Movatterモバイル変換

Network Working Group                                    Craig PartridgeRequest for Comments: 974                 CSNET CIC BBN Laboratories Inc                                                            January 1986MAIL ROUTING AND THE DOMAIN SYSTEMStatus of this Memo   This RFC presents a description of how mail systems on the Internet   are expected to route messages based on information from the domain   system described in RFCs 882, 883 and 973.  Distribution of this memo   is unlimited.Introduction   The purpose of this memo is to explain how mailers are to decide how   to route a message addressed to a given Internet domain name.  This   involves a discussion of how mailers interpret MX RRs, which are used   for message routing.  Note that this memo makes no statement about   how mailers are to deal with MB and MG RRs, which are used for   interpreting mailbox names.   UnderRFC-882 andRFC-883 certain assumptions about mail addresses   have been changed.  Up to now, one could usually assume that if a   message was addressed to a mailbox, for example, at LOKI.BBN.COM,   that one could just open an SMTP connection to LOKI.BBN.COM and pass   the message along.  This system broke down in certain situations,   such as for certain UUCP and CSNET hosts which were not directly   attached to the Internet, but these hosts could be handled as special   cases in configuration files (for example, most mailers were set up   to automatically forward mail addressed to a CSNET host to   CSNET-RELAY.ARPA).   Under domains, one cannot simply open a connection to LOKI.BBN.COM,   but must instead ask the domain system where messages to LOKI.BBN.COM   are to be delivered. And the domain system may direct a mailer to   deliver messages to an entirely different host, such as SH.CS.NET.   Or, in a more complicated case, the mailer may learn that it has a   choice of routes to LOKI.BBN.COM.  This memo is essentially a set of   guidelines on how mailers should behave in this more complex world.   Readers are expected to be familiar with RFCs 882, 883, and the   updates to them (e.g.,RFC-973).Partridge                                                       [Page 1]

RFC 974                                                     January 1986Mail Routing and the Domain SystemWhat the Domain Servers Know   The domain servers store information as a series of resource records   (RRs), each of which contains a particular piece of information about   a given domain name (which is usually, but not always, a host).  The   simplest way to think of a RR is as a typed pair of datum, a domain   name matched with relevant data, and stored with some additional type   information to help systems determine when the RR is relevant.  For   the purposes of message routing, the system stores RRs known as MX   RRs. Each MX matches a domain name with two pieces of data, a   preference value (an unsigned 16-bit integer), and the name of a   host.  The preference number is used to indicate in what order the   mailer should attempt deliver to the MX hosts, with the lowest   numbered MX being the one to try first.  Multiple MXs with the same   preference are permitted and have the same priority.   In addition to mail information, the servers store certain other   types of RR's which mailers may encounter or choose to use.  These   are: the canonical name (CNAME) RR, which simply states that the   domain name queried for is actually an alias for another domain name,   which is the proper, or canonical, name; and the Well Known Service   (WKS) RR, which stores information about network services (such as   SMTP) a given domain name supports.General Routing Guidelines   Before delving into a detailed discussion of how mailers are expected   to do mail routing, it would seem to make sense to give a brief   overview of how this memo is approaching the problems that routing   poses.   The first major principle is derived from the definition of the   preference field in MX records, and is intended to prevent mail   looping.  If the mailer is on a host which is listed as an MX for the   destination host, the mailer may only deliver to an MX which has a   lower preference count than its own host.   It is also possible to cause mail looping because routing information   is out of date or incomplete.  Out of date information is only a   problem when domain tables are changed.  The changes will not be   known to all affected hosts until their resolver caches time out.   There is no way to ensure that this will not happen short of   requiring mailers and their resolvers to always send their queries to   an authoritative server, and never use data stored in a cache.  This   is an impractical solution, since eliminating resolver caching would   make mailing inordinately expensive.  What is more, the out-of-date   RR problem should not happen if, when a domain table is changed,Partridge                                                       [Page 2]

RFC 974                                                     January 1986Mail Routing and the Domain System   affected hosts (those in the list of MXs) have their resolver caches   flushed. In other words, given proper precautions, mail looping as a   result of domain information should be avoidable, without requiring   mailers to query authoritative servers.  (The appropriate precaution   is to check with a host's administrator before adding that host to a   list of MXs).   The incomplete data problem also requires some care when handling   domain queries.  If the answer section of a query is incomplete   critical MX RRs may be left out.  This may result in mail looping, or   in a message being mistakenly labelled undeliverable.  As a result,   mailers may only accept responses from the domain system which have   complete answer sections.  Note that this entire problem can be   avoided by only using virtual circuits for queries, but since this   situation is likely to be very rare and datagrams are the preferred   way to interact with the domain system, implementors should probably   just ensure that their mailer will repeat a query with virtual   circuits should the truncation bit ever be set.Determining Where to Send a Message   The explanation of how mailers should decide how to route a message   is discussed in terms of the problem of a mailer on a host with   domain name LOCAL trying to deliver a message addressed to the domain   name REMOTE. Both LOCAL and REMOTE are assumed to be syntactically   correct domain names.  Furthermore, LOCAL is assumed to be the   official name for the host on which the mailer resides (i.e., it is   not a alias).Issuing a Query   The first step for the mailer at LOCAL is to issue a query for MX RRs   for REMOTE.  It is strongly urged that this step be taken every time   a mailer attempts to send the message.  The hope is that changes in   the domain database will rapidly be used by mailers, and thus domain   administrators will be able to re-route in-transit messages for   defective hosts by simply changing their domain databases.   Certain responses to the query are considered errors:      Getting no response to the query.  The domain server the mailer      queried never sends anything back.  (This is distinct from an      answer which contains no answers to the query, which is not an      error).      Getting a response in which the truncation field of the header isPartridge                                                       [Page 3]

RFC 974                                                     January 1986Mail Routing and the Domain System      set.  (Recall discussion of incomplete queries above).  Mailers      may not use responses of this type, and should repeat the query      using virtual circuits instead of datagrams.      Getting a response in which the response code is non-zero.   Mailers are expected to do something reasonable in the face of an   error.  The behaviour for each type of error is not specified here,   but implementors should note that different types of errors should   probably be treated differently.  For example, a response code of   "non-existent domain" should probably cause the message to be   returned to the sender as invalid, while a response code of "server   failure" should probably cause the message to be retried later.   There is one other special case.  If the response contains an answer   which is a CNAME RR, it indicates that REMOTE is actually an alias   for some other domain name. The query should be repeated with the   canonical domain name.   If the response does not contain an error response, and does not   contain aliases, its answer section should be a (possibly zero   length) list of MX RRs for domain name REMOTE (or REMOTE's true   domain name if REMOTE was a alias).  The next section describes how   this list is interpreted.Interpreting the List of MX RRs   NOTE: This section only discusses how mailers choose which names to   try to deliver a message to, working from a list of RR's.  It does   not discuss how the mailers actually make delivery.  Where ever   delivering a message is mentioned, all that is meant is that the   mailer should do whatever it needs to do to transfer a message to a   remote site, given a domain name for that site.  (For example, an   SMTP mailer will try to get an address for the domain name, which   involves another query to the domain system, and then, if it gets an   address, connect to the SMTP TCP port).  The mechanics of actually   transferring the message over the network to the address associated   with a given domain name is not within the scope of this memo.   It is possible that the list of MXs in the response to the query will   be empty.  This is a special case.  If the list is empty, mailers   should treat it as if it contained one RR, an MX RR with a preference   value of 0, and a host name of REMOTE.  (I.e., REMOTE is its only   MX).  In addition, the mailer should do no further processing on the   list, but should attempt to deliver the message to REMOTE.  The ideaPartridge                                                       [Page 4]

RFC 974                                                     January 1986Mail Routing and the Domain System   here is that if a domain fails to advertise any information about a   particular name we will give it the benefit of the doubt and attempt   delivery.   If the list is not empty, the mailer should remove irrelevant RR's   from the list according to the following steps.  Note that the order   is significant.      For each MX, a WKS query should be issued to see if the domain      name listed actually supports the mail service desired.  MX RRs      which list domain names which do not support the service should be      discarded.  This step is optional, but strongly encouraged.      If the domain name LOCAL is listed as an MX RR, all MX RRs with a      preference value greater than or equal to that of LOCAL's must be      discarded.   After removing irrelevant RRs, the list can again be empty.  This is   now an error condition and can occur in several ways.  The simplest   case is that the WKS queries have discovered that none of the hosts   listed supports the mail service desired.  The message is thus deemed   undeliverable, though extremely persistent mail systems might want to   try a delivery to REMOTE's address (if it exists) before returning   the message. Another, more dangerous, possibility is that the domain   system believes that LOCAL is handling message for REMOTE, but the   mailer on LOCAL is not set up to handle mail for REMOTE.  For   example, if the domain system lists LOCAL as the only MX for REMOTE,   LOCAL will delete all the entries in the list.  But LOCAL is   presumably querying the domain system because it didn't know what to   do with a message addressed to REMOTE. Clearly something is wrong.   How a mailer chooses to handle these situations is to some extent   implementation dependent, and is thus left to the implementor's   discretion.   If the list of MX RRs is not empty, the mailer should try to deliver   the message to the MXs in order (lowest preference value tried   first).  The mailer is required to attempt delivery to the lowest   valued MX.  Implementors are encouraged to write mailers so that they   try the MXs in order until one of the MXs accepts the message, or all   the MXs have been tried.  A somewhat less demanding system, in which   a fixed number of MXs is tried, is also reasonable.  Note that   multiple MXs may have the same preference value.  In this case, all   MXs at with a given value must be tried before any of a higher value   are tried.  In addition, in the special case in which there are   several MXs with the lowest preference value,  all of them should be   tried before a message is deemed undeliverable.Partridge                                                       [Page 5]

RFC 974                                                     January 1986Mail Routing and the Domain SystemMinor Special Issues   There are a couple of special issues left out of the preceding   section because they complicated the discussion.  They are treated   here in no particular order.   Wildcard names, those containing the character '*' in them, may be   used for mail routing.  There are likely to be servers on the network   which simply state that any mail to a domain is to be routed through   a relay. For example, at the time that this RFC is being written, all   mail to hosts in the domain IL is routed through RELAY.CS.NET.  This   is done by creating a wildcard RR, which states that *.IL has an MX   of RELAY.CS.NET.  This should be transparent to the mailer since the   domain servers will hide this wildcard match. (If it matches *.IL   with HUJI.IL for example, a domain server will return an RR   containing HUJI.IL, not *.IL). If by some accident a mailer receives   an RR with a wildcard domain name in its name or data section it   should discard the RR.   Note that the algorithm to delete irrelevant RRs breaks if LOCAL has   a alias and the alias is listed in the MX records for REMOTE.  (E.g.   REMOTE has an MX of ALIAS, where ALIAS has a CNAME of LOCAL).  This   can be avoided if aliases are never used in the data section of MX   RRs.   Implementors should understand that the query and interpretation of   the query is only performed for REMOTE.  It is not repeated for the   MX RRs listed for REMOTE.  You cannot try to support more extravagant   mail routing by building a chain of MXs.  (E.g. UNIX.BBN.COM is an MX   for RELAY.CS.NET and RELAY.CS.NET is an MX for all the hosts in .IL,   but this does not mean that UNIX.BBN.COM accepts any responsibility   for mail for .IL).   Finally, it should be noted that this is a standard for routing on   the Internet.  Mailers serving hosts which lie on multiple networks   will presumably have to make some decisions about which network to   route through. This decision making is outside the scope of this   memo, although mailers may well use the domain system to help them   decide.  However, once a mailer decides to deliver a message via the   Internet it must apply these rules to route the message.Partridge                                                       [Page 6]

RFC 974                                                     January 1986Mail Routing and the Domain SystemExamples   To illustrate the discussion above, here are three examples of how   mailers should route messages.  All examples work with the following   database:      A.EXAMPLE.ORG    IN    MX    10    A.EXAMPLE.ORG      A.EXAMPLE.ORG    IN    MX    15    B.EXAMPLE.ORG      A.EXAMPLE.ORG    IN    MX    20    C.EXAMPLE.ORG      A.EXAMPLE.ORG    IN    WKS   10.0.0.1    TCP    SMTP      B.EXAMPLE.ORG    IN    MX    0      B.EXAMPLE.ORG      B.EXAMPLE.ORG    IN    MX    10     C.EXAMPLE.ORG      B.EXAMPLE.ORG    IN    WKS   10.0.0.2    TCP    SMTP      C.EXAMPLE.ORG    IN    MX    0     C.EXAMPLE.ORG      C.EXAMPLE.ORG    IN    WKS   10.0.0.3    TCP    SMTP      D.EXAMPLE.ORG    IN    MX    0     D.EXAMPLE.ORG      D.EXAMPLE.ORG    IN    MX    0     C.EXAMPLE.ORG      D.EXAMPLE.ORG    IN    WKS   10.0.0.4    TCP    SMTP   In the first example, an SMTP mailer on D.EXAMPLE.ORG is trying to   deliver a message addressed to A.EXAMPLE.ORG. From the answer to its   query, it learns that A.EXAMPLE.ORG has three MX RRs.  D.EXAMPLE.ORG   is not one of the MX RRs and all three MXs support SMTP mail   (determined from the WKS entries), so none of the MXs are eliminated.   The mailer is obliged to try to deliver to A.EXAMPLE.ORG as the   lowest valued MX.  If it cannot reach A.EXAMPLE.ORG it can (but is   not required to) try B.EXAMPLE.ORG. and if B.EXAMPLE.ORG is not   responding, it can try C.EXAMPLE.ORG.   In the second example, the mailer is on B.EXAMPLE.ORG, and is again   trying to deliver a message addressed to A.EXAMPLE.ORG.  There are   once again three MX RRs for A.EXAMPLE.ORG, but in this case the   mailer must discard the RRs for itself and C.EXAMPLE.ORG (because the   MX RR for C.EXAMPLE.ORG has a higher preference value than the RR for   B.EXAMPLE.ORG).  It is left only with the RR for A.EXAMPLE.ORG, and   can only try delivery to A.EXAMPLE.ORG.   In the third example, consider a mailer on A.EXAMPLE.ORG trying to   deliver a message to D.EXAMPLE.ORG.  In this case there are only two   MX RRs, both with the same preference value.  Either MX will accept   messages for D.EXAMPLE.ORG. The mailer should try one MX first (which   one is up to the mailer, though D.EXAMPLE.ORG seems most reasonable),   and if that delivery fails should try the other MX (e.g.   C.EXAMPLE.ORG).Partridge                                                       [Page 7]