RFC3090 - DNS Security Extension Clarification on Zone Status

Network Working Group E. Lewis

Request for Comments: 3090 NAI Labs

Category: Standards Track March 2001

DNS Security Extension Clarification on Zone Status

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Abstract

The definition of a secured zone is presented, clarifying and

updating sections of RFC2535. RFC2535 defines a zone to be secured

based on a per algorithm basis, e.g., a zone can be secured with RSA

keys, and not secured with DSA keys. This document changes this to

define a zone to be secured or not secured regardless of the key

algorithm used (or not used). To further simplify the determination

of a zone's status, "eXPerimentally secure" status is deprecated.

1 IntrodUCtion

Whether a DNS zone is "secured" or not is a question asked in at

least four contexts. A zone administrator asks the question when

configuring a zone to use DNSSEC. A dynamic update server asks the

question when an update request arrives, which may require DNSSEC

processing. A delegating zone asks the question of a child zone when

the parent enters data indicating the status the child. A resolver

asks the question upon receipt of data belonging to the zone.

1.1 When a Zone's Status is Important

A zone administrator needs to be able to determine what steps are

needed to make the zone as secure as it can be. Realizing that due

to the distributed nature of DNS and its administration, any single

zone is at the mercy of other zones when it comes to the appearance

of security. This document will define what makes a zone qualify as

secure.

A name server performing dynamic updates needs to know whether a zone

being updated is to have signatures added to the updated data, NXT

records applied, and other required processing. In this case, it is

conceivable that the name server is configured with the knowledge,

but being able to determine the status of a zone by examining the

data is a desirable alternative to configuration parameters.

A delegating zone is required to indicate whether a child zone is

secured. The reason for this requirement lies in the way in which a

resolver makes its own determination about a zone (next paragraph).

To shorten a long story, a parent needs to know whether a child

should be considered secured. This is a two part question. Under

what circumstances does a parent consider a child zone to be secure,

and how does a parent know if the child conforms?

A resolver needs to know if a zone is secured when the resolver is

processing data from the zone. Ultimately, a resolver needs to know

whether or not to expect a usable signature covering the data. How

this determination is done is out of the scope of this document,

except that, in some cases, the resolver will need to contact the

parent of the zone to see if the parent states that the child is

secured.

1.2 Islands of Security

The goal of DNSSEC is to have each zone secured, from the root zone

and the top-level domains down the hierarchy to the leaf zones.

Transitioning from an unsecured DNS, as we have now, to a fully

secured - or "as much as will be secured" - tree will take some time.

During this time, DNSSEC will be applied in various locations in the

tree, not necessarily "top down."

For example, at a particular instant, the root zone and the "test."

TLD might be secured, but region1.test. might not be. (For

reference, let's assume that region2.test. is secured.) However,

subarea1.region1.test. may have gone through the process of becoming

secured, along with its delegations. The dilemma here is that

subarea1 cannot get its zone keys properly signed as its parent zone,

region1, is not secured.

The colloquial phrase describing the collection of contiguous secured

zones at or below subarea1.region1.test. is an "island of security."

The only way in which a DNSSEC resolver will come to trust any data

from this island is if the resolver is pre-configured with the zone

key(s) for subarea1.region1.test., i.e., the root of the island of

security. Other resolvers (not so configured) will recognize this

island as unsecured.

An island of security begins with one zone whose public key is pre-

configured in resolvers. Within this island are subzones which are

also secured. The "bottom" of the island is defined by delegations

to unsecured zones. One island may also be on top of another -

meaning that there is at least one unsecured zone between the bottom

of the upper island and the root of the lower secured island.

Although both subarea1.region1.test. and region2.test. have both been

properly brought to a secured state by the administering staff, only

the latter of the two is actually "globally" secured - in the sense

that all DNSSEC resolvers can and will verify its data. The former,

subarea1, will be seen as secured by a subset of those resolvers,

just those appropriately configured. This document refers to such

zones as being "locally" secured.

In RFC2535, there is a provision for "certification authorities,"

entities that will sign public keys for zones such as subarea1.

There is another document, [RFC3008], that restricts this activity.

Regardless of the other document, resolvers would still need proper

configuration to be able to use the certification authority to verify

the data for the subarea1 island.

1.2.1 Determining the closest security root

Given a domain, in order to determine whether it is secure or not,

the first step is to determine the closest security root. The

closest security root is the top of an island of security whose name

has the most matching (in order from the root) right-most labels to

the given domain.

For example, given a name "sub.domain.testing.signed.exp.test.", and

given the secure roots "exp.test.", "testing.signed.exp.test." and

"not-the-same.xy.", the middle one is the closest. The first secure

root shares 2 labels, the middle 4, and the last 0.

The reason why the closest is desired is to eliminate false senses of

insecurity because of a NULL key. Continuing with the example, the

reason both "testing..." and "exp.test." are listed as secure root is

presumably because "signed.exp.test." is unsecured (has a NULL key).

If we started to descend from "exp.test." to our given domain

(sub...), we would encounter a NULL key and conclude that sub... was

unsigned. However, if we descend from "testing..." and find keys

"domain...." then we can conclude that "sub..." is secured.

Note that this example assumes one-label deep zones, and assumes that

we do not configure overlapping islands of security. To be clear,

the definition given should exclude "short.xy.test." from being a

closest security root for "short.xy." even though 2 labels match.

Overlapping islands of security introduce no conceptually interesting

ideas and do not impact the protocol in anyway. However, protocol

implementers are advised to make sure their code is not thrown for a

loop by overlaps. Overlaps are sure to be configuration problems as

islands of security grow to encompass larger regions of the name

space.

1.3 Parent Statement of Child Security

In 1.1 of this document, there is the comment "the parent states that

the child is secured." This has caused quite a bit of confusion.

The need to have the parent "state" the status of a child is derived

from the following observation. If you are looking to see if an

answer is secured, that it comes from an "island of security" and is

properly signed, you must begin at the (appropriate) root of the

island of security.

To find the answer you are inspecting, you may have to descend

through zones within the island of security. Beginning with the

trusted root of the island, you descend into the next zone down. As

you trust the upper zone, you need to get data from it about the next

zone down, otherwise there is a vulnerable point in which a zone can

be hijacked. When or if you reach a point of traversing from a

secured zone to an unsecured zone, you have left the island of

security and should conclude that the answer is unsecured.

However, in RFC2535, section 2.3.4, these Words seem to conflict

with the need to have the parent "state" something about a child:

There MUST be a zone KEY RR, signed by its superzone, for every

subzone if the superzone is secure. This will normally appear in

the subzone and may also be included in the superzone. But, in

the case of an unsecured subzone which can not or will not be

modified to add any security RRs, a KEY declaring the subzone to

be unsecured MUST appear with the superzone signature in the

superzone, if the superzone is secure.

The confusion here is that in RFC2535, a secured parent states that

a child is secured by SAYING NOTHING ("may also be" as opposed to

"MUST also be"). This is counter intuitive, the fact that an absence

of data means something is "secured." This notion, while acceptable

in a theoretic setting has met with some discomfort in an operation

setting. However, the use of "silence" to state something does

indeed work in this case, so there hasn't been sufficient need

demonstrated to change the definition.

1.4 Impact on RFC2535

This document updates sections of RFC2535. The definition of a

secured zone is an update to section 3.4 of the RFC. Section 3.4 is

updated to eliminate the definition of experimental keys and

illustrate a way to still achieve the functionality they were

designed to provide. Section 3.1.3 is updated by the specifying the

value of the protocol octet in a zone key.

1.5 "MUST" and other key words

The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY"

in this document are to be interpreted as described in [RFC2119].

Currently, only "MUST" is used in this document.

2 Status of a Zone

In this section, rules governing a zone's DNSSEC status are

presented. There are three levels of security defined: global,

local, and unsecured. A zone is globally secure when it complies

with the strictest set of DNSSEC processing rules. A zone is locally

secured when it is configured in such a way that only resolvers that

are appropriately configured see the zone as secured. All other

zones are unsecured.

Note: there currently is no document completely defining DNSSEC

verification rules. For the purposes of this document, the strictest

rules are assumed to state that the verification chain of zone keys

parallels the delegation tree up to the root zone. (See 2.b below.)

This is not intended to disallow alternate verification paths, just

to establish a baseline definition.

To avoid repetition in the rules below, the following terms are

defined.

2.a Zone signing KEY RR - A KEY RR whose flag field has the value 01

for name type (indicating a zone key) and either value 00 or value 01

for key type (indicating a key permitted to authenticate data). (See

RFC2535, section 3.1.2). The KEY RR also has a protocol octet value

of DNSSEC (3) or ALL (255).

The definition updates RFC2535's definition of a zone key. The

requirement that the protocol field be either DNSSEC or ALL is a new

requirement (a change to section 3.1.3.)

2.b On-tree Validation - The authorization model in which only the

parent zone is recognized to supply a DNSSEC-meaningful signature

that is used by a resolver to build a chain of trust from the child's

keys to a recognized root of security. The term "on-tree" refers to

following the DNS domain hierarchy (upwards) to reach a trusted key,

presumably the root key if no other key is available. The term

"validation" refers to the digital signature by the parent to prove

the integrity, authentication and authorization of the child's key to

sign the child's zone data.

2.c Off-tree Validation - Any authorization model that permits domain

names other than the parent's to provide a signature over a child's

zone keys that will enable a resolver to trust the keys.

2.1 Globally Secured

A globally secured zone, in a nutshell, is a zone that uses only

mandatory to implement algorithms (RFC2535, section 3.2) and relies

on a key certification chain that parallels the delegation tree (on-

tree validation). Globally secured zones are defined by the

following rules.

2.1.a. The zone's apex MUST have a KEY RR set. There MUST be at

least one zone signing KEY RR (2.a) of a mandatory to implement

algorithm in the set.

2.1.b. The zone's apex KEY RR set MUST be signed by a private key

belonging to the parent zone. The private key's public companion

MUST be a zone signing KEY RR (2.a) of a mandatory to implement

algorithm and owned by the parent's apex.

If a zone cannot get a conforming signature from the parent zone, the

child zone cannot be considered globally secured. The only exception

to this is the root zone, for which there is no parent zone.

2.1.c. NXT records MUST be deployed throughout the zone. (Clarifies

RFC2535, section 2.3.2.) Note: there is some operational discomfort

with the current NXT record. This requirement is open to

modification when two things happen. First, an alternate mechanism

to the NXT is defined and second, a means by which a zone can

indicate that it is using an alternate method.

2.1.d. Each RR set that qualifies for zone membership MUST be signed

by a key that is in the apex's KEY RR set and is a zone signing KEY

RR (2.a) of a mandatory to implement algorithm. (Updates 2535,

section 2.3.1.)

Mentioned earlier, the root zone is a special case. The root zone

will be considered to be globally secured provided that if conforms

to the rules for locally secured, with the exception that rule 2.1.a.

be also met (mandatory to implement requirement).

2.2 Locally Secured

The term "locally" stems from the likely hood that the only resolvers

to be configured for a particular zone will be resolvers "local" to

an organization.

A locally secured zone is a zone that complies with rules like those

for a globally secured zone with the following exceptions. The

signing keys may be of an algorithm that is not mandatory to

implement and/or the verification of the zone keys in use may rely on

a verification chain that is not parallel to the delegation tree

(off-tree validation).

2.2.a. The zone's apex MUST have a KEY RR set. There MUST be at

least one zone signing KEY RR (2.a) in the set.

2.2.b. The zone's apex KEY RR set MUST be signed by a private key and

one of the following two subclauses MUST hold true.

2.2.b.1 The private key's public companion MUST be pre-configured in

all the resolvers of interest.

2.2.b.2 The private key's public companion MUST be a zone signing KEY

RR (2.a) authorized to provide validation of the zone's apex KEY RR

set, as recognized by resolvers of interest.

The previous sentence is trying to convey the notion of using a

trusted third party to provide validation of keys. If the domain

name owning the validating key is not the parent zone, the domain

name must represent someone the resolver trusts to provide

validation.

2.2.c. NXT records MUST be deployed throughout the zone. Note: see

the discussion following 2.1.c.

2.2.d. Each RR set that qualifies for zone membership MUST be signed

by a key that is in the apex's KEY RR set and is a zone signing KEY

RR (2.a). (Updates 2535, section 2.3.1.)

2.3 Unsecured

All other zones qualify as unsecured. This includes zones that are

designed to be experimentally secure, as defined in a later section

on that topic.

2.4 Wrap up

The designation of globally secured, locally secured, and unsecured

are merely labels to apply to zones, based on their contents.

Resolvers, when determining whether a signature is expected or not,

will only see a zone as secured or unsecured.

Resolvers that follow the most restrictive DNSSEC verification rules

will only see globally secured zones as secured, and all others as

unsecured, including zones which are locally secured. Resolvers that

are not as restrictive, such as those that implement algorithms in

addition to the mandatory to implement algorithms, will see some

locally secured zones as secured.

The intent of the labels "global" and "local" is to identify the

specific attributes of a zone. The words are chosen to assist in the

writing of a document recommending the actions a zone administrator

take in making use of the DNS security extensions. The words are

explicitly not intended to convey a state of compliance with DNS

security standards.

3 Experimental Status

The purpose of an experimentally secured zone is to facilitate the

migration from an unsecured zone to a secured zone. This distinction

is dropped.

The objective of facilitating the migration can be achieved without a

special designation of an experimentally secure status.

Experimentally secured is a special case of locally secured. A zone

administrator can achieve this by publishing a zone with signatures

and configuring a set of test resolvers with the corresponding public

keys. Even if the public key is published in a KEY RR, as long as

there is no parent signature, the resolvers will need some pre-

configuration to know to process the signatures. This allows a zone

to be secured with in the sphere of the experiment, yet still be

registered as unsecured in the general Internet.

4 IANA Considerations

This document does not request any action from an assigned number

authority nor recommends any actions.

5 Security Considerations

Without a means to enforce compliance with specified protocols or

recommended actions, declaring a DNS zone to be "completely" secured

is impossible. Even if, assuming an omnipotent view of DNS, one can

declare a zone to be properly configured for security, and all of the

zones up to the root too, a misbehaving resolver could be duped into

believing bad data. If a zone and resolver comply, a non-compliant

or subverted parent could interrupt operations. The best that can be

hoped for is that all parties are prepared to be judged secure and

that security incidents can be traced to the cause in short order.

6 Acknowledgements

The need to refine the definition of a secured zone has become

apparent through the efforts of the participants at two DNSSEC

workshops, sponsored by the NIC-SE (.se registrar), CAIRN (a DARPA-

funded research network), and other workshops. Further discussions

leading to the document include Olafur Gudmundsson, Russ Mundy,

Robert Watson, and Brian Wellington. Roy Arends, Ted Lindgreen and

others have contributed significant input via the namedroppers

mailing list.

7 References

[RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",

STD 13, RFC1034, November 1987.

[RFC1035] Mockapetris, P., "Domain Names - Implementation and

Specification", STD 13, RFC1035, November 1987.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate

Requirement Levels", BCP 14, RFC2119, March 1997.

[RFC2136] Vixie, P., (Ed.), Thomson, S., Rekhter, Y. and J. Bound,

"Dynamic Updates in the Domain Name System", RFC2136,

April 1997.

[RFC2535] Eastlake, D., "Domain Name System Security Extensions", RFC

2535, March 1999.

[RFC3007] Wellington, B., "Simple Secure Domain Name System (DNS)

Dynamic Update", RFC3007, November 2000.

[RFC3008] Wellington, B., "Domain Name System Security (DNSSEC)

Signing Authority", RFC3008, November 2000.

10 Author's Address

Edward Lewis

NAI Labs

3060 Washington Road Glenwood

MD 21738

Phone: +1 443 259 2352

EMail: lewis@tislabs.com

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