Survey of Domain Verification Techniques using DNS
draft-sahib-domain-verification-techniques-01
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Shivan Kaul Sahib , Shumon Huque | ||
| Last updated | 2021-04-15 | ||
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draft-sahib-domain-verification-techniques-01
Network Working Group S. Sahib
Internet-Draft S. Huque
Intended status: Informational Salesforce
Expires: 17 October 2021 15 April 2021
Survey of Domain Verification Techniques using DNS
draft-sahib-domain-verification-techniques-01
Abstract
Verification of ownership of domains in the Domain Name System (DNS)
[RFC1034] [RFC1035] often relies on adding or editing DNS records
within the domain. This document surveys various techniques in wide
use today, the pros and cons of each, and possible improvements.
Discussion Venues
This note is to be removed before publishing as an RFC.
Source for this draft and an issue tracker can be found at
https://github.com/ShivanKaul/draft-sahib-domain-verification-
techniques.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 17 October 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Verification Techniques . . . . . . . . . . . . . . . . . . . 3
3.1. TXT based . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1.1. Examples . . . . . . . . . . . . . . . . . . . . . . 4
3.2. CNAME based . . . . . . . . . . . . . . . . . . . . . . . 5
3.2.1. Examples . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Common Patterns . . . . . . . . . . . . . . . . . . . . . 5
3.3.1. Name . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3.2. RDATA . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Targeted Domain Verification . . . . . . . . . . . . . . 6
4.2. TXT vs CNAME . . . . . . . . . . . . . . . . . . . . . . 6
4.3. Continuous checking . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
Many providers on the internet need users to prove that they control
a particular domain before granting them some sort of privilege
associated with that domain. For instance, certificate authorities
like Let's Encrypt [LETSENCRYPT] ask requesters of TLS certificates
to prove that they operate the domain they're requesting the
certificate for. Providers generally allow for several different
ways of proving domain control, some of which include manipulating
DNS records. This document focuses on DNS techniques for domain
verification; other techniques (such as email or HTML verification)
are out-of-scope.
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In practice, DNS-based verification often looks like the provider
generating a random value and asking the requester to create a DNS
record containing this random value and placing it at a location that
the provider can query for. Generally only one temporary DNS record
is sufficient for proving domain ownership.
2. Conventions and Definitions
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 BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Provider: an internet-based provider of a service, for e.g., Let's
Encrypt provides a certificate authority service or GitHub provides
code-hosting services. These services often require a user to verify
that they control a domain.
3. Verification Techniques
3.1. TXT based
TXT record-based DNS domain verification is usually the default
option for DNS verification. The service provider asks the user to
add a DNS TXT record (perhaps through their domain host or DNS
provider) at the domain with a certain value. Then, the service
provider does a DNS TXT query for the domain being verified and
checks that the value exists. For example, this is what a DNS TXT
verification record could look like:
example.com. IN TXT "foo-verification=bar"
Here, the value "bar" for the attribute "foo-verification" serves as
the randomly-generated TXT value being added to prove ownership of
the domain to Foo provider. Although the original DNS protocol
specifications did not associate any semantics with the DNS TXT
record, [RFC1464] describes how to use them to store attributes in
the form of ASCII text key-value pairs for a particular domain. In
practice, there is wide variation in the content of DNS TXT records
used for domain verification, and they often do not follow the key-
value pair model. Even so, the rdata portion of the DNS TXT record
has to contain the value being used to verify the domain. The value
is usually a randomly-generated token in order to guarantee that the
entity who requested that the domain be verified (i.e. the person
managing the account at Foo provider) is the one who has (direct or
delegated) access to DNS records for the domain. The generated token
typically expires in a few days. The TXT record is usually placed at
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the domain being verified ("example.com" in the example above).
After a TXT record has been added, the service provider will usually
take some time to verify that the DNS TXT record with the expected
token exists for the domain.
The same domain name can have multiple distinct TXT records (a TXT
Record Set).
3.1.1. Examples
3.1.1.1. Let's Encrypt
Let's Encrypt [LETSENCRYPT] has a challenge type "DNS-01" that lets a
user prove domain ownership in accordance with the ACME protocol
[RFC8555]. In this challenge, Let's Encrypt asks you to create a TXT
record with a randomly-generated token at "_acme-
challenge.<YOUR_DOMAIN>". For example, if you wanted to prove domain
ownership of "example.com", Let's Encrypt could ask you to create the
DNS record:
_acme-challenge.example.com. IN TXT "cE3A8qQpEzAIYq-T9DWNdLJ1_YRXamdxcjGTbzrOH5L"
[RFC8555] (section 8.4) places requirements on the random value.
3.1.1.2. Google Workspace
[GOOGLE-WORKSPACE-TXT] asks the user to sign in with their
administrative account and obtain their verification token as part of
the setup process for Google Workspace. The verification token is a
68-character string that begins with "google-site-verification=",
followed by 43 characters. Google recommends a TTL of 3600 seconds.
The owner name of the TXT record is the domain or subdomain neme
being verified.
3.1.1.3. GitHub
GitHub asks you to create a DNS TXT record under "_github-challenge-
ORGANIZATION-<YOUR_DOMAIN>", where ORGANIZATION stands for the GitHub
organization name [GITHUB-TXT]. The code is a numeric code that
expires in 7 days.
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3.2. CNAME based
Less commonly than TXT record verification, service providers also
provide the ability to verify domain ownership via CNAME records.
This is used in case the user cannot create TXT records. One common
reason is that the domain name may already have CNAME record that
aliases it to a 3rd-party target domain. CNAMEs have a technical
restriction that no other record types can be placed along side them
at the same domain name ([RFC1034], Section 3.6.2).. The CNAME based
domain verification method typically uses a randomized label
prepended to the domain name being verified.
3.2.1. Examples
3.2.1.1. Google
[GOOGLE-WORKSPACE-CNAME] lets you specify a CNAME record for
verifying domain ownership. The user gets a unique 12-character
string that is added as "Host", with TTL 3600 (or default) and
Destination an 86-character string beginning with "gv-" and ending
with ".domainverify.googlehosted.com.".
To verify a subdomain, the unique 12-character string is appended
with the subdomain name for "Host" field for e.g.
JLKDER712AFP.subdomain where subdomain is the subdomain being
verified.
3.2.1.2. AWS Certificate Manager (ACM)
To get issued a certificate by AWS Certificate Manager (ACM), you can
create a CNAME record to verify domain ownership [ACM-CNAME]. The
record name for the CNAME looks like "_<random-token1>.example.com",
which would point to "_<random-token2>.<random-token3>.acm-
validations.aws."
Note that if there are more than 5 CNAMEs being chained, then this
method does not work.
3.3. Common Patterns
3.3.1. Name
ACME and GitHub have a suffix of "_PROVIDER_NAME-challenge" in the
Name field of the TXT record challenge. For ACME, the full Host is
"_acme-challenge.<YOUR_DOMAIN>", while for GitHub it is "_github-
challenge-ORGANIZATION-<YOUR_DOMAIN>". Both these patterns are
useful for doing targeted domain verification, as discussed in
section (#targeted-domain-verification) because if the provider knows
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what it is looking for (domain in the case of ACME, organization name
+ domain in case of GitHub) it can specifically do a DNS query for
that TXT record, as opposed to having to do a TXT query for the apex.
ACME does the same name construction for CNAME records.
3.3.2. RDATA
One pattern that quite a few providers follow (Dropbox, Atlassian) is
constructing the rdata of the TXT DNS record in the form of
"PROVIDER-SERVICE-domain-verification=" followed by the random value
being checked for. This is in accordance with [RFC1464] which
mandates that attributes must be stored as key-value pairs.
4. Recommendations
4.1. Targeted Domain Verification
The TXT record being used for domain verification is most commonly
placed at the domain name being verified. For example, if
"example.com" is being verified, then the DNS TXT record will have
"example.com" in the Name section.
If many services are attempting to verify the domain name, many
distinct TXT records end up being placed at that name. There is no
way to surgically query only the TXT record for a specific service,
resulting in extra work for a verifying service to sift through the
records for its own domain verification record. In addition, since
DNS Resource Record sets are treated atomically, all TXT records must
be returned to the querier, which leads to a bloating of DNS
responses. This could cause truncation and expensive retrying over
TCP.
A better method is to place the TXT record at a subdomain of the
domain being verified that is specially reserved for use by the
application service in question.
4.2. TXT vs CNAME
TODO
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4.3. Continuous checking
After domain verification is done, there is no need for the TXT or
CNAME record to continue to exist as the existence of the domain
verifying DNS record for a service only implies that a user with
access to the service also has DNS control of the domain at the time
the code was generated. It should be safe to remove the verifying
DNS record once the verification is done. However, despite this,
some services ask the record to exist in perpetuity
[ATLASSIAN-VERIFY].
5. Security Considerations
DNSSEC [RFC4033] should be employed by the domain owner to protect
against domain name spoofing.
6. IANA Considerations
This document has no IANA actions.
7. References
7.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1464] Rosenbaum, R., "Using the Domain Name System To Store
Arbitrary String Attributes", RFC 1464,
DOI 10.17487/RFC1464, May 1993,
<https://www.rfc-editor.org/info/rfc1464>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/info/rfc4033>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
[ACM-CNAME]
AWS, ., "Option 1: DNS Validation", n.d.,
<https://docs.aws.amazon.com/acm/latest/userguide/dns-
validation.html>.
[ATLASSIAN-VERIFY]
Atlassian, ., "Verify over DNS", n.d.,
<https://support.atlassian.com/user-management/docs/
verify-a-domain-to-manage-
accounts/#Verifyadomainforyourorganization-VerifyoverDNS>.
[GITHUB-TXT]
GitHub, ., "Verifying your organization's domain", n.d.,
<https://docs.github.com/en/github/setting-up-and-
managing-organizations-and-teams/verifying-your-
organizations-domain>.
[GOOGLE-WORKSPACE-CNAME]
Google, ., "CNAME record values", n.d.,
<https://support.google.com/a/answer/112038>.
[GOOGLE-WORKSPACE-TXT]
Google, ., "TXT record values", n.d.,
<https://support.google.com/a/answer/2716802>.
[LETSENCRYPT]
Let's Encrypt, ., "Challenge Types: DNS-01 challenge",
2020, <https://letsencrypt.org/docs/challenge-types/#dns-
01-challenge>.
[RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
<https://www.rfc-editor.org/info/rfc8555>.
Acknowledgments
TODO
Authors' Addresses
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Shivan Sahib
Salesforce
Email: shivankaulsahib@gmail.com
Shumon Huque
Salesforce
Email: shuque@gmail.com
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