Network Working Group A. Brotman
Internet-Draft Comcast, Inc
Intended status: Standards Track S. Farrell
Expires: January 9, 2020 Trinity College Dublin
July 8, 2019

Related Domains By DNS


This document outlines a mechanism by which a DNS domain can publicly document the existence or absence of a relationship with a different domain, called "Related Domains By DNS", or "RDBD".

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Table of Contents

1. Introduction

[[Discussion of this draft is taking place on the mailing list. There's a github repo for this draft at <> - issues and PRs are welcome there.]]

Determining relationships between registered domains can be one of the more difficult investigations on the Internet. It is typical to see something such as and and be unsure if there is an actual relationship between those two domains, or if one might be an attacker attempting to impersonate the other. In some cases, anecdotal evidence from the DNS or WHOIS/RDAP may be sufficient. However, service providers of various kinds may err on the side of caution and treat one of the domains as untrustworthy or abusive because it is not clear that the two domains are in fact related. This specification provides a way for one domain to explicitly document a relationship with another, utilizing DNS records.

Possible use cases include:

Similarly, a domain may wish to declare that no relationship exists with some other domain, for example "good.example" may want to declare that it is not associated with "g00d.example" if the latter is currently being used in some cousin-domain style attack. In such cases, it is more likely that there can be a larger list of names (compared to the "positive" use-cases) for which there is a desire to disavow a relationship.

It is not a goal of this specification to provide a high-level of assurance as to whether or not two domains are definitely related, nor to provide fine-grained detail about the kind of relationship that may exist between domains.

Using "Related Domains By DNS", or "RDBD", it is possible to declare that two domains are related, or to disavow such a relationship.

We include an optional digital signature mechanism that can somewhat improve the level of assurance with which an RDBD declaration can be handled. This mechanism is partly modelled on how DKIM [RFC6376] handles public keys and signatures - a public key is hosted at the relating-domain (e.g., and a reference from the related-domain (e.g., contains a signature (verifiable with the public key) over the text representation ('A-label') of the two domain names (plus a couple of other inputs).

RDBD is intended to declare or disavow a relationship between registered domains, not individual hostnames. That is to say that the relationship should exist between and, not and (where those latter two are hosts).

There already exists Vouch By Reference (VBR) [RFC5518], however this only applies to email. RDBD could be a more general purpose solution that could be applied to other use cases, as well as for SMTP transactions.

This document describes the various options, how to create records, and the method of validation, if the option to use digital signatures is chosen.

1.1. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

The following terms are used throughout this document:

2. New Resource Record Types

We define two new RRTYPES, an optional one for the relating-domain (RDBDKEY) to store a public key for when signatures are in use and one for use in related-domains (RDBD).

2.1. RDBDKEY Resource Record Definition

The RDBDKEY record is published at the apex of the relating-domain zone.

The wire and presentation format of the RDBDKEY resource record is identical to the DNSKEY record. [RFC4034]

[[All going well, at some point we'll be able to say...]] IANA has allocated RR code TBD for the RDBDKEY resource record via Expert Review.

The RDBDKEY RR uses the same registries as DNSKEY for its fields. (This follows the precedent set for CDNSKEY in [RFC7344].)

No special processing is performed by authoritative servers or by resolvers, when serving or resolving. For all practical purposes, RDBDKEY is a regular RR type.

The flags field of RDBDKEY records MUST be zero. [[Is that correct/ok? I've no idea really:-)]]

There can be multiple occurrences of the RDBDKEY resource record in the same zone

2.2. RDBD Resource Record Definition

To declare a relationship exists an RDBD resource record is published at the apex of the related-domain zone.

To disavow a relationship an RDBD resource record is published at the apex of the relating-domain zone.

[[All going well, at some point we'll be able to say...]] IANA has allocated RR code TBD for the RDBD resource record via Expert Review.

The RDBD RR is class independent.

The RDBD RR has no special Time to Live (TTL) requirements.

There can be multiple occurrences of the RDBD resource record in the same zone.

The wire format for an RDBD RDATA consists of a two octet rdbd-tag, the relating-domain name(s), and the optional signature fields which are: a two-octet key-tag, a one-octet signature algorithm, and the digital signature bits.

                        1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |           rdbd-tag            |                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               /
   /                        relating-domain name(s)                /
   |    key-tag                    | sig-alg     |                 /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                 /
   /                            signature                          /

We define two possible values for the rdbd-tag in this specification, later specifications can define new rdbd-tag values:

The relating-domain name(s) field contains either a single domain name, or an HTTPS URL. In the latter case, successfully de-referencing that URL results in a JSON object that contains the list of domain names, such as is shown in the figure below.


If an optional signature is included, the sig-alg field MUST contain the signature algorithm used, with the same values used as would be used in an RRSIG. The key-tag MUST match the RDBDKEY RR value for the corresponding public key.

If the optional signature is omitted, then the presentation form of the key-tag, sig-alg and signature fields MAY be omitted. If not omitted then the sig-alg and key-tag fields MUST be zero and the signature field MUST be a an empty string. [[Is that the right way to have optional fields in RRs? Not sure.]]

The input to signing ("to-be-signed" data) is the concatenation of the following linefeed-separated (where linefeed has the value '0x0a') lines:

relating=<relating-domain name>
related=<related-domain name or URL>
rdbd-tag=<rdbd-tag value>

The relating-domain and related-domain values MUST be the 'A-label' representation of these names.

The trailing "." representing the DNS root MUST NOT be included in the to-be-signed data, so a relating-domain value above might be "" but "" MUST NOT be used as input to signing.

A linefeed MUST be included after the "sig-alg" value in the last line.

[[Presentation syntax and to-be-signed details are very liable to change.]]

See the examples in the Appendix for further details.

3. Directionality and Cardinality

RDBD relationships are uni-directional. If bi-directional relationships exist, then both domains can publish RDBD RRs and optionally sign those.

If one domain has relationships with many others, then the relevant RDBD RRs (and RDBDKEY RRs) can be published to represent those or one RDBD RR can contain an HTTPS URL at which one can provide a list of names.

4. Required Signature Algorithms

Consumers of RDBD RRs MAY support signature verification. They MUST be able to parse/process unsigned or signed RDBD RRs even if they cannot cryptographically verify signatures.

Implementations producing RDBD RRs SHOULD support optional signing of those and production of RDBDKEY RRs.

Implementations of this specification that support signing or verifying signatures MUST support use of RSA with SHA256 (sig-alg==8) with at least 2048 bit RSA keys. [RFC5702]

RSA keys SHOULD use a 2048 bit or longer modulus.

Implementations of this specification that support signing or verifying signatures SHOULD support use of Ed25519 (sig-alg==15). [RFC8080][RFC8032]

5. Validation

A validated signature is solely meant to be additional evidence that the relevant domains are related, or that one disavows such a relationship. The existence or disavowal of a relationship does not by itself mean that data or services from any domain should be considered as more or less trustworthy.

6. Security Considerations

6.1. Efficiacy of signatures

The optional signature mechanism defined here offers no protection against an active attack if both the RDBD and RDBDKEY values are accessed via an untrusted path.

If the RDBDKEY value has been cached, or is otherwise known via some sufficiently secure mechanism, then the RDBD signature does confirm that the holder of the private key (presumably the relating-domain) considered that the relationship, or lack thereof, with a related-domain was real at some point in time.


RDBD does not require DNSSEC. Without DNSSEC it is possible for an attacker to falsify DNS query responses for someone investigating a relationship. Conversely, an attacker could delete the response that would normally demonstrate the relationship, causing the investigating party to believe there is no link between the two domains. An attacker could also replay an old RDBD value that is actually no longer published in the DNS by the related-domain.

Deploying signed records with DNSSEC should allow for detection of these kinds of attack.

If the relating-domain has DNSSEC deployed, but the related-domain does not, then the optional signature can (in a sense) extend the DNSSEC chain to cover the RDBD RR in the related-domain's zone.

If both domains have DNSSEC deployed, and if the relating-domain public key has been cached, then the the signature mechanism provides additional protection against active attacks involving a parent of one of the domains. Such attacks may in any case be less likely and detectable in many scenarios as they would be generic attacks against DNSSEC-signing (e.g. if a regisgtry injected a bogus DS for a relating-domain into the registry's signed zone). If the public key from the relevant RDNDKEY RRs is read from the DNS at the same time as a related RDBD RR, then the signature mechanism provided here may provide litle additional value over and above DNSSEC.

6.3. Lookup Loops

It's conceivable that an attacker could create a loop of relationships, such as>>> or similar. This could cause a resource issue for any automated system. A system SHOULD only perform three lookups from the first domain (>>> The related and relating-domains SHOULD attempt to keep links direct and so that only the fewest number of lookups are needed, but it is understood this may not always be possible.

7. IANA Considerations

This document introduces two new DNS RR types, RDBD and RDBDKEY. [[Codepoints for those are not yet allocated by IANA, nor have codepoints been requested so far.]]

[[New rdbd-tag value handling wll need to be defined if we keep that field. Maybe something like: 0-255: RFC required; 256-1023: reserved; 1024-2047: Private use; 2048-65535: FCFS.]]

8. Acknowledgements

Thanks to all who commented on this on the dbound and other lists, in particular to the following who provided comments that caused us to change the draft: Bob Harold, John Levine, Andrew Sullivan, Suzanne Woolf, and Paul Wouters. (We're not implying any of these fine folks actually like this draft btw, but we did change it because of their comments:-) Apologies to anyone we missed, just let us know and we'll add your name here.

9. Informative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D. and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, DOI 10.17487/RFC4034, March 2005.
[RFC5518] Hoffman, P., Levine, J. and A. Hathcock, "Vouch By Reference", RFC 5518, DOI 10.17487/RFC5518, April 2009.
[RFC5702] Jansen, J., "Use of SHA-2 Algorithms with RSA in DNSKEY and RRSIG Resource Records for DNSSEC", RFC 5702, DOI 10.17487/RFC5702, October 2009.
[RFC6376] Crocker, D., Hansen, T. and M. Kucherawy, "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, September 2011.
[RFC7344] Kumari, W., Gudmundsson, O. and G. Barwood, "Automating DNSSEC Delegation Trust Maintenance", RFC 7344, DOI 10.17487/RFC7344, September 2014.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital Signature Algorithm (EdDSA)", RFC 8032, DOI 10.17487/RFC8032, January 2017.
[RFC8080] Sury, O. and R. Edmonds, "Edwards-Curve Digital Security Algorithm (EdDSA) for DNSSEC", RFC 8080, DOI 10.17487/RFC8080, February 2017.

Appendix A. Examples

This appendix provides examples of RDBD-related values. The following names and other values are used in these examples.

The github repo <> has a script in sample/ that generated this appendix.

A.1. Unsigned Examples

; assertion that my-way.example claims to be related 
; to my.example
my-way.example. 3600 IN RDBD 1 my.example.

; assertion that my-way.example claims not to be 
; related to my-bad.example
my-way.example. 3600 IN RDBD 0 my-bad.example.

; assertion that my-way.example claims to be related 
; to whatever is at
my-way.example. 3600 IN RDBD 1

; assertion that my-way.example claims not to be 
; related to whatever is at
my-way.example. 3600 IN RDBD 0


A.2. RSA-signed Example

# HOWTO generate RSA key pair
$ openssl genrsa -out rsa.private 2048
Generating RSA private key, 2048 bit long modulus (2 primes)
e is 65537 (0x010001)
writing RSA key
$ openssl rsa -in rsa.private -out rsa.public -pubout \
     -outform PEM 
$ cat rsa.private
$ cat rsa.public
-----END PUBLIC KEY-----
# To-be-signed data for RSA
$ cat  to-be-signed-8.txt
# Sign that
$ openssl dgst -sha256 -sign rsa.private -out rsa.sig \
# Hexdump of signature
$ hexdump rsa.sig
0000000 8664 bd57 8cbf a8e1 9182 1b5f a4fc 5eb9
0000010 49b4 fe21 f1c7 8097 ed90 44a5 bcb1 543c
0000020 f784 c190 e1d9 2f2b 18ca d3c2 640f 3823
0000030 7f8a e446 d0eb bd14 6077 0597 6015 a82b
0000040 42d7 8677 b1a3 37fa a1e8 8109 07ec ff62
0000050 16b8 3895 66de d992 dc4d ed99 9ec3 0a62
0000060 6a07 3baa 45f2 d528 1e83 a147 60ce 9b25
0000070 a967 4ba0 3fb5 98db 5ff3 b070 058b 4d8f
0000080 f198 6c1f e6b6 7a6c 1e8c ad42 237f 5440
0000090 7856 caac f96c f87d e79c 4dc5 b833 bc03
00000a0 c52e 5603 46a7 59b5 9fe3 fccd 04ee e908
00000b0 71e7 21f8 47ad fea8 40bf 14a5 9e6b b3d4
00000c0 c61a 5b96 c559 3491 4dfa 91a0 4c0b f3ff
00000d0 e460 484c 7e49 5368 85e3 16be fe6b 809a
00000e0 117d c2cb be19 c5ba 7594 2f60 16ad 1132
00000f0 f978 6ca1 5448 180f 8ca7 e73d 1137 7064

; The RDBDKEY RR for my.example is...
my.example. 3600 IN RDBDKEY 0 3 8 (
            TkQgUFVCTElDIEtFWS0tLS0tCg== )
; The RDBD RR to be published by my-way.example is...
my-way.example. 3600 IN RDBD 1 my.example 38501 8 (
            NxFkcA== )

A.3. Ed25519-signed Example

# HOWTO generate an Ed25519 key pair...
$ ./ -s rdbd-example0001rdbd-example0002 \
   -r my.example -d my-way.example
b64pubkey: NT/DHhFoyR8K9l1sJv1EH7fflnGiOnRrs+yGvo01tkg=
keyid: 35988
# hex dump of Ed25519 private
$ hexdump ed25519.priv
0000000 6472 6462 652d 6178 706d 656c 3030 3130
0000010 6472 6462 652d 6178 706d 656c 3030 3230
# hex dump of Ed25519 public
$ hexdump
0000000 3f35 1ec3 6811 1fc9 f60a 6c5d fd26 1f44
0000010 dfb7 7196 3aa2 6b74 ecb3 be86 358d 48b6
# hex dump of Ed25519 signature
$ hexdump ed25519.sig
0000000 bc64 4c44 59e7 94fb fe35 189c eb75 1c24
0000010 c64e 99d0 d85c 8a13 2737 3282 8efc f579
0000020 b83c 80f8 f659 0004 c654 e81b d7cf d43f
0000030 2145 39f7 6294 c78f 01c3 fa35 9a92 0fb0

; The RDBDKEY RR for my.example is...
my.example. 3600 IN RDBDKEY 0 3 15 (
            NT/DHhFoyR8K9l1sJv1EH7fflnGiOnRrs+yGvo01tkg= ) 
; The RDBD RR to be published by my-way.example is...
my-way.example. 3600 IN RDBD 1 my.example 35988 15 (
            WfYEAFTGG+jP1z/URSH3OZRij8fDATX6kpqwDw== ) 

Appendix B. Ed25519 Signing Code

Since OpenSSL does not yet support Ed25519 signing via its command line tool, we generate our example using the python script below, which is called as "" above. This uses the python library from Appendix A of [RFC8032].

#!/usr/bin/env python3
import argparse, sys, binascii
from eddsa2 import Ed25519

# from
def calc_keyid(flags, protocol, algorithm, dnskey):
    st=struct.pack('!HBB', int(flags), int(protocol), int(algorithm))
    cnt = 0
    for idx in range(len(st)):
        s = struct.unpack('B', st[idx:idx+1])[0]
        if (idx % 2) == 0:
            cnt += s << 8
            cnt += s
    return ((cnt & 0xFFFF) + (cnt >> 16)) & 0xFFFF

def main():
    parser=argparse.ArgumentParser(description='Ed25519 signing')
                        dest='secret', help='secret key')
                        dest='relating', help='relating domain')
                        dest='related', help='related domain')
                        dest='negative', help='negative assertion')

    if args.secret is None:
        print("You do need a secret... - exiting")
    # secret has to be 32 octets funnily enuugh:-)
    # e.g. secret="rdbd-example0001rdbd-example0002".encode('utf-8')
    if len(args.secret)!=32:
        print("Secret has to be 32 octets... - exiting")
    if args.relating is None:
        print("You do need a relating domain... - exiting")
    if args.related is None:
        print("You do need a related domain... - exiting")
    privkey,pubkey = Ed25519.keygen(secret)
    print("private:"+ str(binascii.hexlify(privkey)))
    print("public:"+ str(binascii.hexlify(pubkey)))

    print("b64pubkey: " + b64pubkey)

    print("keyid: " + str(keyid))

    if args.negative:

    print("to-be-signed:|" + str(tbs)+"|")

    with open("ed25519.priv", "wb") as privf:
    with open("","wb") as pubf:
    with open("to-be-signed-15.txt","wb") as tbsf:

    signature = Ed25519.sign(privkey, pubkey, msg)
    print("sig:"+ str(binascii.hexlify(signature)))
    with open("ed25519.sig", "wb") as sigf:

if __name__ == "__main__":


Appendix C. Changes and Open Issues

[[RFC editor: please delete this appendix ]]

C.1. Changes from -01 to -02

C.2. Changes from -00 to -01

C.3. Open Issues

Current open github issues include:

These can be seen at: <>

Authors' Addresses

Alex Brotman Comcast, Inc EMail:
Stephen Farrell Trinity College Dublin EMail: