Kevin Schoon 8 months ago

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.

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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

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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.

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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.

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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

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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

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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

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