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

Fellow Task Force members,

We had a Zoom session last week, and discussed all the feedback on the 
document.

I made a bunch of edits (listed below) and think the document is 
basically ready to be send to the RIPE NCC for editing and/or to Mirjam 
for review. Please have a look and let me know what you think!

One minor outstanding thing is that Andronikos said that he would track 
down a reference about BIND connection tracking recommendations, which I 
think is this:

https://kb.isc.org/docs/aa-01183

But I cannot remember in what context this was, or where we wanted to 
put it in the document. Please help clue me in!

Anyway, I made the following edits based on our conversation:

* Added a section explaining that this document does not use RFC 2119
   language.

* Reduced the strength of recommendation for aggressive NSEC caching
   from "should" to "may".

* Reduced the strength of recommendation for interoperable DNS cookies
   from "should" to "may".

* Changed recommendation to disclose the list of blocked websites and
   services to recognize that it is not always possible.

* Added recommendation to disclose the source of block lists when
   possible.

* Reworked RPZ section to be a generic block list section, although
   RPZ is mentioned as a specific technology for distributing such
   lists.

* Added recommendation to include DNS Extended Error codes for blocked
   traffic.

* Replaced the section on anycasting with David Knight's text, which
   covers high availability in general and anycasting as a single case
   within that.

* ZONEMD is now in production, so the section mentioning that it is
   being deployed was updated.

* Noted that privacy policies should mention the sampling rate of DNS
   queries/responses kept.

* Moved the ad policy out of logging considerations.

* Removed the phrase "this is usually enabled by default" describing
   DNSSEC validation, to avoid the chance that resolver operators may
   mistakenly overlook this if it is not enabled.

* De-emphasized ALL DNS resolver operators.

* Changed recommendation from all of DoT+DoH+DoQ to one of them.

* Removed some stray text.

* Removed suggestions that a lower maximum TTL may reduce cache size
   or save memory.

* Increased the strength of the recommendation for trust anchor
   reporting from "may" to "should".

Thank you all for your efforts here. We're almost ready for champagne!

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.

### About Recomendation Text

This is not a standards document, and does not propose any way to
measure compliance or interoperability. It does use words like
"should" or "may be" throughout. These are meant to be interpreted in
the usual English sense, and not as IETF-style RFC 2119 jargon.

## 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).

#### High Availability

This can be considered in terms of local and global scope.

##### Local scope

Inside a single location/region, such as an office, campus, or small
ISP network, the main availability concern is that a resolver is
always reachable.  Client systems can be configured with multiple
resolver addresses, but the failover behaviour of stub resolvers to a
second address can be painful.  Ideally the primary address is highly
available and such fallback rarely required. How much effort is put
into ensuring this is true should probably scale in line with the
number of users, or sensitivity of the clients using that resolver to
delayed resolution.

There are several ways to promote high availability of an individual
resolver address, such as dedicated load balancing equipment, or
network techniques like VRRP, or IP anycast. These generally have in
common a pool of recursive servers and the means to direct queries to
them when a health check has determined them to be capable of
answering those queries.

Dedicated free or commercially produced, hardware or software load
balancing solutions are available. These typically own the resolver IP
address and forward queries to the currently available instances of a
pool of recursive servers.

VRRP enables a technique to make the resolver IP address available on
multiple servers, often used to provide automatic failover between
two.  A pool of recursive servers using this technique must reside in
the same broadcast domain.

IP anycast in the local scope typically involves a pool of recursive
servers advertising a route to a shared resolver IP address into a
routing protocol.  This can be configured in failover or load-sharing
configurations. A load sharing configuration typically requires
network equipment able to balance traffic to a destination over equal
cost paths (ECMP). A pool of recursive servers using this technique
can be distributed in different parts of the network.

##### Global scope

The same concerns as for local service availability are present in the
global scope, with the added issue that DNS resolution over long
distances may be slow. Practically speaking, only multiple resolver
addresses, or IP anycast are useful strategies here. The motivations
for finding better failover solutions than multiple resolver addresses
have been covered above.

IP anycast in the global scope means routing the same IP prefix to
more than one location. This can provide effective solutions for
failover and, when optimally configured for routing client queries to
the topologically least distant recursive server location. IP anycast
in the global scope requires the use of globally routable prefixes. 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.

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

[RFC4786](https://datatracker.ietf.org/doc/html/rfc4786) discuses
operation of anycast services.

##### Generally

Operators of a globally scoped recursive service are encouraged to
also adopt the local scope recommendations in each of the locations
where the service is provisioned.

Though the above deals with the shortcomings of reliance on stub
resolver failover between a list of addresses those recommendations
shouldn’t be seen as an exclusive alternative. Multiple resolver
addresses, where each is provisioned using differing failover
strategies, can provide a resolver of last resort and further improved
resilience.

#### 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.

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

**At least one of 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 may 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.

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 available
for the root zone.

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

### DNS Cookies

**Interoperable DNS Cookies may 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.
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.

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 should 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/).)

    Such policies should explicitly mention the sampling rate of DNS
    queries/responses that are kept, and whether these are anonymized.

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. Encryption: 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

#### Advertisement Policy

If there is any advertising from the service, the policy should be
published as well as how it can potentially affect the users' privacy.

### Filtering and blocking

#### Block Lists

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).

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. This allows updates to occur very quickly. If RPZ
or some other high-speed blocking technology is used, the parties
supplying these sources 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/).

#### 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, when possible (disclosing such a list may not be allowed by
law or regulation). Similarly, the resolver should disclose the source
of such block lists, when possible.

If possible, resolvers should provide information about blocked
responses via the Extended DNS Error with the Blocked, Censored,
Filtered, or Prohibited code - whichever applies best - along with a
text why the response was blocked, censored, filtered, or prohibited.

[RFC 8914](https://www.rfc-editor.org/rfc/rfc8914.html#section-4.16)
provides information about the meanings of the different codes.

**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.

#### 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.
  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.

### About Recomendation Text

This is not a standards document, and does not propose any way to
measure compliance or interoperability. It does use words like
"should" or "may be" throughout. These are meant to be interpreted in
the usual English sense, and not as IETF-style RFC 2119 jargon.

## 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).

#### High Availability

This can be considered in terms of local and global scope.

##### Local scope

Inside a single location/region, such as an office, campus, or small
ISP network, the main availability concern is that a resolver is
always reachable.  Client systems can be configured with multiple
resolver addresses, but the failover behaviour of stub resolvers to a
second address can be painful.  Ideally the primary address is highly
available and such fallback rarely required. How much effort is put
into ensuring this is true should probably scale in line with the
number of users, or sensitivity of the clients using that resolver to
delayed resolution.

There are several ways to promote high availability of an individual
resolver address, such as dedicated load balancing equipment, or
network techniques like VRRP, or IP anycast. These generally have in
common a pool of recursive servers and the means to direct queries to
them when a health check has determined them to be capable of
answering those queries.

Dedicated free or commercially produced, hardware or software load
balancing solutions are available. These typically own the resolver IP
address and forward queries to the currently available instances of a
pool of recursive servers.

VRRP enables a technique to make the resolver IP address available on
multiple servers, often used to provide automatic failover between
two.  A pool of recursive servers using this technique must reside in
the same broadcast domain.

IP anycast in the local scope typically involves a pool of recursive
servers advertising a route to a shared resolver IP address into a
routing protocol.  This can be configured in failover or load-sharing
configurations. A load sharing configuration typically requires
network equipment able to balance traffic to a destination over equal
cost paths (ECMP). A pool of recursive servers using this technique
can be distributed in different parts of the network.

##### Global scope

The same concerns as for local service availability are present in the
global scope, with the added issue that DNS resolution over long
distances may be slow. Practically speaking, only multiple resolver
addresses, or IP anycast are useful strategies here. The motivations
for finding better failover solutions than multiple resolver addresses
have been covered above.

IP anycast in the global scope means routing the same IP prefix to
more than one location. This can provide effective solutions for
failover and, when optimally configured for routing client queries to
the topologically least distant recursive server location. IP anycast
in the global scope requires the use of globally routable prefixes. 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.

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

[RFC4786](https://datatracker.ietf.org/doc/html/rfc4786) discuses
operation of anycast services.

##### Generally

Operators of a globally scoped recursive service are encouraged to
also adopt the local scope recommendations in each of the locations
where the service is provisioned.

Though the above deals with the shortcomings of reliance on stub
resolver failover between a list of addresses those recommendations
shouldn’t be seen as an exclusive alternative. Multiple resolver
addresses, where each is provisioned using differing failover
strategies, can provide a resolver of last resort and further improved
resilience.

#### 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.

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

**At least one of 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 may 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.

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 available
for the root zone.

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

### DNS Cookies

**Interoperable DNS Cookies may 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.
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.

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 should 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/).)

    Such policies should explicitly mention the sampling rate of DNS
    queries/responses that are kept, and whether these are anonymized.

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. Encryption: 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

#### Advertisement Policy

If there is any advertising from the service, the policy should be
published as well as how it can potentially affect the users' privacy.

### Filtering and blocking

#### Block Lists

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).

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. This allows updates to occur very quickly. If RPZ
or some other high-speed blocking technology is used, the parties
supplying these sources 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/).

#### 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, when possible (disclosing such a list may not be allowed by
law or regulation). Similarly, the resolver should disclose the source
of such block lists, when possible.

If possible, resolvers should provide information about blocked
responses via the Extended DNS Error with the Blocked, Censored,
Filtered, or Prohibited code - whichever applies best - along with a
text why the response was blocked, censored, filtered, or prohibited.

[RFC 8914](https://www.rfc-editor.org/rfc/rfc8914.html#section-4.16)
provides information about the meanings of the different codes.

**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.

#### 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.
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