| DNS-Based Authentication of Named Entities (DANE) | T. Finch |
| Internet-Draft | University of Cambridge |
| Intended status: Standards Track | June 2012 |
| Expires: December 01, 2012 |
Secure SMTP with TLS, DNSSEC and TLSA records.
draft-fanf-dane-smtp-03
SMTP has a STARTTLS extension, but (especially in the case of inter-domain mail transfer) it only provides very limited security because it does not specify how to authenticate the server's certificate. This memo specifies how TLSA records in the DNS can be used for proper SMTP server authentication.
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The specification for SMTP over TLS [RFC3207] does not describe how to authenticate a server: which identity relating to the connection ought to be authenticated by the server's certificate. In practice, most certificates presented by publicly-referenced SMTP servers either cannot be validated with respect to a well-known certification authority, or do not verify any identity expected by the client.
As a result, inter-domain SMTP clients cannot require working server authentication if they want to successfully send mail using TLS. Therefore TLS currently provides only a limited amount of additional security for inter-domain SMTP. Its encryption protects against on-path passive eavesdropping; but it does not protect against an active attack, since the client has no way to detect when an attacker is spoofing the server.
This memo describes how to fix this using DNSSEC [RFC4033] and TLSA records [I-D.ietf-dane-protocol] with owner names of the form "_25._tcp.hostname".
We use DNSSEC to secure the association between a mail domain and its SMTP server host names, and between the host names and their certificates. Connections to servers are authenticated by their TLS certificates.
As well as its normal function of providing an association between a domain name and a certificate, we are also using the existance of a TLSA record to signal to the client that it can expect the server to offer TLS with a valid certificate.
The security situation is better for intra-domain SMTP, because in this case the client and server can be configured with prior knowledge of how to authenticate each other. This specification can also be used for authenticating servers in intra-domain SMTP.
This memo does not cover message submission [RFC4409] [RFC5068] [RFC6186], nor does it cover LMTP [RFC2033], since they use the DNS in a different way than MTA-to-MTA SMTP.
The protocol described in this memo adds new security checks that can cause email delivery to be delayed when a security failure is detected. We specify that clients treat a problems as a "temporary failure", causing the message to be queued for a later delivery attempt, in the hope that the attack (or configuration error) will have been dealt with.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this memo are to be interpreted as described in [RFC2119].
In the following we describe some additions to the usual MX resolution algorithm described in [RFC5321] section 5. If there is any conflict between this memo and the other specifications cited in this section, that is an error in this memo.
The client SHALL look up the MX RRset for the mail domain. There are three succesful results that yield a list of SMTP server host names:
If the lookup is not successful, the client SHALL proceed as described in [RFC5321] section 5.
If any of the responses is "bogus" according to DNSSEC validation ([RFC4033] section 5) the client MUST treat this as a temporary error.
For this protocol to take effect, all of these DNS RRsets MUST be "secure" according to DNSSSEC validation. In the case of an implicit MX record, there MUST be a secure denial of existence of an MX RRset for the mail domain. In the case of a (chain of) CNAME or DNAME RRs, the whole chain MUST be secure as well as the ultimate target.
If they are not all secure, this protocol has not been fully deployed. The client SHOULD fall back to insecure delivery (which might be over unauthenticated TLS).
The client now has an authentic list of SMTP server host names and priority values. It processes this list as described in [RFC5321] section 5 (sorting the host names etc.) without regard to the presence or absence of DNSSEC or TLSA records.
This sub-section applies to each SMTP server host name individually.
When connecting to a server, the client SHALL look up the server's TLSA RRset as described in [I-D.ietf-dane-protocol] section 3. That is, the TLSA RRset owner name SHALL be "_25._tcp.hostname" where "hostname" is the SMTP server host name. The response can be one of the following (as listed in [I-D.ietf-dane-protocol] section 4.1):
The client now has one or more TLSA records for the server it is connecting to.
The client MUST ensure that the server offers the STARTTLS service extension [RFC3207] in its response to the client's EHLO command ([RFC5321] section 4.1.1.1).
The client SHALL then issue the STARTTLS command which MUST be successful. It then proceeds with TLS negotiation [RFC5246]. If the client uses the Server Name Indication TLS extension ([RFC6066] section 3) it MUST use the SMTP server host name as the value for the ServerName field.
The client SHALL validate the server's certificate as described in [I-D.ietf-dane-protocol] section 2.1.
The client SHALL verify the server's identity as described in [RFC6125] section 6. Its list of reference identifiers SHOULD include the SMTP server host name with type DNS-ID, and MAY include a second copy of the host name with type CN-ID.
If any of these checks fail, the client MUST disconnect from the server and treat this as a temporary failure.
The client can now proceed to deliver mail securely.
Mail transmission within an ADMD can be based on MX records (such as when delivering incoming mail to its destination host) or on statically configured host names (such as when routing outgoing mail via a border relay).
When routing internal mail using MX records, Section 3 applies the same as for inter-domain SMTP.
When routing mail using host names, the MX lookup step is skipped and only Section 3.2 applies.
The client MAY wish to insert a Transmitted: header field at the start of the message header just before transmitting the message. This records the result of the checks specified in the previous section. (See Section 7 for some comments on its utility or lack thereof.) It is a client-side counterpart to the Received: header field ([RFC5321] section 4.4) and has very similar syntax. It SHOULD be treated as a trace field.
The syntax of the Transmitted: header field is described using ABNF [RFC5234]. Non-terminal syntax rules not defined in this memo are defined in [RFC5321], or [RFC5322], or [RFC5234].
Transmitted-line = "Transmitted:" FWS To-domain By-domain
Opt-info [CFWS] ";" date-time CRLF
To-domain = "TO" FWS Extended-Domain
A <Transmitted-line> SHALL include:
The "with" protocol type registry includes a number of keywords that indicate the use of SMTP with or without TLS and/or AUTH [RFC3848]. When these types appear in a Transmitted: header field "with" clause they indicate that the client did not authenticate the server as described in Section 3.
These new keywords are not for use in Received: header fields since the server cannot tell whether or not the client authenticated it.
There are no keywords corresponding to a client trying and failing to authenticate the server, since in this case no message transmission occurs.
Delivery status notifications [RFC3464] can include a Remote-MTA field recording an SMTP server host name. When this has been authenticated according to Section 3 the reporting MTA MAY use an MTA-type-name of "dane".
This memo provides only conditional security. It allows a server to publish in the DNS the details of how it can be authenticated. Clients that implement this protocol can use it to provide a strong guarantee that they are sending mail to the correct place. If either of these is missing, mail delivery will be insecure.
There is no secure way for a server to tell if a client has authenticated it using this protocol. This is a general limitation of TLS. The Transmitted: header field records this information for tracing and debugging and measuring deployment, not for security purposes.
We do not specify that clients check that all of a mail domain's SMTP server host names consistently have or do not have TLSA records. This is so that partial or incremental deployment does not break mail delivery. Different levels of deployment are likely if a domain has a third-party backup MX, for example.
The MX sorting rules are unchanged; in particular they have not been altered in order to prioritize secure servers over insecure servers. If a site wants to be secure it needs to deploy this protocol completely; a partial deployment is not secure and we make no special effort to support it.
By signing their zone with DNSSEC, a mail domain owner implicitly instructs SMTP clients to check their SMTP server TLSA records. This implies another point in the trust relationship between mail domain owner and smtp server operator. Most of the setup requirements for this protocol fall on the SMTP server operator: installing a TLS certificate with the correct name, and publishing a TLSA record under that name. If these are not correct then mail delivery from TLSA-aware clients might be delayed.
Many provisioning failures in SMTP cause "permanent" failures, that is the immediate and final rejection of the message. This includes missing DNS records, an SMTP server that is not configured to accept mail for the recipient domain, and so forth.
In this protocol, provisioning an incorrect TLS certificate triggers a temporary error. This is because we want to minimise the damage that occurs when an on-path attacker intercepts the TCP connection between an SMTP client and server. An attacker can cause delays, but is not able to trigger immediate delivery failures.
We do not specify that clients check the DNSSEC state of the SMTP server address records. This is not necessary since the certificate checks ensure that the client has connected to the correct server. (The address records will normally have the same security state as the TLSA records, but they can differ if there are CNAME or DNAME indirections.)
This memo does not specify any changes to SMTP client authentication. Inter-domain SMTP client authentication remains extremely weak. Intra-domain SMTP can be configured as strong as necessary (using SMTP AUTH or TLS client certificates, for instance) but that is out of scope for this memo.
Thanks to Mark Andrews for arguing that authenticating the SMTP server host name is the right thing, and that we should rely on DNSSEC to secure the MX lookup. Thanks to Ned Freed, Olafur Gudmundsson, Paul Hoffman, Phil Pennock, Hector Santos, and Alessandro Vesely for helpful suggestions.
| [RFC2033] | Myers, J.G., "Local Mail Transfer Protocol", RFC 2033, October 1996. |
| [RFC4409] | Gellens, R. and J. Klensin, "Message Submission for Mail", RFC 4409, April 2006. |
| [RFC5068] | Hutzler, C., Crocker, D., Resnick, P., Allman, E. and T. Finch, "Email Submission Operations: Access and Accountability Requirements", BCP 134, RFC 5068, November 2007. |
| [RFC5598] | Crocker, D., "Internet Mail Architecture", RFC 5598, July 2009. |
| [RFC6186] | Daboo, C., "Use of SRV Records for Locating Email Submission/Access Services", RFC 6186, March 2011. |
In the following, most of the DNS resource data is elided for simplicity.
; mail domain
example.com. MX 1 mx.example.net.
example.com. RRSIG MX ...
; SMTP server host name
mx.example.net. A 192.0.2.1
mx.example.net. AAAA 2001:db8:212:8::e:1
; TLSA resource record
_25._tcp.mx.example.net. TLSA ...
_25._tcp.mx.example.net. RRSIG TLSA ...
Mail for addresses at example.com is delivered by SMTP to mx.example.net. Connections to mx.example.net port 25 that use STARTTLS will get a server certificate that authenticates the name mx.example.net.
There are a number of reasons for the certificate to authenticate the SMTP server host name rather than the mail domain.
SMTP allows a client to transfer mail to recipients at multiple domains in the same connection. If the certificate identifies the host name then it does not need to list all the possible mail domains.
It is not in general feasible for the server to select a mail domain certificate based on the recipient domains when the connection is established (using Server Name Indication, [RFC6066] section 3), because an SMTP client might not know all of the recipients when it establishes the connection.
Outgoing SMTP relays and message submission servers handle mail for any domain, so in those cases the only sensible option is for the certificate to contain the host name. It is more consistent for incoming MX server certificates to match.
It is common for SMTP servers to act in multiple roles, as outgoing relays or as incoming MX servers, depending on the client identity. It is simpler if the server can present the same certificate regardless of the role in which it is to act.
Sometimes the server does not know its role until the client has authenticated, which usually occurs after TLS has been established.
This protocol does not provide an option for directly authenticating the mail domain because that would add complexity without providing any benefit, and security protocols are best kept simple. As described above, there are real-world cases where authenticating the mail domain cannot be made to work, so there are complicated criteria for when mail domain TLSA records might be used and when they cannot. This is all avoided by authenticating the SMTP server host name.
Finally, this protocol only affects the logic in the SMTP client and requires no additional SMTP server functionality, such as support for the TLS Server Name Indication extension.
Clarify how to use SNI with this protocol.
Clarify lack of changes to MX sorting rules.
Mention DNAME as well as CNAME.
An example.
Clarify the wording that describes how a client determines that this protocol is in effect.
Divide the security considerations into sub-sections, and add a subsection on denial of service.
Clarify intro, mentioning TLSA owner name format.
Extend the scope to cover MTA-to-MTA mail within an ADMD as well as between ADMDs.
More about why not to authenticate mail domains in the rationale.
Change DNS-ID requirement from MUST to SHOULD to follow RFC 6125.
Acknowledgments section.
Transmitted: header trace field. Not sure if this is a good idea; feedback wanted.
"dane" MTA-name-type for use in DSNs. Even less sure if this is a good idea.