--- 1/draft-ietf-uta-mta-sts-10.txt 2017-11-09 17:13:55.788526966 -0800 +++ 2/draft-ietf-uta-mta-sts-11.txt 2017-11-09 17:13:55.840528210 -0800 @@ -1,24 +1,24 @@ Using TLS in Applications D. Margolis Internet-Draft M. Risher Intended status: Standards Track Google, Inc -Expires: April 1, 2018 B. Ramakrishnan +Expires: May 12, 2018 B. Ramakrishnan Yahoo!, Inc A. Brotman Comcast, Inc J. Jones Microsoft, Inc - September 28, 2017 + November 8, 2017 SMTP MTA Strict Transport Security (MTA-STS) - draft-ietf-uta-mta-sts-10 + draft-ietf-uta-mta-sts-11 Abstract SMTP Mail Transfer Agent Strict Transport Security (MTA-STS) is a mechanism enabling mail service providers to declare their ability to receive Transport Layer Security (TLS) secure SMTP connections, and to specify whether sending SMTP servers should refuse to deliver to MX hosts that do not offer TLS with a trusted server certificate. Status of This Memo @@ -29,72 +29,76 @@ Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." - This Internet-Draft will expire on April 1, 2018. + This Internet-Draft will expire on May 12, 2018. Copyright Notice Copyright (c) 2017 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents - 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 + 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Related Technologies . . . . . . . . . . . . . . . . . . . . 3 3. Policy Discovery . . . . . . . . . . . . . . . . . . . . . . 4 3.1. MTA-STS TXT Records . . . . . . . . . . . . . . . . . . . 4 3.2. MTA-STS Policies . . . . . . . . . . . . . . . . . . . . 5 3.3. HTTPS Policy Fetching . . . . . . . . . . . . . . . . . . 8 3.4. Policy Selection for Smart Hosts and Subdomains . . . . . 9 - 3.5. MX Certificate Validation . . . . . . . . . . . . . . . . 9 + 3.5. MX Certificate Validation . . . . . . . . . . . . . . . . 10 4. Policy Application . . . . . . . . . . . . . . . . . . . . . 10 4.1. Policy Application Control Flow . . . . . . . . . . . . . 11 5. Reporting Failures . . . . . . . . . . . . . . . . . . . . . 11 - 6. Operational Considerations . . . . . . . . . . . . . . . . . 12 - 6.1. Policy Updates . . . . . . . . . . . . . . . . . . . . . 12 - 6.2. Policy Delegation . . . . . . . . . . . . . . . . . . . . 12 - 6.3. Removing MTA-STS . . . . . . . . . . . . . . . . . . . . 13 - 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 - 7.1. Well-Known URIs Registry . . . . . . . . . . . . . . . . 13 - 7.2. MTA-STS TXT Record Fields . . . . . . . . . . . . . . . . 13 - 7.3. MTA-STS Policy Fields . . . . . . . . . . . . . . . . . . 14 - 8. Security Considerations . . . . . . . . . . . . . . . . . . . 14 - 8.1. Obtaining a Signed Certificate . . . . . . . . . . . . . 15 - 8.2. Preventing Policy Discovery . . . . . . . . . . . . . . . 15 - 8.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 16 - 8.4. Weak Policy Constraints . . . . . . . . . . . . . . . . . 16 - 8.5. Compromise of the Web PKI System . . . . . . . . . . . . 17 - 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 17 - 10. Appendix 1: MTA-STS example record & policy . . . . . . . . . 17 - 11. Appendix 2: Message delivery pseudocode . . . . . . . . . . . 18 - 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 21 - 12.1. Normative References . . . . . . . . . . . . . . . . . . 21 - 12.2. Informative References . . . . . . . . . . . . . . . . . 22 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 + 6. Interoperability Considerations . . . . . . . . . . . . . . . 12 + 6.1. SNI Support . . . . . . . . . . . . . . . . . . . . . . . 12 + 6.2. Minimum TLS Version Support . . . . . . . . . . . . . . . 12 + 7. Operational Considerations . . . . . . . . . . . . . . . . . 12 + 7.1. Policy Updates . . . . . . . . . . . . . . . . . . . . . 13 + 7.2. Policy Delegation . . . . . . . . . . . . . . . . . . . . 13 + 7.3. Removing MTA-STS . . . . . . . . . . . . . . . . . . . . 14 + 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 + 8.1. Well-Known URIs Registry . . . . . . . . . . . . . . . . 14 + 8.2. MTA-STS TXT Record Fields . . . . . . . . . . . . . . . . 15 + 8.3. MTA-STS Policy Fields . . . . . . . . . . . . . . . . . . 15 + 9. Security Considerations . . . . . . . . . . . . . . . . . . . 15 + 9.1. Obtaining a Signed Certificate . . . . . . . . . . . . . 16 + 9.2. Preventing Policy Discovery . . . . . . . . . . . . . . . 16 + 9.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 17 + 9.4. Weak Policy Constraints . . . . . . . . . . . . . . . . . 18 + 9.5. Compromise of the Web PKI System . . . . . . . . . . . . 18 + 10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 18 + 11. Appendix 1: MTA-STS example record & policy . . . . . . . . . 19 + 12. Appendix 2: Message delivery pseudocode . . . . . . . . . . . 19 + 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 22 + 13.1. Normative References . . . . . . . . . . . . . . . . . . 22 + 13.2. Informative References . . . . . . . . . . . . . . . . . 23 + 13.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 24 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24 1. Introduction The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and hosts to negotiate the use of a TLS channel for encrypted mail transmission. While this opportunistic encryption protocol by itself provides a high barrier against passive man-in-the-middle traffic interception, any attacker who can delete parts of the SMTP session (such as the @@ -106,23 +110,23 @@ policies specifying: o whether MTAs sending mail to this domain can expect PKIX- authenticated TLS support o what a conforming client should do with messages when TLS cannot be successfully negotiated 1.1. Terminology - The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, - SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this - document, are to be interpreted as described in [RFC2119]. + The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", + "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this + document are to be interpreted as described in [RFC2119]. We also define the following terms for further use in this document: o MTA-STS Policy: A commitment by the Policy Domain to support PKIX authenticated TLS for the specified MX hosts. o Policy Domain: The domain for which an MTA-STS Policy is defined. This is the next-hop domain; when sending mail to "alice@example.com" this would ordinarily be "example.com", but this may be overridden by explicit routing rules (as described in @@ -130,21 +134,21 @@ 2. Related Technologies The DANE TLSA record [RFC7672] is similar, in that DANE is also designed to upgrade unauthenticated encryption or plaintext transmission into authenticated, downgrade-resistant encrypted transmission. DANE requires DNSSEC [RFC4033] for authentication; the mechanism described here instead relies on certificate authorities (CAs) and does not require DNSSEC, at a cost of risking malicious downgrades. For a thorough discussion of this trade-off, see - Section 8, "Security Considerations". + Section 9, "Security Considerations". In addition, MTA-STS provides an optional report-only mode, enabling soft deployments to detect policy failures; partial deployments can be achieved in DANE by deploying TLSA records only for some of a domain's MXs, but such a mechanism is not possible for the per-domain policies used by MTA-STS. The primary motivation of MTA-STS is to provide a mechanism for domains to upgrade their transport security even when deploying DNSSEC is undesirable or impractical. However, MTA-STS is designed @@ -317,21 +321,22 @@ sts-policy-max-age-value sts-policy-max-age-field = %s"max_age" sts-policy-max-age-value = 1*10(DIGIT) sts-policy-extension = sts-policy-ext-name field-delim ; additional sts-policy-ext-value ; extension ; fields -sts-policy-ext-name = (ALPHA / DIGIT) *31(ALPHA / DIGIT / "_" / "-" / ".") +sts-policy-ext-name = (ALPHA / DIGIT) + *31(ALPHA / DIGIT / "_" / "-" / ".") sts-policy-ext-value = 1*(%x21-3A / %x3C / %x3E-7E) ; chars excluding ; "=", ";", SP, ; and control ; chars Parsers MUST accept TXT records and policy files which are syntactically valid (i.e. valid key/value pairs separated by semi- colons for TXT records) and but containing additional key/value pairs not specified in this document, in which case unknown fields SHALL be @@ -353,21 +358,21 @@ considerations: o Matching is performed only against the DNS-ID and CN-ID identifiers. o DNS domain names in server certificates MAY contain the wildcard character '*' as the complete left-most label within the identifier. The certificate MAY be checked for revocation via the Online - Certificate Status Protocol (OCSP) [RFC2560], certificate revocation + Certificate Status Protocol (OCSP) [RFC6960], certificate revocation lists (CRLs), or some other mechanism. Policies fetched via HTTPS are only valid if the HTTP response code is 200 (OK). HTTP 3xx redirects MUST NOT be followed, and HTTP caching (as specified in [RFC7234]) MUST NOT be used. Senders may wish to rate-limit the frequency of attempts to fetch the HTTPS endpoint even if a valid TXT record for the recipient domain exists. In the case that the HTTPS GET fails, we suggest implementions may limit further attempts to a period of five minutes @@ -420,21 +425,21 @@ policies which specify mode values of "report" or "none" MUST NOT be interpreted as delivery failures, as described in Section 4, "Policy Application". 3.5. MX Certificate Validation The certificate presented by the receiving MX MUST chain to a root CA that is trusted by the sending MTA and be non-expired. The certificate MUST have a CN-ID ([RFC6125]) or subject alternative name (SAN, [RFC5280]) with a DNS-ID matching the "mx" pattern. The MX's - certificate MAY also be checked for revocation via OCSP [RFC2560], + certificate MAY also be checked for revocation via OCSP [RFC6960], CRLs [RFC6818], or some other mechanism. Because the "mx" patterns are not hostnames, however, matching is not identical to other common cases of X.509 certificate authentication (as described, for example, in [RFC6125]). Consider the example policy given above, with an "mx" pattern containing ".example.com". In this case, if the MX server's X.509 certificate contains a SAN matching "*.example.com", we are required to implement "wildcard-to- wildcard" matching. @@ -465,21 +470,21 @@ 1. "enforce": In this mode, sending MTAs MUST NOT deliver the message to hosts which fail MX matching or certificate validation. 2. "report": In this mode, sending MTAs which also implement the TLSRPT specification (TODO: add ref) merely send a report indicating policy application failures (so long as TLSRPT is also implemented by the recipient domain). 3. "none": In this mode, sending MTAs should treat the policy domain - as though it does not have any active policy; see Section 6.3, + as though it does not have any active policy; see Section 7.3, "Removing MTA-STS", for use of this mode value. When a message fails to deliver due to an "enforce" policy, a compliant MTA MUST NOT permanently fail to deliver messages before checking for the presence of an updated policy at the Policy Domain. (In all cases, MTAs SHOULD treat such failures as transient errors and retry delivery later.) This allows implementing domains to update long-lived policies on the fly. 4.1. Policy Application Control Flow @@ -516,43 +521,76 @@ o HTTPS policy fetch failures when a valid TXT record is present. o Policy fetch failures of any kind when a valid policy exists in the policy cache, except if that policy's mode is "none". o Delivery attempts in which a contacted MX does not support STARTTLS or does not present a certificate which validates according to the applied policy, except if that policy's mode is "none". -6. Operational Considerations +6. Interoperability Considerations -6.1. Policy Updates +6.1. SNI Support + + To ensure that the server sends the right certificate chain, the SMTP + client MUST have support for the TLS SNI extension [RFC6066]. When + connecting to a HTTP server to retrieve the MTA-STS policy, the SNI + extension MUST contain the name of the policy host (e.g. "mta- + sts.example.com"). When connecting to an SMTP server, the SNI + extension MUST contain the MX hostname. + + HTTP servers used to deliver MTA-STS policies MUST have support for + the TLS SNI extension and MAY rely on SNI to determine which + certificate chain to present to the client. In either case, HTTP + servers MUST respond with a certificate chain that matches the policy + hostname or abort the TLS handshake if unable to do so. + + SMTP servers MUST have support for the TLS SNI extension and MAY rely + on SNI to determine which certificate chain to present to the client. + If the client sends no SNI extension or sends an SNI extension for an + unsupported server name, the server MUST simply send a fallback + certificate chain of its choice. The reason for not enforcing strict + matching of the requested SNI hostname is that MTA-STS TLS clients + may be typically willing to accept multiple server names but can only + send one name in the SNI extension. The server's fallback + certificate may match a different name that is acceptable to the + client, e.g., the original next-hop domain. + +6.2. Minimum TLS Version Support + + MTAs supporting MTA-STS MUST have support for TLS version 1.2 + [RFC5246] or higher. The general TLS usage guidance in [RFC7525] + SHOULD be followed. + +7. Operational Considerations +7.1. Policy Updates Updating the policy requires that the owner make changes in two places: the "_mta-sts" TXT record in the Policy Domain's DNS zone and at the corresponding HTTPS endpoint. As a result, recipients should expect a policy will continue to be used by senders until both the HTTPS and TXT endpoints are updated and the TXT record's TTL has passed. In other words, a sender who is unable to successfully deliver a message while applying a cache of the recipient's now-outdated policy may be unable to discover that a new policy exists until the DNS TTL has passed. Recipients should therefore ensure that old policies continue to work for message delivery during this period of time, or risk message delays. Recipients should also prefer to update the HTTPS policy body before updating the TXT record; this ordering avoids the risk that senders, seeing a new TXT record, mistakenly cache the old policy from HTTPS. -6.2. Policy Delegation +7.2. Policy Delegation Domain owners commonly delegate SMTP hosting to a different organization, such as an ISP or a Web host. In such a case, they may wish to also delegate the MTA-STS policy to the same organization which can be accomplished with two changes. First, the Policy Domain must point the "_mta-sts" record, via CNAME, to the "_mta-sts" record maintained by the hosting organization. This allows the hosting organization to control update signaling. @@ -571,21 +609,31 @@ DNS: _mta-sts.user.com. IN CNAME _mta-sts.provider.com. Policy: > GET /.well-known/mta-sts.txt > Host: mta-sts.user.com < HTTP/1.1 200 OK # Response proxies content from https://mta-sts.provider.com -6.3. Removing MTA-STS + Note that while sending MTAs MUST NOT use HTTP caching when fetching + policies via HTTPS, such caching may nonetheless be useful to a + reverse proxy configured as described in this section. An HTTPS + policy endpoint expecting to be proxied for multiple hosted domains-- + as with a large mail hosting provider or similar--may wish to + indicate an HTTP Cache-Control "max-age" response directive (as + specified in [RFC7234]) of 60 seconds as a reasonable value to save + reverse proxies an unnecessarily high-rate of proxied policy + fetching. + +7.3. Removing MTA-STS In order to facilitate clean opt-out of MTA-STS by implementing policy domains, and to distinguish clearly between failures which indicate attacks and those which indicate such opt-outs, MTA-STS implements the "none" mode, which allows validated policies to indicate authoritatively that the policy domain wishes to no longer implement MTA-STS and may, in the future, remove the MTA-STS TXT and policy endpoints entirely. A suggested workflow to implement such an opt out is as follows: @@ -594,62 +642,62 @@ "max_age" (e.g. one day). 2. Publish a new TXT record to trigger fetching of the new policy. 3. When all previously served policies have expired--normally this is the time the previously published policy was last served plus that policy's "max_age", but note that older policies may have been served with a greater "max_age", allowing overlapping policy caches--safely remove the TXT record and HTTPS endpoint. -7. IANA Considerations +8. IANA Considerations -7.1. Well-Known URIs Registry +8.1. Well-Known URIs Registry A new .well-known URI will be registered in the Well-Known URIs registry as described below: URI Suffix: mta-sts.txt Change Controller: IETF -7.2. MTA-STS TXT Record Fields +8.2. MTA-STS TXT Record Fields IANA is requested to create a new registry titled "MTA-STS TXT Record Fields". The initial entries in the registry are: +------------+--------------------+------------------------+ | Field Name | Description | Reference | +------------+--------------------+------------------------+ | v | Record version | Section 3.1 of RFC XXX | | id | Policy instance ID | Section 3.1 of RFC XXX | +------------+--------------------+------------------------+ New fields are added to this registry using IANA's "Expert Review" policy. -7.3. MTA-STS Policy Fields +8.3. MTA-STS Policy Fields IANA is requested to create a new registry titled "MTA-STS Policy Fields". The initial entries in the registry are: +------------+----------------------+------------------------+ | Field Name | Description | Reference | +------------+----------------------+------------------------+ | version | Policy version | Section 3.2 of RFC XXX | | mode | Enforcement behavior | Section 3.2 of RFC XXX | | max_age | Policy lifetime | Section 3.2 of RFC XXX | | mx | MX identities | Section 3.2 of RFC XXX | +------------+----------------------+------------------------+ New fields are added to this registry using IANA's "Expert Review" policy. -8. Security Considerations +9. Security Considerations SMTP MTA Strict Transport Security attempts to protect against an active attacker who wishes to intercept or tamper with mail between hosts who support STARTTLS. There are two classes of attacks considered: o Foiling TLS negotiation, for example by deleting the "250 STARTTLS" response from a server or altering TLS session negotiation. This would result in the SMTP session occurring over plaintext, despite both parties supporting TLS. @@ -661,29 +709,29 @@ for the recipient domain, or by redirecting client connections intended for the legitimate recipient server (for example, by altering BGP routing tables). MTA-STS can thwart such attacks only if the sender is able to previously obtain and cache a policy for the recipient domain, and only if the attacker is unable to obtain a valid certificate that complies with that policy. Below, we consider specific attacks on this model. -8.1. Obtaining a Signed Certificate +9.1. Obtaining a Signed Certificate SMTP MTA-STS relies on certificate validation via PKIX based TLS identity checking [RFC6125]. Attackers who are able to obtain a valid certificate for the targeted recipient mail service (e.g. by compromising a certificate authority) are thus able to circumvent STS authentication. -8.2. Preventing Policy Discovery +9.2. Preventing Policy Discovery Since MTA-STS uses DNS TXT records for policy discovery, an attacker who is able to block DNS responses can suppress the discovery of an MTA-STS Policy, making the Policy Domain appear not to have an MTA- STS Policy. The sender policy cache is designed to resist this attack by decreasing the frequency of policy discovery and thus reducing the window of vulnerability; it is nonetheless a risk that attackers who can predict or induce policy discovery--for example, by inducing a victim sending domain to send mail to a never-before- contacted recipient while carrying out a man-in-the-middle attack-- @@ -701,27 +749,28 @@ string against the TXT record on each successful send, or in a background task that runs daily or weekly), an attacker would have to foil policy discovery consistently over the lifetime of a cached policy to prevent a successful refresh. Additionally, MTAs should alert administrators to repeated policy refresh failures long before cached policies expire (through warning logs or similar applicable mechanisms), allowing administrators to detect such a persistent attack on policy refresh. (However, they should not implement such alerts if the cached policy has a "none" - mode, to allow clean MTA-STS removal, as described in Section 6.3.) + mode, to allow clean MTA-STS removal, as described in Section 7.3.) + Resistance to downgrade attacks of this nature--due to the ability to authoritatively determine "lack of a record" even for non- participating recipients--is a feature of DANE, due to its use of DNSSEC for policy discovery. -8.3. Denial of Service +9.3. Denial of Service We additionally consider the Denial of Service risk posed by an attacker who can modify the DNS records for a victim domain. Absent MTA-STS, such an attacker can cause a sending MTA to cache invalid MX records, but only for however long the sending resolver caches those records. With MTA-STS, the attacker can additionally advertise a new, long-"max_age" MTA-STS policy with "mx" constraints that validate the malicious MX record, causing senders to cache the policy and refuse to deliver messages once the victim has resecured the MX records. @@ -742,56 +791,56 @@ In these cases, there is a risk that untrusted users would be able to serve custom content at the "mta-sts" host, including serving an illegitimate MTA-STS policy. We believe this attack is rendered more difficult by the need for the attacker to also serve the "_mta-sts" TXT record on the same domain--something not, to our knowledge, widely provided to untrusted users. This attack is additionally mitigated by the aforementioned ability for a victim domain to update an invalid policy at any future date. -8.4. Weak Policy Constraints +9.4. Weak Policy Constraints Even if an attacker cannot modify a served policy, the potential exists for configurations that allow attackers on the same domain to receive mail for that domain. For example, an easy configuration option when authoring an MTA-STS Policy for "example.com" is to set the "mx" equal to ".example.com"; recipient domains must consider in this case the risk that any user possessing a valid hostname and CA- signed certificate (for example, "dhcp-123.example.com") will, from the perspective of MTA-STS Policy validation, be a valid MX host for that domain. -8.5. Compromise of the Web PKI System +9.5. Compromise of the Web PKI System A host of risks apply to the PKI system used for certificate authentication, both of the "mta-sts" HTTPS host's certificate and the SMTP servers' certificates. These risks are broadly applicable within the Web PKI ecosystem and are not specific to MTA-STS; nonetheless, they deserve some consideration in this context. Broadly speaking, attackers may compromise the system by obtaining certificates under fraudulent circumstances (i.e. by impersonating the legitimate owner of the victim domain), by compromising a Certificate Authority or Delegate Authority's private keys, by obtaining a legitimate certificate issued to the victim domain, and similar. One approach commonly employed by Web browsers to help mitigate against some of these attacks is to allow for revocation of - compromised or fraudulent certificates via OCSP [RFC2560] or CRLs + compromised or fraudulent certificates via OCSP [RFC6960] or CRLs [RFC6818]. Such mechanisms themselves represent tradeoffs and are not universally implemented; we nonetheless recommend implementors of MTA-STS to implement revocation mechanisms which are most applicable to their implementations. -9. Contributors +10. Contributors Nicolas Lidzborski Google, Inc nlidz (at) google (dot com) Wei Chuang Google, Inc weihaw (at) google (dot com) Brandon Long Google, Inc blong (at) google (dot com) Franck Martin LinkedIn, Inc fmartin (at) linkedin (dot com) Klaus Umbach 1&1 Mail & Media Development & Technology GmbH @@ -789,48 +838,46 @@ Nicolas Lidzborski Google, Inc nlidz (at) google (dot com) Wei Chuang Google, Inc weihaw (at) google (dot com) Brandon Long Google, Inc blong (at) google (dot com) Franck Martin LinkedIn, Inc fmartin (at) linkedin (dot com) Klaus Umbach 1&1 Mail & Media Development & Technology GmbH klaus.umbach (at) 1und1 (dot de) - Markus Laber 1&1 Mail & Media Development & Technology GmbH markus.laber (at) 1und1 (dot de) -10. Appendix 1: MTA-STS example record & policy +11. Appendix 1: MTA-STS example record & policy The owner of "example.com" wishes to begin using MTA-STS with a policy that will solicit reports from senders without affecting how the messages are processed, in order to verify the identity of MXs that handle mail for "example.com", confirm that TLS is correctly used, and ensure that certificates presented by the recipient MX validate. MTA-STS policy indicator TXT RR: _mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;" - MTA-STS Policy file served as the response body at + MTA-STS Policy file served as the response body at [1] version: STSv1 mode: report mx: mx1.example.com mx: mx2.example.com mx: mx.backup-example.com max_age: 12345678 -11. Appendix 2: Message delivery pseudocode +12. Appendix 2: Message delivery pseudocode Below is pseudocode demonstrating the logic of a compliant sending MTA. While this pseudocode implementation suggests synchronous policy retrieval in the delivery path, in a working implementation that may be undesirable, and we expect some implementers to instead prefer a background fetch that does not block delivery if no cached policy is present. @@ -860,21 +905,22 @@ return true } } return false } func certMatches(connection, policy) { // Assume a handy function to return CN and DNS-ID SANs. for san in getDnsIdSansAndCnFromCert(connection) { for mx in policy.mx { - // Return if the server certificate from "connection" matches the "mx" host. + // Return if the server certificate from "connection" matches the "mx" + // host. if san[0] == '*' { // Invalid wildcard! if san[1] != '.' continue san = san[1:] } if isWildcardMatch(san, mx) || isWildcardMatch(mx, san) { return true } } } @@ -936,113 +982,135 @@ if policy { cachePolicy(domain, policy) } else { policy = tryGetCachedPolicy(domain) } if policy { return tryWithPolicy(message, domain, policy) } // Try to deliver the message normally (i.e. without MTA-STS). } -12. References -12.1. Normative References +13. References + +13.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate - Requirement Levels", BCP 14, RFC 2119, - DOI 10.17487/RFC2119, March 1997, . + Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ + RFC2119, March 1997, + . [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003, - . + . + + [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security + (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/ + RFC5246, August 2008, . [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, - . + . [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, - DOI 10.17487/RFC5321, October 2008, . + DOI 10.17487/RFC5321, October 2008, + . [RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known - Uniform Resource Identifiers (URIs)", RFC 5785, - DOI 10.17487/RFC5785, April 2010, . + Uniform Resource Identifiers (URIs)", RFC 5785, DOI 10 + .17487/RFC5785, April 2010, + . + + [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) + Extensions: Extension Definitions", RFC 6066, DOI 10 + .17487/RFC6066, January 2011, . [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March - 2011, . + 2011, . [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer - Protocol (HTTP/1.1): Semantics and Content", RFC 7231, - DOI 10.17487/RFC7231, June 2014, . + Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI + 10.17487/RFC7231, June 2014, . - [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", - RFC 7405, DOI 10.17487/RFC7405, December 2014, - . + [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC + 7405, DOI 10.17487/RFC7405, December 2014, + . -12.2. Informative References + [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, + "Recommendations for Secure Use of Transport Layer + Security (TLS) and Datagram Transport Layer Security + (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May + 2015, . - [RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. - Adams, "X.509 Internet Public Key Infrastructure Online - Certificate Status Protocol - OCSP", RFC 2560, - DOI 10.17487/RFC2560, June 1999, . +13.2. Informative References [RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207, - February 2002, . + February 2002, . [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. - Rose, "DNS Security Introduction and Requirements", - RFC 4033, DOI 10.17487/RFC4033, March 2005, - . + Rose, "DNS Security Introduction and Requirements", RFC + 4033, DOI 10.17487/RFC4033, March 2005, + . - [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, - DOI 10.17487/RFC5322, October 2008, . + [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI + 10.17487/RFC5322, October 2008, + . [RFC5891] Klensin, J., "Internationalized Domain Names in - Applications (IDNA): Protocol", RFC 5891, - DOI 10.17487/RFC5891, August 2010, . + Applications (IDNA): Protocol", RFC 5891, DOI 10.17487/ + RFC5891, August 2010, + . [RFC6818] Yee, P., "Updates to the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 6818, DOI 10.17487/RFC6818, January 2013, . + [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A., + Galperin, S., and C. Adams, "X.509 Internet Public Key + Infrastructure Online Certificate Status Protocol - OCSP", + RFC 6960, DOI 10.17487/RFC6960, June 2013, . + [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", - RFC 7234, DOI 10.17487/RFC7234, June 2014, - . + RFC 7234, DOI 10.17487/RFC7234, June 2014, . [RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via Opportunistic DNS-Based Authentication of Named Entities - (DANE) Transport Layer Security (TLS)", RFC 7672, - DOI 10.17487/RFC7672, October 2015, . + (DANE) Transport Layer Security (TLS)", RFC 7672, DOI 10 + .17487/RFC7672, October 2015, + . + +13.3. URIs + + [1] https://mta-sts.example.com/.well-known/mta-sts.txt: Authors' Addresses Daniel Margolis Google, Inc Email: dmargolis (at) google.com + Mark Risher Google, Inc Email: risher (at) google (dot com) Binu Ramakrishnan Yahoo!, Inc Email: rbinu (at) yahoo-inc (dot com)