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rfc:rfc1985

Network Working Group J. De Winter Request for Comments: 1985 Wildbear Consulting, Inc. Category: Standards Track August 1996

                       SMTP Service Extension
                 for Remote Message Queue Starting

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.

Abstract

 This memo defines an extension to the SMTP service whereby an SMTP
 client and server may interact to give the server an opportunity to
 start the processing of its queues for messages to go to a given
 host.  This extension is meant to be used in startup conditions as
 well as for mail nodes that have transient connections to their
 service providers.

1. Introduction

 The TURN command was a valid attempt to address the problem of having
 to start the processing for the mail queue on a remote machine.
 However, the TURN command presents a large security loophole.  As
 there is no verification of the remote host name, the TURN command
 could be used by a rogue system to download the mail for a site other
 than itself.
 Therefore, this memo introduces the ETRN command.  This command uses
 the mechanism defined in [4] to define extensions to the SMTP service
 whereby a client ("sender-SMTP") may request that the server
 ("receiver-SMTP") start the processing of its mail queues for
 messages that are waiting at the server for the client machine.  If
 any messages are at the server for the client, then the server should
 create a new SMTP session and send the messages at that time.

De Winter Standards Track [Page 1] RFC 1985 SMTP Service Extension - ETRN August 1996

2. Framework for the ETRN Extension

 The following service extension is therefore defined:
 (1) the name of the SMTP service extension is "Remote Queue
 Processing Declaration";
 (2) the EHLO keyword value associated with this extension is "ETRN",
 with no associated parameters;
 (3) one additional verb, ETRN, with a single parameter that
 specifies the name of the client(s) to start processing for;
 (4) no additional SMTP verbs are defined by this extension.
 The remainder of this memo specifies how support for the extension
 affects the behavior of an SMTP client and server.

3. The Remote Queue Processing Declaration service extension

 To save money, many small companies want to only maintain transient
 connections to their service providers.  In addition, there are some
 situations where the client sites depend on their mail arriving
 quickly, so forcing the queues on the server belonging to their
 service provider may be more desirable than waiting for the retry
 timeout to occur.
 Both of these situations could currently be fixed using the TURN
 command defined in [1], if it were not for a large security loophole
 in the TURN command.  As it stands, the TURN command will reverse the
 direction of the SMTP connection and assume that the remote host is
 being honest about what its name is.  The security loophole is that
 there is no documented stipulation for checking the authenticity of
 the remote host name, as given in the HELO or EHLO command.  As such,
 most SMTP and ESMTP implementations do not implement the TURN command
 to avoid this security loophole.
 This has been addressed in the design of the ETRN command.  This
 extended turn command was written with the points in the first
 paragraph in mind, yet paying attention to the problems that
 currently exist with the TURN command.  The security loophole is
 avoided by asking the server to start a new connection aimed at the
 specified client.
 In this manner, the server has a lot more certainty that it is
 talking to the correct SMTP client.  This mechanism can just be seen
 as a more immediate version of the retry queues that appear in most
 SMTP implementations.  In addition, as this command will take a

De Winter Standards Track [Page 2] RFC 1985 SMTP Service Extension - ETRN August 1996

 single parameter, the name of the remote host(s) to start the queues
 for, the server can decide whether it wishes to respect the request
 or deny it for any local administrative reasons.

4. Definitions

 Remote queue processing means that using an SMTP or ESMTP connection,
 the client may request that the server start to process parts of its
 messaging queue.  This processing is performed using the existing
 SMTP infrastructure and will occur at some point after the processing
 is initiated.
    The server host is the node that is responding to the ETRN
    command.
    The client host is the node that is initiating the ETRN command.
 The remote host name is defined to be a plain-text field that
 specifies a name for the remote host(s).  This remote host name may
 also include an alias for the specified remote host or special
 commands to identify other types of queues.

5. The extended ETRN command

 The extended ETRN command is issued by the client host when it wishes
 to start the SMTP queue processing of a given server host.  The
 syntax of this command is as follows:
    ETRN [<option character>]<node name><CR><LF>
 This command may be issued at any time once a session is established,
 as long as there is not a transaction occuring.  Thus, this command
 is illegal between a MAIL FROM: command and the end of the DATA
 commands and responses.
 The specified node name must be a fully qualified domain name for the
 node, which may refer to a CNAME or MX pointer in the DNS.  If an
 alias is used for the node, multiple ETRN commands may be needed to
 start the processing for the node as it may be listed at the remote
 site under multiple names.  This can also be addressed using the
 options discussed in section 5.3.
 The option character under normal circumstances is not used.

De Winter Standards Track [Page 3] RFC 1985 SMTP Service Extension - ETRN August 1996

5.1 Server action on receipt of the extended ETRN command

 When the server host receives the ETRN command, it should have a look
 at the node name that is specified in the command and make a local
 decision if it should honour the request.  If not, the appropriate
 error codes should be returned to the client.
 Otherwise, the server host should force its retry queues to start
 sending messages to that remote site, using another SMTP connection.
 At the moment, there is no requirement that a connection must occur,
 or that the connection must occur within a given time frame.  This
 should be noted in the case where there are no messages for the
 client host at the server host and only the 250 response is used.
 Since the processing of the queues may take an indeterminate amount
 of time, this command should return immediately with a response to
 the client host.  The valid return codes for this command are:
 250 OK, queuing for node <x> started
 251 OK, no messages waiting for node <x>
 252 OK, pending messages for node <x> started
 253 OK, <n> pending messages for node <x> started
 458 Unable to queue messages for node <x>
 459 Node <x> not allowed: <reason>
 500 Syntax Error
 501 Syntax Error in Parameters
 The 250 response code does not indicate that messages will be sent to
 the system in question, just that the queue has been started and some
 action will occur.  If the server is capable of supporting it, the
 251, 252 or 253 response codes should be used to give more
 information to the client side.  In this case, if there are messages
 waiting for the client side node, a check can be performed using
 these responses codes as an indication of when there are no more
 pending messages in the queue for that node.
 The 458 and 459 result codes should be used to give more information
 back to the client host as to why the action was not performed.  If
 the syntax of the request is not correct, then the 500 and 501 result
 codes should be used.

5.2 Client action on receiving response to extended ETRN command

 If one of the 500 level error codes (550 or 551) are sent, the client
 should assume that the protocol is not supported in the remote host
 or that the protocol has not been implemented correctly on either the
 client or server host.  In this case, multiple ETRN commands (dealing
 with the aliases for the system) should not be sent.

De Winter Standards Track [Page 4] RFC 1985 SMTP Service Extension - ETRN August 1996

 If the 250 response is received, then the client host can assume that
 the server host found its request to be satisfactory and it will send
 any queued messages.  This process may involve going through a very
 large retry queue, and may take some time.
 If the 400 level response is received, then the client can assume
 that the server supports the command, but for some local reason does
 not want to accept the ETRN command as is.  In most cases, it will
 mean that there is a list of nodes that it will accept the command
 from and the current client is not on that list.  The 459 response
 code is presented to allow for a more in-depth reason as to why the
 remote queuing cannot be started.

5.3 Use Of ETRN to release mail for a subdomain or queue

 If the requesting server wishes to release all of the mail for a
 given subdomain, a variation on the ETRN command can be used.  To
 perform this request, the option character '@' should be used in
 front of the node name.  In this manner, any domain names that are
 formed with a suffix of the specified node name are released.
 For example, if the command ETRN @foo.com was issued, then any
 accumulated mail for fred.foo.com, a.b.c.d.e.f.g.foo.com or foo.com
 may be released.  It should be noted that the receiving side of the
 ETRN command should make a decision based on the client in question
 and only allow certain combinations for each of the nodes.  This is
 more of a security issue than anything else.
 In a similar vein, it might be necessary under some circumstances to
 release a certain queue, where that queue does not correspond to a
 given domain name.  To this end, the option character '#' can be used
 to force the processing of a given queue.  In this case, the node
 name would be used as a queue name instead, and its syntactical
 structure would be dependant on the receiving server.  An example of
 this would be using the command ETRN #uucp to force the flush of a
 UUCP queue.  Note that the use of this option is entirely a local
 matter and there is no way for a client to find a list of any such
 queues that exist.

6. Minimal usage

 A "minimal" client may use this extension with its host name to start
 the queues on the server host.  This minimal usage will not handle
 cases where mail for 'x.y' is sent to 's.x.y'.
 A minimal server may use this extensions to start the processing of
 the queues for all remote sites.  In this case, the 458 error
 response will not be seen, and it should always return the 250

De Winter Standards Track [Page 5] RFC 1985 SMTP Service Extension - ETRN August 1996

 response as it will always try and start the processing for any
 request.

7. Example

 The following example illustrates the use of remote queue processing
 with some permanent and temporary failures.
 S: <wait for connection on TCP port 25>
 C: <open connection to server>
 S: 220 sigurd.innosoft.com -- Server SMTP (PMDF V4.2-6 #1992)
 C: EHLO ymir.claremont.edu
 S: 250-sigurd.innosoft.com
 S: 250-EXPN
 S: 250-HELP
 S: 250 ETRN
 C: ETRN
 S: 500 Syntax Error
 C: ETRN localname
 S: 501 Syntax Error in Parameters
 C: ETRN uu.net
 S: 458 Unable to queue messages for node uu.net
 ...
 C: ETRN sigurd.innosoft.com
 S: 250 OK, queuing for node sigurd.innosoft.com started
 C: ETRN innosoft.com
 S: 250 OK, queuing for node innosoft.com started
 OR
 C: ETRN sigurd.innosoft.com
 S: 251 OK, no messages waiting for node sigurd.innosoft.com
 C: ETRN innosoft.com
 S: 252 OK, pending messages for node innosoft.com started
 C: ETRN mysoft.com
 S: 253 OK, 14 pending messages for node mysoft.com started
 ...
 C: ETRN foo.bar
 S: 459 Node foo.bar not allowed: Unable to resolve name.
 ...
 C: QUIT
 S: 250 Goodbye

De Winter Standards Track [Page 6] RFC 1985 SMTP Service Extension - ETRN August 1996

8. Security Considerations

 This command does not compromise any security considerations of any
 existing SMTP or ESMTP protocols as it merely shortens the time that
 a client needs to wait before their messages are retried.
 Precautions should be taken to make sure that any client server can
 only use the @ and # option characters for systems that make sense.
 Failure to implement some kind of sanity checking on the parameters
 could lead to congestion.  This would be evident if a person asking
 to release @com, which would release mail for any address that ended
 with com.

9. Acknowledgements

 This document was created with lots of support from the users of our
 products, who have given some input to the functionality that they
 would like to see in the software that they bought.

10. References

 [1] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
     821, August 1982.
 [2] Klensin, J., WG Chair, Freed, N., Editor, Rose, M., Stefferud,
     E., and D. Crocker, "SMTP Service Extensions" RFC 1425, United
     Nations University, Innosoft International, Inc., Dover Beach
     Consulting, Inc., Network Management Associates, Inc., The Branch
     Office, February 1993.

11. Author's Address

 Jack De Winter
 Wildbear Consulting, Inc.
 17 Brock Street
 Kitchener, Ontario, Canada
 N2M 1X2
 Phone: +1 519 576 3873
 EMail: jack@wildbear.on.ca

De Winter Standards Track [Page 7]

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