Mail Working Group Jack De Winter, Director
Internet Draft Wildbear Consulting, Inc.
Expires in six months
SMTP Service Extension
for Remote Message Queue Starting
January 26, 1996
1. Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
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2. 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 is an attempt to fix the security problems with the TURN
by creating a new command ETRN that places the emphasis on the server
instead of the connection. This extension is meant to be used in
startup conditions as well as for mail nodes that have transient
connections to their service providers.
3. 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.
4. 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";
(3) one additional command, ETRN, with a single parameter that
specifies the name of the client 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.
5. 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 stipulation for checking the authenticity of the remote
host name, as given in the HELO or EHLO (for the ESMTP extensions in
[4]).
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 placing the focus on 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
single parameter, the name of the remote host to start the queues for,
the server can make a decision as to whether it wishes to respect the
request or deny it for administrative reasons.
6. Definitions
The term remote queue processing is meant to reflect the manner in
which a server implementation of the SMTP protocol maintains its
queue of messages that it currently cannot send to the proper hosts.
This method reflects the wishes of that remote host, as reflected by
the MX records, and most likely will reflect that the local host is
listed in the MX records or that there is no MX record for the remote
host.
The remote host name is defined to be a plain-text field that
specifies a fully qualified domain name for the remote host. This
remote host name may also include an alias for the specified remote
host.
7. The extended ETRN command
The extended ETRN command is issued by a client when it wishes to
start the SMTP queue processing of a remote host. The syntax of this
command is as follows:
ETRN [<option character>]<node name><CR><LF>
This command may only 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 to the end of
the DATA commands and responses.
The specified node name must be a fully qualified domain name for
the node, but may include an alias for the local node. 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. The option character under normal circumstances
is not used.
7.1 Server action on receipt of the extended ETRN command
When the server 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 respect the node name. If not, the
appropriate error codes should be returned to the client.
Otherwise, the server should start force its retry queues to start
sending messages to that remote site, using another SMTP connection.
At the moment, there is no definition that a connection must occur.
This should be noted in the case where there are no messages for the
client at the server site.
Seeing as the processing of the queues may take an indeterminate
amount of time, this command should return immediately with a
response to the client side. The valid return codes for this command
are:
250 OK, queuing for node <x> started
251 OK, no messages waiting for node <x>
458 Unable to queue messages for node <x>
459 Node <x> not allowed: <reason>
500 Syntax Error
501 Syntax Error in Parameters
7.2 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
site or that the protocol has not been implemented correctly on
either the client or server site. In this case, multiple ETRN
commands (dealing with the aliases for the system) should not be
sent.
If the 250 response is received, then the client can assume that
the server 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.
7.3 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 node names that
are formed of a prefix of characters and 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 node 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 containing information. 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.
8. Minimal usage
A "minimal" client may use this extension to simply force its
empirical name to be used to start the queues on the remote host.
This minimal usage will not handle cases where mail for 'x.y' is sent
to 's.x.y' which is aliased through a firewall or simple for local
site management reasons.
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
response as it will always try and start the processing for any
request.
9. 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
...
C: ETRN foo.bar
S: 459 Node foo.bar not allowed: Unable to resolve name.
...
C: QUIT
S: 250 Goodbye
10. 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 should
only use the @ and # option characters for systems that make sense.
Failure to implement some kind of secure sanity checking on the
parameters could lead to congestion. This would be evident of a person
asking to release #com, which would release mail for any address that
ended with com.
11. 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.
11. References
[1] J. B. Postel. Simple Mail Transfer Protocol. Request for Comments
821, August 1982.
[2] D. H. Crocker. Standard for the Format of ARPA Internet Text
Messages. Request for Comments 822, August 1982.
[3] K. Moore. Representation of Non-ASCII Text in Internet Message
Headers. Request for Comments 1522, September 1993.
[4] M. T. Rose, E. A. Stefferud, D. H. Crocker, John C. Klensin, Ned
Freed. SMTP Service Extensions. Internet-draft, April 1995.
[5] C. Partridge. Mail Routing and the Domain System. Request for
Comments 974, January 1986.
12. Chair, editor, and author addresses
John Klensin, WG Chair
MCI
2100 Reston Parkway
Reston, VA 22091
tel: +1 703 715-7361 fax: +1 703 715-7436
email: klensin@mci.net
Ned Freed, Editor
Innosoft International, Inc.
1050 East Garvey Avenue South
West Covina, CA 91790
USA
tel: +1 818 919 3600 fax: +1 818 919 3614
email: ned@innosoft.com
Jack De Winter
Wildbear Consulting, Inc.
8-589 Beechwood Drive
Waterloo, Ontario, Canada
N2T 2K9
tel: +1 519 886 3683
email: jack@wildside.kwnet.on.ca