Network Working Group Craig Partridge
Request for Comments: 974 CSNET CIC BBN Laboratories Inc
January 1986
MAIL ROUTING AND THE DOMAIN SYSTEM
Status 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.
Under RFC-882 and RFC-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).
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Mail Routing and the Domain System
What 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,
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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 is
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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 idea
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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.
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Minor 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.
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Examples
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).
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RFC, FYI, BCP