Movatterモバイル変換

Network Working Group                                       J. De WinterRequest for Comments: 1985                     Wildbear Consulting, Inc.Category: Standards Track                                    August 1996SMTP Service Extensionfor Remote Message Queue StartingStatus 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 19962.  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 aDe 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 insection 5.3.   The option character under normal circumstances is not used.De Winter                   Standards Track                     [Page 3]

RFC 1985             SMTP Service Extension - ETRN           August 19965.1  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 250De 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 GoodbyeDe Winter                   Standards Track                     [Page 6]

RFC 1985             SMTP Service Extension - ETRN           August 19968. 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,RFC821, 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.caDe Winter                   Standards Track                     [Page 7]