Network Working Group A. Vesely
Internet-Draft June 16, 2010
Intended status: Standards Track
Expires: December 18, 2010
Verified Hello SMTP extension
draft-vesely-vhlo-06
Abstract
Verified Hello (VHLO) is an SMTP extension for managing authorization
by policy, as done for whitelisting messages. The VHLO command verb
provides for weak authentication of SMTP clients and policy
negotiation.
Policies and reputation are being increasingly used to identify
messages worthiness. However, they are currently enforced by
rejecting SMTP transactions, or discarding messages. Feedback is
scarce, also because reply codes are difficult to interpret
automatically. Negotiation is not provided for. VHLO is designed so
that servers can provide feedback to their clients about which
vouching services or authentication methods they require.
Credentials can also be negotiated on the fly, in order to allow
clients to learn whether messages will be whitelisted by the
receiving server before actually transmitting them. Negotiation and
feedback are intended to ease rapid diffusion of popular reputation
systems and authentication methods. A IANA register is defined for
extending the set of available methods.
The VHLO command is similar to EHLO, but accepts a series of
parameters. The sender communicates the mail domain name of the
organization on whose behalf it operates, along with any vouching
services (VBR) for its reputation. On the other hand, the sending
host's affiliation with that mail domain is checked by DNS lookups
(MX, PTR, or SPF) or using DKIM. DNSBLs and Greylisting are also
considered.
Weakly authenticated clients enjoy an intermediate level of trust:
they have no relying privileges, but may attempt to deliver mail to
local users, are whitelisted from some filters, and may receive DSNs
and feedback-loop abuse reports as needed. However, failing to
succesfully negotiate VHLO authentication does not preclude a
client's ability to relay mail: It may relay as usual; that is to
say, without knowing whether the credentials it tries to provide have
any meaning for the receiving server.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Prime delivery . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Domains as branding . . . . . . . . . . . . . . . . . . . 5
1.3. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. Definition and Registration of the VHLO Extension . . . . . . 5
3. Behavior of SMTP client and server . . . . . . . . . . . . . . 6
3.1. Syntax of the VHLO command . . . . . . . . . . . . . . . . 7
3.2. Server side checks on the Domain . . . . . . . . . . . . . 8
3.2.1. Greylisting check . . . . . . . . . . . . . . . . . . 8
3.2.2. DNSBL check . . . . . . . . . . . . . . . . . . . . . 9
3.2.3. SPF check . . . . . . . . . . . . . . . . . . . . . . 9
3.2.4. MX check . . . . . . . . . . . . . . . . . . . . . . . 9
3.2.5. PTR and 'iprev' checks . . . . . . . . . . . . . . . . 9
3.2.6. VBR check . . . . . . . . . . . . . . . . . . . . . . 10
3.2.7. DKIM check . . . . . . . . . . . . . . . . . . . . . . 11
3.3. Responses to the VHLO command . . . . . . . . . . . . . . 12
3.3.1. Overview of possible responses . . . . . . . . . . . . 12
3.3.2. Positive response . . . . . . . . . . . . . . . . . . 13
3.3.2.1. VHLO parameter and MAIL FROM command . . . . . . . 13
3.3.3. Transient error responses . . . . . . . . . . . . . . 13
3.3.4. Negative responses . . . . . . . . . . . . . . . . . . 14
3.3.5. Diagnosis of failed VHLO commands . . . . . . . . . . 14
3.4. Restrictions and further server side checks . . . . . . . 15
3.4.1. MAIL FROM restriction . . . . . . . . . . . . . . . . 16
3.4.2. VBR restriction . . . . . . . . . . . . . . . . . . . 16
3.4.3. DKIM-Signature headers existence and verification . . 16
3.4.4. Greylisting restrictions . . . . . . . . . . . . . . . 17
4. Forwarding of messages accepted under VHLO . . . . . . . . . . 17
5. Submission strategy . . . . . . . . . . . . . . . . . . . . . 18
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
6.1. IANA Mail Parameters . . . . . . . . . . . . . . . . . . . 19
6.2. IANA VHLO methods . . . . . . . . . . . . . . . . . . . . 19
7. Security Considerations . . . . . . . . . . . . . . . . . . . 20
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
8.1. Normative References . . . . . . . . . . . . . . . . . . . 22
8.2. Informative References . . . . . . . . . . . . . . . . . . 22
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 23
A.1. Prime delivery message transfer . . . . . . . . . . . . . 23
A.2. Failure after DNSBL check . . . . . . . . . . . . . . . . 24
A.3. Failure on the MAIL FROM restriction check . . . . . . . . 24
A.4. Automatically finding a common vouching service . . . . . 24
A.5. Reattempting Greylisted transmission . . . . . . . . . . . 25
A.6. Mandating DKIM usage . . . . . . . . . . . . . . . . . . . 25
A.7. Requiring extra DKIM tags . . . . . . . . . . . . . . . . 26
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 26
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1. Introduction
The SMTP extension defined by this memo provides a VHLO command verb
that takes a registered domain name, instead of the client identity
taken by EHLO. The declared Domain identifies the organization that
is responsible for sending the messages. Two kinds of verifications
are required to validate the VHLO command:
the Domain is trustworthy, and
the SMTP client is affiliated with the Domain.
Not all mail messages are amenable to be transmitted in the framework
of a VHLO command, only those transmitted on behalf of the weakly
authenticated Domain. If weak authentication succeeds, the client
can transmit messages that will enjoy prime delivery (Section 1.1).
If it fails, the client is told what requirements it misses, so that
its administrators know exactly what to do in order to gain
acceptance. Such feedback is deemed necessary and sufficient for
triggering widespread deployment of domain whitelists, a.k.a. RHSWL
(right hand side whitelists), as discussed below (Section 1.2).
1.1. Prime delivery
The term "prime delivery" is used to indicate that a message is not
tagged as spam, quarantined, silently dropped, or delivered to junk
folders. Here, a junk folder is one from where unread messages are
normally deleted, or moved to another junk folder, without human
intervention. In addition, prime delivery implies that messages are
not altered in such a way as to make them less visible or discourage
users from displaying their content.
In case the message has to be forwarded to another internal or
external server, its transmission SHOULD attempt to preserve the
trust and reputation that was granted on acceptance, as detailed in
Section 4, always reporting failure to relay.
End users may operate their own content filtering. They can do so
within their clients, or setting up their own filtering recipes
within per-user sections of the Mail Delivery Agent configuration.
Prime delivery only concerns stock filters that operate for all
users. In case users can configure their mailboxes by making on/off
decisions about specific content filters, implementing prime delivery
involves dynamically turning off the relevant filters. For the sake
of reliability, the delivery agent SHOULD ensure that prime delivery
is consistently flagged by Authentication Status [RFC5451] headers,
and known IMAP keywords. Administrators should educate their users
on how to appropriately whitelist messages flagged that way.
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1.2. Domains as branding
DNS domain names can be used as a brand, and reputation records based
on them last longer than those based on IP addresses. While a domain
is not formally required for sending email messages, Verified Hello
provides for a framework where only messages sent on behalf of an
authenticated domain are accepted. In this respect, this extension
is only useful for relaying messages across domain boundaries, which
typically happens after Message Submission [RFC4409].
1.3. Requirements Language
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 RFC 2119 [RFC2119].
2. Definition and Registration of the VHLO Extension
According to [RFC5321] provisions, the definition for this extension
goes as follows:
o the textual name of this extension is "Verified Hello";
o the EHLO keyword associated with the extension is "VHLO";
o the parameter associated with the EHLO keyword is a random value
up to 16 octets long (see Section 3.3.2.1);
o this extension defines one additional verb, VHLO, whose only
mandatory parameter is the Domain name of the sender, possibly
followed by one parameter for each reputation tag (see
Section 3.1);
o VHLO is also defined as one additional parameter to the MAIL verb
(see Section 3.3.2.1), no parameters are defined for the RCPT
verb;
o supporting the extension affects the behavior of a server and
client SMTP as described in Section 3; and
o the maximum length of the MAIL command is increased by 22 octets,
while the RCPT command is not affected.
Finally, as required by [RFC4409], this extension is NOT RECOMMENDED
on the Submission port.
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3. Behavior of SMTP client and server
The VHLO command is used by a client to request prime delivery
(Section 1.1) of messages. If the server accepts the command by
giving a positive response (see Section 3.3.2), the messages
transmitted thereafter are said to be in the framework of that VHLO
command.
The framework terminates on any of the following, whichever comes
first:
the end of the SMTP session,
a further successful VHLO command, or
a further successful EHLO command.
An SMTP session contains zero or more VHLO frameworks, and each VHLO
framework contains zero or more transactions.
An SMTP client MAY issue the VHLO command as part of a session
initiation, before initiating a mail transaction. That is to say,
right after the EHLO command, or instead of it. (In the latter case,
of course, the client has to infer that the server supports this
extension by some other means.) Clients MAY attempt the VHLO command
various times with different parameters, as long as the receiving
server allows further retries (see Section 3.3.4).
If no EHLO command has been issued by the client, the server assumes
an EHLO command with an address literal matching the remote address.
However, if the client specified the PTR parameter, the server MAY
assume an EHLO command with the resolved host name.
Clients failing to issue a successful VHLO command MAY transmit
messages using regular transactions, outside of any VHLO framework.
If the server supports VHLO, and issued reply codes 550 or 553 to
indicate that relaying from the given Domain is not wanted in the
current state, then if the client's configuration includes a list of
alternative MSAs that it may use as smart hosts in such cases, then
the client SHOULD relay through an alternative MSA instead (see
Section 5).
After successfully transmitting one or more messages in the framework
of a successful VHLO command, a client MAY issue another VHLO command
to transmit more messages. At any time in a VHLO framework, except
during a transaction, a client MAY issue an EHLO command to transmit
messages outside of any VHLO framework. Changing framework is
required when new messages are transmitted on behalf of a different
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Domain, or with different VHLO parameters.
Messages transmitted in a VHLO framework are subject to the MAIL FROM
restriction, and, possibly, to the DKIM-Signature headers existence
and verification, the VBR restriction, and any Greylisting
restrictions (see Section 3.4). A message satisfying all those
restrictions is said to be compatible with the VHLO parameters.
By giving a positive reply to VHLO, a server commits itself to accept
messages compatible with the given VHLO parameters, and grants them
prime delivery. Prime delivery overrides any other policy that might
otherwise encourage the server to discard messages, such as ADSP
[RFC5617]. While this section indicates circumstances for the
failure of each single check, it is up to the local policy to
establish what combinations of successful checks yield positive
responses. Missing requirements are communicated to the client as
described below (Section 3.3.5).
3.1. Syntax of the VHLO command
The only mandatory argument to VHLO is the Domain. The syntax is as
follows:
vhlo = "VHLO" SP Domain *( SP auth-rept-claim) CRLF
auth-rept-claim = auth-rept-tag [ ":" tag-spec-param ]
auth-rept-tag = "GID" / "MX" / "PTR" / "VBR" / "DKIM" / further-tag
tag-spec-param = gid-param / vbr-param / dkim-param / further-param
where the Domain is the fully-qualified DNS domain name delegated to
the entity or organization that is responsible for sending the
message(s) that will be transmitted in the framework of this command.
Note that, unlike the EHLO command, the Domain is not necessarily the
host name of the SMTP client.
The maximum line length of the VHLO command is 1000 octets, including
the terminating CRLF.
When the authentication method corresponding to a VBR or DKIM auth-
rept-tag fails, it may be recovered automatically as described in
Section 3.3.5. The remaining methods defined in this document don't
provide for negotiation.
The GID auth-rept-tag and its associated gid-param SHOULD be supplied
in the special cases described in sections Greylisting check, and
Greylisting restrictions.
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The remaining arguments MAY be supplied to authenticate the domain
name or provide hints for its reputation. These arguments are
supplied spontaneously by the client, up to the maximum line length.
3.2. Server side checks on the Domain
The receiving server SHOULD check that the supplied domain is valid
and reckon its reputation. The server is not limited by the checking
methods indicated in the parameters. Checks that are normally
carried out anyway don't even have a corresponding auth-rept-tag, but
are mentioned below.
Some circumstances may require to terminate a VHLO framework and
start a new one, with varied Domain or parameters. Typically, only a
part of the checks need to be carried out again.
3.2.1. Greylisting check
The GID auth-rept-tag provides the value of a VHLO framework that had
been given by this same server or a related MX during a previous SMTP
session:
gid-param = original-vhlo-string
The receiving server SHOULD check that the original-vhlo-string
corresponds to the value that it or a related MX has given as random-
string in response to a successful VHLO command. Use of the GID
auth-rept-tag is reserved for retrying the transmission of messages
that suffered a transient failure in the framework of the
corresponding VHLO command, as described in Section 3.3.2.1.
If the server applies Greylisting[greylisting], it MAY use the
provided gid-param, if supplied, as an additional key to a group of
messages, besides other data items used to implement Greylisting. If
using this parameter, the server MUST still check that the other data
items correspond, and that the sender accomplishes the directives
described in Greylisting restrictions.
The server SHOULD NOT issue a negative response for improper usage of
this parameter. However, if bad faith can be ascertained, the server
MAY add that knowledge to the sending Domain's reputation. On the
other hand, using this parameter eases the task of verifying that a
Domain's servers adopt a regular retrying behavior. Such knowledge
MAY also be added to the Domain's reputation. It is RECOMMENDED that
Domains with enough reputation are whitelisted from Greylisting.
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3.2.2. DNSBL check
The server SHOULD check any relevant DNSBL, and, if a DNSBL that the
server, according to its policy, considers trustworthy for either
rejecting messages or degrading their worthiness, gives a positive
match, then the server SHOULD issue a negative 550 response to VHLO.
See [RFC5782] for details on this check.
3.2.3. SPF check
Checking SPF SHOULD be omitted when the MX or DKIM parameters are
specified by the client. Otherwise, if the server carries out SPF
checks, it SHOULD check the supplied Domain using the method
described in [RFC4408], and, if that results in a "fail" or
"permerror", the server SHOULD issue a negative 550 response. For
"temperror" see Section 3.3.3. According to its policy, the server
MAY issue a negative response when the result is anything but "pass".
However, if the client specified the PTR parameter, then the "none",
"neutral", and "softfail" SPF results SHOULD also be accepted.
Administrators of a Domain who do not take responsibility for
messages transmitted by specific hosts, even if those hosts would be
related to the Domain according to the PTR and 'iprev' checks, SHOULD
use SPF to exclude those addresses, so that the SPF check results in
"fail".
Note that the so-called "helo check" often gets a result of "none"
because [RFC4408] does not provide for SPF (or TXT) RRs to be valid
for a whole zone, and many hostmasters omit to define an SPF policy
for each host. Unlike EHLO, the Domain argument taken by VHLO points
to the sending domain, not the host. Because of the MAIL FROM
restriction (Section 3.4.1), no further SPF checks are required for
transactions in the framework of this VHLO command.
3.2.4. MX check
The MX auth-rept-tag suggests that the client is connecting from an
IP address that belongs to one of the Domain's MX servers. The
receiving server SHOULD lookup the MX records of the given Domain and
successively lookup the addresses (A or AAAA depending on the
connection) of each of the hosts listed therein, until it finds a
matching address or the list is exhausted. If no match was found,
the server SHOULD issue a negative 550 response.
3.2.5. PTR and 'iprev' checks
The PTR auth-rept-tag suggests that the client is connecting from an
IP address that can be resolved backward to an host name under the
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given Domain's hierarchy.
The receiving server SHOULD lookup the PTR records for the connecting
address and verify that at least one of the returned RRs, after
resolving any CNAME, results in a host name whose rightmost part
matches the Domain. If no match was found, the server SHOULD issue a
negative 550 response.
The server SHOULD also check that the name found thereby resolves
forward, possibly through a CNAME, to the connecting address, as
indicated by the 'iprev' Authentication Method described in
[RFC5451]. In case the 'iprev' check fails, the server SHOULD issue
a negative 550 response.
3.2.6. VBR check
The VBR auth-rept-tag provides a list of vouching services:
vbr-param = [ "mc=" type-string ";" "mv=" ] certifier-list
certifier-list = domain-name *( ":" domain-name )
If the receiving server has a list of trusted vouching services, it
SHOULD carry out the VBR validation process as it would be done for a
VBR-Info header containing the corresponding elements, see [RFC5518].
In particular, the type-string defaults to "all", and the domain to
certify is the given Domain. The server SHALL remove from the
certifier-list provided by the client any certifier not mentioned in
its list of trusted vouching services. If the resulting list is
empty, the server SHOULD issue a negative 555 response, passing its
full list of trusted vouching services as indicated in Section 3.3.5.
Otherwise, the server SHOULD proceed with querying one or more
services in the resulting list. If any of those queries fails for
non-transient reasons, the server SHOULD issue a 550 response. If
all the services in the resulting list fail for a transient reason,
the server SHOULD issue either a 455 response (formatted as if the
failed services were not trusted) or a 450 or 451 response.
The meaning of the list of trusted vouching services configured
within the server is that any single vouch suffices. In case a more
complicated logic is needed, e.g. service A and either B or C or else
service D, it has to be implemented as an ad-hoc mashup of vouching
services to be presented as a single service.
Domains within a closed set may enjoy mutual whitelisting by setting
up their own ad-hoc vouching server.
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3.2.7. DKIM check
The DKIM auth-rept-tag asserts that all messages transmitted in the
given VHLO framework have a valid DKIM-Signature header field whose
domain (d) tag matches the Domain in the VHLO command.
The parameter contains additional properties of such signatures:
See [RFC4871] for imported ABNF
dkim-param = sig-s-tag *( ";" sig-tag )
where the sig-s-tag is the s=selector string, while the optional sig-
tag's are selected parts of the DKIM-Signature header field. Note
that the parameter MUST NOT contain any whitespace, although it is
allowed in the signature header. At least the sig-s-tag for the
selector (and the sig-q-tag if a query method different than "dns/
txt" is used) MUST be provided. In addition, in case multiple
signatures are present whose domain (d) tag equals the Domain, then
the client MUST include a sig-b-tag, that is a b=base64data string
containing the signature data, or its first bytes, so that the
receiving server can unambiguously identify which signature
determines a message's compatibility with the VHLO framework. Any
other tag from the DKIM-Signature MAY be present in the parameter.
The receiving server MAY fetch the public key required to verify the
DKIM signatures. If the key does not exist, the server SHOULD issue
a negative 550 response.
The receiving server may be picky about DKIM signatures; that is to
say, it may reject or drop messages based on there being too few
signed fields, too weak algorithm used for signing, too old signature
timestamp, and similar policy requirements. In such case, it SHOULD
verify that the tags accompanying the parameter are sufficient to
make a decision, and issue a negative 555 response otherwise, passing
the full list of the tags it needs, as indicated in Section 3.3.5.
The machine readable part of such response SHALL contain any required
sig-tag, with or without a value. Values given for header list,
signature timestamp, and expiration date are not meant to be exact,
but to specify minimal requirements, and the client may retry with
compatible values.
Header fields (h) are compatible if the dkim-param contains more
fields than required by the 555 reply. The dkim-param may contain
fields required by the reply even if they are not present in the
actual DKIM-Signature, provided that they are not present in the
message's header.
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Timestamp (t) is compatible if it is actually more recent in the
dkim-param than required in the 555 reply.
Expiration time (x) is compatible if it is actually more
permissive than required in the 555 reply.
In case the server can determine from the content of the tags present
in the parameter that the DKIM-Signature is not adequate for its
policy, it SHOULD issue a negative response 550, 553, or 555.
3.3. Responses to the VHLO command
An organization's servers accept incoming mail messages according to
some policies. The requisites for according a positive reply to a
VHLO command SHOULD NOT be less strict than those for accepting an
incoming message. In particular, if a policy states that certain
conditions imply that a message would be accepted with some reserves,
it should likely state that VHLO is denied under the same conditions.
When processing the optional auth-rept-claim's parameters, the server
MUST ignore any parameter whose tag it does not support or
understand.
In case of unsuccessful response, the server retains its previous
state.
3.3.1. Overview of possible responses
250 Domain OK, greetings and extension list
450 VHLO temporarily unavailable
451 VHLO aborted: error in processing
455 Parameter temporarily unverifiable
500 Syntax error, command unrecognized
501 Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
550 Missing required qualification
553 Domain rejected by policy
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555 Failed for recoverable reason
3.3.2. Positive response
If the checks carried out on the Domain and the connection indicate
that the server will wholeheartedly accept messages from the client,
the server returns a 250 reply code. The response is a multi-line
response with the same format as the EHLO response (ehlo-ok-rsp in
[RFC5321]), with the keywords for all the SMTP extensions available
as a consequence of entering this VHLO framework.
Upon a positive response, the client MUST reset any flags and
variables associated to SMTP extensions that it may have since
previous EHLO or VHLO commands in the same session.
3.3.2.1. VHLO parameter and MAIL FROM command
The server response to the VHLO and EHLO commands includes the VHLO
keyword along with a randomly generated token of up to 16 octets.
The format of the relevant line is as follows:
ehlo-line = "VHLO" SP random-string
random-string = 1*16( %d33-60 / %d62-126 )
; any CHAR excluding "=", SP, and control
; characters.
The random string supplied by the server MUST be repeated by the
client as the value of the VHLO parameter to the MAIL command, for
each transaction in the framework of this VHLO command. This is
meant to guard against blind attacks and to ease Greylisting checks.
3.3.3. Transient error responses
If the the server is temporarily unable to carry out any required
check on the Domain, it SHOULD return the 451 reply code. Then, the
client SHOULD quit the session and retry at a later time.
The server MAY return the 450 reply code to indicate that it is not
able or willing to reckon the client's reputation during this
session, irrespectively of any parameter supplied. In this case, the
client MAY try an EHLO command instead, to transmit messages outside
of any VHLO framework.
The server MAY return the 455 reply code to indicate that it is
temporarily unable to carry out the checks implied by one or more
specific parameters. It is possible that a positive response is
given if the client repeats the command using different auth-rept-
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claim's or different tag-spec-param's. The text of the response
SHOULD indicate the parameters that are still available as described
in Section 3.3.5.
3.3.4. Negative responses
If the the server cannot grant prime delivery (Section 1.1) because
of a missing parameter or parameter's value in the VHLO command, it
SHOULD return the 550 or 555 reply codes indicating the missing
parameters and arguments as described in Section 3.3.5.
The server MAY return the 553 reply code to indicate that it will
never grant prime delivery for the given Domain to the current
client, whatever auth-rept-claim's the client may supply.
The server MUST return the 503 reply code (bad sequence of commands)
if a VHLO command is issued while a transaction is active.
Servers that don't support this extension MAY return the 500 or 502
reply codes.
After a 555 reply code, the client MAY retry a VHLO command with the
parameters modified accordingly. Otherwise, if it is unable to
satisfy the server requirements, the client SHOULD proceed as if it
obtained a 500 reply code. It is RECOMMENDED that the client
application logs the missing requirements, so that administrators
know how to gain access to the given server.
After reply codes 500, 502, 550, and 553, the client MUST NOT attempt
more VHLO commands during the current session. In addition, after
reply codes 550 and 553, the client SHOULD NOT ever attempt any
further VHLO command to an MX server of the current target for
messages originating from the given Domain; this implies caching the
domains pair in a buffer that will be cleared by either configuration
updates or overrun (in theory, VHLO should not be retried until the
relevant datum changes in any of the involved servers, including
third parties).
After reply codes 500, 502, 550, 553, and 555, the client MAY quit
the session and send the message through an alternative relay as
described in Section 5. Alternatively, the client MAY try an EHLO
command, if it hasn't issued one already, and transmit messages
outside of any VHLO framework.
3.3.5. Diagnosis of failed VHLO commands
Normally, a client supplies all the claims that can possibly result
in increased reputation, except for line length limitations. VBR's
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certifier-list's, for example, might grow quite long and clients may
be unable to store them on a single line. However, servers can issue
multi-line responses containing the complete list, so that a client
can select the correct certifiers to include in the next attempt. As
some failures can be worked around automatically, failure responses
SHALL contain both human readable text and machine readable text.
Formally, reply codes 455, 550, and 555 to the VHLO verb have the
following syntax:
See [RFC5234] for imported ABNF
Failure-resp = *( Failure-code "-" [ diag-text ] CRLF )
Failure-code [ SP diag-text ] CRLF
Failure-code = %x34-35 %x35 %x35
diag-text = hread-text / (":" mread-text)
hread-text = 1*( %d09 / %d32-57 / %d59-126 ) *VCHAR
; the first character must not be ":"
mread-text = auth-rept-claim / check-failed
check-failed = check-keyword ":" check-spec-info
check-keyword = auth-rept-tag / "SPF"
check-spec-info = hread-text
; a column separated domain name list for VBR,
; a domain name or URL for DNSBL,
; required result or failure reason for SPF,
; required tags or failure reason for DKIM,
; n/a for MX, GID, PTR
A server SHOULD NOT vary its requirements during a given session.
If a client manages to issue a successful VHLO command for a given
Domain after a previous attempt failed, it MAY store the parameters
for future reuse. However, the server requirements MAY be changed in
future sessions.
3.4. Restrictions and further server side checks
Messages transmitted in the framework of a successful VHLO command
are subject to the restrictions detailed in this section. Clients
MUST NOT attempt to break these restrictions. Servers SHOULD check
that clients comply.
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3.4.1. MAIL FROM restriction
Non-empty arguments of the MAIL FROM commands are restricted to
addresses whose domain part consists of the authenticated Domain.
In addition, the server MUST check that the VHLO parameter is
included and that the corresponding value matches the random string
that the server generated on giving the positive response to the VHLO
command.
3.4.2. VBR restriction
If the VHLO command in whose framework the message is received
contained a VBR tag, the message MAY have a VBR-Info header. If that
header is present, it MUST be compatible with the given vbr-param.
Compatible here means that it mentions at least the certifier that
the server trusts and verified before accepting the relevant VHLO
command.
If a VBR-Info header is not present, the receiving server MAY add one
based on the Domain given, the certifiers it trusts and verified, and
its guess of the type of content among those mentioned in the RR(s)
obtained during the verification query.
3.4.3. DKIM-Signature headers existence and verification
If the VHLO command in whose framework the message is received
contained a DKIM tag, the message MUST have a valid DKIM-Signature
header field, compatible with the given dkim-param. Compatible here
means that the domain (d) of the DKIM-Signature is the same, the
selector (s) is the same one given in the parameter, and any sig-tag
on the signature that was also present in the VHLO parameter has a
value compatible with what has been given in the parameter. Again,
compatible means different things for different tags:
Header fields (h) are compatible if the actual signature contains
more fields than dkim-param. However, the signature may not
contain some fields, present in the dkim-param, but not actually
present in the message header, and still be compatible.
Timestamp (t) is compatible if it is actually more recent than
advertised in dkim-param.
Expiration time (x) is compatible if it is actually more
permissive than advertised in dkim-param.
Signature data (b) is compatible if the actual signature begins
with the same sequence of bytes contained in the dkim-param, and
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if it this unambiguously identifies it among signatures by the
given Domain.
If the server verifies signatures on the fly, the verification fails,
and such failure would prevent the message from having a prime
delivery (Section 1.1), the server SHOULD reject the message instead.
Note that the server does not need to verify more than one signature.
3.4.4. Greylisting restrictions
If transmission of a message in the framework of a VHLO command fails
due to transient conditions (4xx reply codes), and the transmission
was not itself a retry, the sending server SHOULD annotate the
current VHLO parameter in the message's meta data while it queues the
message for further retries. We refer to this piece of data as
original-vhlo-string. Typically, a message's meta data includes the
envelope and possibly the failure reason, and is used by a server to
devise a sending strategy as described in section 4.5.4.1 of
[RFC5321]. (Note that we are talking about transient failures in the
transmission of a message, i.e. after MAIL, RCPT, DATA, or data
completion by <CRLF>.<CRLF>; not the VHLO command.)
The current VHLO parameter should be added to meta data only after
the very first failure; in particular, not if a previous attempt to
transmit the message has happened before, whether in the framework of
a VHLO command or not. This implies that use of VHLO is restricted
to hosts who are able to discern new messages from retried attempts.
When attempting to retransmit a queued message that has this
original-vhlo-string in its meta data, the sending client SHOULD
transmit such string using the GID auth-rept-tag with
gid-param = original-vhlo-string
Only messages that share the same original-vhlo-string may be
transmitted in the framework of a VHLO command that used the GID
auth-rept-tag with that value. This implies that the sending client
MUST terminate the current VHLO framework in case the next message's
original-vhlo-string differs from the gid-param used to establish it
(where no gid-param matches an empty original-vhlo-string.)
4. Forwarding of messages accepted under VHLO
A message accepted in the framework of a VHLO command deserves prime
delivery (Section 1.1). However, the receiving server possibly does
not host the mailboxes of the relevant recipients directly. For
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example, it may be a boundary or secondary exchanger, a vanity
address server, or it may be following user-specific forwarding
instructions. For this specification, we just distinguish if the
message is forwarded within the same organization or to an external
domain.
If the message is forwarded internally, all hosts MUST be configured
so as to honor the promise of prime delivery that border or secondary
exchangers grant on their behalf. If, for whatever reason, prime
delivery is not possible, a failure notification MUST be sent to the
Return-Path address, if any. Even if sending notifications is
expected to be fairly safe at this point, it is RECOMMENDED that any
organization-wide policy that can be applied on acceptance produces
an on-line rejection rather than a delayed failure notification.
If the message is forwarded to an external domain, the SMTP client
SHOULD attempt to transmit it in the framework of a VHLO command,
unless either it can determine that the target host does not
implement this SMTP extension, or it has some other arrangement with
the target host that grants prime delivery (e.g. using strong
authentication as provided by [ff]).
VHLO may be used for forwarding in two different ways:
If the forwarder is affiliated with the original Domain or if the
message contains a valid DKIM signature from it, then the message
can be sent using the original envelope's originator address. The
Domain declared as VHLO parameter is the original one. (This is
as "Alias expansion".)
Otherwise, the Domain in the VHLO parameter is the forwarder's
domain and the originator address MUST be changed (e.g. using
[srs]). (This is as "List expansion".)
Failure to relay MUST always be reported as indicated by [RFC5321].
In particular, any reason that may be considered valid for not
issuing a failure notification SHOULD be ruled out before giving a
positive reply to the VHLO command.
5. Submission strategy
In order to avoid hassles, several smaller MTAs are configured to use
external Mail Submission Agents (MSAs) as smart hosts. One
collateral advantage of using Verified Hello is that falling back to
smart hosts can be confined to specific cases, depending on the
outcome of the weak authentication process. The postmasters of a
sending domain can resort to smart hosts while they collect feedback.
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Then, for the increased privacy and efficiency that direct delivery
yields, they'll have the ability to select what combination of
mechanisms and brands will satisfy the majority of their targets, and
decide to implement those requirements.
The VHLO command, by allowing to check deliverability in advance,
enables clients to use smart hosts optionally. Rather than
configuring a fixed mail-out path for certain target domains, relays
can dynamically adjust their strategy according to the target host's
response to the VHLO command. The list of preferred VBR certifiers
provided by a 555 negative response may be used as keys to build a
corresponding list of smart hosts that can be used as Mail Submission
Agents, provided that the certifiers of each smart host are known.
To implement this strategy, a relay's configuration needs a list of
alternative MSAs, consisting in one or more entries containing a host
name, a username/password pair, and an optional list of VBR
certifiers of that MSA. The latter field should be updated
dynamically whenever it does not correspond to the list returned with
a 555 negative reply from the smart host; it is RECOMMENDED to log
such updates as appropriate. Other means to dynamically select an
MSA, and how to determine the default one MAY also be provided for.
6. IANA Considerations
6.1. IANA Mail Parameters
This extension will have to be inserted in the mail-parameters
assignments IANA registry. The keyword VHLO should appear
o as a Registry Keyword, along with the "Verified Hello"
description, this document's reference, and a "+" for SMTP only,
and
o as an SMTP extension keyword that has a parameter, after the
"Verified Hello" description column, before the "Random ID"
parameter description and this document's reference that terminate
its row.
Formally, VHLO is not a service type, as it requires or assumes EHLO.
6.2. IANA VHLO methods
A registry is needed for tracking the auth-rept-tag / check-keyword
that must be unique in the diagnostic text. New methods may be
defined publishing their own RFCs where semantic and syntactic
details are explained, including error response and diagnosis. This
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document defines
+---------+----------------------------------+-----------+
| Keyword | Parameter/Description | Reference |
+---------+----------------------------------+-----------+
| DKIM | Key selector, query method, etc. | [this] |
| DNSBL | None. Diagnostic only. | [this] |
| GID | Greylisting ID. | [this] |
| MX | None. DNS lookup. | [this] |
| PTR | None. rDNS lookup. | [this] |
| SPF | None. Diagnostic only. | [this] |
| VBR | Certifier list. | [this] |
+---------+----------------------------------+-----------+
Initial registry values
7. Security Considerations
Global communications require that SMTP servers accept mail coming
from unknown hosts. This requirement rules out strong authentication
schemes, because, by definition, it is not possible to authenticate
unknown entities. Historically, Internet protocols granted some
trust to any host, since sporting a global IP address was deemed a
sufficient credential. When more restrictive criteria became
required, a number of mechanisms have emerged for identifying the
sender. DNS and rDNS are used to check the relationship between the
sender's IP address and its domain. However, using EHLO, the
sender's domain can only be guessed at. Some mechanisms, e.g. rDNS,
are not universally available, and, although good senders try and
facilitate the identification of themselves by setting up DNS as well
as they can, receivers provide no feedback on their effort. Since
senders don't know which mechanism, if any, would satisfy the
requirements of a target server, they can only follow generic
guidelines, outdated static policy pages, and rare support team's
hints whose validity is not imperishable.
This document proposes an intermediate level of trust. An SMTP
client is being authenticated based on weak evidence, originating
from the DNS and the TCP layer:
o The IP address of the remote client is known from the TCP layer.
Verification of the random string implies it is fairly difficult
to forge it.
o Any of the MX, PTR, or SPF checks confirms that the IP address is
somehow authorized by the organization who owns the Domain.
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o The DNSBL check implies that the IP address is not that of a known
attacker.
The two remaining checks, DKIM and VBR, may provide two additional
characterizations of the messages being transmitted. DKIM ensures
that messages have passed through the domain's signing process, which
presumably implies that any sender's local policy has been enforced.
In this respect, DKIM can be regarded as an open authorization to
impersonate the original Domain for the purpose of forwarding a
signed message. See [RFC5617], [RFC5672], and [RFC5863] for further
insight on DKIM semantics.
VBR, depending on the certifier's policy, may generically ensure that
the sending domain is well behaved. A vouching service may
scrutinize the DNS settings of a given domain, verify its whois
record, check their spam rate using honeypots, investigate the
domain's users, receive and process copies of the abuse reports
issued against messages emitted by that domain, verify that reported
spammers get blocked according to some policy, or otherwise establish
the domain reputation. The possibility to communicate the preferred
vouching services may work as an incentive for the advertised service
providers.
The authentication provided by this extension is weaker than SMTP
Authentication [RFC4954]. Therefore, it SHOULD NOT be used instead
of it.
Diagnostic messages provided with negative responses to the VHLO
command may disclose acceptance policies of the target domain. This
is not considered harmful, since such policies are usually public.
Letting a sender know which mechanism failed is a risk only in case
of security through obscurity. Mechanisms that are secure by design
don't have to be kept secret. The mechanisms considered in this memo
only involve DNSBL, SPF, MX, PTR, VBR, and DKIM. However, Verified
Hello provides for extensibility of this authentication/reputation
(auth-rept) mechanisms base. Giving feedback is important for
mechanism management, as it allows popular mechanisms to gain
momentum. In addition, some mechanisms reference a different domain
that makes explicit assertions about the reputation of the sender's
domain. This is where the branding practice comes into play. As the
number of domains that give reputation indications may grow much more
quickly than the number of mechanisms, feedback is specially
important for spreading their popularity. In this respect, Verified
Hello is not yet another authentication mechanism. It is a framework
for managing those mechanisms.
However, in case a Domain's security structure depends on keeping
that information secret, the server should carefully consider what
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diagnostic messages it provides to what clients. It is possible to
provide VHLO services to selected domains only, and discarding the
rest with the reply code 553.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC5518] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
Reference", RFC 5518, April 2009.
8.2. Informative References
[RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
for Authorizing Use of Domains in E-Mail, Version 1",
RFC 4408, April 2006.
[RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail",
RFC 4409, April 2006.
[RFC4954] Siemborski, R. and A. Melnikov, "SMTP Service Extension
for Authentication", RFC 4954, July 2007.
[RFC5451] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, April 2009.
[RFC5617] Allman, E., Fenton, J., Delany, M., and J. Levine,
"DomainKeys Identified Mail (DKIM) Author Domain Signing
Practices (ADSP)", RFC 5617, August 2009.
[RFC5672] Crocker, D., "RFC 4871 DomainKeys Identified Mail (DKIM)
Signatures -- Update", RFC 5672, August 2009.
[RFC5782] Levine, J., "DNS Blacklists and Whitelists", RFC 5782,
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February 2010.
[RFC5863] Hansen, T., Siegel, E., Hallam-Baker, P., and D. Crocker,
"DomainKeys Identified Mail (DKIM) Development,
Deployment, and Operations", RFC 5863, May 2010.
[ff] FixForwarding.org, "solution proposed", 2009,
<http://FixForwarding.org/wiki/solution_proposed>.
[greylisting]
Greylisting.org, "Greylisting.org - a great weapon against
spammers", 2009, <http://www.greylisting.org/>.
[srs] Libsrs2.org, "libsrs2 - Home", 2004,
<http://www.libsrs2.org/>.
Appendix A. Examples
Some examples showing the relevant snippet of client-server dialog.
A.1. Prime delivery message transfer
Complete example where the client successfully transfers a message
S: 220 example.com SMTP server ready
C: VHLO example.net
S: 250-example.com greetings example.net
250 VHLO 0123456789ABCDEF
C: MAIL FROM:<author@example.net> VHLO=0123456789ABCDEF
S: 250 Ok
C: RCPT TO:<dest@example.com>
S: 250 Ok
C: DATA
S: 354 Go ahead
S: From: author@example.net
To: dest@example.com
Subject: test
This is transmitted with prime delivery!
.
S: 250 Ok
C: QUIT
S: 221 Bye
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A.2. Failure after DNSBL check
Colons have been replaced in the automatic message to formally
preserve machine readability
C: VHLO example.net
S: 555-You are blacklisted
555 :DNSBL:see http_//www.dnsbl.example/query/bl?ip=192.0.2.3
C: QUIT
S: 221 Bye
Alternatively, the failure can be signaled as usual. Since feedback
plays a minor role for negative (black) vouching, the following is
likely to get an equivalent effect.
C: VHLO example.net
S: 550-You are blacklisted
550 see http://www.dnsbl.example/query/bl?ip=192.0.2.3
C: QUIT
S: 221 Bye
A.3. Failure on the MAIL FROM restriction check
In this snippet, the domain names are mismatched
C: VHLO example.net
S: 250-example.com greetings example.net
250 VHLO 0123456789ABCDEF
C: MAIL FROM:<user@example.org> VHLO=0123456789ABCDEF
S: 550 Domain origin mismatch
C: QUIT
S: 221 Bye
A.4. Automatically finding a common vouching service
In this snippet, the client finds a valid VBR name
C: VHLO example.net MX VBR:vouch1.example:vouch2.example
S: 555-we only accept these :VBR:vouch97.example:vouch98.example
555-:VBR:vouch99.example:vouch100.example:vouch101:example
555 :VBR:vouch102:example:vouch103:example:vouch104:example
C: VHLO example.net MX VBR:vouch100.example:vouch101.example
S: 250-example.com greetings example.net
250 VHLO 0123456789ABCDEF
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A.5. Reattempting Greylisted transmission
On a first attempt the client got greylisted
S: 220 example.com SMTP server ready
C: VHLO example.net
S: 250-example.com greetings example.net
250 VHLO FirstTime
C: MAIL FROM:<author@example.net> VHLO=FirstTime
S: 250 Ok
C: RCPT TO:<dest@example.com>
S: 450 You are greylisted, retry after 5 mins.
C: QUIT
S: 221 Bye
... 5 minutes later ...
S: 220 example.com SMTP server ready
C: VHLO example.net GID:FirstTime
S: 250-example.com greetings example.net
250 VHLO SecondTime
C: MAIL FROM:<author@example.net> VHLO=SecondTime
S: 250 Ok
C: RCPT TO:<dest@example.com>
S: 250 Ok
C: DATA
S: 354 Go ahead
S: From: author@example.net
To: dest@example.com
Subject: test
This is transmitted after greylisting delay!
.
S: 250 Ok
C: QUIT
S: 221 Bye
A.6. Mandating DKIM usage
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In this snippet, the server requires DKIM signature for specific
headers. The client might have turned for alternative delivery, EHLO
or alternative MSA, if it could not comply. In addition, a common
vouching service is automatically found as in the example above
(Appendix A.4).
C: VHLO example.net VBR:v1.example:v2.example
S: 555-we only accept these :VBR:v97.example:v98.example
555-:VBR:v99.example:v100.example:v101:example
555-:VBR:v102:example:v103:example:v104:example
555 :DKIM:h=to:from:cc:date
C: VHLO example.net VBR:v100.example DKIM:s=mail;h=to:from:cc:date
S: 250-example.com greetings example.net
250 VHLO 0123456789ABCDEF
A.7. Requiring extra DKIM tags
In this snippet, the server wants to know the timestamp and
expiration of the signature. Note that this almost certainly will
require a new VHLO framework in case further message for that Domain
have to be relayed.
C: VHLO example.net DKIM:s=mail
S: 555-we want to check signature timestamp and expiration time
555 :DKIM:t=;x=
C: VHLO example.net DKIM:s=mail;t=1117574938;x=1118006938
S: 250-example.com greetings example.net
250 VHLO 0123456789ABCDEF
Author's Address
Alessandro Vesely
v. L. Anelli 13
Milano, MI 20122
IT
Email: vesely@tana.it
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