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rfc:rfc7817

Internet Engineering Task Force (IETF) A. Melnikov Request for Comments: 7817 Isode Ltd Updates: 2595, 3207, 3501, 5804 March 2016 Category: Standards Track ISSN: 2070-1721

Updated Transport Layer Security (TLS) Server Identity Check Procedure

                    for Email-Related Protocols

Abstract

 This document describes the Transport Layer Security (TLS) server
 identity verification procedure for SMTP Submission, IMAP, POP, and
 ManageSieve clients.  It replaces Section 2.4 (Server Identity Check)
 of RFC 2595 and updates Section 4.1 (Processing After the STARTTLS
 Command) of RFC 3207, Section 11.1 (STARTTLS Security Considerations)
 of RFC 3501, and Section 2.2.1 (Server Identity Check) of RFC 5804.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7817.

Copyright Notice

 Copyright (c) 2016 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.

Melnikov Standards Track [Page 1] RFC 7817 TLS Server Identity Check for Email March 2016

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
 2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
 3.  Email Server Certificate Verification Rules . . . . . . . . .   3
 4.  Compliance Checklist for Certification Authorities  . . . . .   5
   4.1.  Notes on Handling of Delegated Email Services by
         Certification Authorities . . . . . . . . . . . . . . . .   5
 5.  Compliance Checklist for Mail Service Providers and
     Certificate Signing Request Generation Tools  . . . . . . . .   6
   5.1.  Notes on Hosting Multiple Domains . . . . . . . . . . . .   7
 6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
 7.  Operational Considerations  . . . . . . . . . . . . . . . . .   9
 8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
 9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
   9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
 Appendix A.  Changes to RFCs 2595, 3207, 3501, and 5804 . . . . .  12
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  13
 Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1. Introduction

 Use of TLS by SMTP Submission, IMAP, POP, and ManageSieve clients is
 described in [RFC3207], [RFC3501], [RFC2595], and [RFC5804],
 respectively.  Each of the documents describes slightly different
 rules for server certificate identity verification (or doesn't define
 any rules at all).  In reality, email client and server developers
 implement many of these protocols at the same time, so it would be
 good to define modern and consistent rules for verifying email server
 identities using TLS.
 This document describes the updated TLS server identity verification
 procedure for SMTP Submission [RFC6409] [RFC3207], IMAP [RFC3501],
 POP [RFC1939], and ManageSieve [RFC5804] clients.  Section 3 of this
 document replaces Section 2.4 of [RFC2595].
 Note that this document doesn't apply to use of TLS in MTA-to-MTA
 SMTP.
 This document provides a consistent TLS server identity verification
 procedure across multiple email-related protocols.  This should make
 it easier for Certification Authorities (CAs) and ISPs to deploy TLS
 for email use and would enable email client developers to write more
 secure code.

Melnikov Standards Track [Page 2] RFC 7817 TLS Server Identity Check for Email March 2016

2. Conventions Used in This Document

 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].
 The following terms or concepts are used through the document:
 reference identifier:  One of the domain names that the email client
    (an SMTP, IMAP, POP3, or ManageSieve client) associates with the
    target email server.  For some identifier types, the identifier
    also includes an application service type.  Reference identifiers
    are used for performing name checks on server certificates.  (This
    term is formally defined in [RFC6125].)
 CN-ID, DNS-ID, SRV-ID, and URI-ID are identifier types (see [RFC6125]
 for details).  For convenience, their short definitions from
 [RFC6125] are listed below:
 CN-ID:  A Relative Distinguished Name (RDN) in the certificate
    subject field that contains one and only one attribute-type-and-
    value pair of type Common Name (CN), where the value matches the
    overall form of a domain name (informally, dot-separated, letter-
    digit-hyphen labels).
 DNS-ID:  A subjectAltName entry of type dNSName
 SRV-ID:  A subjectAltName entry of type otherName whose name form is
    SRVName
 URI-ID:  A subjectAltName entry of type uniformResourceIdentifier
    whose value includes both (i) a "scheme" and (ii) a "host"
    component (or its equivalent) that matches the "reg-name" rule
    (where the quoted terms represent the associated [RFC5234]
    productions from [RFC3986]).

3. Email Server Certificate Verification Rules

 During a TLS negotiation, an email client (i.e., an SMTP, IMAP, POP3,
 or ManageSieve client) MUST check its understanding of the server
 identity (client's reference identifiers) against the server's
 identity as presented in the server Certificate message in order to
 prevent man-in-the-middle attacks.  This check is only performed
 after the server certificate passes certification path validation as
 described in Section 6 of [RFC5280].  Matching is performed according
 to the rules specified in Section 6 of [RFC6125], including the
 relative order of matching of different identifier types,
 "certificate pinning", and the procedure on failure to match.  The

Melnikov Standards Track [Page 3] RFC 7817 TLS Server Identity Check for Email March 2016

 following inputs are used by the verification procedure used in
 [RFC6125]:
 1.  For DNS-ID and CN-ID identifier types, the client MUST use one or
     more of the following as "reference identifiers": (a) the domain
     portion of the user's email address, (b) the hostname it used to
     open the connection (without CNAME canonicalization).  The client
     MAY also use (c) a value securely derived from (a) or (b), such
     as using "secure" DNSSEC [RFC4033] [RFC4034] [RFC4035] validated
     lookup.
 2.  When using email service discovery procedure specified in
     [RFC6186], the client MUST also use the domain portion of the
     user's email address as another "reference identifier" to compare
     against an SRV-ID identifier in the server certificate.
 The rules and guidelines defined in [RFC6125] apply to an email
 server certificate with the following supplemental rules:
 1.  Support for the DNS-ID identifier type (subjectAltName of dNSName
     type [RFC5280]) is REQUIRED in email client software
     implementations.
 2.  Support for the SRV-ID identifier type (subjectAltName of SRVName
     type [RFC4985]) is REQUIRED for email client software
     implementations that support [RFC6186].  A list of SRV-ID types
     for email services is specified in [RFC6186].  For the
     ManageSieve protocol, the service name "sieve" is used.
 3.  A URI-ID identifier type (subjectAltName of
     uniformResourceIdentifier type [RFC5280]) MUST NOT be used by
     clients for server verification, as URI-IDs were not historically
     used for email.
 4.  For backward compatibility with deployed software, a CN-ID
     identifier type (CN attribute from the subject name, see
     [RFC6125]) MAY be used for server identity verification.
 5.  Email protocols allow use of certain wildcards in identifiers
     presented by email servers.  The "*" wildcard character MAY be
     used as the left-most name component of a DNS-ID or CN-ID in the
     certificate.  For example, a DNS-ID of "*.example.com" would
     match "a.example.com", "foo.example.com", etc., but would not
     match "example.com".  Note that the wildcard character MUST NOT
     be used as a fragment of the left-most name component (e.g.,
     "*oo.example.com", "f*o.example.com", or "foo*.example.com").

Melnikov Standards Track [Page 4] RFC 7817 TLS Server Identity Check for Email March 2016

4. Compliance Checklist for Certification Authorities

 1.  CAs MUST support issuance of server certificates with a DNS-ID
     identifier type (subjectAltName of dNSName type [RFC5280]).
     (Note that some DNS-IDs may refer to domain portions of email
     addresses, so they might not have corresponding A/AAAA DNS
     records.)
 2.  CAs MUST support issuance of server certificates with an SRV-ID
     identifier type (subjectAltName of SRVName type [RFC4985]) for
     each type of email service.  See Section 4.1 for more discussion
     on what this means for CAs.
 3.  For backward compatibility with a deployed client base, CAs MUST
     support issuance of server certificates with a CN-ID identifier
     type (CN attribute from the subject name, see [RFC6125]).
 4.  CAs MAY allow "*" (wildcard) as the left-most name component of a
     DNS-ID or CN-ID in server certificates it issues.

4.1. Notes on Handling of Delegated Email Services by Certification

    Authorities
 [RFC6186] provides an easy way for organizations to autoconfigure
 email clients.  It also allows for delegation of email services to an
 email hosting provider.  When connecting to such delegated hosting
 service, an email client that attempts to verify TLS server identity
 needs to know that if it connects to "imap.hosting.example.net", such
 server is authorized to provide email access for an email such as
 alice@example.org.  In absence of SRV-IDs, users of compliant email
 clients would be forced to manually confirm exceptions because the
 TLS server certificate verification procedures specified in this
 document would result in failure to match the TLS server certificate
 against the expected domain(s).  One way to provide such
 authorization is for the TLS certificate for
 "imap.hosting.example.net" to include SRV-ID(s) (or a DNS-ID) for the
 "example.org" domain.  Note that another way is for DNS Service
 Record (SRV) lookups to be protected by DNSSEC, but this solution
 depends on ubiquitous use of DNSSEC and availability of DNSSEC-aware
 APIs and thus is not discussed in this document.  A future update to
 this document might rectify this.
 A CA that receives a Certificate Signing Request containing multiple
 unrelated DNS-IDs and/or SRV-IDs (e.g., a DNS-ID of "example.org" and
 a DNS-ID of "example.com") needs to verify that the entity that
 supplied such Certificate Signing Request is authorized to provide
 email service for all requested domains.

Melnikov Standards Track [Page 5] RFC 7817 TLS Server Identity Check for Email March 2016

 The ability to issue certificates that contain an SRV-ID (or a DNS-ID
 for the domain part of email addresses) implies the ability to verify
 that entities requesting them are authorized to run email service for
 these SRV-IDs/DNS-IDs.  In particular, CAs that can't verify such
 authorization (whether for a particular domain or in general) MUST
 NOT include such email SRV-IDs/DNS-IDs in certificates they issue.
 This document doesn't specify exact mechanism(s) that can be used to
 achieve this.  However, a few special case recommendations are listed
 below.
 A CA willing to sign a certificate containing a particular DNS-ID
 SHOULD also support signing a certificate containing one or more of
 the email SRV-IDs for the same domain because the SRV-ID effectively
 provides more restricted access to an email service for the domain
 (as opposed to unrestricted use of any services for the same domain,
 as specified by the DNS-ID).
 A CA that also provides DNS service for a domain can use DNS
 information to validate SRV-IDs/DNS-IDs for the domain.
 A CA that is also a Mail Service Provider for a hosted domain can use
 that knowledge to validate SRV-IDs/DNS-IDs for the domain.

5. Compliance Checklist for Mail Service Providers and Certificate

  Signing Request Generation Tools
 Mail Service Providers and Certificate Signing Request generation
 tools:
 1.  MUST include the DNS-ID identifier type in Certificate Signing
     Requests for the host name(s) where the email server(s) are
     running.  They SHOULD include the DNS-ID identifier type in
     Certificate Signing Requests for the domain portion of served
     email addresses.
 2.  MUST include the SRV-ID identifier type for each type of email
     service in Certificate Signing Requests if the email services
     provided are discoverable using DNS SRV as specified in
     [RFC6186].
 3.  SHOULD include the CN-ID identifier type for the host name where
     the email server(s) is running in Certificate Signing Requests
     for backward compatibility with deployed email clients.  (Note, a
     certificate can only include a single CN-ID, so if a mail service
     is running on multiple hosts, either each host has to use
     different certificate with its own CN-ID, a single certificate
     with multiple DNS-IDs, or a single certificate with wildcard in a
     CN-ID can be used).

Melnikov Standards Track [Page 6] RFC 7817 TLS Server Identity Check for Email March 2016

 4.  MAY include "*" (wildcard) as the left-most name component of a
     DNS-ID or CN-ID in Certificate Signing Requests.

5.1. Notes on Hosting Multiple Domains

 A server that hosts multiple domains needs to do one of the following
 (or some combination thereof):
 1.  Use DNS SRV records to redirect each hosted email service to a
     fixed domain, deploy TLS certificate(s) for that single domain,
     and instruct users to configure their clients with appropriate
     pinning (unless the SRV records can always be obtained via
     DNSSEC).  Some email clients come with preloaded lists of pinned
     certificates for some popular domains; this can avoid the need
     for manual confirmation.
 2.  Use a single TLS certificate that includes a complete list of all
     the domains it is serving.
 3.  Serve each domain on its own IP/port, using separate TLS
     certificates on each IP/port.
 4.  Use the Server Name Indication (SNI) TLS extension [RFC6066] to
     select the right certificate to return during TLS negotiation.
     Each domain has its own TLS certificate in this case.
 Each of these deployment choices have their scaling disadvantages
 when the list of domains changes.  Use of DNS SRV without an SRV-ID
 requires manual confirmation from users.  While preloading pinned
 certificates avoids the need for manual confirmation, this
 information can get stale quickly or would require support for a new
 mechanism for distributing preloaded pinned certificates.  A single
 certificate (the second choice) requires that when a domain is added,
 then a new Certificate Signing Request that includes a complete list
 of all the domains needs to be issued and passed to a CA in order to
 generate a new certificate.  A separate IP/port can avoid
 regenerating the certificate but requires more transport layer
 resources.  Use of TLS SNI requires each email client to use it.
 Several Mail Service Providers host hundreds and even thousands of
 domains.  This document, as well as its predecessors, RFCs 2595,
 3207, 3501, and 5804, don't address scaling issues caused by use of
 TLS in multi-tenanted environments.  Further work is needed to
 address this issue, possibly using DNSSEC or something like PKIX over
 Secure HTTP (POSH) [RFC7711].

Melnikov Standards Track [Page 7] RFC 7817 TLS Server Identity Check for Email March 2016

6. Examples

 Consider an IMAP-accessible email server that supports both IMAP and
 IMAP-over-TLS (IMAPS) at the host "mail.example.net" servicing email
 addresses of the form "user@example.net".  A certificate for this
 service needs to include DNS-IDs of "example.net" (because it is the
 domain portion of emails) and "mail.example.net" (this is what a user
 of this server enters manually if not using [RFC6186]).  It might
 also include a CN-ID of "mail.example.net" for backward compatibility
 with deployed infrastructure.
 Consider the IMAP-accessible email server from the previous paragraph
 that is additionally discoverable via DNS SRV lookups in domain
 "example.net" (using DNS SRV records "_imap._tcp.example.net" and
 "_imaps._tcp.example.net").  In addition to the DNS-ID/CN-ID identity
 types specified above, a certificate for this service also needs to
 include SRV-IDs of "_imap.example.net" (when STARTTLS is used on the
 IMAP port) and "_imaps.example.net" (when TLS is used on IMAPS port).
 See [RFC6186] for more details.  (Note that unlike DNS SRV there is
 no "_tcp" component in SRV-IDs).
 Consider the IMAP-accessible email server from the first paragraph
 that is running on a host also known as "mycompany.example.com".  In
 addition to the DNS-ID identity types specified above, a certificate
 for this service also needs to include a DNS-ID of
 "mycompany.example.com" (this is what a user of this server enters
 manually if not using [RFC6186]).  It might also include a CN-ID of
 "mycompany.example.com" instead of the CN-ID "mail.example.net" for
 backward compatibility with deployed infrastructure.  (This is so,
 because a certificate can only include a single CN-ID)
 Consider an SMTP Submission server at the host "submit.example.net"
 servicing email addresses of the form "user@example.net" and
 discoverable via DNS SRV lookups in domain "example.net" (using DNS
 SRV record "_submission._tcp.example.net").  A certificate for this
 service needs to include SRV-IDs of "_submission.example.net" (see
 [RFC6186]) along with DNS-IDs of "example.net" and
 "submit.example.net".  It might also include a CN-ID of
 "submit.example.net" for backward compatibility with deployed
 infrastructure.
 Consider a host "mail.example.net" servicing email addresses of the
 form "user@example.net" and discoverable via DNS SRV lookups in
 domain "example.net", which runs SMTP Submission, IMAPS and POP3S
 (POP3-over-TLS), and ManageSieve services.  Each of the servers can
 use their own certificate specific to their service (see examples
 above).  Alternatively, they can all share a single certificate that
 would include SRV-IDs of "_submission.example.net",

Melnikov Standards Track [Page 8] RFC 7817 TLS Server Identity Check for Email March 2016

 "_imaps.example.net", "_pop3s.example.net", and "_sieve.example.net"
 along with DNS-IDs of "example.net" and "mail.example.net".  It might
 also include a CN-ID of "mail.example.net" for backward compatibility
 with deployed infrastructure.

7. Operational Considerations

 Section 5 covers operational considerations (in particular, use of
 DNS SRV for autoconfiguration) related to generating TLS certificates
 for email servers so that they can be successfully verified by email
 clients.  Additionally, Section 5.1 talks about operational
 considerations related to hosting multiple domains.

8. Security Considerations

 The goal of this document is to improve interoperability and thus
 security of email clients wishing to access email servers over TLS-
 protected email protocols by specifying a consistent set of rules
 that email service providers, email client writers, and CAs can use
 when creating server certificates.
 The TLS server identity check for email relies on use of trustworthy
 DNS hostnames when constructing "reference identifiers" that are
 checked against an email server certificate.  Such trustworthy names
 are either entered manually (for example, if they are advertised on a
 Mail Service Provider's website), explicitly confirmed by the user
 (e.g., if they are a target of a DNS SRV lookup), or derived using a
 secure third party service (e.g., DNSSEC-protected SRV records that
 are verified by the client or trusted local resolver).  Future work
 in this area might benefit from integration with DNS-Based
 Authentication of Named Entities (DANE) [RFC6698], but it is not
 covered by this document.

9. References

9.1. Normative References

 [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
            STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
            <http://www.rfc-editor.org/info/rfc1939>.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.

Melnikov Standards Track [Page 9] RFC 7817 TLS Server Identity Check for Email March 2016

 [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
            Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
            February 2002, <http://www.rfc-editor.org/info/rfc3207>.
 [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
            4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
            <http://www.rfc-editor.org/info/rfc3501>.
 [RFC4985]  Santesson, S., "Internet X.509 Public Key Infrastructure
            Subject Alternative Name for Expression of Service Name",
            RFC 4985, DOI 10.17487/RFC4985, August 2007,
            <http://www.rfc-editor.org/info/rfc4985>.
 [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,
            <http://www.rfc-editor.org/info/rfc5280>.
 [RFC5804]  Melnikov, A., Ed. and T. Martin, "A Protocol for Remotely
            Managing Sieve Scripts", RFC 5804, DOI 10.17487/RFC5804,
            July 2010, <http://www.rfc-editor.org/info/rfc5804>.
 [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
            Extensions: Extension Definitions", RFC 6066,
            DOI 10.17487/RFC6066, January 2011,
            <http://www.rfc-editor.org/info/rfc6066>.
 [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, <http://www.rfc-editor.org/info/rfc6125>.
 [RFC6186]  Daboo, C., "Use of SRV Records for Locating Email
            Submission/Access Services", RFC 6186,
            DOI 10.17487/RFC6186, March 2011,
            <http://www.rfc-editor.org/info/rfc6186>.
 [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
            STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
            <http://www.rfc-editor.org/info/rfc6409>.

Melnikov Standards Track [Page 10] RFC 7817 TLS Server Identity Check for Email March 2016

9.2. Informative References

 [RFC2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP",
            RFC 2595, DOI 10.17487/RFC2595, June 1999,
            <http://www.rfc-editor.org/info/rfc2595>.
 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66,
            RFC 3986, DOI 10.17487/RFC3986, January 2005,
            <http://www.rfc-editor.org/info/rfc3986>.
 [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,
            <http://www.rfc-editor.org/info/rfc4033>.
 [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "Resource Records for the DNS Security Extensions",
            RFC 4034, DOI 10.17487/RFC4034, March 2005,
            <http://www.rfc-editor.org/info/rfc4034>.
 [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "Protocol Modifications for the DNS Security
            Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
            <http://www.rfc-editor.org/info/rfc4035>.
 [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234,
            DOI 10.17487/RFC5234, January 2008,
            <http://www.rfc-editor.org/info/rfc5234>.
 [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
            of Named Entities (DANE) Transport Layer Security (TLS)
            Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
            2012, <http://www.rfc-editor.org/info/rfc6698>.
 [RFC7711]  Miller, M. and P. Saint-Andre, "PKIX over Secure HTTP
            (POSH)", RFC 7711, DOI 10.17487/RFC7711, November 2015,
            <http://www.rfc-editor.org/info/rfc7711>.

Melnikov Standards Track [Page 11] RFC 7817 TLS Server Identity Check for Email March 2016

Appendix A. Changes to RFCs 2595, 3207, 3501, and 5804

 This section lists detailed changes this document applies to RFCs
 2595, 3207, 3501, and 5804.
 The entire Section 2.4 of RFC 2595 is replaced with the following
 text:
    During the TLS negotiation, the client checks its understanding of
    the server identity against the provided server's identity as
    specified in Section 3 of [RFC7817].
 The 3rd paragraph (and its subparagraphs) in Section 11.1 of RFC 3501
 is replaced with the following text:
    During the TLS negotiation, the IMAP client checks its
    understanding of the server identity against the provided server's
    identity as specified in Section 3 of [RFC7817].
 The 3rd paragraph (and its subparagraphs) in Section 4.1 of RFC 3207
 is replaced with the following text:
    During the TLS negotiation, the Submission client checks its
    understanding of the server identity against the provided server's
    identity as specified in Section 3 of [RFC7817].
 Sections 2.2.1 and 2.2.1.1 of RFC 5804 are replaced with the
 following text:
    During the TLS negotiation, the ManageSieve client checks its
    understanding of the server identity against the server's identity
    as specified in Section 3 of [RFC7817].  When the reference
    identity is an IP address, the iPAddress subjectAltName SHOULD be
    used by the client for comparison.  The comparison is performed as
    described in Section 2.2.1.2 of RFC 5804.

Melnikov Standards Track [Page 12] RFC 7817 TLS Server Identity Check for Email March 2016

Acknowledgements

 Thank you to Chris Newman, Viktor Dukhovni, Sean Turner, Russ
 Housley, Alessandro Vesely, Harald Alvestrand, and John Levine for
 comments on this document.
 The editor of this document copied lots of text from RFCs 2595 and
 6125, so the hard work of editors of these documents is appreciated.

Author's Address

 Alexey Melnikov
 Isode Ltd
 14 Castle Mews
 Hampton, Middlesex  TW12 2NP
 United Kingdom
 EMail: Alexey.Melnikov@isode.com

Melnikov Standards Track [Page 13]

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