This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.
The following 'Verified' errata have been incorporated in this document:
Network Working Group P. Hoffman
Request for Comments: 5518 J. Levine
Category: Standards Track Domain Assurance Council
Vouch By Reference
Status of This Memo
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This document describes the Vouch By Reference (VBR) protocol. VBR
is a protocol for adding third-party certification to email. It
permits independent third parties to certify the owner of a domain
name that is associated with received mail.
Table of Contents
1. Introduction ....................................................3
1.1. Definitions ................................................4
2. Use of the VBR-Info Header Field ................................4
3. Validation Process ..............................................4
4. The VBR-Info Header Field .......................................5
4.1. Syntax of VBR-Info Header Fields ...........................5
5. DNS Query .......................................................6
6. Types of Message Content ........................................7
6.1. All ........................................................8
6.2. List .......................................................8
6.3. Transaction ................................................8
7. Obtaining a Useful Domain Name ..................................8
7.1. DKIM .......................................................8
7.2. DomainKeys .................................................9
7.3. SPF ........................................................9
7.4. Sender ID .................................................10
8. Security Considerations ........................................10
9. IANA Considerations ............................................10
10. References ....................................................11
10.1. Normative References .....................................11
10.2. Informative References ...................................11
Appendix A. Acknowledgements .....................................12
Vouch By Reference, or VBR, is a protocol for adding third-party
certification to email. Specifically, VBR permits independent third
parties to certify the owner of a domain name that is associated with
received mail. VBR may be performed anywhere along the email transit
path, by any capable receiving module, either within the handling
service or by end-user software.
VBR accomplishes this with a two-part protocol:
o In the first part, a sender affixes VBR information to email
messages. The VBR information says which domain certification
services the sender believes will vouch for email traffic
associated with that sender.
o In the second part, the receiver queries one or more certification
services to obtain information about the identity that has been
associated with a received message. This latter protocol uses the
DNS to distribute the certification information.
A sender provides certification attestations through the use of a new
RFC 5322 ([RFC5322]) mail header field, "VBR-Info:". This header
field contains the names of services that the sender claims will
vouch for it, and the particular type of content of the message. A
queried, third-party, DNS-based certification service can respond
with a list of the types of message content it will vouch for, such
as "transactional mail from somebank.example" and/or "all mail from
A prerequisite for successful VBR operation is validation of the
identity associated with the message. VBR is based on the use of
domain names as identifiers, and permits multiple methods of
obtaining and validating domain names. The validation methods are
described in the "Obtaining a Useful Domain Name" section below.
The sender performs two steps:
1. Adds a VBR-Info header field to its message
2. Protects the message, as appropriate
If a recipient uses the results of vouching to adjust spam scores on
incoming email, that recipient is placing a great deal of operational
trust and power in the vouching service. Therefore, recipients need
to select such services with care. Further, such recipients may want
to select more than one vouching service in order to avoid a single
point of failure for setting spam scores.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Use of the VBR-Info Header Field
A sender uses VBR to indicate which domain certification services the
sender believes will vouch for a particular piece of mail. The
certification service uses VBR to state for which signatures it will
vouch. This protocol uses the DNS to distribute the certification
A message may have multiple VBR-Info header fields. This means that,
in the terminology of RFC 5322, VBR-Info is a "trace header field"
and SHOULD be added at the top of the header fields.
The content of the VBR-Info header field is a list of three elements:
o The accountable domain
o The type of content in the message
o A list of domain names of services that the sender expects to
vouch for that kind of content
The accountable domain is given as md= followed by a domain name.
The content type is given as mc= followed by a string; the defined
values of that string are found below. The list of services is given
as mv= followed by a colon-separated list of domain names.
The formal syntax of the header field is defined in Section 4.
3. Validation Process
A message receiver uses VBR to determine certification status by
following these steps:
1. Extracts the domain to certify and the type of message content
2. Verifies legitimate use of that domain using one or more
authentication mechanisms as described herein
3. Obtains the name of a vouching service that it trusts, either
from among the set supplied by the sender or from a locally
defined set of preferred vouching services
4. Queries the vouching service to determine whether the vouching
service actually vouches for that type of content for that
4. The VBR-Info Header Field
The VBR-Info header field has the following format:
VBR-Info: md=<domain>; mc=<type-string>; mv=<certifier-list>;
where <domain> is the domain for which vouching is offered, <type-
string> is the content type of the message, and <certifier-list> is a
list of domain names of certification providers that the sender
asserts will vouch for this particular message. The structure of the
<certifier-list> is one or more domain names with a colon (":")
between each. The elements in the <domain>, <type-string>, and
<certifier-list> must not have any white space in them.
For example, assume that the signer has two companies that are
willing to vouch for its transactional notices: certifier-a.example
and certifier-b.example. The signer would add the following to the
header of its outgoing message.
VBR-Info: md=somebank.example; mc=transaction;
All three header parameters in the VBR-Info header are mandatory. In
particular, there is no default for the md= domain.
Upper and lowercase characters in a VBR-Info header field are
equivalent, although conventionally the contents are all in lower
case. For upward compatibility, verifiers MUST accept the fields in
any order and SHOULD ignore any fields other than the three defined
If a message has more than one VBR-Info header field, verifiers
SHOULD check each in turn or in parallel until either a satisfactory
certifier is found or all the header fields have been checked. All
of the VBR-Info header fields in a single message MUST have identical
4.1. Syntax of VBR-Info Header Fields
In the ABNF below, the ALPHA and DIGIT tokens are imported from
[RFC5234], and the FWS and domain-name tokens are imported from
vbr-info-header = "VBR-Info:" 1*([FWS] element [FWS] ";")
element = md-element / mc-element / mv-element
md-element = "md=" [FWS] domain-name
mc-element = "mc=" [FWS] type-string
type-string = "all" / "list" / "transaction"
mv-element = "mv=" [FWS] certifier-list
certifier-list = domain-name *(":" domain-name)
5. DNS Query
When a recipient wants to check whether a certification claim is
valid, it compares the list in the message to the list of services it
trusts. For each service that is on the intersection of the two
lists, it marshals a domain name to look up that consists of the
following DNS labels (from left to right):
o the domain name that asserts it can be certified
o _vouch (a string literal)
o the host name of the vouching service
This domain name is queried for a DNS TXT record. The recipient
looks up the domain name in the DNS in the exact same manner it looks
up all other domain names.
For example, if a message signed by somebank.example contained the
VBR-Info header field above, the receiver might look up either or
both of the following names, depending on which vouching service it
If the DNS TXT record exists, it contains a space-delimited list of
all the types that the service certifies, given as lowercase ASCII.
For example, the contents of the TXT record might be:
In the example above, the receiver checks whether or not either
certifier vouches for "transaction" mail. That would be indicated by
either of the following types: "all" or "transaction" ("all"
indicates that the certifier vouches for all message types sent by
the domain in question). If either of those types appear in either
TXT record, the certifier has vouched for the validity of the
message. Of course, the recipient needs to ignore services that it
does not trust; otherwise, a bad actor could just add an authority
that it has set up so that it can vouch for itself.
The name for the label _vouch was chosen because any domain name that
includes it as one of its labels cannot be a valid host name. There
will never be any accidental overlap with a valid host name.
Further, it is safe to create a rule that says that a TXT DNS record
that comes from a domain name that includes a _vouch label will
always have the structure defined in this document.
If the RDATA in the TXT record contains multiple character-strings
(as defined in Section 3.3 of [RFC1035]), the code handling that
reply from DNS MUST assemble all of these marshaled text blocks into
a single one before any syntactical verification takes place.
Verifiers MUST then check that the TXT record consists of strings of
lowercase letters separated by spaces, and discard any records not in
that format. This defends against misconfigured records and
irrelevant records synthesized from DNS wildcards.
The VBR record MUST have only one TXT record.
This query method relies on the considerable advantages of existing
DNS efficiencies, reliability, and experience. The lookup is very
efficient, and certifiers can add and delete client records as
quickly as they want. The lookup also leverages the DNS's negative
6. Types of Message Content
This section describes the types of content for which a certifier can
vouch. While the rest of the VBR specification is mostly technical
and precise, describing the types of contents in mail messages is
inherently open to interpretation. Thus, this section makes
distinctions as specifically as possible, but the reader needs to
understand that these semantic definitions can be interpreted in very
different ways by different people.
Note that the value in the mc= element is self-asserted. The purpose
of this element is for auditing. There will likely be cases where a
certifier will vouch for one type of a sender's mail (such as
transactional mail) but not another type (such as advertising). A
sender who cannot get anyone to certify its advertising mail, but has
a certifier for its transactional mail, might be tempted to cheat and
mislabel it as transactional. The mc= element creates an the audit
trail to help their certifiers catch such cheating and allow the
removal of the certification for the transactional mail.
Three types of content are defined.
"all" means all mail from the sender.
"list" is the category for email sent to multiple recipients where
each piece of mail is identical or is very similar to the others.
"transaction" is the category for transactional messages. This is a
response to a specific action of the user, or a notice about an event
in the user's account at the sender.
7. Obtaining a Useful Domain Name
VBR relies on having a domain name that specifies a party that is
accountable for the message. This requires obtaining the domain name
and possessing a strong basis for believing that the use of the
domain name is valid, that is, that it has not been spoofed.
There are different ways to achieve this and this section discusses
the allowed mechanisms. Senders SHOULD use Domain Keys Identified
Mail (DKIM) (and MAY use DomainKeys, Sender Policy Framework (SPF),
or SenderID) to give an accountable identity for the sender.
DomainKeys Identified Mail (DKIM), [RFC4871], defines an accountable
identity by associating a domain name with the message. It provides
assurance that the association is valid through a public-key-based
o When DKIM is the validation mechanism, VBR's md= MUST match the
domain name taken from one of the DKIM-Signature header fields.
The domain name from the DKIM signature d= field is used.
EID 2583 (Verified) is as follows:Section: 7.1
If the DKIM signature contains an i= field, the domain name from
that field is used; otherwise, the domain name from the DKIM
signature d= field is used.
The domain name from the DKIM signature d= field is used.
RFC 5672 clarifies that the d= domain is the primary identity in a DKIM signature.
o The VBR-Info header field SHOULD be included in the set of header
fields protected by DKIM to prevent a malicious party from
changing the contents of the VBR-Info header field or adding bogus
VBR-Info header fields.
o The VBR-Info header field SHOULD be added in the header
immediately below the corresponding DKIM-Signature header field.
If the DKIM signature validates, the domain name taken from that
signature is valid for use with VBR.
DomainKeys (DK), [RFC4870], defines an accountable identity by
associating a domain name with the message in the d= tag of the
DomainKey-Signature header field. It provides assurance that the
association is valid through a public-key-based authentication
o When DomainKeys is the validation mechanism, VBR's md= MUST be the
same value as the domain name found in the DomainKey-Signature d=
o The VBR-Info header field SHOULD be included in the set of header
fields protected by DK to prevent a malicious party from changing
the contents of the VBR-Info header field or adding bogus VBR-Info
o The VBR-Info header field SHOULD be added immediately below the
corresponding DomainKey-Signature header field.
If the DomainKeys signature validates, the domain in the d= tag is
valid for use with VBR.
Sender Policy Framework (SPF), [RFC4408], defines an accountable
identity by using an existing message address and querying the DNS to
discover whether it is valid for SPF use.
When SPF is the validation mechanism, VBR's md= MUST be the same
value as the domain name in the <reverse-path> address that is the
first parameter to the SMTP MAIL command.
A domain is valid for use with VBR only when the SPF process produces
a "pass" result.
7.4. Sender ID
Sender ID, [RFC4406], defines an accountable identity by using an
existing message address known as the Purported Responsible Address
([RFC4407]) and querying the DNS to discover whether it is valid for
Sender ID use.
When Sender ID is the validation mechanism, VBR's md= MUST be the
same value as the domain name in the Purported Responsible Address in
A domain is valid for use with VBR only when the Sender ID process
produces a "pass" result.
8. Security Considerations
VBR is used to allow users to trust independent third parties to
certify the owner of a domain name that is associated with received
mail. The party validating the mail might use that trust
relationship to perform actions that affect the security of their
The receiver of a message with a VBR-Info header field MUST ignore
certifiers that it does not trust; otherwise, a bad actor could just
add an authority that it has set up so that it can vouch for itself.
Implementations SHOULD limit the number of VBR-Info header fields
they process in a single message in order to protect themselves from
a make-work or denial-of-service attack.
9. IANA Considerations
IANA registered the VBR-Info header field in the Message Header
Fields Registry ([RFC3864]) as follows:
Header field name: VBR-Info
Applicable protocol: mail
Author/Change controller: IETF
Specification document(s): RFC 5518
Related information: none
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
10.2. Informative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
[RFC4406] Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
RFC 4406, April 2006.
[RFC4407] Lyon, J., "Purported Responsible Address in E-Mail
Messages", RFC 4407, April 2006.
[RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
for Authorizing Use of Domains in E-Mail, Version 1",
RFC 4408, April 2006.
[RFC4870] Delany, M., "Domain-Based Email Authentication Using
Public Keys Advertised in the DNS (DomainKeys)", RFC 4870,
[RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.
Appendix A. Acknowledgements
Many members of the Domain Assurance Council contributed to the
design of the protocol and the wording of this document. In
addition, constructive suggestions were received from Jim Fenton and
Domain Assurance Council
Domain Assurance Council