Client SMTP Validation (CSV)
draft-ietf-marid-csv-intro-01

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Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

Internet mail relies on exchanges between systems that have made no prior arrangement with each other. Widespread abuse of the email system has led operators to demand accountability for the email their receiving SMTP servers are being asked to process. Client SMTP Validation (CSV) provides an economical service that permits a receiving SMTP server to decide whether a sending SMTP client is likely to produce well-behaved traffic, or at least to decide whether the client is sufficiently accountable for its actions. CSV provides a small, simple and useful improvement to Internet mail service accountability. It builds upon the existing practice of service providers that accredit the networks from which sending systems are connecting.

Table of Contents

1  Overview
2  Background
3  Service Goal
4  Requirements
 4.1  Assessing Authorization
 4.2  Assessing Accreditation
5  Client SMTP Validation Details
 5.1  Assessing Authorization
 5.2  Assessing Accreditation
6  Security Considerations
§  Normative References
§  Informative References
§  Author's Addresses
A  Acknowledgements
B  Host Name Authentication
 B.1  DNS-based Mapping
 B.2  Reverse DNS
 B.3  Forward DNS Lookup
§  Intellectual Property and Copyright Statements

1  Overview

CSV considers two questions at the start of each SMTP session:

• Does a domain's management authorize this MTA to be sending email?
• Do independent accreditation services consider that domain's policies and practices sufficient for controlling email abuse?

To validate an SMTP session from an unknown sending SMTP client using CSV, the receiving SMTP server:

1. Obtains the remote IP address of the TCP connection.
2. Extracts the domain name from the EHLO command sent by the SMTP client
3. Queries a chosen Accreditation Service for the EHLO domain name (see Domain Name Accreditation (DNA) [ID-CSVDNA])
4. Queries DNS for a SRV record under the EHLO domain name (see Client SMTP Authorization (CSA) [ID-CSVCSA])
5. Checks the SRV reply for flags returned, and checks for a match in the list of returned IP addresses
6. Determines the level of trust to give to the sending SMTP client, based on the results of (3) and (5)

If the level of trust is high enough, process all email from that session in the traditional manner, delivering or forwarding without the need for further validation.

If the level of trust is too low, return an error showing the reason for not trusting the sending SMTP client.

If the level of trust is in between, document the result in a header in each email delivered or forwarded, and/or perform additional checks.

Terminology:

Terminology conforms to [ID-mail-arch].

Discussion:

The venue for discussing this proposal is the IETF MARID working group<http://ietf.org/html.charters/marid-charter.html>.

2  Background

Internet mail suffers from the operation of hosts acting as mail transfer agents (MTA) without any meaningful cross-net accountability. This makes it impossible to vet MTAs or find recourse when their operations cause problems. Many of these hosts have been compromised and have been turned into unwilling participants in large networks of hostile MTAs that send spam and worms, and contribute to denial of service attacks.

When a server MTA receives a connection, it decides whether to accept the message traffic that is being sent to it, trusting that delivery of that traffic's messages will not be problematic to the operation of the provider or their users. How can it do this, when operating in the open Internet? Client SMTP Validation (CSV) defines a service that permits the receiving SMTP server to decide whether messages transmitted by the sending SMTP client are likely to be well-behaved, or at least to decide whether the client is sufficiently accountable for its actions.

The process of deciding on this trust of the client requires performing a series of conceptually discrete steps:

Identification:

What is the "name" of the client to be trusted? How is it referenced?

CSV uses the domain name supplied by a client in the SMTP HELO/EHLO.

Authentication:

Is the client MTA legitimately associated with that name? Can we prove that the client is who it purports to be?

By finding the sending SMTP client's actual IP address, in the list of IP addresses returned by a DNS Address query on the EHLO domain-name, CSV satisfies the minimal authentication needs of this task.

Authorization:

Is the remote host permitted to act as a sending SMTP client? Has the domain management authorized it to perform this function?

CSV specifies a DNS-based record that states whether an associated host has permission to operate as a client MTA.

Accreditation:

What is the trust that is to be extended to the entity that authorized the sending SMTP client? Does the receiving SMTP server have a basis for deciding that the entity providing authorization for the client MTA can, itself, be trusted to make accountable authorizations?

CSV defines a DNS record that permits domains to announce the accreditation services in which they are listed. It also defines a separate record by which accreditation services publish their assessments of sending domains.

An implementation well might combine some of these steps. However it is important to consider them independently, in order to ensure that they are specified in a valid manner, or at least that the constraints of the proposal are clear for each of these conceptual functions. This specification is based on validation of the EHLO domain name. The mechanism is small, simple and useful. In particular it permits detecting machines that are prohibited from acting as Client MTAs and those that are permitted. The mechanism is designed to be useful between peer MTAs and only requires use of well-established mechanisms.

Address-based Accreditation:

Service providers often maintain lists of remote networks that are known to be trustworthy or untrustworthy as sending SMTP clients. Typically, these lists are based on the use of IP Addresses of the clients. The IP Addresses serve as identifiers. The list specifies positive or negative authorization, and the source of the list is an organization that the operator of the receiving SMTP server deems worthy to assess other sites.

When used in this way, IP Addresses are authenticated by relying on their use in the IP routing infrastructure. Packets are routed to the specified IP Address, over the open Internet. A continuing TCP session using that IP Address is therefore presumed to be an interaction with the host legitimately associated with that IP Address.

Increased topological, transfer and access complexities on the Internet are making IP Addresses increasingly problematic for use as persistent identifiers. Instead they are viewed as appropriate only for the most transient task of delivering individual packets.

CSV builds upon this popular model. Besides the considerable benefit of having operational practice, the model can be extremely efficient. It permits the service provider to assess the source of an entire message stream, rather than having to evaluate each message. Also, CSV makes its assessment before messages cross the Internet, thereby saving bandwidth and reducing the impact of a distributed denial of service attack.

3  Service Goal

CSV verifies that a host is authorized to act as an SMTP client and that the client is likely to be operated acceptably. CSV enhances current practice with:

• Identification by persistent domain name rather than transient IP Address.
• A standardized method of documenting authorization to operate as a sending SMTP client.
• A standardized method of referencing accreditation services.
• A standardized method of querying an accreditation service.

4  Requirements

4.1  Assessing Authorization

For a receiving SMTP server to determine whether a host has authorization to act as a sending SMTP client, it is necessary to identify the host and verify its association with that identity. Given that, a DNS query on the name can return an explicit authorization.

Identification

The means of identifying a remote host or service anchor=""requires uniqueness and is aided by persistence. The identifier must not be ambiguous and its use is made far more efficient if it is stable over time. The two usual choices are IP Addresses and Domain Names.

An IP Address typically refers to a single host and can change relatively frequently, as the host's connection to the Internet changes. IP Addresses are reported by the Internet infrastructure and for simple security requirements, transactional use of an IP Address through the Internet's routing fabric is taken as validation of the Address.

Domain Names are longer-lived but require new administrative effort. They can be used to refer to multiple hosts simultaneously. The DNS administrator for a domain will maintain record(s) listing one or more IP addresses associated with that name, even though reverse-DNS records (not controlled by the same DNS administrator) may give conflicting information. The forward-DNS is considered the valid authority for CSV purposes. Therefore, authentication of a domain name's reference to a particular IP Address requires an explicit authentication step.

Authentication

If the sending SMTP client of a connection can be authenticated, then it is possible to develop an accountability mechanism based on that authentication. MUA-MSA exchanges have a substantial number of useful authentication mechanisms available. These are often very strong, and involve significant prior arrangement. The same holds true for MDA-MUA exchanges, and often for MSA-MTA and MTA-MDA exchanges, such as within an organization's local network.

What is missing is a useful means of authenticating MTA-MTA exchanges over the open Internet. Prior arrangement between such a pair of MTAs is antithetical to the history and operation of Internet mail. Spontaneous communications are at the core of Internet design and operation. So the challenge is to develop an authentication mechanism that permits the necessary amount of accountability, without imposing undue overhead or restrictions.

A number of strong authentication mechanisms are possible, but none has yet attained widespread adoption among MTAs with no prior relationship. CSV specifies a weaker authentication scheme that meets the modest requirements for this service. Stronger methods can be supported later, if necessary. However they must be tied to a domain name and must not require any prior relationship.

Authorization

Internet operation has typically required no public mechanism for restricting or permitting particular hosts to operate clients or servers for particular services on behalf of particular domains. The DNS MX record states where to route email that is destined for a specific domain; this implies a degree of authorization for the host referenced in the MX. However the record is really for routing and there is no equivalent means of specifying authorization of other hosts that might act as email relays. Similarly there is no means for checking the authorization of World Wide Web servers, DNS servers, telnet clients or other Internet applications.

What is missing is an open, interoperable means by which accountable domain management can announce its authorization of a particular host to operate a particular service. CSV defines such a mechanism for sending SMTP clients.

4.2  Assessing Accreditation

This portion of CSV determines accreditations for the sending SMTP client or for the administration under which it operates. The basis for deciding that an authorizing agency is, itself, to be trusted can be highly varied. Often, well-established practices are not that well-understood. This makes it difficult to predict what methods of accreditation will be most appropriate and successful for Internet mail. It is expected that this portion of an Internet mail validation service will therefore need to support be a variety of accreditation service styles.

What is needed is a standard means for:

• referencing different accreditation services, and
• querying a service to obtain information about the domain it is accrediting.

5  Client SMTP Validation Details

CSV defines a mechanism for session-time, domain-based validation of a sending SMTP client. It is useful across the open Internet, between MTAs that have made no prior arrangement with each other. Validation establishes that the operation of the MTA is authorized by an accredited administrator of the declared domain name.

The validation requirements are modest, because the system does not seek to provide long-term vetting of the client host, nor does it assess the actual content being exchanged. Techniques that would be wholly inadequate for classic, strong authentication and validation can be entirely sufficient for CSV's needs.

Validation has two separate phases: assessing authorization and assessing accreditation. The first is performed between the receiving SMTP server and the sending SMTP client. The second is performed between the receiving SMTP server and one or more accrediting services.

5.1  Assessing Authorization

This phase provides a means for a network administrator to publicly state what hosts are authorized by it to act as client MTAs. Absent such a statement of authority, it is possible that the client is a rogue or compromised host.

The assessment requires three steps:

Identification:

The sending SMTP client host is identified by a Domain Name. The domain name serves as a unique, topologically-independent, persistent identifier that is registered in the Domain Name Service.

A sending SMTP client MUST supply a published domain name as the parameter to an SMTP HELO or EHLO. The sending SMTP client MAY issue multiple EHLO's over the course of a session, such as for isolating email flows for accreditation, with different domain names to represent different users on the client system. If an EHLO is issued, the entire CSV process MUST be restarted without needing to make a new connection.

For CSV, a sending SMTP client places the domain name into the <Domain> field specified for a SMTP HELO or EHLO [RFC2821]. The domain name is any name under which it is claiming authorization to act as a sending SMTP client. A receiving SMTP server will extract this name and use it as the identification for the client seeking to send email, upon which CSV assessments are then made.

Authentication:

There is no universal, strong method to authenticate that a host is correctly identifying itself. For most email transport purposes, it will be sufficient to show that the EHLO domain name forward-resolves to the IP address of the sending SMTP client.

The response to a CSV authentication query usually includes the list of associated IP addresses in the Additional Information section. Formally, this additional information is the same as would be obtained from additional queries for that information. A server includes it in the CSV query for efficiency, to avoid additional DNS queries.

If the list is returned and the actual IP address of the sending SMTP client is in it, the receiving SMTP server SHOULD consider the EHLO domain name to be authenticated. Conversely, if the list is returned and the actual IP address is not in it, the assertion of the EHLO domain name SHOULD be considered incorrect, and result in an error being returned.

Authorization:

In CSV, the purpose of authorization is to establish that an accountable authority has given permission for the sending SMTP client host to operate in that role.

CSV participants MUST use the method defined in Client SMTP Authorization (CSA) [ID-CSVCSA]. It specifies a DNS record that is associated with the domain name offered by the sending SMTP client host.

5.2  Assessing Accreditation

The CSV authorization phase provides a basis for trusting that the sending SMTP client is under the control of a domain's management; but this says nothing about the policies and practices of that management. Separate accreditation services are needed for that. It is expected that there will be numerous services that provide accreditation. CSV is intended to support use of any accreditation service that gains credibility among operators of SMTP servers.

An initial set of capabilities for specifying CSV-related accreditation services is specified in Domain Name Accreditation (DNA) [ID-CSVDNA]. Sending SMTP clients SHOULD publish CSV records referring to accreditation services in which they are listed. Accreditation services MUST publish DNA-conformant records.

6  Security Considerations

CSV defines a security mechanism. The nature of the security requirements for CSV are significantly different from typical, "strong" methods required for most Internet security functions.

The proposal relies on the integrity and authenticity of DNS data.

Normative References

Informative References

Author's Addresses

A  Acknowledgements

This proposal is similar to DRIP [ID-brand-drip], however it uses a different DNS [RFC1035] record.

Review comments and suggestions, on previous versions of CSV, have been made by: Tony Finch, Carl Hutzler, Meng Weng Wong, Greg Connor.

B  Host Name Authentication

The routing infrastructure of the Internet distinguishes hosts by their topological attachment, noted as its IP Address. Because IP Addresses change periodically and users prefer references that can be mnemonic, hosts on the Internet generally have one or more Domain Names (DNS) [RFC1035] assigned to them. A Domain Name is globally unique. The core function of the DNS is to map from a name supplied by the user, to an IP Address associated with that name. Internet protocols often permit a host to identify itself with its domain name.

But what if a host is programmed incorrectly, or even maliciously? We need a way to authenticate that a host is reporting its name correctly. Establishing this authentication is separate from determining its authorization to perform any particular service. Until the relationship is authenticated, we cannot apply policies associated with the name.

A number of methods for authenticating the relationship between the host and its reported name might be used. The current CSV specification supports authentication through Domain Name Service mappings between a domain name and an IP Address. Other equally valid methods are possible. However none has yet proved practical for authenticating a client to a server, without prior arrangement between them.

B.1  DNS-based Mapping

The Domain Name System has a common mapping mechanism that can be used in a variety of ways, based on the schema for assigning names and the types of data listed under those names. The two most popular schemas are forward mapping and Reverse-DNS. Forward looks up a "regular" domain name and receives information about it, such as a list of IP Addresses associated with that name. Reverse DNS starts with an IP Address and maps it to a pointer to a "regular" domain name.

Often when contacted by a remote host, a host uses a reverse-DNS query to get the name of the remote host. This can be followed by a forward-DNS query to see if the name reported by the reverse-DNS query matches an IP address reported by the forward-DNS query. If so, this is generally considered an authentication of the relationship of the name to the host. This method is often used by receiving SMTP servers to decide whether to trust the sending SMTP client.

Closing the circle in this manner permits verifying both that the domain assigning the name and the service provider assigning IP addresses agree that this is the appropriate name for that remote host. Although this process has known limitations, it is considered sufficient for many basic uses.

Use of an IP Address returned by the DNS is sufficient for CSV-related authentication requirements. However it MUST NOT be considered a strong form of authentication for allowing otherwise privileged access. The use of this mechanism is to aid selection of accreditation services, such as whether to query using the domain name or the client address. Other measures may be taken intended to limit exposure to unknown clients but are beyond the scope of this specification.

B.2  Reverse DNS

Reverse DNS can be used by itself to associate a domain name with an IP address. It indicates that the entity responsible for allocating that block of IP addresses has designated an IP address to be used by the domain name. Unfortunately, the reverse-IP branch of the DNS has a long history of being poorly maintained, and often does not match the forward-DNS information even when the relationship of host to name is genuine.

Reverse DNS by itself SHOULD NOT be considered sufficient authentication.

B.3  Forward DNS Lookup

An isolated forward lookup is sufficient for simple sending SMTP client authentication, if an IP Address returned for that name matches the IP Address reported by the underlying IP service for that remote host. This indicates that the domain in question currently designates that IP Address as an IP address entitled to respond for that domain name.

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