could have a different mode from the global one. If the request may use
TRR, then we dispatch a request in _nsHostResolver::TrrLookup_. Since we
usually resolve both IPv4 and IPv6 names, a **TRRQuery** object is
created to perform and combine both responses.

Once done, _nsHostResolver::CompleteLookup_ is called. If the DoH server
returned a valid response we use it, otherwise we report a failure in
TRR-only mode, or try Do53 in TRR-first mode.

**TRRService** controls the global state and settings of the feature.
Each individual request is performed by the **TRR** class.

Since HTTP channels in Firefox normally work on the main thread, TRR
uses a special implementation called **TRRServiceChannel** to avoid
congestion on the main thread.

## Dynamic Blocklist

In order to improve performance TRR service manages a dynamic blocklist
for host names that can\'t be resolved with DoH but work with the native
resolver. Blocklisted entries will not be retried over DoH for one
minute (See _network.trr.temp\_blocklist\_duration\_sec_
pref). When a domain is added to the blocklist, we also check if there
is an NS record for its parent domain, in which case we add that to the
blocklist. This feature is controlled by the
_network.trr.temp\_blocklist_ pref.

## TRR Confirmation

TRR requests normally have a 1.5 second timeout. If for some reason we
do not get a response in that time we fall back to Do53. To avoid this
delay for all requests when the DoH server is not accessible, we perform
a confirmation check. If the check fails, we conclude that the server is
not usable and will use Do53 directly. The confirmation check is retried
periodically to check if the TRR connection is functional again.

The confirmation state has one of the following values:

-   CONFIRM\_OFF: TRR is turned off, so the service is not active.
-   CONFIRM\_TRING\_OK: TRR in on, but we are not sure yet if the
        DoH server is accessible. We optimistically try to resolve via
        DoH and fall back to Do53 after 1.5 seconds. While in this state
        the TRRService will be performing NS record requests to the DoH
        server as a connectivity check. Depending on a successful
        response it will either transition to the CONFIRM\_OK or
        CONFIRM\_FAILED state.
-   CONFIRM\_OK: TRR is on and we have confirmed that the DoH server
        is behaving adequately. Will use TRR for all requests (and fall
        back to Do53 in case of timeout, NXDOMAIN, etc).
-   CONFIRM\_FAILED: TRR is on, but the DoH server is not
        accessible. Either we have no network connectivity, or the
        server is down. We don\'t perform DoH requests in this state
        because they are sure to fail.
-   CONFIRM\_TRYING\_FAILED: This is equivalent to CONFIRM\_FAILED,
        but we periodically enter this state when rechecking if the DoH
        server is accessible.
-   CONFIRM\_DISABLED: We are in this state if the browser is in
        TRR-only mode, or if the confirmation was explicitly disabled
        via pref.

The state machine for the confirmation is defined in the
_HandleConfirmationEvent_ method in _TRRService.cpp_

If strict fallback mode is enabled, Confirmation will set a flag to
refresh our connection to the provider.

## Excluded Domains

Some domains will never be resolved via TRR. This includes:

-   domains listed in the **network.trr.builtin-excluded-domains** pref
(normally domains that are equal or end in *localhost* or *local*)
-   domains listed in the **network.trr.excluded-domains** pref (chosen by the user)
-   domains that are subdomains of the network\'s DNS suffix
(for example if the network has the **lan** suffix, domains such as **computer.lan** will not use TRR)
-   requests made by Firefox to check for the existence of a captive-portal
-   requests made by Firefox to check the network\'s IPv6 capabilities
-   domains listed in _/etc/hosts_

## Steering


A small set of TRR providers are only available on certain networks.
Detection is performed in DoHHeuristics.sys.mjs followed by a call to
_TRRService::SetDetectedURI_. This causes Firefox to use the
network specific TRR provider until a network change occurs.

## User Choice

The TRR feature is designed to prioritize user choice before user agent
decisions. That means the user may explicitly disable TRR by setting
**network.trr.mode** to **5** (TRR-disabled), and that
_doh-rollout_ will not overwrite user settings. Changes to
the TRR URL or TRR mode by the user will disable heuristics use the user
configured settings.
