Internet Draft
Category: Experimental G. Fecyk
Document: draft-fecyk-dsprotocol-03.txt Pan-Am Internet Services
Expires: September 2003 July 2003
Designated Mailers Protocol
A Way to Identify Hosts Authorized to Send SMTP Traffic
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Changes Since Last Revision
Testing revealed an inconsistency with wildcard handling between
various DNS implementations and RFC 1034 4.3.2. This draft no
longer requires wildcard records at all.
Throughout the document, added new DNS nodes for different transport
protocols. Introduced the nodes "in-addr" and "ip6" so-named for
their nodes in the arpa domain. DMP is now extensible to any
transport that supports address-to-name mapping through DNS.
5. Changed the TXT RR values to "dmp=deny" and "dmp=allow" in
accordance with RFC 1464, which describes a way to store arbitrary
string values using TXT RR records. The values for the "dmp"
attribute were chosen for their English meanings.
Throughout this document, renamed the Protocol to "Designated
Mailers Protocol" and changed the acronym for the Protocol to "dmp"
to avoid confusion with record types available in DNSSEC.
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Throughout the document, changed DNS node used for the Protocol to
"${addr-type}._smtp-client". This avoids conflicts and confusion
with SRV RR records.
5. Template, description and examples rewritten to use TXT RR
records instead of A RR records.
6. Protocol Flowchart redesigned to demonstrate changes to the
Protocol. Of most import: The second Protocol lookup, eliminated in
the second draft, was restored.
7 and 9. Example SMTP Conversations changed to accommodate changes
to Protocol, Template and Record Type use.
8. An additional mail forwarding problem, Source Routing, is
described.
Abstract
This document describes a proposed standard for identifying host
computer systems designated as Simple Mail Transfer Protocol (SMTP)
clients for an Internet domain or host through Domain Name System
(DNS). This identification allows SMTP servers to verify if a
connecting client is allowed to make outgoing SMTP connections on
behalf of the client's domain.
Conventions used in this document
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
In examples, the domain name "example.net" represents a fictional
domain receiving SMTP communications with servers, and "example.com"
represents a fictional domain transmitting SMTP communications with
clients.
All name record examples use BIND 4 syntax. The majority of DNS
server software supports this syntax. If yours does not, you will
need to translate the examples into the correct syntax for your DNS
server. Wildcard capability is desired, but not required.
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 [1].
Table of Contents
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1. Introduction...................................................3
2. Why Identify Sending Hosts as well as Receiving Hosts..........4
2.1 Junk E-mail................................................4
2.2 Viruses and Worms..........................................4
2.3 Account Fraud..............................................4
2.4 "Joe-Jobbers"..............................................5
3. What Participating in the Designated Mailers Protocol does NOT
Prevent...........................................................5
3.1 Junk E-mail with Correct Envelope Information..............5
3.2 "Joe-Jobbing" Within the Same Domain.......................5
3.3 Viruses and Worms using the Infected Party's SMTP Server...6
4. Background.....................................................6
5. Designating SMTP Clients.......................................6
5.1 By Internet Protocol v4 (IPv4) Address.....................9
5.2 Example Designations by IPv4 Address.......................9
5.3 By Internet Protocol v6 (IPv6) Address....................10
6. Protocol Flowchart............................................11
7. Example SMTP Conversations....................................13
8. Mail Forwarding...............................................15
8.1 Authorized Mail Relay.....................................15
8.2 Secondary Mail Exchangers.................................15
8.3 Mailing List Servers......................................16
8.4 Mail Forwarding such as .forward files on *ix systems.....16
8.5 SMTP Source Routing.......................................17
9. Accepting Mail from the Null Sender Envelope: MAIL FROM:<>....17
10. Security Considerations......................................18
10.1 DNS Security.............................................18
10.2 Mail Transfer Agent Security.............................18
10.3 DNS Outages..............................................18
10.4 Answering RFC 2821 7.1: Mail Security and Spoofing.......18
Appendix A: Why the Default Record was Restored..................19
Appendix B: How to Avoid Looking Up the Default Record...........20
Appendix C: Stupid Spammer Tricks................................21
C.1: Bogus Subdomains.........................................21
References.......................................................21
Acknowledgments..................................................22
Author's Addresses...............................................22
Full Copyright Statement.........................................22
1. Introduction
My ultimate goal in fighting junk e-mail is bringing accountability
back to its senders. With accountability, spammers must answer for
their e-mail abuse or stop sending junk e-mail. My first and most
successful attempt at doing so was the Orca DUL Project.
Even with projects like these and many others, spammers continued to
find ways to avoid accountability. This document is my latest
attempt to restore it, and with accountability restored, strengthen
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the myriad of anti-spam measures, policies and projects already
available.
This author wants to remind readers that implementing and using the
Designated Mailers Protocol is entirely OPTIONAL and NOT REQUIRED to
operate SMTP services. Also, readers are reminded that recipients
have the right to refuse any communication from anyone.
2. Why Identify Sending Hosts as well as Receiving Hosts
You want to identify your outgoing SMTP hosts so you can effectively
audit e-mail sent in the name of your domain. While it is still
possible to send authorized e-mail and still be abusive, it becomes
far easier to audit the abuse and identify the senders.
2.1 Junk E-mail
"Spammers," not wishing be identified or have their abused resources
identified, routinely falsify their e-mail's sender information,
including Sender Envelope (MAIL FROM: envelope), From: header and
Reply-To: header. Spammers often impersonate large, popular domains
such as hotmail.com[tm] when they do this. If such domains
participated in this Protocol, other participating domains would not
receive junk e-mail from spammers falsifying their sender
information.
Recipients using the Protocol could reject any e-mail sent in this
manner, regardless of how a resource is exploited. This includes
open relay, insecure proxy, dynamic IP, insecure and unrelated
resources such as older Formmail versions, and yet-undiscovered
exploits.
2.2 Viruses and Worms
While many viruses use the infected machine's sender address, those
same viruses rarely use the sender domain's resources to send copies
of itself, instead running their own SMTP engines.
Domains participating in this Protocol may reject copies of viruses
sent in this manner.
2.3 Account Fraud
Members of both financial institutions such as PayPal, and non-
financial account maintainers such as America On-Line, routinely
receive communication from confidence-artists and scammers
falsifying the membership host's domain in their e-mail. They do
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this to entice the recipients into divulging critical account
information to the scam artist.
Account maintainers participating in this Protocol can discourage
would-be scam artists from falsifying their domain, as participating
recipients may reject e-mail from them.
2.4 "Joe-Jobbers"
People wishing to harm other people on the Internet often falsify a
specific person's e-mail address to direct blame on that other
person. The first well-known instance of this abuse put the domain
joes.com out of business, hence the phrase "Being Joe'd" or "Joe-
Jobbing."
Domains participating in this Protocol could greatly diminish their
chances of being "Joe'd," as participating recipients could reject
such e-mail.
3. What Participating in the Designated Mailers Protocol does NOT
Prevent
Designated Mailers Protocol addresses a specific e-mail problem, and
does not stop "authorized" abuse. It does, however, make
"authorized" abuse far easier to track.
3.1 Junk E-mail with Correct Envelope Information
The Protocol would not stop a spammer from using envelope
information they are authorized to use. However, maintainers of a
spammer's domain could audit their activity more effectively, as the
spammer is forced to use their correct information.
If spammers maintain their own domains, domain-based blocking
becomes more effective. In extreme cases, where a spammer hosts
multiple domains, blocking the authoritative DNS servers for the
spammer's domains becomes very effective, as the recipient could
reject all Protocol lookups on the spammers' DNS servers. They would
effectively reject all e-mail from domains hosted there.
3.2 "Joe-Jobbing" Within the Same Domain
If a user on example.com wanted to falsify e-mail coming from
someone else within example.com, the Protocol would still allow for
it, since the sender envelope information would be correct.
However, maintainers of said domain could audit this e-mail and
identify the sender.
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3.3 Viruses and Worms using the Infected Party's SMTP Server
If a user's computer runs such a virus that, instead of using its
own SMTP engine, uses their outgoing mail server, using the Protocol
would not stop it. However, domain maintainers could audit this e-
mail and identify the user with the infected computer.
4. Background
At least one similar document [2] describes identifying outgoing
SMTP hosts by name using SRV RR records, and defines the sub-domain
"_CLIENT._SMTP._TCP". [4] However, there was no quick way to use the
sending host's IP address in the query. As the connecting IP
address is the most readily available bit of information we have on
a host, I propose using it, instead, in the query to see if it is a
designated SMTP mailer.
Many anti-spam projects use a variant of the addressing scheme
employed by the IN-ADDR.ARPA domain, where a name record for a given
IPv4 address may be found by querying each dotted-quad value as a
sub-domain. For example, we should find a PTR RR record in the IN-
ADDR.ARPA domain for 192.168.0.1 here:
1.0.168.192.in-addr.arpa.
Anti-spam projects using IN-ADDR.ARPA-like naming to identify
addresses to avoid receiving SMTP communication, do so by creating A
RR records, and use an otherwise meaningless value for the A RR
itself:
1.0.168.192.evil-spammers-list.example.com. A xxx.xxx.xxx.xxx
A subscriber to this project would look up 192.168.0.1 by querying
this name for a valid A RR record. If it found one, it would refuse
further SMTP connectivity from there.
The Designated Mailers Protocol defines a unique set of DNS resource
records to identify a domain as a participant of the Protocol. It
further defines a way to identify the addresses that are designated
to make SMTP connections on behalf of the sender's domain or host.
5. Designating SMTP Clients
All domains participating in DMP set up TXT RR records in a sub-
domain called "_smtp-client". The first is a placeholder which
identifies the domain as a participant. The second is a default
lookup result for servers that support wildcard records:
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;REQUIRED: DMP Placeholder
_smtp-client.${DOMAINNAME}. TXT "dmp="
;RECOMMENDED: Default record
*._smtp-client.${DOMAINNAME}. TXT "dmp=deny"
Additional records are OPTIONAL and identify the network hosts
designated as SMTP clients:
;OPTIONAL: DMP records
${ADDRESS}.${ADDR-TYPE}._smtp-client.${DOMAINNAME}. TXT "dmp=allow"
${ADDRESS} represents a network address in inverse form as used in
the "arpa" node. For instance, IPv4 addresses are represented as
they are in the in-addr.arpa node, IPv6 addresses are represented as
they are in ip6.arpa. ${ADDR-TYPE} represents the address type as
already used in the "arpa" node, such as "in-addr" for IPv4 and
"ip6" for IPv6. ${DOMAINNAME} represents the right-hand side of a
SMTP mail sender envelope as used in the MAIL FROM: command. For
example, the sender envelope "user@example.com" has a right-hand
side of "example.com". If there is no domain part in a sender
envelope, or the domain is "localhost", no DMP lookup is needed as
it is a local delivery.
Using inverse addresses in the zone records allows us to use
wildcards in the records, reducing the number of records needed for
a large bank of SMTP clients.
The TXT RR record was chosen for its flexibility. TXT RR records
are supported by all DNS server software. The values were selected
for their English meanings. The record strings are case-
insensitive, so "DMP=ALLOW" is the same as "dmp=allow".
Software designed to look up RFC 1464 [8] type records may be used
to look up DMP records, but ANY algorithm that looks up TXT RR
records will work. For example, a function written in C to RFC
1464's example would have the following parameters:
snprintf(dmpRecordName, bufsize, "%s.in-addr._smtp-client.%s",
reversedIP, domainName);
getattributebyname(dmpRecordName, "dmp", result, bufsize);
To accommodate messages using a Null Reverse Path (MAIL FROM:<>),
participating sites MUST create a minimum of two DMP records for
each of their servers in addition to the records for their domain:
;REQUIRED: DMP Placeholder and single record
_smtp-client.${FQDN}. TXT "dmp="
${ADDRESS}.{ADDR-TYPE}._smtp-client.${FQDN}. TXT "dmp=allow"
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; RECOMMENDED: Default DMP record for FQDN
*._smtp-client.${FQDN}. TXT "dmp=deny"
...where ${FQDN} represents the MTA's fully qualified domain name.
Multiple records, or Wildcards if supported, may be used for a range
of hosts with the same FQDN.
A lookup of a network address will generate one of four results:
- A successful retrieval of a TXT RR record from a valid DMP
domain node, and a value of "dmp=allow" indicating the connecting
IP is a Designated Mailer for the domain,
- A successful retrieval of a TXT RR record from a valid DMP
domain node, and a value of "dmp=deny" indicating the connecting
IP is NOT a Designated Mailer for the domain,
- A permanent error, either NXDOMAIN, CNAME only, multiple
conflicting DMP records or non-DMP TXT RR record retrieved,
indicating the domain in the MAIL command does not participate in
the Protocol, or
- A temporary error, or failure to retrieve a record (SERVFAIL).
A lookup of a domain itself, to see if it participates in the
Protocol, will generate one of three possible results:
- A successful retrieval of a TXT RR record from a valid DMP
domain node, and a value of "dmp=" indicating the domain in the
MAIL FROM: command participates in the Protocol,
- A permanent error, either NXDOMAIN, CNAME only, DMP record with
a value different from "dmp=", multiple conflicting DMP records or
non-DMP TXT RR record retrieved, indicating the domain in the MAIL
command does not participate in the Protocol, or
- A temporary error, or failure to retrieve a record (SERVFAIL).
Participating servers MUST distinguish between all possible results.
Notably, SERVFAIL is a temporary error and not a permanent one, and
a subsequent lookup of the same record may succeed.
A participating domain MUST have one TXT RR record indicating they
participate in the Protocol, and that record MUST have a "null"
value for the "dmp" attribute:
_smtp-client.${DOMAIN_NAME}. TXT "dmp="
The "dmp=" record was introduced to operate correctly with DNS
servers that may handle wildcards differently from another, or not
handle wildcards at all. A full explanation appears in Appendix A.
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A participating domain SHOULD also have an additional TXT RR record
providing a default lookup result, if the DNS server supports
wildcards:
*._smtp-client.${DOMAIN_NAME}. TXT "dmp=deny"
A participating domain MAY have NO additional Designated Mailer
records, thereby indicating no SMTP traffic will originate from
their domain. The default of "deny" ensures that any lookups for
non-designated mailers will return this value.
This author understands that site administrators will want to permit
SMTP client connections from any host on a network. If a site
administrator insists on doing this, the site MUST have its default
"allow" record set per address type:
*.${ADDR-TYPE}._smtp-client.${DOMAINNAME}. TXT "dmp=allow"
Site administrators really, really SHOULD NOT do this, and instead
SHOULD designate their own hosts and networks. It is not difficult
to add and remove DMP records for dynamically addressed hosts,
testing hosts, temporary relays, third-party relays and so on.
5.1 By Internet Protocol v4 (IPv4) Address
A host or domain participating in Designated Mailers would create
TXT RR records in their DNS zone with this template [3]:
;REQUIRED: DMP placeholder
_smtp-client.${DOMAINNAME} TXT "dmp="
;RECOMMENDED: DMP default record
*._smtp-client.${DOMAINNAME} TXT "dmp=deny"
;OPTIONAL: DMP records
${REVERSEDIP_1}.in-addr._smtp-client.${DOMAINNAME}. TXT "dmp=allow"
${REVERSEDIP_2}.in-addr._smtp-client.${DOMAINNAME}. TXT "dmp=allow"
[...]
${REVERSEDIP_N}.in-addr._smtp-client.${DOMAINNAME}. TXT "dmp=allow"
${REVERSEDIP_N} represents the IPv4 address in reverse-dotted-quad
order. For example, the IPv4 address 192.168.0.1 would become
"1.0.168.192" in reverse-dotted-quad order, as used in IN-ADDR.ARPA.
5.2 Example Designations by IPv4 Address
Here is an example portion of a DNS zone file for example.com, where
the hosts at IPv4 addresses 192.168.0.10 and 192.168.1.110 are the
Designated Mailers for example.com:
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$ORIGIN example.com.
_smtp-client TXT "dmp="
*._smtp-client TXT "dmp=deny"
10.0.168.192.in-addr._smtp-client TXT "dmp=allow"
110.1.168.192.in-addr._smtp-client TXT "dmp=allow"
Here is an example of example.com designating a rack of load-
balancing hosts in the network of 192.168.1.0/24 as Designated
Mailers:
$ORIGIN example.com.
_smtp-client TXT "dmp="
*._smtp-client TXT "dmp=deny"
*.1.168.192.in-addr._smtp-client TXT "dmp=allow"
Here is an example of example.com stating that no SMTP traffic will
originate from their domain:
$ORIGIN example.com.
_smtp-client TXT "dmp="
*._smtp-client TXT "dmp=deny"
Finally, here is an example of a stand-alone host with its own Fully
Qualified Domain Name, "lonehost.example.com":
$ORIGIN example.com.
lonehost A 192.168.0.1
_smtp-client.lonehost TXT "dmp="
*._smtp-client.lonehost TXT "dmp=deny"
1.0.168.192.in-addr._smtp-client.lonehost TXT "dmp=allow"
Alternately:
$ORIGIN lonehost.example.com.
@ A 192.168.0.1
_smtp-client TXT "dmp="
*._smtp-client TXT "dmp=deny"
1.0.168.192.in-addr._smtp-client TXT "dmp=allow"
5.3 By Internet Protocol v6 (IPv6) Address
We can trivially extend the Protocol to identify authorized IPv6
addressed hosts. The template for identifying IPv6 hosts as
Designated Mailers is identical to the template for IPv4, only using
reversed IPv6 addresses as part of the record names.
;Required: DMP placeholder
_smtp-client.${DOMAINNAME} TXT "dmp="
;Recommended: DMP default record
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*._smtp-client.${DOMAINNAME} TXT "dmp=deny"
;Optional: DMP records
${REVERSIPv6_1}.ip6._smtp-client.${DOMAINNAME}. TXT "dmp=allow"
${REVERSIPv6_2}.ip6._smtp-client.${DOMAINNAME}. TXT "dmp=allow"
[...]
${REVERSIPv6_N}.ip6._smtp-client.${DOMAINNAME}. TXT "dmp=allow"
${REVERSIPv6_N} represents a reverse nybble of the IPv6 address as
used in ip6.arpa. For instance, The IPv6 address
2345:00C1:CA11:0001:1234:5678:9ABC:DEF0 becomes
"0.F.E.D.C.B.A.9.8.7.6.5.4.3.2.1.1.0.0.0.1.1.A.C.1.C.0.0.5.4.3.2".
The remainder of this document will use IPv4 addressing and records
in its examples. Aside from the template, the actions for IPv6
participants are identical to that of IPv4 participants.
6. Protocol Flowchart
The flowchart starts when the client issues the MAIL command. It
ends with an acknowledgement or error before the client issues the
RCPT command.
====================================
| Client issues MAIL FROM: command |
====================================
|
=============================================================
| Server checks local Allow and Auth rules on connecting IP |
=============================================================
|
=============================
| Allowed or Authenticated? |-------Yes------------|
============================= |
| ============
No | 250 OK |
| | Resume |
| | Normally |
| ============
=======================================
| Server performs DMP Protocol lookup |
=======================================
|
=============
| Response? |-----------|---------------|---------------|
============= | | |
| | | |
Success Success NXDOMAIN or SERVFAIL
Valid DMP Record Valid DMP Record Not Valid DMP or |
"dmp=allow" "dmp=deny" No TXT RR Record |
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| | | |
| | | |
| | ================ ===============
| | | Do I accept | | Do I accept |
| | | mail from | | mail from |
| | | non-DMP site?| | (possibly) |
| | | | |non-DMP site?|
| | ================ ===============
| | | |
| |-----No--------| |---|
| | | | |
| | Yes Yes No
| | | | |
| | =================== | |
| | | Server Performs | | |
| | | dmp= lookup | | |
| | =================== | |
| | | | |
| | =================== | |
| | | dmp= found? | | |
| | =================== | |
| | | | |
| |------Yes------| | |
| | | | |
| | No | |
| | | | |
| | | |--------| |
| | | | |
=========== ============= =========== ==============
| 250 OK | | 550 ERROR | | 250 OK | | 451 ERROR |
| Resume | | Need DMP | | Resume | | Cannot see |
| Normally| | Records | | Normally| | DMP records|
=========== ============= =========== ==============
Designated Mailers Protocol uses error code 550 for a permanent
error, indicating a policy reason, and 451 for a temporary error,
indicating a local processing error. Error code 451 is the only
temporary error response allowed to the MAIL command. [5]
Previous drafts eliminated this second Protocol lookup to determine
if a domain participates or not. Because of discoveries in testing
this had to be restored. However, there are ways to avoid
performing this lookup; see Appendix B.
Participating servers MAY bypass the Protocol altogether for chosen
addresses, for instance, their "allow relay" list for client IPs, or
for client IPs running a Mail User Agent that supports some form of
authentication for sending e-mail. Example authentication protocols
include POPAUTH, SMTP AUTH, client certificates and so on. Doing
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this avoids requiring dynamically created records for dial-up users
and roaming users.
When a non-participating client connects to a participating server,
it SHOULD respond as though the Protocol didn't exist. However, as
the Protocol gains acceptance, it MAY respond as its operators wish.
For instance, the server MAY refuse e-mail until the senders
participate in the Protocol, or accept the message and include
additional headers in the message to inform the recipient.
7. Example SMTP Conversations
These examples introduce new error conditions in the SMTP or ESMTP
protocol after the MAIL command. RFC 2821 section 4.3.2 states this
is allowed. [5]
An example of a successful SMTP session between hosts in
participating domains:
S: 220 mail.example.net MMS SMTPRCV service v0.95
C: HELO clientmachine.example.com
S: 250 mail.example.net Hello clientmachine.example.com
[192.168.0.1]
C: MAIL FROM:<user@example.com>
(server looks up 1.0.168.192.in-addr._smtp-client.example.com and
receives "dmp=allow")
S: 250 OK client at 192.168.0.1 verified as authorized sender for
example.com
[resume normally]
An example of a rejected (dmp=deny) session from an undesignated
host claiming to be part of a participating domain to a
participating host:
S: 220 mail.example.net MMS SMTPRCV service v0.95
C: HELO not-clientmachine.example.com
S: 250 mail.example.net Hello not-clientmachine.example.com
[192.168.1.1]
C: MAIL FROM:<user@example.com>
(server looks up 1.1.168.192.in-addr._smtp-client.example.com and
receives "dmp=deny")
S: 550 ERROR client at 192.168.1.1 is not a Designated Mailer for
example.com
C: RCPT TO:<receipient@foo.bar>
S: 550 ERROR must issue MAIL FROM: command first
C: DATA
S: 554 ERROR must issue MAIL FROM: and RCPT TO: first
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An example of a rejected (NXDOMAIN and dmp=) session from an
undesignated host claiming to be part of a participating domain,
only where the first lookup returns NXDOMAIN:
S: 220 mail.example.net MMS SMTPRCV service v0.95
C: HELO not-clientmachine.example.com
S: 250 mail.example.net Hello not-clientmachine.example.com
[192.168.1.1]
C: MAIL FROM:<user@example.com>
(server looks up 1.1.168.192.in-addr._smtp-client.example.com and
returns NXDOMAIN)
(server looks up _smtp-client.example.com and receives "dmp=")
S: 550 ERROR client at 192.168.1.1 is not a Designated Mailer for
example.com
An example of a successful session from a host belonging to a non-
participating domain (NXDOMAIN), where the participating server acts
as though the protocol didn't exist:
S: 220 mail.example.net MMS SMTPRCV service v0.95
C: HELO not-clientmachine.example.com
S: 250 mail.example.net Hello not-clientmachine.example.com
[192.168.1.1]
C: MAIL FROM:<user@example.com>
(server looks up 1.1.168.192.in-addr._smtp-client.example.com and
returns NXDOMAIN)
(server looks up _smtp-client.example.com and returns NXDOMAIN)
S: 250 OK, mail from user@example.com.
[resume normally]
An example of a rejected (NXDOMAIN) session from a host belonging to
a non-participating domain, where the participating server refuses
SMTP traffic from non-participating domains:
S: 220 mail.example.net MMS SMTPRCV service v0.95
C: HELO not-clientmachine.example.com
S: 250 mail.example.net Hello not-clientmachine.example.com
[192.168.1.1]
C: MAIL FROM:<user@example.com>
(server looks up 1.1.168.192.in-addr._smtp-client.example.com and
returns NXDOMAIN)
(Note that, in this case, a lookup for _smtp-client.example.com
isn't necessary)
S: 550 ERROR cannot verify 192.168.1.1 as sender for example.com.
An example of a failed (SERVFAIL) session from a host belonging to,
possibly, a non-participating domain, where the participating server
refuses SMTP traffic from, possibly, non-participating domains:
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S: 220 mail.example.net MMS SMTPRCV service v0.95
C: HELO not-clientmachine.example.com
S: 250 mail.example.net Hello not-clientmachine.example.com
[192.168.1.1]
C: MAIL FROM:<user@example.com>
(server looks up 1.1.168.192.in-addr._smtp-client.example.com and
returns SERVFAIL)
(Note that, in this case, a lookup for _smtp-client.example.com is
not necessary)
S: 451 ERROR cannot verify 192.168.1.1 as sender for example.com at
this time.
A DNS outage may be responsible for a SERVFAIL response. For this
reason, participating servers MUST use a 451 error message
indicating a temporary failure, and not a 550 series message
indicating a permanent failure, when any lookup returns SERVFAIL.
This is a separate and distinct condition from where the server can
determine (through a NXDOMAIN response) that the client is not from
a participating domain.
8. Mail Forwarding
Mail forwarding without altering the sender envelope in the MAIL
command becomes difficult with the Designated Mailers Protocol. The
host acting as the forwarding agent rarely is a Designated Mailer
for the sender's domain. This section covers how to accommodate
mail forwarding under the Protocol.
8.1 Authorized Mail Relay
Most mail user agents (MUAs) use an external SMTP server as a relay
agent or "outgoing mail server." The MUA's IP address is almost
never a Designated Mailer for the sender's domain.
The relaying MTA only has to use their "Allow Relay" list or an
authentication mechanism to permit SMTP traffic from the MUA, as is
accepted practice. [6] Hosts running MUAs do not need their own DMP
records if they use a relay or smart host that has them.
8.2 Secondary Mail Exchangers
Larger Internet providers use a network of peer mail exchangers and
directly inject e-mail from the peer into their internal mail
system, so these are largely unaffected. Smaller providers use a
single Primary host and one or more Secondary hosts as relays to the
Primary, and the Secondary is almost never a Designated Mailer for a
sender's domain.
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Participants in the Protocol that use Secondary mail exchangers can
use one of several ways to accept mail from their Secondary hosts
again. Here are two possibilities:
- Add the Secondary host's IP address to the Primary's "Allow
Relay" list, similar to 8.1 above.
- Program the Secondary to alter the sender envelope, and
designate the Secondary as a Designated Mailer for itself, similar
to 5.2 above. An altered sender envelope could contain the
original sender's information, for instance including the original
sender envelope directly:
MAIL FROM:<originalsender(at)example.com@secondarymx.example.net>
...or encode the original sender into the new sender envelope
somehow, such as with a two-way hash:
MAIL FROM:<236FA24C@secondarymx.example.net>
Whatever method is used, a Secondary MUST return mail back to the
sender if the Primary rejects it.
This author understands that most MTAs relaying as a Secondary do
not modify the sender envelope. While modifying the envelope is
more secure, adding a Secondary to the Primary's "allow" list is
acceptable.
8.3 Mailing List Servers
Some list server implementations use the original envelope sender
when forwarding mail from a sender to the list members, but most
modern list software alters or replaces the sender envelope somehow.
For example, list software uses the list-owner address in the sender
envelope.
Maintainers of mailing lists that do not alter the sender envelope
SHOULD NOT to participate in the Protocol until they upgrade their
mailing list software.
8.4 Mail Forwarding such as .forward files on *ix systems
This author recognizes that every MTA using .forward files on *ix
hosts or similar configurations do not alter the sender envelope
when forwarding mail. However, commercial and proprietary mail-
forwarding systems such as mail.com[tm] alter the sender envelope,
and these are by far the most popular pure mail-forwarding
instances.
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There is no fix for .forward usage aside from altering the sender
envelope and designating the forwarding host as a Designated Mailer
for itself, as described in 5.2 above.
8.5 SMTP Source Routing
While depreciated, few SMTP sites still support source routing.
Consider the following sender envelope:
MAIL FROM:<@host.one,@host.two:user@host.three>
If a participating site supports source routing, the site SHOULD
perform its Protocol lookup on "host.one" as the client should be
from that domain. Otherwise, the site MUST perform its Protocol
lookup on "host.three".
9. Accepting Mail from the Null Sender Envelope: MAIL FROM:<>
As there is no domain to perform a DMP Protocol lookup in a Null
Sender envelope, it is difficult to tell if the sender of such a
message is authorized to do so. However, there are other ways to
ensure the sender is authorized to send it.
According to RFC 2821 section 4.5.5, the Null reverse-path
accompanies Delivery Status Notifications, Message Disposition
Notifications, and other messages which are notifications about
previous, non-null-enveloped messages. These types of messages are
always generated by mail delivery software only, and not by users.
We can ensure that null reverse-path messages only come from hosts
running MTAs and not from users. The simplest is to make the sender
host a Designated Mailer for itself as described in 5 and 5.2 above.
To strengthen this, a recipient server MAY inspect the forward or
reverse DNS records, or both, on the client host's HELO or EHLO
identifier:
S: 220 mail.example.net MMS SMTPRCV service v0.95
C: HELO clientmachine.example.com
S: 250 mail.example.net Hello clientmachine.example.com
[192.168.0.1]
C: MAIL FROM:<>
(server looks up 1.0.168.192.in-addr._smtp-
client.clientmachine.example.com and receives "dmp=allow")
(optional: server looks up clientmachine.example.com and receives
192.168.0.1, compares A RR record to connecting IP address and they
match)
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(optional: server looks up 1.0.168.192.in-addr.arpa for PTR and
receives clientmachine.example.com, compares PTR RR record to HELO'd
name and they match)
S: 250 OK Null reverse-path message coming from
clientmachine.example.com
With the exception of using the HELO or EHLO host name as the domain
to perform the DMP lookup on, and the optional comparisons suggested
above, the flowchart for handling null reverse-path messages is
identical to that for handing other messages.
Note that a HELO or EHLO host name is not necessarily a mail domain.
DMP only treats it as one for the purpose of performing a Protocol
lookup, and only for messages with null reverse-paths.
10. Security Considerations
10.1 DNS Security
Designated Mailers Protocol relies solely on DNS to verify
Designated Mailer hosts. Any compromise of a domain's DNS records
would make that domain vulnerable to "spoofing" in a run by
spammers, but the spammers would need to insert necessary name
records into your DNS zone. Likewise, a compromised server at the
tier below a domain could allow spammers to insert false Designated
Mailer records for that domain. Best current practices for DNS
server security will prevent these and similar abuses. DMP records
may reside on DNSSEC servers without changes to the Protocol.
10.2 Mail Transfer Agent Security
Compromised hosts already identified as Designated Mailers may be
used to send unauthorized e-mail in the name of the designator's
domain. Best current practices for SMTP MTAs in general will prevent
these and similar abuses.
10.3 DNS Outages
A domain may still be successfully spoofed if the sender domain's
records are unreachable (SERVFAIL), AND if the participating server
accepts SMTP traffic from, possibly, non-participating domains.
Participating sites SHOULD refuse SMTP traffic based on this case,
using a 400 series error indicating a temporary failure, even if
they choose to accept SMTP traffic from domains that decidedly not
(NXDOMAIN) participate, to avoid this.
10.4 Answering RFC 2821 7.1: Mail Security and Spoofing
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From 7.1 of RFC 2821:
Efforts to make it more difficult for users to set envelope return
path and header "From" fields to point to valid addresses other
than their own are largely misguided: they frustrate legitimate
applications in which mail is sent by one user on behalf of
another or in which error (or normal) replies should be directed
to a special address.
[...]
This specification does not further address the authentication
issues associated with SMTP other than to advocate that useful
functionality not be disabled in the hope of providing some small
margin of protection against an ignorant user who is trying to
fake mail.
Designated Mailers Protocol still allows a user to send mail on
behalf of another within the same domain. It also still allows
mailing list systems to send mail to subscribers on behalf of other
subscribers in different domains.
DMP does not affect the From or Reply-to headers, or body of a
message. It still allows sending mail on behalf of another, while
using an envelope authorized for use within the sender's domain.
We are no longer looking at one or two ignorant users trying to fake
mail. Today we are looking at an entire industry built on the
unwilling backs of recipients, based on several hundreds of
thousands of deliberately ignorant users who fake mail for fun and
profit. This author can live with losing a little otherwise-useful
functionality for a very large margin of protection against them.
Appendix A: Why the Default Record was Restored
Those who have followed the Protocol's development to this point
will note that this author tried to eliminate the need for more than
one DNS query using DNS wildcard records. Testing revealed an
inconsistency between implementations of DNS concerning wildcards.
RFC 1034 4.3.2 describes the algorithm with which DNS servers are to
parse a name record file. That algorithm allows for wildcard nodes,
but allows for sub-nodes below the wildcard in the DNS tree to
override the wildcard. For instance, with the following name
records in place:
$ORIGIN _smtp-client.example.com.
* TXT "dmp=deny"
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1.1.168.192.in-addr TXT "dmp=allow"
...a lookup of x.x.x.192.in-addr._smtp-client.example.com (where "x"
could be any RFC 1034-legal string) would return NXDOMAIN, and not
the originally intended default record of "dmp=deny". A lookup of
other records outside *.192 would return the intended default
record.
This is clearly defined in RFC 1034, and later clarified in a work-
in-progress titled draft-ietf-dnsext-wcard-clarify-00.txt. However,
most DNS server implementations treat a wildcard record of this
nature as override ALL other possible lookups that aren't explicitly
defined, such as 1.1.168.192 above.
This author had to restore the second lookup in order to accommodate
RFC 1034's requirements, and not depend on any specific
implementation's algorithm. This is in spite of an overwhelming
number of DNS sites that violate RFC 1034 4.3.2 in this author's
favour. However, this change effectively eliminates the need for
wildcard records, avoiding the wildcard problem entirely.
Sites that participate in the Protocol MAY choose DNS
implementations that work to their best advantage, so they can
reduce the number of maintained records. However, no one DNS
implementation is required to participate.
Appendix B: How to Avoid Looking Up the Default Record
The second Protocol lookup, originally introduced in the first draft
and restored in this draft, will impact the DNS system harder at
first. This author expects this impact will diminish as sites adopt
the Protocol.
However, participating sites and MTA authors can take steps to avoid
performing this lookup immediately. For example:
- A participating MTA MAY maintain a list of domains known to
participate. As the MTA retrieves "dmp=allow" from participating
domains, it could store the domain name in a local list to check
against in case future Protocol lookups on the domain return
NXDOMAIN. The lifetime for storing this record COULD be as long as
the DNS record's lifetime as returned by the participating site's
DNS server.
- A participating site's DNS server COULD use a DNS implementation
that treats wildcard records slightly differently from RFC 1034
4.3.2, that is, allow the wildcard to substitute for any node not
explicitly defined in the DNS zone file.
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- Over time, participating sites MAY choose to bypass this second
lookup altogether. This will depend on the adoption rate of DMP.
Appendix C: Stupid Spammer Tricks
C.1: Bogus Subdomains
If example.com participates in the Protocol, a spammer could still
spoof a subdomain of example.com. Sites that accept mail from non-
DMP sites could accept mail from the spammer.
Already, spam-aware MTAs could perform MX and A RR lookups on the
domain in the MAIL FROM: command, to ensure that the domain exists
and normally handles SMTP. This kind of check could occur before
any DMP lookup.
Of course, as DMP is adopted, a participating site could refuse mail
from non-DMP sites, eliminating this problem entirely.
References
1. Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
2. Credit where due: B. Gingery first brought up the topic on the
SPAM-L mailing list, and it had been discussed earlier on the
Spamtools mailing list. There are other documents describing
similar approaches to this problem.
3. Derek J. Balling <dredd@megacity.org> for initial design of the
DNS record template and explanation, and "der Mouse"
<mouse@rodents.montreal.qc.ca> for generalizing it into the
current form.
4. Gulbrandsen, A., Vixie, P., Esibov, L., "A DNS RR for specifying
the location of services (DNS SRV)", RFC 2782, February 2000
5. 4.3.2 of Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001
6. 2.1 of Lindenberg, G., "Anti-Spam Recommendations for SMTP MTAs",
RFC 2505, February 1999
7. 4.5.5 of Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001
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8. Rosenbam, R., "Using the Domain Name System To Store Arbitrary
String Attributes", RFC 1464, May 1993
Acknowledgments
Derek J. Balling <dredd@megacity.org> for initial design of the DNS
record template and explanation, and "der Mouse"
<mouse@rodents.montreal.qc.ca> for generalizing it into the
current form.
Michael A. Smith for the hint to RFC 1464.
Jack Bates for SMTP Source Routing hints.
Steve Atkins and Bill Cole for DNS Clue.
"der Mouse" for IPv6 hints and implementation testing.
"Zefram" for first uncovering the wildcard problem, and everyone
else already mentioned for detailing it.
Author's Addresses
Gordon Fecyk
Pan-Am Internet Services
24 - 482 Young Street
Winnipeg, MB R3B 2S6
Canada
Phone: (204) 292-9970
Email: gordonf@pan-am.ca
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