RFC 8904: DNS Whitelist (DNSWL) Email Authentication Method Extension

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.¶

This is a contribution to the RFC Series, independently of any other RFC stream. The RFC Editor has chosen to publish this document at its discretion and makes no statement about its value for implementation or deployment. Documents approved for publication by the RFC Editor are not candidates for any level of Internet Standard; see Section 2 of RFC 7841.¶

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8904.¶

Copyright Notice

Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.¶

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document.¶

2. Method Details

The result of the method states how the query did, up to the interpretation of the returned data.¶

The method has four possible results:¶

pass:

The query successfully returned applicable records. This result is usually accompanied by one or both of the policy properties described below. Since the list is configured as a DNSWL, agents unable to interpret list-specific properties can still derive a positive value from the fact that the sender is whitelisted.¶

none:

The query worked but yielded no A record or returned NXDOMAIN, so the sender is not whitelisted.¶

temperror:

The DNS evaluation could not be completed due to some error that is likely transient in nature, such as a temporary DNS error (e.g., a DNS RCODE of 2, commonly known as SERVFAIL) or other error condition. A later attempt may produce a final result.¶

permerror:

The DNS evaluation cannot work because test entries don't work (that is, DNSWL is broken) or because queries are over quota (reported by a DNS RCODE of 5, commonly known as REFUSED, or by a DNSWL-specific property (policy.ip, defined below) with the same meaning). A later attempt is unlikely to produce a final result. Human intervention is required.¶

Note that there is no "fail" result.¶

The following ptype.property items define how the data provided by the whitelist lookup can be saved.¶

dns.zone:

DNSWL query root domain, which defines the meaning of the policy.ip property below. Note that an MTA can use a local mirror with a different name. The name stored here has to be the best available reference for all foreseeable downstream consumers. Setting dns.zone to the global zone makes the result intelligible even if the message is handed outside of the internal network.¶

policy.ip:

The bit mask value received in type A response, in dotted quad notation. Multiple entries can be arranged in a quoted, comma-separated list (quotes are necessary because commas are not allowed in a token).¶

policy.txt:

The TXT record, if any. Multiple records are concatenated in the usual way (explained, for example, in Section 3.3 of [RFC7208]). See Section 3 for the resulting content and query options.¶

dns.sec:

This is a generic property stating whether the relevant data was validated using DNSSEC [RFC4033]. For the present method, the relevant data consists of the reported policy properties above or, if the method result is "none", its nonexistence. This property has three possible values:¶

yes:

DNSSEC validation confirms the integrity of data. Section 5.2 considers how that is related to the DNS response.¶

no:

The data is not signed. See Section 5.2.¶

na:

Not applicable. No DNSSEC validation can be performed, possibly because the lookup is run through a different means than a security-aware DNS resolver. This does not necessarily imply less security. In particular, "na" is used if the data was downloaded in bulk and then loaded on a local nameserver, which is the case of an MTA querying a local zone different from the reported dns.zone. DNS errors, including validation errors, can also report "na". This is also the value assumed by default.¶

3. TXT Record Contents

According to [RFC5782], TXT records describe the reason why IP addresses are listed in a DNSWL. An example of a DNSWL whose TXT records contain the domain name of the organization assignee of the sending IP is given in Appendix B. The domain name would correspond to the DNS domain name used by or within the Administrative Management Domain (ADMD) operating the relevant MTA, sometimes called the "organizational domain". In that case, the authentication provided by this method is equivalent to a DomainKeys Identified Mail (DKIM) signature [RFC6376] or an SPF check host [RFC7208], if the DNSWL is trusted.¶

According to a DNSWL's policy, attributing responsibility of an IP address to an organization may require something more than a mere PTR record consistency. If no domain names can be responsibly associated to a given IP address, for example, because the IP address was added without direct involvement of the organization concerned, DNSWLs can use a subdomain of .INVALID [RFC2606] where the leftmost label hints at why an address is whitelisted. For example, if the address 192.0.2.38 was added by the list managers solely based on their knowledge, the corresponding TXT record might be AUTOPROMOTED.INVALID so as to avoid explicitly identifying an entity that didn't opt in.¶

Following the example of Multicast DNS (see the second paragraph of Section 16 of [RFC6762]), names containing non-ASCII characters can be encoded in UTF-8 [RFC3629] using the Normalization Form C [NFC], as described in "Unicode Format for Network Interchange" [RFC5198]. Inclusion of unaltered UTF-8 TXT values in the header entails an environment compatible with Email Address Internationalization (EAI) [RFC6530].¶

DNS queries with a QTYPE of ANY may lead to inconsistent replies, depending on the cache status. In addition, ANY is not "all", and the provisions for queries that have QTYPE=ANY [RFC8482] don't cover DNSxLs. A mail server can issue two simultaneous queries, A and TXT. Otherwise, a downstream filter can issue a TXT query on its own, if it knows that an A query was successful and that the DNSWL serves useful TXT records. It is unlikely that TXT records exist if a query for QTYPE A brought a result of "none".¶

Appendix A. Example

Delivered-To: recipient@example.org
Return-Path: <sender@example.com>
Authentication-Results: mta.example.org;
  dkim=pass (whitelisted) header.i=@example.com
Authentication-Results: mta.example.org;
  dnswl=pass dns.zone=list.dnswl.example dns.sec=na
  policy.ip=127.0.10.1
  policy.txt="fwd.example https://dnswl.example/?d=fwd.example"
Received-SPF: fail (Address does not pass Sender Policy Framework)
  client-ip=2001:db8::2:1;
  envelope-from="sender@example.com";
  helo=mail.fwd.example;
  receiver=mta.example.org;
Received: from mail.fwd.example (mail.fwd.example [2001:db8::2:1])
  (TLS: TLSv1/SSLv3,128bits,ECDHE-RSA-AES128-GCM-SHA256)
  by mta.example.org with ESMTPS; Thu, 03 Oct 2019 19:23:11 +0200
  id 00000000005DC044.000000005702D87C.000007FC

The message went through a third party, fwd.example, which forwarded it to the final MTA. The mail path was not arranged beforehand with the involved MTAs; it emerged spontaneously. This message would not have made it to the target without whitelisting, because:¶

However, the target also implemented the last paragraph of Appendix D.3 of [RFC7208]. Its behavior hinges on the following DNS entries:¶

  1.0.0.0.2.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.d.b.8.2.0.0.1.
                                               list.dnswl.example.
       IN  A    127.0.10.1
       IN  TXT  "fwd.example https://dnswl.example/?d=fwd.example"

If mail.fwd.example had connected from address 192.0.2.1, then the query name would have been 1.2.0.192.list.dnswl.example. See full description in [RFC5782].¶

At connection time, because the remote IP address is whitelisted, the target MTA did not reject the message before DATA. Instead, it recorded the SPF fail result and indicated the local policy mechanism that was applied in order to override that result. Subsequent filtering verified DKIM [RFC6376].¶

At later stages, mail filters can reject or quarantine the message based on its content. A deeper knowledge of the policy values obtained from dnswl.example allows interpreting the values of policy.ip and weighing them against other factors so as to make better decisions.¶

Appendix B. Known Implementation

Implementation details mentioned in this section have been stable for several years. Yet, this description is necessarily superficial, version dependent, and subject to change.¶

Courier-MTA [Courier-MTA] can be configured to look up DNSBLs and DNSWLs, with similar command-line switches:¶

-block=zone[=displayzone][,var[/n.n.n.n][,msg]]
-allow=zone[=displayzone][,var[/n.n.n.n[,]]]

"zone" is the zone to be queried.¶

"displayzone" is only used for -allow; it is the value to be set in the dns.zone property.¶

"var" stands for the environment variable whose existence triggers a special action. The default variable names result in a conventional behavior implemented by Courier-MTA. By setting different environment variables, users can customize the behavior. Conventional behavior differs widely between -block and -allow. The former rejects the message; the latter produces Authentication-Results header fields.¶

The n.n.n.n IP address requires a precise A record response. If not given, any response results in setting the corresponding variable. If given, variables are set only if the response matches exactly. Such syntax provides for a very limited interpretation of the information encoded in A records. However, it is considered to be too complicated already. Even specifying a range, an enumeration of values, or a regular expression would require something beyond what a normal user would be willing to manage.¶

Finally, the trailing message, which overrides the 5xx SMTP reply for -block, is not used for -allow, except that its mere presence requires querying TXT records to be registered in policy.txt.¶

SPF is part of Courier-MTA's core. It is configured separately and provides for an "allowok" keyword to indicate the choice to override rejection in case of SPF failure and -allow whitelisting.¶

A customary whitelist is defined by DNSWL.org [DNSWL]. It serves A records encoded as follows:¶

1st octet:

127.¶

2nd octet:

0.¶

3rd octet:

Category of business, 15 values.¶

4th octet:

Trustworthiness/score, 4 values.¶

They also serve TXT records containing the domain name followed by a URL pointing to further information about the relevant organization, such as what other IP addresses of theirs are being whitelisted. They don't use UTF-8.¶

DNSWL.org provides for free registration and free access below 100,000 queries per day. They use a special return code, 127.0.0.255 as exemplified above, to signal that the quota has been exceeded. Although Courier-MTA itself does not recognize this return code, it has a mail filter (zdkimfilter, named after its main usage) that hard codes recognition of this code and the code for trustworthiness in the 4th octet.¶

Author's Address