[dns-resolver-tf] Draft of RIPE DNS Resolver Best Common Practices

Colleagues,

Here is a draft of the RIPE DNS Resolver Best Common Practices document
that we have been working on.

My intention is to collect feedback during the RIPE 87 meeting and
afterwards, and either publish a RIPE document or another draft based on
that.

I'm going to send a copy of this to the DNS working group list, and
mention that to the RIPE list and the cooperation working group list.

Cheers,

--
Shane

# DNS Resolver Recommendations

About the DNS Resolver Best Common Practice Task Force
  https://www.ripe.net/participate/ripe/tf/dns-resolver-best-common-practice-task-force

## Terminology

* Open Resolver: A DNS resolver that accepts queries from any client.
  Often the result of misconfiguration.

* Public Resolver: A resolver intentionally configured to be an open
  resolver.

## Introduction

### What Is This Document? Who Is It For?

This document presents recommendations and best current practices for
operating DNS resolvers, both public and non-public ones. It covers
technical aspects of operations and provides best practice
recommendations for data management, with a particular focus on user
privacy, security, and resilience.

The document serves as guidance for the wider Internet community,
offering input to:

* Those running public DNS resolver services, and
* Those who want to make informed choices between such services.

Its purpose is to provide clear guidance and promote effective
practices in DNS resolver operation.

The intended audience is not the entire DNS community. Advice here is
probably not useful for operators of authoritative servers, domain
registrars, and so on. It is also not meant to be an introductory or
educational document. There are many documents which cover the basics
of DNS and the roles of organizations in it; a good overview is:

Addressing the challenges of modern DNS - a comprehensive tutorial
by van der Toorn et al.
  https://ris.utwente.nl/ws/files/282427879/1_s2.0_S1574013722000132_main.pdf

The document does not consider how to measure adherence to these
recommendations. So it is not intended to be used for certification,
although certification created based on the principles here is
possible.

### How Is This Document Organized?

This document has a number of sections, and specific recommendations
in each section. The intent is for each recommendations to have clear
guidance at the top, and then background and discussion related to the
recommendation afterwards. Each recommendation indicates whether it is
mostly for operators of public resolvers or for operators of any
resolver.

## System and Network Hardening

### Infrastructure considerations

Running any Internet service requires attention to the infrastructure
used to operate it. This section discusses various approaches that can
be used to run a DNS resolver. Everything applies to both public and
non-public DNS resolvers.

#### Bare metal or public cloud

All DNS resolver software can run either on dedicated servers (rented
or colocated), or in virtualized clouds, or in a combination of those.
Every approach has pros and cons. Most of these are not specific to
running DNS resolvers, however, some of them are.

**Running DNS resolver instances as OS level daemons on bare metal
hosts:**

Pros:

  - Performance: Bare metal servers have direct access to the
    underlying hardware, and can offer superior performance/cost
    balance by avoiding the overhead associated with virtualization.
    Moreover, you have full control over the server's configurations,
    down to the hardware level, which can be beneficial for
    performance and cost optimization once you get the understanding
    of your typical work load during peak hours.

  - Data Security: Since you are in control of the physical servers,
    there is no risk of data leakage that can occur due to
    vulnerabilities in multi-tenant virtualization platforms,
    including CPU cache-based side-channel vulnerabilities. It could
    be argued that attacks targeting such issues are rare, and their
    impact on a DNS resolver service is low, but potential breaches
    may have significant privacy impact. It is advised to evaluate
    this against your organisation's risk model, or to discuss this
    with your information security compliance experts.

  - Predictability: Because there is no virtualization layer and no
    "noisy neighbours" on the host, the performance of your servers is
    more predictable.

Cons:

  - Cost of failure: If you pick hardware configuration that is not
    optimal for the workload of your DNS resolver, you may need to
    upgrade and replace hardware components afterwards. Ways to reduce
    this risk include renting servers instead of buying them, carrying
    load testing with data similar to production workloads, and
    providing limited beta access to the service before it fully
    enters the production phase.

  - Scalability: Scaling up with physical servers means acquiring or
    renting, installing, and configuring new hardware, which will take
    more time than provisioning new virtual servers in a cloud
    environment. Moreover, most cloud environments will provide you
    with cluster autoscaling features, which could barely be achieved
    in bare metal.

  - Maintenance: You will be responsible for all server maintenance
    tasks, including hardware issues, which can require significant
    effort and specific expertise.

  - Redundancy: Setting up high availability and disaster recovery
    strategies can be more complex and time consuming compared to the
    cloud, where these features are often provided as value added
    products. See the Redundancy section for more details.

**Running DNS resolver instances in containers in a public cloud:**

Pros:

  - Scalability: Clouds excel at scaling applications. You can scale
    up and down rapidly based on load, which is important for a DNS
    resolver that needs to handle variable query loads. In case of
    regional or geographically distributed resolvers, in every region
    where the resolver would be deployed, daily periodicity is likely
    to be observed, for example peak hour is likely to occur around
    19:00 local time, and off-peak hours may begin at around
    01:00-03:00. In a situation like that, using cluster autoscaling
    features and tools, you can run less instances in the night and
    more instances throughout the day, which may help to optimize your
    cloud hosting costs.

  - Fault Tolerance and High Availability: Most clouds have built-in
    strategies, features, and products for handling node failures,
    which can increase your service's availability.

  - Deployment and Management: Cloud providers offer built-in methods
    to deploy and manage applications, which can simplify operations
    and reduce the likelihood of human errors if your infrastructure
    management department is already familiar with these tools.

  - Cost: While this largely depends on your specific usage, cloud
    services can sometimes be more cost-effective than managing your
    own physical servers, especially when you consider the total cost
    of ownership, including power, cooling, and maintenance.

Cons:

  - Performance: The virtualization layer of public clouds can impact
    performance. While this certainly could be mitigated through
    scaling the number of virtual hosts, the cost would also increase
    accordingly.

  - Complexity: Advanced cloud technologies are complex systems which
    come with a steep learning curve. Without prior experience,
    properly configuring and managing a cloud-based compute cluster
    can be challenging.

  - Cost Variability: While the cloud can be cheaper, it can also be
    more expensive if not properly managed. Costs can rise
    unexpectedly based on traffic. Make sure to always set some limits
    on how much may be spent on hosting in the cloud control panel,
    and to set up notifications to be sent to you when these
    thresholds are about to be triggered.

  - Multi-tenancy Risks: In a public cloud environment, the "noisy
    neighbour" problem could potentially affect your service's
    performance. Additionally, even though cloud providers take steps
    to isolate tenant environments, vulnerabilities could potentially
    expose sensitive data (see the previous section for a detailed
    explanation).

**Additional considerations**

  - In today's environments, Kubernetes and Terraform are sometimes
    used as a substitute for cloud APIs when it comes to production
    services' management. When running a DNS resolver in a Kubernetes
    cluster on top of a public cloud environment, all the pros and
    cons of the public cloud apply; basically, Kubernetes becomes your
    public cloud provider. If you have significant prior experience
    running services in Kubernetes in production, you may successfully
    replicate your experience with the DNS resolver software.
    Otherwise, we would advise against Kubernetes in this case.

  - The only reason we may find to run a DNS resolver in a Kubernetes
    cluster on top of self-hosted dedicated servers is when you have
    significant hands-on experience with Kubernetes and it is natural
    for you to manage applications this way. Otherwise, running DNS
    resolver daemons in containers brings little, if any, benefit.
    Autoscaling features are not available to you in this case, and
    neither horizontal nor vertical pod autoscaling is of any use,
    because DNS resolver software typically scales in-host by itself
    just fine.

  - When designing a cluster of resolvers for autoscaling, keep in
    mind that newly spawned resolver machines would need to populate
    resolver cache first before they are fully useful. Your DNS
    resolver software may provide cache replication mechanisms.
    Otherwise, it is safe to overprovision clusters somewhat under
    heavy load, and discarding excessive instances once all the caches
    are populated and the average load of a compute instance
    decreases. In addition, it may be worthwhile to consider sharing
    cache data between instances.

  - It is always advised to prefer environments your infrastructure
    management team is familiar with.

### Software considerations

#### Open Source

**Recommendation**: Choose any well-maintained DNS software you are
comfortable using. Regardless of which software you choose, ensure you
have somewhere to go for support. In the case of open source software,
consider providing financial support to ensure continued development.
Some open source maintainers take donations, while others offer
support contracts.

There are both open source and proprietary implementations of DNS
resolver software. Mixing these is also possible, for example, by
using proprietary extensions with open source software or deploying
open source software modified in-house.

General observations:
  - Software licensing is orthogonal to software security. Neither is
    proprietary software less secure on principle nor are
    contributions by "unknown" developers more of a risk in open
    source.

Benefits of open source:
  - Open source allows for inspection, independent auditing, and
troubleshooting.
  - Open source can avoid vendor lock-in.
  - Open source can aid internet standards development.
    Widely-deployed open source implementations allow proponents of
    standards drafts to contribute proof of concept implementations
    without permission or cooperation of vendors.

Drawbacks of open source
  - Both open source and proprietary software require skilled
    maintenance, which has costs. Proprietary licensed software or
    appliances typically come with license fees to cover these. In
    contrast, open source licenses decouple usage by operators from
    monetary compensation to developers. It is up to operators to
    consider the financial sustainability of continued maintenance of
    the open source DNS software they depend upon.

Please also consider deploying different software implementations to
ensure diversity, as discussed in the diversity section below.

### Networking considerations

#### IPv4 and IPv6

**If available, both IPv4 and IPv6 must be deployed.**

Large parts of the authoritative DNS are only accessible via IPv4, so
the resolver must be able to originate IPv4 queries. Authoritative DNS
that is only accessible via IPv6 is very rare.

Depending on the connectivity of clients, a resolver may be IPv4-only,
IPv6-only, or support IPv4 and IPv6.

#### Addressing

**Using multiple IP addresses for the service address should be
considered.**

Using 2 or more IPv4 addresses and 2 or more IPv6 addresses from
different RIR will allow resilience in failure at an RIR, either
governance, security, or technical. Note that support for multiple
addresses for recursive resolvers varies and some clients perform
poorly if any address does not respond normally.

There is no need to pick an IPv4 address with all octets the same,
like 2.2.2.2 or 11.11.11.11.

**Publishing a list of back-end addresses used for resolving should be
considered.**

Publishing a list of back-end addresses used for resolving can be
useful for other network & DNS operators (for example, geo-IP
location, making sure data is getting to correct places, and so on).

#### Anycasting

**Anycasting may be considered**

Anycasting means routing the same IP prefix to more than one location.
As mentioned above for addressing, client support for multiple
addresses is not always good; with anycasting you can use a single IP
address and have redundancy from different sites. This will often
allow you to place sites close to the user - although it is tricky to
get optimal routing with BGP.

For a resolver service with a single site there is no benefit. For a
resolver service with multiple sites, it may be better to configure
clients with different IP addresses rather than use anycasting.

[RFC7094](https://www.rfc-editor.org/rfc/rfc7094.html) discusses
anycast in detail, including references to various other RFC which
discuss anycasting in general and to DNS in particular.

If a separate prefix is to be used for anycasting, usually this means
a /24 in IPv4 and a /48 in IPv6, as those are the smallest sizes that
will be widely propagated in BGP. A common practice is to use a
covering prefix (/23 in IPv4 or /47 in IPv6) for fallback, and a
more-specific prefix (/24 or /48) for the traffic. The more-specific
prefix can then be withdrawn to send traffic to a backup site; this
will happen automatically if the site is disconnected from routing.

#### Ingress Filtering

**Ingress Filtering to follow BCP 38 should be deployed.**

DNS normally uses UDP traffic, which makes it a common vector of both
[reflection](https://en.wikipedia.org/wiki/Reflection_attack) and
[amplification](https://www.cisa.gov/news-events/alerts/2014/01/17/udp-based-amplification-attacks)
attacks. To minimize the amount of spoofed traffic that a resolver
responds to, the network should be configured as recommended in 
[BCP 38](https://www.rfc-editor.org/rfc/rfc2827.html).

#### RPKI Sign Advertised Routes

**Route Advertisements should be signed using RPKI**

Using RPKI to sign any route advertisements - either toward
authoritative servers or toward DNS clients - is straightforward to do
and will reduce the impact of BGP misconfigurations and some BGP
hijacking attempts.

RPKI validation is also possible, although the effort is greater. It
is possible that the hosting provider or the transit provider for your
service validates BGP; asking and making this part of your selection
criteria is reasonable.

#### (D)DoS measures

Denial-of-Service (DoS) attacks, both distributed (DDoS) and not are a
threat to any Internet service. Network operators for a service
providing any DNS service must be prepared for large amounts of attack
traffic.

In addition to attacks on the service itself, a resolver may be used
both as an attack reflector and as an attack amplifier.

Active monitoring of network and service usage, careful logging, and a
security team that is able to respond to problem reports is necessary.
Mitigation techniques will include filtering or rate-limiting traffic,
both on the authoritative and client side of the resolver.

### Capacity planning

#### Server capacity

If using a model that is easy to scale (cloud based, or Kubernetes
based, or similar), then getting server capacity correct is largely a
question of budgeting. If using a less-flexible model (bare metal for
example), then under-estimating will mean problems delivering service.

Hardware performance varies widely, as does operating system and
resolver performance. Some lab testing will be necessary to estimate
the number of systems needed.

#### Network capacity

Since DNS is mostly UDP-based, it is often easy to generate large
amounts of spoofed traffic to and from DNS servers. DNS traffic is
small compared to application traffic (videos and other content), but
still significant. Authoritative server operators often build their
networks and servers to handle 10 times their normal load. Recursive
server operators may need to do the same, although the service only
accepts traffic from IP addresses that cannot be spoofed (for example
users within a network that operated by the same company) then this
can be reduced, for example to 3 times normal load. To estimate
expected load, the best approach is to examine historical usage for
the actual expected users of the system.

### Resilience

#### System Diversity

In addition to the software considerations above, operators should
consider whether to use different server implementations to provide
service. This allows continued operation if a critical vulnerability
is found in one implementation, by shifting traffic to other
implementations.

Placing resolvers and control systems in different physical locations
will allow continued operation in the event of a disaster or other
problem that impacts a single location. In addition, ensuring diverse
connectivity to other networks will prevent single points of failure
on the network side. Ensuring network diversity may take some care, as
it is not always obvious what fate is shared between any given path;
this may be physical, virtual, or organizational, and my sometimes be
hidden.

#### Security

In addition to the DNS-specific security considerations, normal
security best practices for any Internet service should be followed,
including updating software updated regularly, patching software as
soon as possible for any known security vulnerabilities, following
CERT announcements and so on.

#### Certification

It may be useful or required for an organization to follow specific
certifications, such as ISO or ITIL. These can be government-defined
or industry-defined. For end users there is typically not much direct
value, but business customers will often look for services that are
operated by organizations meeting such standards.

## DNS configuration knobs

The DNS is an old protocol that has a lot of settings that can be
tweaked. This section reviews these and provides recommendations on
which should be used for a resolver.

### DNSSEC validation

**DNSSEC validation should be enabled.**

For: All DNS resolver operators.

DNSSEC validation is the best way to ensure that the answers from the
owner of domain name being queried are returned.

The root KSK must be updated when it changes. While
[RFC5011](https://www.rfc-editor.org/rfc/rfc5011.html) defines an
automated way to do this, a resolver operator will probably either
manage this trust anchor directly or have it updated via OS updates.

[RFC9364](https://www.rfc-editor.org/rfc/rfc9364.html) provides a lot
of useful information, and links to further documents about DNSSEC.
However, operators usually do not need to know the details, and can
simply ensure that DNSSEC validation is enabled in their software;
this is usually enabled by default.

Resolver software that does not support DNSSEC validation should be
avoided.

### DNS Transport Protocols

**UDP and TCP must be supported.**

For: ALL DNS resolver operators.

UDP is what most clients use, and TCP is necessary for DNS answers
that are too large for a single UDP packet.

[RFC7766](https://www.rfc-editor.org/rfc/rfc7766.html) explains why
TCP is necessary in more detail.

### Packet Fragmentation Avoidance

**Servers should be configured to avoid fragmentation.**

For: ALL DNS resolver operators.

Packet fragmentation can cause issues with DNS over UDP, especially
over IPv6. These issues can be minimized by choosing implementations
that set IP options to avoid this, and by taking care with EDNS0
message sizes.

Recommendations are available in
[draft-ietf-dnsop-avoid-fragmentation](https://datatracker.ietf.org/doc/draft-ietf-dnsop-avoid-fragmentation/).

### Encrypted DNS

**DNS-over-TLS (DoT), DNS-over-HTTPS (DoH), and DNS-over-QUIC (DoQ)
should be supported.**

For: All DNS resolver operators.

DoT, DoH, and DoQ are different technologies that all provide an
encrypted channel between the resolver and the authoritative server.
DoT is the oldest, and provides encrypted DNS using TLS. DoH uses HTTP
over TLS as a way to transmit queries and answers, and is widely
supported by web browsers. DoQ is the newest, and provides advanced
features such as separate streams for each query, avoiding the "head
of line" blocking problem common with all protocols layered on top of
TCP (such as DoT and DoH).

- DoT
  - [RFC7858](https://www.rfc-editor.org/rfc/rfc7858.html)
- DoH
  - [RFC8484](https://www.rfc-editor.org/rfc/rfc8484.html)
- DoQ
  - [RFC9250](https://www.rfc-editor.org/rfc/rfc9250.html)

**Discovery of DNS Designated Resolvers**

There are new mechanisms that allow DNS clients to use DNS records to
discover encrypted DNS configurations.  Resolvers should publish DNS
records to assist clients finding encrypted resolvers.

- Discovery of Designated Resolvers
  - [RFC9462](https://www.rfc-editor.org/rfc/rfc9462.html)

QUESTION: Do we need to publish certificate in other ways that via the
DDR mechanisms?

### QNAME Minimization

**QNAME minimization should be enabled.**

For: All DNS resolver operators.

Using QNAME minimization, a resolver does not send the full name that
it is trying to resolver to authoritative servers higher in the DNS
hierarchy. So, for example, when querying "atlas.ripe.net" the servers
for ".net" would only be asked for "ripe.net". This improves privacy
for the end user querying the name.

[RFC7816](https://www.rfc-editor.org/rfc/rfc7816.html) covers QNAME
minimization.

### Aggressive NSEC caching

**Aggressive NSEC caching should be enabled.**

For: Public resolver operators.

"Aggressive NSEC caching", meaning negative caching based on NSEC and
NSEC3 values, can reduce traffic greatly. It is important to protect
against random subdomain attacks.

This style of caching takes advantage of the way that NSEC and NSEC3
records cover a range of names in a zone. A resolver can know that a
query falls within such a range without sending any further queries,
by remembering the NSEC or NSEC3 redords that is has seen as answers
to earlier queries.

Aggressive NSEC caching is almost always a good idea. However enabling
this is less important for DNS resolver operators who have a close
relationship with users, since they can stop attacks by blocking users
or otherwise directly dealing with the source of abusive queries.

[RFC8189](https://www.rfc-editor.org/rfc/rfc8189.html) describes
negative caching in detail.

### Local Root

**Local root should be used.**

For: Public resolver operators.

Since the root zone is DNSSEC signed,

Running a local root has several benefits, but it is an additional
component to maintain. For public resolver operators this is
definitely worth the cost, but other resolver operators may choose to
simply send all queries to the well-distributed root name servers.

[RFC8806](https://www.rfc-editor.org/rfc/rfc8806.html) describes local
root, including several example configurations.

In the future it will be possible to use ZONEMD to validate the copy
of the root zone obtained before using it. This is currently being
deployed for the root zone, but not yet available.

[RFC8976](https://www.rfc-editor.org/rfc/rfc8976.html) describes ZONEMD.

### DNS Cookies

**Interoperable DNS Cookies should be supported.**

For: Public resolver operators.

DNS cookies provide some improved security over plain UDP, and are
designed to be more lightweight than TCP. If more than one server is
responding for a given IP address, then the Server Secret must be
shared by all servers, and the answer must be constructed in a
consistent manner by all server implementations.

Since client-side support for DNS cookies is not very widespread, and
since managing server-side secrets involves some work, the costs may
outweigh the benefits for some non-public resolver operators.

[RFC7873](https://www.rfc-editor.org/rfc/rfc7873.html) describes DNS
cookies, and [RFC9018](https://www.rfc-editor.org/rfc/rfc9018.html)
standardizes shared DNS cookies.

### TTL Recommendations

**TTL limits may be adjusted.**

For: All DNS resolver operators.

Software typically defaults to a maximum stored TTL of 1 or 2 days.
This may be lowered to reduce the cache size. A lower TTL will mean
removing rarely-used records that have long TTL, and should not have
much operational impact from a CPU or network point of view, but may
save memory.

It is possible to set a minimum TTL in many implementations. This is a
violation of the DNS protocol, although may be useful to reduce load
from records with very low TTL (less than 5 seconds).

Note that software may set different maximum and minimum TTL
independent of the results that the resolver returns. That may have a
significant impact on queries as well, but resolver operators cannot
influence that.

### TTL-based Record Pre-Fetch

**TTL record pre-fetch should be enabled when available.**

For: All DNS resolver operators.

Some resolvers have the ability to look up a record before it has
expired from cache, in order to refresh the value and extend the TTL.
This way there is never a time when the records are missing from the
cache. This is not currently standardized, but a form of this was
proposed in the IETF as
[DNS
Hammer](https://datatracker.ietf.org/doc/html/draft-wkumari-dnsop-hammer-03).
We recommend turning this feature on if available.

### EDNS Client Subnet (ECS)

**ECS may be enabled.**

For: All DNS resolver operators.

EDNS Client Subnet (ECS) allows the resolver to include information
about the IP address of the client querying it when sending messages
to authoritative servers. This may allow authoritative servers to
provide different answers which are more appropriate for the client.
However, ECS will increase the amount of cache space required by
resolvers, may reduce DNS performance, and may have privacy
implications.

A resolver operator that has clients that are limited to a specific
region may see no benefit. A resolver operator that has a widely
distributed anycast network may not have much benefit from ECS, since
the locations that initiate the query will be close to the client. But
a resolver operator that answers client queries only from a few
locations, and expects clients to come from a wide area, may provide
better service for end-users by supporting ECS.

EDNS client subnet is described in
[RFC7871](https://www.rfc-editor.org/rfc/rfc7871.html), an
informational RFC.

### Extended DNS Errors

**Extended DNS errors should be enabled.**

For: All DNS resolver operators.

DNS traditionally provides very broad error reporting, SERVFAIL being
the most common. This makes diagnosing and fixing problems difficult.
Extended DNS errors provide extra information about failures, for
example expired DNSSEC signatures. They also allow resolver operators
to report administrative reasons for DNS failures, such as blocks due
to legal requirements.

[RFC8914](https://www.rfc-editor.org/rfc/rfc8914.html) defines
extended DNS errors.

### Negative Trust Anchors

**Negative trust anchors may be deployed.**

For: All DNS resolver operators.

Negative trust anchors (NTA) allow a resolver operator to handle a
case where an authoritative server has a DNSSEC problem and becomes
inaccessible. They basically disable DNSSEC checking for a domain.
When this is warranted is difficult to know with certainty, and will
usually requires some manual checking. Since DNSSEC validation is now
widespread, DNSSEC failures on the authoritative side will impact many
resolvers.

Because of these reasons this document does not recommend NTA, but
also does not recommend that a deployment avoid NTA if it makes sense
for that environment.

Negative trust anchors are documented in
[RFC7646](https://www.rfc-editor.org/rfc/rfc7646.html).

### DNS Error Reporting

**DNS error reporting may be enabled.**

For: All DNS resolver operators.

DNS error reporting is a way for resolver operators to let
authoritative operators know about problems in authoritative servers
or zones. It provides little direct value for the resolver operators,
but over time should improve the overall quality of the DNS ecosystem.
It is neither widely deployed nor standardized, but hopefully will be
both soon. Resolver operators are encouraged to enable DNS error
reporting when it is available.

DNS error reporting is proposed in
[draft-ietf-dnsop-dns-error-reporting](https://datatracker.ietf.org/doc/draft-ietf-dnsop-dns-error-reporting).

### Trust Anchor Reporting

**Trust anchor reporting may be enabled.**

For: All DNS resolver operators.

Trust anchor reporting is a way for resolver operators to convey their
DNSSEC trust anchor configuration to the operator of the zone that it
is for. For most resolvers this is only the root zone. This
information is intended to be used during a root KSK rollover to
ensure that it is safe to proceed. In the future ICANN is planning an
algorithm roll for the root KSK, and this information could be
helpful. Resolver operators are encouraged to enable trust anchor
reporting.

[RFC8145](https://www.rfc-editor.org/rfc/rfc8145.html) covers trust
anchor reporting, in both possibilities available.

## Privacy, Filtering, Transparency

### Privacy & anonymity

Operators are advised to apply
[RFC8932](https://www.rfc-editor.org/rfc/rfc8932.html)

"Recommendations for DNS Privacy Service Operators" as follows:

  1. its operational and policy guidance related to DNS encrypted
  transports and data handling, by applying all "Threat mitigations"
  (thereby by meeting its level of "minimally compliant") and
  additionally by applying the "Optimizations" on EDNS Client Subnet
  listed in section 5.3.1.

  2. its framework on a Recursive operator Privacy Statement, by
  publishing a privacy statement on their website that is compliant
  with Section 6.

#### Logging considerations

1. Public privacy policy: DNS resolvers are recommended to publish
   their privacy policies transparently on their website. It can be a
   brief privacy commitment as well or be more elaborate on how the
   privacy policy was made. (See for example
   [Cloudflare's
statement](https://developers.cloudflare.com/1.1.1.1/privacy/public-dns-resolver)
or [Quad9's privacy page](https://www.quad9.net/service/privacy/).)

2. Third party access to personal data: it seems that the only
   critical personal data that DNS resolvers collect are IP addresses
   and the queries that are resolved. The other meta data collected
   can be used to have an understanding of for example which user
   accessed which website which can reveal information about a
   person’s health, lifestyle and other personal preferences (we call
   this profiling). For example, resolving the website for alcoholics
   anonymous may tell you something about the health of a person
   behind an IP address. IP addresses are personally identifiable
   information. Follow the applicable privacy laws or privacy
   principles when receiving third party requests to access. Resolvers
   should only comply with such requests when balancing legitimate
   third party interest with other fundamental rights.

3. Access to data for researchers: how it is done, who has access and
   who can request access, how the resolver makes a decision to give
   access (validated and credible researchers, what they can access
   and other issues)

4. Data minimization: do not collect personal information not needed
   for critical operations.  Only retain or use what is being asked
   (the query). If collecting data to make the service more private
   and secure, explain the rationale for each piece of data (data
   collection purpose)

5. Ad policy and encryption: explain the ad policy and how it can
   potentially affect the users privacy. If data is encrypted, explain
   how it has been encrypted (DoH, DoT, or so on).

6. Data security and retention: when to delete the data and how it is
   stored

### Filtering and blocking

#### Legal blocking:
**Legal requests and blocking and filtering laws:** DNS resolvers
should not filter content and block access to web-services. When the
local law requires blocking, and the law applies to the resolver, the
resolver should transparently disclose a list of blocked websites and
services.

**Community governance of blocklists:** blocklists, if mandatory, have
to be audited and assessed by third parties and there should be a
right to appeal for those blocked. The Internet community can vet the
blocklists from time to time to avoid blocking access to websites that
are mistakenly blocked. During crisis - when mistakes can have drastic
effects on accessing a critical service - preferably filtering and
blocking should not be used.

#### RPZ-based filtering

Response Policy Zone (RPZ) allows a way to both document specific
modifications that resolvers will make to DNS answers, and send the
rules to resolvers. Resolvers can be directed to block or modify
answers in various ways. Blocklists may be provided by governments,
communities, or other parties (for example security firms) using RPZ.
This allows updates to occur very quickly. These source must be highly
trusted, as changes to blocklists will usually immediately impact user
queries.

RPZ is not standardized, but there is an IETF draft,
[draft-vixie-dnsop-dns-rpz](https://datatracker.ietf.org/doc/draft-vixie-dnsop-dns-rpz/00/).

#### Opt-in/Opt-out Mechanisms

End users may choose to use a DNS resolver that filters specific kinds
of traffic. For example, they may wish to avoid potential malware web
sites. Or resolver operators may be required to default to filtering
but allowed for to provide an unfiltered DNS resolver service.

Depending on the specific requirements, a resolver service may publish
different IP addresses and what type of filtering applies to each
address. It is also possible to perform client authentication and
authorization, using IP-based authentication, TSIG keys, or
client-side TLS certificates.

### Transparency

DNS resolvers usually provide transparency reports once a year. The
reports inform the public about disclosure of user information and
removal of content required by law enforcement and other government
agencies.

Transparency reports should (to the extent that the law allows)
indicate which government agencies and law enforcement agencies
request access on what basis.

It should also be clear from the transparency reports what kind of
data has been requested and if content removal and content blocking
have been requested. Categories of data include: Content Data, Basic
Subscriber Data, Other Non-Content Data and Content Blocking.

#### Voluntary certificates and standards

Some DNS resolvers opt for obtaining certificates in security and
privacy. Some also undertake audits on their privacy practices. See
for example:https://www.cloudflare.com/trust-hub/compliance-resources/

#### Human rights considerations

DNS resolvers can opt for declaring their understanding of their
responsibilities regarding human rights from the Universal Declaration
of Human Rights. Specifically, Quad9 mentions rights to freedoms
without distinction made on the basis of country, no interference with
privacy, the right to freedom of opinion and expression, the right to
peaceful assembly, and the right to freely participate in the cultural
life of the community.

See
[Quad9's Human Rights
Considerations](https://www.quad9.net/privacy/human-rights-considerations/)
for the full statement.

It also invokes other human rights related solutions other than
[UDHR](https://www.un.org/en/universal-declaration-human-rights/) such
as Articles 8 and 9 of Resolution 42/15 of the United Nations Human
Rights Council on the right to privacy in the digital age of 26
September 2019 more directly define the responsibilities of the
private sector toward the furtherance of human rights in modern terms.
They also follow the Guidelines for Human Rights Protocol and
Architecture Consideration of the Human Rights Protocol Considerations
Research Group at Internet Research Task Force.

The latest version of the IRTF
[Guidelines for
HRPC](https://tools.ietf.org/html/draft-irtf-hrpc-guidelines) may be
considered for all network operators.

## Appendix A: Why Did RIPE Write This Document?

There is increasing concern that large open DNS resolvers will become
centralised points of DNS operations on the Internet. In order to
address this, the European Commission issued the
[DNS4EU](https://hadea.ec.europa.eu/calls-proposals/equipping-backbone-networks-high-performance-and-secure-dns-resolution-infrastructures-works_en)
proposal. However, such an initiative could lead to centralised
guidance or regulation which might interfere with the decentralised
way the Internet infrastructure works - including the DNS. See for
reference the
[RIPE NCC Open House
discussion](https://labs.ripe.net/author/chrisb/dns4eu-ripe-ncc-open-house-discussion/)
on this topic.

Rather than attempting to respond to the EC proposal or organize
specific DNS resolver deployments, the RIPE community has decided that
it is best able to provide advice and guidance. The RIPE Community is
well positioned to provide a set of Best Current Practices that
operators of Open DNS Resolvers will be encouraged to subscribe to.