As per Relevance of the word extension, we have this rfc below:

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


SMTP Service
for Remote Message Queue

Status of this

This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited



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

1.

The TURN command was a valid attempt to address the problem of
to start the processing for the mail queue on a remote machine
However, the TURN command presents a large security loophole.
there is no verification of the remote host name, the TURN
could be used by a rogue system to download the mail for a site
than itself

Therefore, this memo introduces the ETRN command. This command
the mechanism defined in [4] to define extensions to the SMTP
whereby a client ("sender-SMTP") may request that the
("receiver-SMTP") start the processing of its mail queues
messages that are waiting at the server for the client machine.
any messages are at the server for the client, then the server
create a new SMTP session and send the messages at that time










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RFC 1985 SMTP Service Extension - ETRN August 1996


2. Framework for the ETRN

The following service extension is therefore defined

(1) the name of the SMTP service extension is "Remote
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
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
affects the behavior of an SMTP client and server

3. The Remote Queue Processing Declaration service

To save money, many small companies want to only maintain
connections to their service providers. In addition, there are
situations where the client sites depend on their mail
quickly, so forcing the queues on the server belonging to
service provider may be more desirable than waiting for the
timeout to occur

Both of these situations could currently be fixed using the
command defined in [1], if it were not for a large security
in the TURN command. As it stands, the TURN command will reverse
direction of the SMTP connection and assume that the remote host
being honest about what its name is. The security loophole is
there is no documented stipulation for checking the authenticity
the remote host name, as given in the HELO or EHLO command. As such
most SMTP and ESMTP implementations do not implement the TURN
to avoid this security loophole

This has been addressed in the design of the ETRN command.
extended turn command was written with the points in the
paragraph in mind, yet paying attention to the problems
currently exist with the TURN command. The security loophole
avoided by asking the server to start a new connection aimed at
specified client

In this manner, the server has a lot more certainty that it
talking to the correct SMTP client. This mechanism can just be
as a more immediate version of the retry queues that appear in
SMTP implementations. In addition, as this command will take



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RFC 1985 SMTP Service Extension - ETRN August 1996


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

4.

Remote queue processing means that using an SMTP or ESMTP connection
the client may request that the server start to process parts of
messaging queue. This processing is performed using the
SMTP infrastructure and will occur at some point after the
is initiated

The server host is the node that is responding to the
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
specifies a name for the remote host(s). This remote host name
also include an alias for the specified remote host or
commands to identify other types of queues

5. The extended ETRN

The extended ETRN command is issued by the client host when it
to start the SMTP queue processing of a given server host.
syntax of this command is as follows

ETRN [character>]
This command may be issued at any time once a session is established
as long as there is not a transaction occuring. Thus, this
is illegal between a MAIL FROM: command and the end of the
commands and responses

The specified node name must be a fully qualified domain name for
node, which may refer to a CNAME or MX pointer in the DNS. If
alias is used for the node, multiple ETRN commands may be needed
start the processing for the node as it may be listed at the
site under multiple names. This can also be addressed using
options discussed in section 5.3.

The option character under normal circumstances is not used








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RFC 1985 SMTP Service Extension - ETRN August 1996


5.1 Server action on receipt of the extended ETRN

When the server host receives the ETRN command, it should have a
at the node name that is specified in the command and make a
decision if it should honour the request. If not, the
error codes should be returned to the client

Otherwise, the server host should force its retry queues to
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.
should be noted in the case where there are no messages for
client host at the server host and only the 250 response is used

Since the processing of the queues may take an indeterminate
of time, this command should return immediately with a response
the client host. The valid return codes for this command are

250 OK, queuing for node
251 OK, no messages waiting for node 252 OK, pending messages for node
253 OK, pending messages for node
458 Unable to queue messages for node 459 Node not allowed: 500 Syntax
501 Syntax Error in

The 250 response code does not indicate that messages will be sent
the system in question, just that the queue has been started and
action will occur. If the server is capable of supporting it,
251, 252 or 253 response codes should be used to give
information to the client side. In this case, if there are
waiting for the client side node, a check can be performed
these responses codes as an indication of when there are no
pending messages in the queue for that node

The 458 and 459 result codes should be used to give more
back to the client host as to why the action was not performed.
the syntax of the request is not correct, then the 500 and 501
codes should be used

5.2 Client action on receiving response to extended ETRN

If one of the 500 level error codes (550 or 551) are sent, the
should assume that the protocol is not supported in the remote
or that the protocol has not been implemented correctly on either
client or server host. In this case, multiple ETRN commands (
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
the server host found its request to be satisfactory and it will
any queued messages. This process may involve going through a
large retry queue, and may take some time

If the 400 level response is received, then the client can
that the server supports the command, but for some local reason
not want to accept the ETRN command as is. In most cases, it
mean that there is a list of nodes that it will accept the
from and the current client is not on that list. The 459
code is presented to allow for a more in-depth reason as to why
remote queuing cannot be started

5.3 Use Of ETRN to release mail for a subdomain or

If the requesting server wishes to release all of the mail for
given subdomain, a variation on the ETRN command can be used.
perform this request, the option character '@' should be used
front of the node name. In this manner, any domain names that
formed with a suffix of the specified node name are released

For example, if the command ETRN @foo.com was issued, then
accumulated mail for fred.foo.com, a.b.c.d.e.f.g.foo.com or foo.
may be released. It should be noted that the receiving side of
ETRN command should make a decision based on the client in
and only allow certain combinations for each of the nodes. This
more of a security issue than anything else

In a similar vein, it might be necessary under some circumstances
release a certain queue, where that queue does not correspond to
given domain name. To this end, the option character '#' can be
to force the processing of a given queue. In this case, the
name would be used as a queue name instead, and its
structure would be dependant on the receiving server. An example
this would be using the command ETRN #uucp to force the flush of
UUCP queue. Note that the use of this option is entirely a
matter and there is no way for a client to find a list of any
queues that exist

6. Minimal

A "minimal" client may use this extension with its host name to
the queues on the server host. This minimal usage will not
cases where mail for 'x.y' is sent to 's.x.y'.

A minimal server may use this extensions to start the processing
the queues for all remote sites. In this case, the 458
response will not be seen, and it should always return the 250



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RFC 1985 SMTP Service Extension - ETRN August 1996


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

7.

The following example illustrates the use of remote queue
with some permanent and temporary failures

S: connection on TCP port 25>
C: connection to server
S: 220 sigurd.innosoft.com -- Server SMTP (PMDF V4.2-6 #1992)
C: EHLO ymir.claremont.
S: 250-sigurd.innosoft.
S: 250-
S: 250-
S: 250
C:
S: 500 Syntax
C: ETRN
S: 501 Syntax Error in
C: ETRN uu.
S: 458 Unable to queue messages for node uu.
...

C: ETRN sigurd.innosoft.
S: 250 OK, queuing for node sigurd.innosoft.com
C: ETRN innosoft.
S: 250 OK, queuing for node innosoft.com



C: ETRN sigurd.innosoft.
S: 251 OK, no messages waiting for node sigurd.innosoft.
C: ETRN innosoft.
S: 252 OK, pending messages for node innosoft.com
C: ETRN mysoft.
S: 253 OK, 14 pending messages for node mysoft.com

...
C: ETRN foo.
S: 459 Node foo.bar not allowed: Unable to resolve name
...
C:
S: 250







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RFC 1985 SMTP Service Extension - ETRN August 1996


8. Security

This command does not compromise any security considerations of
existing SMTP or ESMTP protocols as it merely shortens the time
a client needs to wait before their messages are retried

Precautions should be taken to make sure that any client server
only use the @ and # option characters for systems that make sense
Failure to implement some kind of sanity checking on the
could lead to congestion. This would be evident if a person
to release @com, which would release mail for any address that
with com

9.

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

10.

[1] Postel, J., "Simple Mail Transfer Protocol", STD 10,
821, August 1982.

[2] Klensin, J., WG Chair, Freed, N., Editor, Rose, M., Stefferud
E., and D. Crocker, "SMTP Service Extensions" RFC 1425,
Nations University, Innosoft International, Inc., Dover
Consulting, Inc., Network Management Associates, Inc., The
Office, February 1993.

11. Author's

Jack De
Wildbear Consulting, Inc
17 Brock
Kitchener, Ontario,
N2M 1X

Phone: +1 519 576 3873
EMail: jack@wildbear.on.











De Winter Standards Track [Page 7]

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if you see any problems within the linking, don't worry be happy,
this is version 0.1 of the Relevance System and you gotta expect some crappy subroutines sometimes,
just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.



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