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RFC2487 - SMTP Service Extension for Secure SMTP over TLS

王朝other·作者佚名  2008-05-31
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Network Working Group P. Hoffman

Request for Comments: 2487 Internet Mail Consortium

Category: Standards Track January 1999

SMTP Service Extension for Secure SMTP over TLS

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1999). All Rights Reserved.

1. Abstract

This document describes an extension to the SMTP service that allows

an SMTP server and client to use transport-layer security to provide

private, authenticated communication over the Internet. This gives

SMTP agents the ability to protect some or all of their

communications from eavesdroppers and attackers.

2. IntrodUCtion

SMTP [RFC-821] servers and clients normally communicate in the clear

over the Internet. In many cases, this communication goes through one

or more router that is not controlled or trusted by either entity.

Such an untrusted router might allow a third party to monitor or

alter the communications between the server and client.

Further, there is often a desire for two SMTP agents to be able to

authenticate each others' identities. For example, a secure SMTP

server might only allow communications from other SMTP agents it

knows, or it might act differently for messages received from an

agent it knows than from one it doesn't know.

TLS [TLS], more commonly known as SSL, is a popular mechanism for

enhancing TCP communications with privacy and authentication. TLS is

in wide use with the HTTP protocol, and is also being used for adding

security to many other common protocols that run over TCP.

2.1 Terminology

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 [RFC-2119].

3. STARTTLS Extension

The STARTTLS extension to SMTP is laid out as follows:

(1) the name of the SMTP service defined here is STARTTLS;

(2) the EHLO keyword value associated with the extension is STARTTLS;

(3) the STARTTLS keyword has no parameters;

(4) a new SMTP verb, "STARTTLS", is defined;

(5) no additional parameters are added to any SMTP command.

4. The STARTTLS Keyword

The STARTTLS keyword is used to tell the SMTP client that the SMTP

server allows use of TLS. It takes no parameters.

5. The STARTTLS Command

The format for the STARTTLS command is:

STARTTLS

with no parameters.

After the client gives the STARTTLS command, the server responds with

one of the following reply codes:

220 Ready to start TLS

501 Syntax error (no parameters allowed)

454 TLS not available due to temporary reason

A publicly-referenced SMTP server MUST NOT require use of the

STARTTLS extension in order to deliver mail locally. This rule

prevents the STARTTLS extension from damaging the interoperability of

the Internet's SMTP infrastructure. A publicly-referenced SMTP server

is an SMTP server which runs on port 25 of an Internet host listed in

the MX record (or A record if an MX record is not present) for the

domain name on the right hand side of an Internet mail address.

Any SMTP server may refuse to accept messages for relay based on

authentication supplied during the TLS negotiation. An SMTP server

that is not publicly referenced may refuse to accept any messages for

relay or local delivery based on authentication supplied during the

TLS negotiation.

A SMTP server that is not publicly referenced may choose to require

that the client perform a TLS negotiation before accepting any

commands. In this case, the server SHOULD return the reply code:

530 Must issue a STARTTLS command first

to every command other than NOOP, EHLO, STARTTLS, or QUIT. If the

client and server are using the ENHANCEDSTATUSCODES ESMTP extension

[RFC-2034], the status code to be returned SHOULD be 5.7.0.

After receiving a 220 response to a STARTTLS command, the client

SHOULD start the TLS negotiation before giving any other SMTP

commands.

If the SMTP client is using pipelining as defined in RFC1854, the

STARTTLS command must be the last command in a group.

5.1 Processing After the STARTTLS Command

After the TLS handshake has been completed, both parties MUST

immediately decide whether or not to continue based on the

authentication and privacy achieved. The SMTP client and server may

decide to move ahead even if the TLS negotiation ended with no

authentication and/or no privacy because most SMTP services are

performed with no authentication and no privacy, but some SMTP

clients or servers may want to continue only if a particular level of

authentication and/or privacy was achieved.

If the SMTP client decides that the level of authentication or

privacy is not high enough for it to continue, it SHOULD issue an

SMTP QUIT command immediately after the TLS negotiation is complete.

If the SMTP server decides that the level of authentication or

privacy is not high enough for it to continue, it SHOULD reply to

every SMTP command from the client (other than a QUIT command) with

the 554 reply code (with a possible text string such as "Command

refused due to lack of security").

The decision of whether or not to believe the authenticity of the

other party in a TLS negotiation is a local matter. However, some

general rules for the decisions are:

- A SMTP client would probably only want to authenticate an SMTP

server whose server certificate has a domain name that is the

domain name that the client thought it was connecting to.

- A publicly-referenced SMTP server would probably want to accept

any certificate from an SMTP client, and would possibly want to

put distinguishing information about the certificate in the

Received header of messages that were relayed or submitted from

the client.

5.2 Result of the STARTTLS Command

Upon completion of the TLS handshake, the SMTP protocol is reset to

the initial state (the state in SMTP after a server issues a 220

service ready greeting). The server MUST discard any knowledge

oBTained from the client, such as the argument to the EHLO command,

which was not obtained from the TLS negotiation itself. The client

MUST discard any knowledge obtained from the server, such as the list

of SMTP service extensions, which was not obtained from the TLS

negotiation itself. The client SHOULD send an EHLO command as the

first command after a successful TLS negotiation.

The list of SMTP service extensions returned in response to an EHLO

command received after the TLS handshake MAY be different than the

list returned before the TLS handshake. For example, an SMTP server

might not want to advertise support for a particular SASL mechanism

[SASL] unless a client has sent an appropriate client certificate

during a TLS handshake.

Both the client and the server MUST know if there is a TLS session

active. A client MUST NOT attempt to start a TLS session if a TLS

session is already active. A server MUST NOT return the TLS extension

in response to an EHLO command received after a TLS handshake has

completed.

6. Usage Example

The following dialog illustrates how a client and server can start a

TLS session:

S: <waits for connection on TCP port 25>

C: <opens connection>

S: 220 mail.imc.org SMTP service ready

C: EHLO mail.ietf.org

S: 250-mail.imc.org offers a warm hug of welcome

S: 250 STARTTLS

C: STARTTLS

S: 220 Go ahead

C: <starts TLS negotiation>

C & S: <negotiate a TLS session>

C & S: <check result of negotiation>

C: <continues by sending an SMTP command>

. . .

7. Security Considerations

It should be noted that SMTP is not an end-to-end mechanism. Thus, if

an SMTP client/server pair decide to add TLS privacy, they are not

securing the transport from the originating mail user agent to the

recipient. Further, because delivery of a single piece of mail may

go between more than two SMTP servers, adding TLS privacy to one pair

of servers does not mean that the entire SMTP chain has been made

private. Further, just because an SMTP server can authenticate an

SMTP client, it does not mean that the mail from the SMTP client was

authenticated by the SMTP client when the client received it.

Both the STMP client and server must check the result of the TLS

negotiation to see whether acceptable authentication or privacy was

achieved. Ignoring this step completely invalidates using TLS for

security. The decision about whether acceptable authentication or

privacy was achieved is made locally, is implementation-dependant,

and is beyond the scope of this document.

The SMTP client and server should note carefully the result of the

TLS negotiation. If the negotiation results in no privacy, or if it

results in privacy using algorithms or key lengths that are deemed

not strong enough, or if the authentication is not good enough for

either party, the client may choose to end the SMTP session with an

immediate QUIT command, or the server may choose to not accept any

more SMTP commands.

A server announcing in an EHLO response that it uses a particular TLS

protocol should not pose any security issues, since any use of TLS

will be at least as secure as no use of TLS.

A man-in-the-middle attack can be launched by deleting the "250

STARTTLS" response from the server. This would cause the client not

to try to start a TLS session. An SMTP client can protect against

this attack by recording the fact that a particular SMTP server

offers TLS during one session and generating an alarm if it does not

appear in the EHLO response for a later session. The lack of TLS

during a session SHOULD NOT result in the bouncing of email, although

it could result in delayed processing.

Before the TLS handshake has begun, any protocol interactions are

performed in the clear and may be modified by an active attacker. For

this reason, clients and servers MUST discard any knowledge obtained

prior to the start of the TLS handshake upon completion of the TLS

handshake.

The STARTTLS extension is not suitable for authenticating the author

of an email message unless every hop in the delivery chain, including

the submission to the first SMTP server, is authenticated. Another

proposal [SMTP-AUTH] can be used to authenticate delivery and MIME

security multiparts [MIME-SEC] can be used to authenticate the author

of an email message. In addition, the [SMTP-AUTH] proposal offers

simpler and more flexible options to authenticate an SMTP client and

the SASL EXTERNAL mechanism [SASL] MAY be used in conjunction with

the STARTTLS command to provide an authorization identity.

A. References

[RFC-821] Postel, J., "Simple Mail Transfer Protocol", RFC821,

August 1982.

[RFC-1869] Klensin, J., Freed, N, Rose, M, Stefferud, E. and D.

Crocker, "SMTP Service Extensions", STD 10, RFC1869,

November 1995.

[RFC-2034] Freed, N., "SMTP Service Extension for Returning Enhanced

Error Codes", RFC2034, October 1996.

[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate

Requirement Levels", BCP 14, RFC2119, March 1997.

[SASL] Myers, J., "Simple Authentication and Security Layer

(SASL)", RFC2222, October 1997.

[SMTP-AUTH] "SMTP Service Extension for Authentication", Work in

Progress.

[TLS] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",

RFC2246, January 1999.

B. Author's Address

Paul Hoffman

Internet Mail Consortium

127 Segre Place

Santa Cruz, CA 95060

Phone: (831) 426-9827

EMail: phoffman@imc.org

C. Full Copyright Statement

Copyright (C) The Internet Society (1999). All Rights Reserved.

This document and translations of it may be copied and furnished to

others, and derivative works that comment on or otherwise eXPlain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

 
 
 
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