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RFC1123 MAIL -- SMTP & RFC-822 October 1989
CNAME.
5.2.3 VRFY and EXPN Commands: RFC-821 Section 3.3
A receiver-SMTP MUST implement VRFY and SHOULD implement EXPN
(this requirement overrides RFC-821). However, there MAY be
configuration information to disable VRFY and EXPN in a
particular installation; this might even allow EXPN to be
disabled for selected lists.
A new reply code is defined for the VRFY command:
252 Cannot VRFY user (e.g., info is not local), but will
take message for this user and attempt delivery.
DISCUSSION:
SMTP users and administrators make regular use of these
commands for diagnosing mail delivery problems. With the
increasing use of multi-level mailing list expansion
(sometimes more than two levels), EXPN has been
increasingly important for diagnosing inadvertent mail
loops. On the other hand, some feel that EXPN represents
a significant privacy, and perhaps even a security,
exposure.
5.2.4 SEND, SOML, and SAML Commands: RFC-821 Section 3.4
An SMTP MAY implement the commands to send a message to a
user's terminal: SEND, SOML, and SAML.
DISCUSSION:
It has been suggested that the use of mail relaying
through an MX record is inconsistent with the intent of
SEND to deliver a message immediately and directly to a
user's terminal. However, an SMTP receiver that is unable
to write directly to the user terminal can return a "251
User Not Local" reply to the RCPT following a SEND, to
inform the originator of possibly deferred delivery.
5.2.5 HELO Command: RFC-821 Section 3.5
The sender-SMTP MUST ensure that the <domain> parameter in a
HELO command is a valid principal host domain name for the
client host. As a result, the receiver-SMTP will not have to
perform MX resolution on this name in order to validate the
HELO parameter.
The HELO receiver MAY verify that the HELO parameter really
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corresponds to the IP address of the sender. However, the
receiver MUST NOT refuse to accept a message, even if the
sender's HELO command fails verification.
DISCUSSION:
Verifying the HELO parameter requires a domain name lookup
and may therefore take considerable time. An alternative
tool for tracking bogus mail sources is suggested below
(see "DATA Command").
Note also that the HELO argument is still required to have
valid <domain> syntax, since it will appear in a Received:
line; otherwise, a 501 error is to be sent.
IMPLEMENTATION:
When HELO parameter validation fails, a suggested
procedure is to insert a note about the unknown
authenticity of the sender into the message header (e.g.,
in the "Received:" line).
5.2.6 Mail Relay: RFC-821 Section 3.6
We distinguish three types of mail (store-and-) forwarding:
(1) A simple forwarder or "mail exchanger" forwards a message
using private knowledge about the recipient; see section
3.2 of RFC-821.
(2) An SMTP mail "relay" forwards a message within an SMTP
mail environment as the result of an explicit source route
(as defined in section 3.6 of RFC-821). The SMTP relay
function uses the "@...:" form of source route from RFC-
822 (see Section 5.2.19 below).
(3) A mail "gateway" passes a message between different
environments. The rules for mail gateways are discussed
below in Section 5.3.7.
An Internet host that is forwarding a message but is not a
gateway to a different mail environment (i.e., it falls under
(1) or (2)) SHOULD NOT alter any existing header fields,
although the host will add an appropriate Received: line as
required in Section 5.2.8.
A Sender-SMTP SHOULD NOT send a RCPT TO: command containing an
explicit source route using the "@...:" address form. Thus,
the relay function defined in section 3.6 of RFC-821 should
not be used.
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DISCUSSION:
The intent is to discourage all source routing and to
abolish explicit source routing for mail delivery within
the Internet environment. Source-routing is unnecessary;
the simple target address "user@domain" should always
suffice. This is the result of an explicit architectural
decision to use universal naming rather than source
routing for mail. Thus, SMTP provides end-to-end
connectivity, and the DNS provides globally-unique,
location-independent names. MX records handle the major
case where source routing might otherwise be needed.
A receiver-SMTP MUST accept the explicit source route syntax in
the envelope, but it MAY implement the relay function as
defined in section 3.6 of RFC-821. If it does not implement
the relay function, it SHOULD attempt to deliver the message
directly to the host to the right of the right-most "@" sign.
DISCUSSION:
For example, suppose a host that does not implement the
relay function receives a message with the SMTP command:
"RCPT TO:<@ALPHA,@BETA:joe@GAMMA>", where ALPHA, BETA, and
GAMMA represent domain names. Rather than immediately
refusing the message with a 550 error reply as suggested
on page 20 of RFC-821, the host should try to forward the
message to GAMMA directly, using: "RCPT TO:<joe@GAMMA>".
Since this host does not support relaying, it is not
required to update the reverse path.
Some have suggested that source routing may be needed
occasionally for manually routing mail around failures;
however, the reality and importance of this need is
controversial. The use of explicit SMTP mail relaying for
this purpose is discouraged, and in fact it may not be
successful, as many host systems do not support it. Some
have used the "%-hack" (see Section 5.2.16) for this
purpose.
5.2.7 RCPT Command: RFC-821 Section 4.1.1
A host that supports a receiver-SMTP MUST support the reserved
mailbox "Postmaster".
The receiver-SMTP MAY verify RCPT parameters as they arrive;
however, RCPT responses MUST NOT be delayed beyond a reasonable
time (see Section 5.3.2).
Therefore, a "250 OK" response to a RCPT does not necessarily
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imply that the delivery address(es) are valid. Errors found
after message acceptance will be reported by mailing a
notification message to an appropriate address (see Section
5.3.3).
DISCUSSION:
The set of conditions under which a RCPT parameter can be
validated immediately is an engineering design choice.
Reporting destination mailbox errors to the Sender-SMTP
before mail is transferred is generally desirable to save
time and network bandwidth, but this advantage is lost if
RCPT verification is lengthy.
For example, the receiver can verify immediately any
simple local reference, such as a single locally-
registered mailbox. On the other hand, the "reasonable
time" limitation generally implies deferring verification
of a mailing list until after the message has been
transferred and accepted, since verifying a large mailing
list can take a very long time. An implementation might
or might not choose to defer validation of addresses that
are non-local and therefore require a DNS lookup. If a
DNS lookup is performed but a soft domain system error
(e.g., timeout) occurs, validity must be assumed.
5.2.8 DATA Command: RFC-821 Section 4.1.1
Every receiver-SMTP (not just one that "accepts a message for
relaying or for final delivery" [SMTP:1]) MUST insert a
"Received:" line at the beginning of a message. In this line,
called a "time stamp line" in RFC-821:
* The FROM field SHOULD contain both (1) the name of the
source host as presented in the HELO command and (2) a
domain literal containing the IP address of the source,
determined from the TCP connection.
* The ID field MAY contain an "@" as suggested in RFC-822,
but this is not required.
* The FOR field MAY contain a list of <path> entries when
multiple RCPT commands have been given.
An Internet mail program MUST NOT change a Received: line that
was previously added to the message header.
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DISCUSSION:
Including both the source host and the IP source address
in the Received: line may provide enough information for
tracking illicit mail sources and eliminate a need to
explicitly verify the HELO parameter.
Received: lines are primarily intended for humans tracing
mail routes, primarily of diagnosis of faults. See also
the discussion under 5.3.7.
When the receiver-SMTP makes "final delivery" of a message,
then it MUST pass the MAIL FROM: address from the SMTP envelope
with the message, for use if an error notification message must
be sent later (see Section 5.3.3). There is an analogous
requirement when gatewaying from the Internet into a different
mail environment; see Section 5.3.7.
DISCUSSION:
Note that the final reply to the DATA command depends only
upon the successful transfer and storage of the message.
Any problem with the destination address(es) must either
(1) have been reported in an SMTP error reply to the RCPT
command(s), or (2) be reported in a later error message
mailed to the originator.
IMPLEMENTATION:
The MAIL FROM: information may be passed as a parameter or
in a Return-Path: line inserted at the beginning of the
message.
5.2.9 Command Syntax: RFC-821 Section 4.1.2
The syntax shown in RFC-821 for the MAIL FROM: command omits
the case of an empty path: "MAIL FROM: <>" (see RFC-821 Page
15). An empty reverse path MUST be supported.
5.2.10 SMTP Replies: RFC-821 Section 4.2
A receiver-SMTP SHOULD send only the reply codes listed in
section 4.2.2 of RFC-821 or in this document. A receiver-SMTP
SHOULD use the text shown in examples in RFC-821 whenever
appropriate.
A sender-SMTP MUST determine its actions only by the reply
code, not by the text (except for 251 and 551 replies); any
text, including no text at all, must be acceptable. The space
(blank) following the reply code is considered part of the
text. Whenever possible, a sender-SMTP SHOULD test only the
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first digit of the reply code, as specified in Appendix E of
RFC-821.
DISCUSSION:
Interoperability problems have arisen with SMTP systems
using reply codes that are not listed explicitly in RFC-
821 Section 4.3 but are legal according to the theory of
reply codes explained in Appendix E.
5.2.11 Transparency: RFC-821 Section 4.5.2
Implementors MUST be sure that their mail systems always add
and delete periods to ensure message transparency.
5.2.12 WKS Use in MX Processing: RFC-974, p. 5
RFC-974 [SMTP:3] recommended that the domain system be queried
for WKS ("Well-Known Service") records, to verify that each
proposed mail target does support SMTP. Later experience has
shown that WKS is not widely supported, so the WKS step in MX
processing SHOULD NOT be used.
The following are notes on RFC-822, organized by section of that
document.
5.2.13 RFC-822 Message Specification: RFC-822 Section 4
The syntax shown for the Return-path line omits the possibility
of a null return path, which is used to prevent looping of
error notifications (see Section 5.3.3). The complete syntax
is:
return = "Return-path" ":" route-addr
/ "Return-path" ":" "<" ">"
The set of optional header fields is hereby expanded to include
the Content-Type field defined in RFC-1049 [SMTP:7]. This
field "allows mail reading systems to automatically identify
the type of a structured message body and to process it for
display accordingly". [SMTP:7] A User Agent MAY support this
field.
5.2.14 RFC-822 Date and Time Specification: RFC-822 Section 5
The syntax for the date is hereby changed to:
date = 1*2DIGIT month 2*4DIGIT
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All mail software SHOULD use 4-digit years in dates, to ease
the transition to the next century.
There is a strong trend towards the use of numeric timezone
indicators, and implementations SHOULD use numeric timezones
instead of timezone names. However, all implementations MUST
accept either notation. If timezone names are used, they MUST
be exactly as defined in RFC-822.
The military time zones are specified incorrectly in RFC-822:
they count the wrong way from UT (the signs are reversed). As
a result, military time zones in RFC-822 headers carry no
information.
Finally, note that there is a typo in the definition of "zone"
in the syntax summary of appendix D; the correct definition
occurs in Section 3 of RFC-822.
5.2.15 RFC-822 Syntax Change: RFC-822 Section 6.1
The syntactic definition of "mailbox" in RFC-822 is hereby
changed to:
mailbox = addr-spec ; simple address
/ [phrase] route-addr ; name & addr-spec
That is, the phrase preceding a route address is now OPTIONAL.
This change makes the following header field legal, for
example:
From: <craig@nnsc.nsf.net>
5.2.16 RFC-822 Local-part: RFC-822 Section 6.2
The basic mailbox address specification has the form: "local-
part@domain". Here "local-part", sometimes called the "left-
hand side" of the address, is domain-dependent.
A host that is forwarding the message but is not the
destination host implied by the right-hand side "domain" MUST
NOT interpret or modify the "local-part" of the address.
When mail is to be gatewayed from the Internet mail environment
into a foreign mail environment (see Section 5.3.7), routing
information for that foreign environment MAY be embedded within
the "local-part" of the address. The gateway will then
interpret this local part appropriately for the foreign mail
environment.
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DISCUSSION:
Although source routes are discouraged within the Internet
(see Section 5.2.6), there are non-Internet mail
environments whose delivery mechanisms do depend upon
source routes. Source routes for extra-Internet
environments can generally be buried in the "local-part"
of the address (see Section 5.2.16) while mail traverses
the Internet. When the mail reaches the appropriate
Internet mail gateway, the gateway will interpret the
local-part and build the necessary address or route for
the target mail environment.
For example, an Internet host might send mail to:
"a!b!c!user@gateway-domain". The complex local part
"a!b!c!user" would be uninterpreted within the Internet
domain, but could be parsed and understood by the
specified mail gateway.
An embedded source route is sometimes encoded in the
"local-part" using "%" as a right-binding routing
operator. For example, in:
user%domain%relay3%relay2@relay1
the "%" convention implies that the mail is to be routed
from "relay1" through "relay2", "relay3", and finally to
"user" at "domain". This is commonly known as the "%-
hack". It is suggested that "%" have lower precedence
than any other routing operator (e.g., "!") hidden in the
local-part; for example, "a!b%c" would be interpreted as
"(a!b)%c".
Only the target host (in this case, "relay1") is permitted
to analyze the local-part "user%domain%relay3%relay2".
5.2.17 Domain Literals: RFC-822 Section 6.2.3
A mailer MUST be able to accept and parse an Internet domain
literal whose content ("dtext"; see RFC-822) is a dotted-
decimal host address. This satisfies the requirement of
Section 2.1 for the case of mail.
An SMTP MUST accept and recognize a domain literal for any of
its own IP addresses.
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5.2.18 Common Address Formatting Errors: RFC-822 Section 6.1
Errors in formatting or parsing 822 addresses are unfortunately
common. This section mentions only the most common errors. A
User Agent MUST accept all valid RFC-822 address formats, and
MUST NOT generate illegal address syntax.
o A common error is to leave out the semicolon after a group
identifier.
o Some systems fail to fully-qualify domain names in
messages they generate. The right-hand side of an "@"
sign in a header address field MUST be a fully-qualified
domain name.
For example, some systems fail to fully-qualify the From:
address; this prevents a "reply" command in the user
interface from automatically constructing a return
address.
DISCUSSION:
Although RFC-822 allows the local use of abbreviated
domain names within a domain, the application of
RFC-822 in Internet mail does not allow this. The
intent is that an Internet host must not send an SMTP
message header containing an abbreviated domain name
in an address field. This allows the address fields
of the header to be passed without alteration across
the Internet, as required in Section 5.2.6.
o Some systems mis-parse multiple-hop explicit source routes
such as:
@relay1,@relay2,@relay3:user@domain.
o Some systems over-qualify domain names by adding a
trailing dot to some or all domain names in addresses or
message-ids. This violates RFC-822 syntax.
5.2.19 Explicit Source Routes: RFC-822 Section 6.2.7
Internet host software SHOULD NOT create an RFC-822 header
containing an address with an explicit source route, but MUST
accept such headers for compatibility with earlier systems.
DISCUSSION:
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In an understatement, RFC-822 says "The use of explicit
source routing is discouraged". Many hosts implemented
RFC-822 source routes incorrectly, so the syntax cannot be
used unambiguously in practice. Many users feel the
syntax is ugly. Explicit source routes are not needed in
the mail envelope for delivery; see Section 5.2.6. For
all these reasons, explicit source routes using the RFC-
822 notations are not to be used in Internet mail headers.
As stated in Section 5.2.16, it is necessary to allow an
explicit source route to be buried in the local-part of an
address, e.g., using the "%-hack", in order to allow mail
to be gatewayed into another environment in which explicit
source routing is necessary. The vigilant will observe
that there is no way for a User Agent to detect and
prevent the use of such implicit source routing when the
destination is within the Internet. We can only
discourage source routing of any kind within the Internet,
as unnecessary and undesirable.
5.3 SPECIFIC ISSUES
5.3.1 SMTP Queueing Strategies
The common structure of a host SMTP implementation includes
user mailboxes, one or more areas for queueing messages in
transit, and one or more daemon processes for sending and
receiving mail. The exact structure will vary depending on the
needs of the users on the host and the number and size of
mailing lists supported by the host. We describe several
optimizations that have proved helpful, particularly for
mailers supporting high traffic levels.
Any queueing strategy MUST include:
o Timeouts on all activities. See Section 5.3.2.
o Never sending error messages in response to error
messages.
5.3.1.1 Sending Strategy
The general model of a sender-SMTP is one or more processes
that periodically attempt to transmit outgoing mail. In a
typical system, the program that composes a message has some
method for requesting immediate attention for a new piece of
outgoing mail, while mail that cannot be transmitted
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immediately MUST be queued and periodically retried by the
sender. A mail queue entry will include not only the
message itself but also the envelope information.
The sender MUST delay retrying a particular destination
after one attempt has failed. In general, the retry
interval SHOULD be at least 30 minutes; however, more
sophisticated and variable strategies will be beneficial
when the sender-SMTP can determine the reason for non-
delivery.
Retries continue until the message is transmitted or the
sender gives up; the give-up time generally needs to be at
least 4-5 days. The parameters to the retry algorithm MUST
be configurable.
A sender SHOULD keep a list of hosts it cannot reach and
corresponding timeouts, rather than just retrying queued
mail items.
DISCUSSION:
Experience suggests that failures are typically
transient (the target system has crashed), favoring a
policy of two connection attempts in the first hour the
message is in the queue, and then backing off to once
every two or three hours.
The sender-SMTP can shorten the queueing delay by
cooperation with the receiver-SMTP. In particular, if
mail is received from a particular address, it is good
evidence that any mail queued for that host can now be
sent.
The strategy may be further modified as a result of
multiple addresses per host (see Section 5.3.4), to
optimize delivery time vs. resource usage.
A sender-SMTP may have a large queue of messages for
each unavailable destination host, and if it retried
all these messages in every retry cycle, there would be
excessive Internet overhead and the daemon would be
blocked for a long period. Note that an SMTP can
generally determine that a delivery attempt has failed
only after a timeout of a minute or more; a one minute
timeout per connection will result in a very large
delay if it is repeated for dozens or even hundreds of
queued messages.
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When the same message is to be delivered to several users on
the same host, only one copy of the message SHOULD be
transmitted. That is, the sender-SMTP should use the
command sequence: RCPT, RCPT,... RCPT, DATA instead of the
sequence: RCPT, DATA, RCPT, DATA,... RCPT, DATA.
Implementation of this efficiency feature is strongly urged.
Similarly, the sender-SMTP MAY support multiple concurrent
outgoing mail transactions to achieve timely delivery.
However, some limit SHOULD be imposed to protect the host
from devoting all its resources to mail.
The use of the different addresses of a multihomed host is
discussed below.
5.3.1.2 Receiving strategy
The receiver-SMTP SHOULD attempt to keep a pending listen on
the SMTP port at all times. This will require the support
of multiple incoming TCP connections for SMTP. Some limit
MAY be imposed.
IMPLEMENTATION:
When the receiver-SMTP receives mail from a particular
host address, it could notify the sender-SMTP to retry
any mail pending for that host address.
5.3.2 Timeouts in SMTP
There are two approaches to timeouts in the sender-SMTP: (a)
limit the time for each SMTP command separately, or (b) limit
the time for the entire SMTP dialogue for a single mail
message. A sender-SMTP SHOULD use option (a), per-command
timeouts. Timeouts SHOULD be easily reconfigurable, preferably
without recompiling the SMTP code.
DISCUSSION:
Timeouts are an essential feature of an SMTP
implementation. If the timeouts are too long (or worse,
there are no timeouts), Internet communication failures or
software bugs in receiver-SMTP programs can tie up SMTP
processes indefinitely. If the timeouts are too short,
resources will be wasted with attempts that time out part
way through message delivery.
If option (b) is used, the timeout has to be very large,
e.g., an hour, to allow time to expand very large mailing
lists. The timeout may also need to increase linearly
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with the size of the message, to account for the time to
transmit a very large message. A large fixed timeout
leads to two problems: a failure can still tie up the
sender for a very long time, and very large messages may
still spuriously time out (which is a wasteful failure!).
Using the recommended option (a), a timer is set for each
SMTP command and for each buffer of the data transfer.
The latter means that the overall timeout is inherently
proportional to the size of the message.
Based on extensive experience with busy mail-relay hosts, the
minimum per-command timeout values SHOULD be as follows:
o Initial 220 Message: 5 minutes
A Sender-SMTP process needs to distinguish between a
failed TCP connection and a delay in receiving the initial
220 greeting message. Many receiver-SMTPs will accept a
TCP connection but delay delivery of the 220 message until
their system load will permit more mail to be processed.
o MAIL Command: 5 minutes
o RCPT Command: 5 minutes
A longer timeout would be required if processing of
mailing lists and aliases were not deferred until after
the message was accepted.
o DATA Initiation: 2 minutes
This is while awaiting the "354 Start Input" reply to a
DATA command.
o Data Block: 3 minutes
This is while awaiting the completion of each TCP SEND
call transmitting a chunk of data.
o DATA Termination: 10 minutes.
This is while awaiting the "250 OK" reply. When the
receiver gets the final period terminating the message
data, it typically performs processing to deliver the
message to a user mailbox. A spurious timeout at this
point would be very wasteful, since the message has been
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successfully sent.
A receiver-SMTP SHOULD have a timeout of at least 5 minutes
while it is awaiting the next command from the sender.
5.3.3 Reliable Mail Receipt
When the receiver-SMTP accepts a piece of mail (by sending a
"250 OK" message in response to DATA), it is accepting
responsibility for delivering or relaying the message. It must
take this responsibility seriously, i.e., it MUST NOT lose the
message for frivolous reasons, e.g., because the host later
crashes or because of a predictable resource shortage.
If there is a delivery failure after acceptance of a message,
the receiver-SMTP MUST formulate and mail a notification
message. This notification MUST be sent using a null ("<>")
reverse path in the envelope; see Section 3.6 of RFC-821. The
recipient of this notification SHOULD be the address from the
envelope return path (or the Return-Path: line). However, if
this address is null ("<>"), the receiver-SMTP MUST NOT send a
notification. If the address is an explicit source route, it
SHOULD be stripped down to its final hop.
DISCUSSION:
For example, suppose that an error notification must be
sent for a message that arrived with:
"MAIL FROM:<@a,@b:user@d>". The notification message
should be sent to: "RCPT TO:<user@d>".
Some delivery failures after the message is accepted by
SMTP will be unavoidable. For example, it may be
impossible for the receiver-SMTP to validate all the
delivery addresses in RCPT command(s) due to a "soft"
domain system error or because the target is a mailing
list (see earlier discussion of RCPT).
To avoid receiving duplicate messages as the result of
timeouts, a receiver-SMTP MUST seek to minimize the time
required to respond to the final "." that ends a message
transfer. See RFC-1047 [SMTP:4] for a discussion of this
problem.
5.3.4 Reliable Mail Transmission
To transmit a message, a sender-SMTP determines the IP address
of the target host from the destination address in the
envelope. Specifically, it maps the string to the right of the
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"@" sign into an IP address. This mapping or the transfer
itself may fail with a soft error, in which case the sender-
SMTP will requeue the outgoing mail for a later retry, as
required in Section 5.3.1.1.
When it succeeds, the mapping can result in a list of
alternative delivery addresses rather than a single address,
because of (a) multiple MX records, (b) multihoming, or both.
To provide reliable mail transmission, the sender-SMTP MUST be
able to try (and retry) each of the addresses in this list in
order, until a delivery attempt succeeds. However, there MAY
also be a configurable limit on the number of alternate
addresses that can be tried. In any case, a host SHOULD try at
least two addresses.
The following information is to be used to rank the host
addresses:
(1) Multiple MX Records -- these contain a preference
indication that should be used in sorting. If there are
multiple destinations with the same preference and there
is no clear reason to favor one (e.g., by address
preference), then the sender-SMTP SHOULD pick one at
random to spread the load across multiple mail exchanges
for a specific organization; note that this is a
refinement of the procedure in [DNS:3].
(2) Multihomed host -- The destination host (perhaps taken
from the preferred MX record) may be multihomed, in which
case the domain name resolver will return a list of
alternative IP addresses. It is the responsibility of the
domain name resolver interface (see Section 6.1.3.4 below)
to have ordered this list by decreasing preference, and
SMTP MUST try them in the order presented.
DISCUSSION:
Although the capability to try multiple alternative
addresses is required, there may be circumstances where
specific installations want to limit or disable the use of
alternative addresses. The question of whether a sender
should attempt retries using the different addresses of a
multihomed host has been controversial. The main argument
for using the multiple addresses is that it maximizes the
probability of timely delivery, and indeed sometimes the
probability of any delivery; the counter argument is that
it may result in unnecessary resource use.
Note that resource use is also strongly determined by the
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sending strategy discussed in Section 5.3.1.
5.3.5 Domain Name Support
SMTP implementations MUST use the mechanism defined in Section
6.1 for mapping between domain names and IP addresses. This
means that every Internet SMTP MUST include support for the
Internet DNS.
In particular, a sender-SMTP MUST support the MX record scheme
[SMTP:3]. See also Section 7.4 of [DNS:2] for information on
domain name support for SMTP.
5.3.6 Mailing Lists and Aliases
An SMTP-capable host SHOULD support both the alias and the list
form of address expansion for multiple delivery. When a
message is delivered or forwarded to each address of an
expanded list form, the return address in the envelope
("MAIL FROM:") MUST be changed to be the address of a person
who administers the list, but the message header MUST be left
unchanged; in particular, the "From" field of the message is
unaffected.
DISCUSSION:
An important mail facility is a mechanism for multi-
destination delivery of a single message, by transforming
or "expanding" a pseudo-mailbox address into a list of
destination mailbox addresses. When a message is sent to
such a pseudo-mailbox (sometimes called an "exploder"),
copies are forwarded or redistributed to each mailbox in
the expanded list. We classify such a pseudo-mailbox as
an "alias" or a "list", depending upon the expansion
rules:
(a) Alias
To expand an alias, the recipient mailer simply
replaces the pseudo-mailbox address in the envelope
with each of the expanded addresses in turn; the rest
of the envelope and the message body are left
unchanged. The message is then delivered or
forwarded to each expanded address.
(b) List
A mailing list may be said to operate by
"redistribution" rather than by "forwarding". To
Internet Engineering Task Force [Page 65]
RFC1123 MAIL -- SMTP & RFC-822 October 1989
expand a list, the recipient mailer replaces the
pseudo-mailbox address in the envelope with each of
the expanded addresses in turn. The return address in
the envelope is changed so that all error messages
generated by the final deliveries will be returned to
a list administrator, not to the message originator,
who generally has no control over the contents of the
list and will typically find error messages annoying.
5.3.7 Mail Gatewaying
Gatewaying mail between different mail environments, i.e.,
different mail formats and protocols, is complex and does not
easily yield to standardization. See for example [SMTP:5a],
[SMTP:5b]. However, some general requirements may be given for
a gateway between the Internet and another mail environment.
(A) Header fields MAY be rewritten when necessary as messages
are gatewayed across mail environment boundaries.
DISCUSSION:
This may involve interpreting the local-part of the
destination address, as suggested in Section 5.2.16.
The other mail systems gatewayed to the Internet
generally use a subset of RFC-822 headers, but some
of them do not have an equivalent to the SMTP
envelope. Therefore, when a message leaves the
Internet environment, it may be necessary to fold the
SMTP envelope information into the message header. A
possible solution would be to create new header
fields to carry the envelope information (e.g., "X-
SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this would
require changes in mail programs in the foreign
environment.
(B) When forwarding a message into or out of the Internet
environment, a gateway MUST prepend a Received: line, but
it MUST NOT alter in any way a Received: line that is
already in the header.
DISCUSSION:
This requirement is a subset of the general
"Received:" line requirement of Section 5.2.8; it is
restated here for emphasis.
Received: fields of messages originating from other
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environments may not conform exactly to RFC822.
However, the most important use of Received: lines is
for debugging mail faults, and this debugging can be
severely hampered by well-meaning gateways that try
to "fix" a Received: line.
The gateway is strongly encouraged to indicate the
environment and protocol in the "via" clauses of
Received field(s) that it supplies.
(C) From the Internet side, the gateway SHOULD accept all
valid address formats in SMTP commands and in RFC-822
headers, and all valid RFC-822 messages. Although a
gateway must accept an RFC-822 explicit source route
("@...:" format) in either the RFC-822 header or in the
envelope, it MAY or may not act on the source route; see
Sections 5.2.6 and 5.2.19.
DISCUSSION:
It is often tempting to restrict the range of
addresses accepted at the mail gateway to simplify
the translation into addresses for the remote
environment. This practice is based on the
assumption that mail users have control over the
addresses their mailers send to the mail gateway. In
practice, however, users have little control over the
addresses that are finally sent; their mailers are
free to change addresses into any legal RFC-822
format.
(D) The gateway MUST ensure that all header fields of a
message that it forwards into the Internet meet the
requirements for Internet mail. In particular, all
addresses in "From:", "To:", "Cc:", etc., fields must be
transformed (if necessary) to satisfy RFC-822 syntax, and
they must be effective and useful for sending replies.
(E) The translation algorithm used to convert mail from the
Internet protocols to another environment's protocol
SHOULD try to ensure that error messages from the foreign
mail environment are delivered to the return path from the
SMTP envelope, not to the sender listed in the "From:"
field of the RFC-822 message.
DISCUSSION:
Internet mail lists usually place the address of the
mail list maintainer in the envelope but leave the
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RFC1123 MAIL -- SMTP & RFC-822 October 1989
original message header intact (with the "From:"
field containing the original sender). This yields
the behavior the average recipient expects: a reply
to the header gets sent to the original sender, not
to a mail list maintainer; however, errors get sent
to the maintainer (who can fix the problem) and not
the sender (who probably cannot).
(F) Similarly, when forwarding a message from another
environment into the Internet, the gateway SHOULD set the
envelope return path in accordance with an error message
return address, if any, supplied by the foreign
environment.
5.3.8 Maximum Message Size
Mailer software MUST be able to send and receive messages of at
least 64K bytes in length (including header), and a much larger
maximum size is highly desirable.
DISCUSSION:
Although SMTP does not define the maximum size of a
message, many systems impose implementation limits.
The current de facto minimum limit in the Internet is 64K
bytes. However, electronic mail is used for a variety of
purposes that create much larger messages. For example,
mail is often used instead of FTP for transmitting ASCII
files, and in particular to transmit entire documents. As
a result, messages can be 1 megabyte or even larger. We
note that the present document together with its lower-
layer companion contains 0.5 megabytes.
Internet Engineering Task Force [Page 68]
RFC1123 MAIL -- SMTP & RFC-822 October 1989
5.4 SMTP REQUIREMENTS SUMMARY
| | | | |S| |
| | | | |H| |F
| | | | |O|M|o
| | |S| |U|U|o
| | |H| |L|S|t
| |M|O| |D|T|n
| |U|U|M| | |o
| |S|L|A|N|N|t
| |T|D|Y|O|O|t
FEATURE |SECTION | | | |T|T|e
-----------------------------------------------|----------|-|-|-|-|-|--
| | | | | | |
RECEIVER-SMTP: | | | | | | |
Implement VRFY |5.2.3 |x| | | | |
Implement EXPN |5.2.3 | |x| | | |
EXPN, VRFY configurable |5.2.3 | | |x| | |
Implement SEND, SOML, SAML |5.2.4 | | |x| | |
Verify HELO parameter |5.2.5 | | |x| | |
Refuse message with bad HELO |5.2.5 | | | | |x|
Accept explicit src-route syntax in env. |5.2.6 |x| | | | |
Support "postmaster" |5.2.7 |x| | | | |
Process RCPT when received (except lists) |5.2.7 | | |x| | |
Long delay of RCPT responses |5.2.7 | | | | |x|
| | | | | | |
Add Received: line |5.2.8 |x| | | | |
Received: line include domain literal |5.2.8 | |x| | | |
Change previous Received: line |5.2.8 | | | | |x|
Pass Return-Path info (final deliv/gwy) |5.2.8 |x| | | | |
Support empty reverse path |5.2.9 |x| | | | |
Send only official reply codes |5.2.10 | |x| | | |
Send text from RFC-821 when appropriate |5.2.10 | |x| | | |
Delete "." for transparency |5.2.11 |x| | | | |
Accept and recognize self domain literal(s) |5.2.17 |x| | | | |
| | | | | | |
Error message about error message |5.3.1 | | | | |x|
Keep pending listen on SMTP port |5.3.1.2 | |x| | | |
Provide limit on recv concurrency |5.3.1.2 | | |x| | |
Wait at least 5 mins for next sender cmd |5.3.2 | |x| | | |
Avoidable delivery failure after "250 OK" |5.3.3 | | | | |x|
Send error notification msg after accept |5.3.3 |x| | | | |
Send using null return path |5.3.3 |x| | | | |
Send to envelope return path |5.3.3 | |x| | | |
Send to null address |5.3.3 | | | | |x|
Strip off explicit src route |5.3.3 | |x| | | |
Minimize acceptance delay (RFC-1047) |5.3.3 |x| | | | |
-----------------------------------------------|----------|-|-|-|-|-|--
Internet Engineering Task Force [Page 69]
RFC1123 MAIL -- SMTP & RFC-822 October 1989
| | | | | | |
SENDER-SMTP: | | | | | | |
Canonicalized domain names in MAIL, RCPT |5.2.2 |x| | | | |
Implement SEND, SOML, SAML |5.2.4 | | |x| | |
Send valid principal host name in HELO |5.2.5 |x| | | | |
Send explicit source route in RCPT TO: |5.2.6 | | | |x| |
Use only reply code to determine action |5.2.10 |x| | | | |
Use only high digit of reply code when poss. |5.2.10 | |x| | | |
Add "." for transparency |5.2.11 |x| | | | |
| | | | | | |
Retry messages after soft failure |5.3.1.1 |x| | | | |
Delay before retry |5.3.1.1 |x| | | | |
Configurable retry parameters |5.3.1.1 |x| | | | |
Retry once per each queued dest host |5.3.1.1 | |x| | | |
Multiple RCPT's for same DATA |5.3.1.1 | |x| | | |
Support multiple concurrent transactions |5.3.1.1 | | |x| | |
Provide limit on concurrency |5.3.1.1 | |x| | | |
| | | | | | |
Timeouts on all activities |5.3.1 |x| | | | |
Per-command timeouts |5.3.2 | |x| | | |
Timeouts easily reconfigurable |5.3.2 | |x| | | |
Recommended times |5.3.2 | |x| | | |
Try alternate addr's in order |5.3.4 |x| | | | |
Configurable limit on alternate tries |5.3.4 | | |x| | |
Try at least two alternates |5.3.4 | |x| | | |
Load-split across equal MX alternates |5.3.4 | |x| | | |
Use the Domain Name System |5.3.5 |x| | | | |
Support MX records |5.3.5 |x| | | | |
Use WKS records in MX processing |5.2.12 | | | |x| |
-----------------------------------------------|----------|-|-|-|-|-|--
| | | | | | |
MAIL FORWARDING: | | | | | | |
Alter existing header field(s) |5.2.6 | | | |x| |
Implement relay function: 821/section 3.6 |5.2.6 | | |x| | |
If not, deliver to RHS domain |5.2.6 | |x| | | |
Interpret 'local-part' of addr |5.2.16 | | | | |x|
| | | | | | |
MAILING LISTS AND ALIASES | | | | | | |
Support both |5.3.6 | |x| | | |
Report mail list error to local admin. |5.3.6 |x| | | | |
| | | | | | |
MAIL GATEWAYS: | | | | | | |
Embed foreign mail route in local-part |5.2.16 | | |x| | |
Rewrite header fields when necessary |5.3.7 | | |x| | |
Prepend Received: line |5.3.7 |x| | | | |
Change existing Received: line |5.3.7 | | | | |x|
Accept full RFC-822 on Internet side |5.3.7 | |x| | | |
Act on RFC-822 explicit source route |5.3.7 | | |x| | |
Internet Engineering Task Force [Page 70]
RFC1123 MAIL -- SMTP & RFC-822 October 1989
Send only valid RFC-822 on Internet side |5.3.7 |x| | | | |
Deliver error msgs to envelope addr |5.3.7 | |x| | | |
Set env return path from err return addr |5.3.7 | |x| | | |
| | | | | | |
USER AGENT -- RFC-822 | | | | | | |
Allow user to enter <route> address |5.2.6 | | | |x| |
Support RFC-1049 Content Type field |5.2.13 | | |x| | |
Use 4-digit years |5.2.14 | |x| | | |
Generate numeric timezones |5.2.14 | |x| | | |
Accept all timezones |5.2.14 |x| | | | |
Use non-num timezones from RFC-822 |5.2.14 |x| | | | |
Omit phrase before route-addr |5.2.15 | | |x| | |
Accept and parse dot.dec. domain literals |5.2.17 |x| | | | |
Accept all RFC-822 address formats |5.2.18 |x| | | | |
Generate invalid RFC-822 address format |5.2.18 | | | | |x|
Fully-qualified domain names in header |5.2.18 |x| | | | |
Create explicit src route in header |5.2.19 | | | |x| |
Accept explicit src route in header |5.2.19 |x| | | | |
| | | | | | |
Send/recv at least 64KB messages |5.3.8 |x| | | | |
Internet Engineering Task Force [Page 71]
RFC1123 SUPPORT SERVICES -- DOMAINS October 1989
6. SUPPORT SERVICES
6.1 DOMAIN NAME TRANSLATION
6.1.1 INTRODUCTION
Every host MUST implement a resolver for the Domain Name System
(DNS), and it MUST implement a mechanism using this DNS
resolver to convert host names to IP addresses and vice-versa
[DNS:1, DNS:2].
In addition to the DNS, a host MAY also implement a host name
translation mechanism that searches a local Internet host
table. See Section 6.1.3.8 for more information on this
option.
DISCUSSION:
Internet host name translation was originally performed by
searching local copies of a table of all hosts. This
table became too large to update and distribute in a
timely manner and too large to fit into many hosts, so the
DNS was invented.
The DNS creates a distributed database used primarily for
the translation between host names and host addresses.
Implementation of DNS software is required. The DNS
consists of two logically distinct parts: name servers and
resolvers (although implementations often combine these
two logical parts in the interest of efficiency) [DNS:2].
Domain name servers store authoritative data about certain
sections of the database and answer queries about the
data. Domain resolvers query domain name servers for data
on behalf of user processes. Every host therefore needs a
DNS resolver; some host machines will also need to run
domain name servers. Since no name server has complete
information, in general it is necessary to obtain
information from more than one name server to resolve a
query.
6.1.2 PROTOCOL WALK-THROUGH
An implementor must study references [DNS:1] and [DNS:2]
carefully. They provide a thorough description of the theory,
protocol, and implementation of the domain name system, and
reflect several years of experience.
Internet Engineering Task Force [Page 72]
RFC1123 SUPPORT SERVICES -- DOMAINS October 1989
6.1.2.1 Resource Records with Zero TTL: RFC-1035 Section 3.2.1
All DNS name servers and resolvers MUST properly handle RRs
with a zero TTL: return the RR to the client but do not
cache it.
DISCUSSION:
Zero TTL values are interpreted to mean that the RR can
only be used for the transaction in progress, and
should not be cached; they are useful for extremely
volatile data.
6.1.2.2 QCLASS Values: RFC-1035 Section 3.2.5
A query with "QCLASS=*" SHOULD NOT be used unless the
requestor is seeking data from more than one class. In
particular, if the requestor is only interested in Internet
data types, QCLASS=IN MUST be used.
6.1.2.3 Unused Fields: RFC-1035 Section 4.1.1
Unused fields in a query or response message MUST be zero.
6.1.2.4 Compression: RFC-1035 Section 4.1.4
Name servers MUST use compression in responses.
DISCUSSION:
Compression is essential to avoid overflowing UDP
datagrams; see Section 6.1.3.2.
6.1.2.5 Misusing Configuration Info: RFC-1035 Section 6.1.2
Recursive name servers and full-service resolvers generally
have some configuration information containing hints about
the location of root or local name servers. An
implementation MUST NOT include any of these hints in a
response.
DISCUSSION:
Many implementors have found it convenient to store
these hints as if they were cached data, but some
neglected to ensure that this "cached data" was not
included in responses. This has caused serious
problems in the Internet when the hints were obsolete
or incorrect.
Internet Engineering Task Force [Page 73]
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6.1.3 SPECIFIC ISSUES
6.1.3.1 Resolver Implementation
A name resolver SHOULD be able to multiplex concurrent
requests if the host supports concurrent processes.
In implementing a DNS resolver, one of two different models
MAY optionally be chosen: a full-service resolver, or a stub
resolver.
(A) Full-Service Resolver
A full-service resolver is a complete implementation of
the resolver service, and is capable of dealing with
communication failures, failure of individual name
servers, location of the proper name server for a given
name, etc. It must satisfy the following requirements:
o The resolver MUST implement a local caching
function to avoid repeated remote access for
identical requests, and MUST time out information
in the cache.
o The resolver SHOULD be configurable with start-up
information pointing to multiple root name servers
and multiple name servers for the local domain.
This insures that the resolver will be able to
access the whole name space in normal cases, and
will be able to access local domain information
should the local network become disconnected from
the rest of the Internet.
(B) Stub Resolver
A "stub resolver" relies on the services of a recursive
name server on the connected network or a "nearby"
network. This scheme allows the host to pass on the
burden of the resolver function to a name server on
another host. This model is often essential for less
capable hosts, such as PCs, and is also recommended
when the host is one of several workstations on a local
network, because it allows all of the workstations to
share the cache of the recursive name server and hence
reduce the number of domain requests exported by the
local network.
Internet Engineering Task Force [Page 74]
RFC1123 SUPPORT SERVICES -- DOMAINS October 1989
At a minimum, the stub resolver MUST be capable of
directing its requests to redundant recursive name
servers. Note that recursive name servers are allowed
to restrict the sources of requests that they will
honor, so the host administrator must verify that the
service will be provided. Stub resolvers MAY implement
caching if they choose, but if so, MUST timeout cached
information.
6.1.3.2 Transport Protocols
DNS resolvers and recursive servers MUST support UDP, and
SHOULD support TCP, for sending (non-zone-transfer) queries.
Specifically, a DNS resolver or server that is sending a
non-zone-transfer query MUST send a UDP query first. If the
Answer section of the response is truncated and if the
requester supports TCP, it SHOULD try the query again using
TCP.
DNS servers MUST be able to service UDP queries and SHOULD
be able to service TCP queries. A name server MAY limit the
resources it devotes to TCP queries, but it SHOULD NOT
refuse to service a TCP query just because it would have
succeeded with UDP.
Truncated responses MUST NOT be saved (cached) and later
used in such a way that the fact that they are truncated is
lost.
DISCUSSION:
UDP is preferred over TCP for queries because UDP
queries have much lower overhead, both in packet count
and in connection state. The use of UDP is essential
for heavily-loaded servers, especially the root
servers. UDP also offers additional robustness, since
a resolver can attempt several UDP queries to different
servers for the cost of a single TCP query.
It is possible for a DNS response to be truncated,
although this is a very rare occurrence in the present
Internet DNS. Practically speaking, truncation cannot
be predicted, since it is data-dependent. The
dependencies include the number of RRs in the answer,
the size of each RR, and the savings in space realized
by the name compression algorithm. As a rule of thumb,
truncation in NS and MX lists should not occur for
answers containing 15 or fewer RRs.
Internet Engineering Task Force [Page 75]
RFC1123 SUPPORT SERVICES -- DOMAINS October 1989
Whether it is possible to use a truncated answer
depends on the application. A mailer must not use a
truncated MX response, since this could lead to mail
loops.
Responsible practices can make UDP suffice in the vast
majority of cases. Name servers must use compression
in responses. Resolvers must differentiate truncation
of the Additional section of a response (which only
loses extra information) from truncation of the Answer
section (which for MX records renders the response
unusable by mailers). Database administrators should
list only a reasonable number of primary names in lists
of name servers, MX alternatives, etc.
However, it is also clear that some new DNS record
types defined in the future will contain information
exceeding the 512 byte limit that applies to UDP, and
hence will require TCP. Thus, resolvers and name
servers should implement TCP services as a backup to
UDP today, with the knowledge that they will require
the TCP service in the future.
By private agreement, name servers and resolvers MAY arrange
to use TCP for all traffic between themselves. TCP MUST be
used for zone transfers.
A DNS server MUST have sufficient internal concurrency that
it can continue to process UDP queries while awaiting a
response or performing a zone transfer on an open TCP
connection [DNS:2].
A server MAY support a UDP query that is delivered using an
IP broadcast or multicast address. However, the Recursion
Desired bit MUST NOT be set in a query that is multicast,
and MUST be ignored by name servers receiving queries via a
broadcast or multicast address. A host that sends broadcast
or multicast DNS queries SHOULD send them only as occasional
probes, caching the IP address(es) it obtains from the
response(s) so it can normally send unicast queries.
DISCUSSION:
Broadcast or (especially) IP multicast can provide a
way to locate nearby name servers without knowing their
IP addresses in advance. However, general broadcasting
of recursive queries can result in excessive and
unnecessary load on both network and servers.
Internet Engineering Task Force [Page 76]
RFC1123 SUPPORT SERVICES -- DOMAINS October 1989
6.1.3.3 Efficient Resource Usage
The following requirements on servers and resolvers are very
important to the health of the Internet as a whole,
particularly when DNS services are invoked repeatedly by
higher level automatic servers, such as mailers.
(1) The resolver MUST implement retransmission controls to
insure that it does not waste communication bandwidth,
and MUST impose finite bounds on the resources consumed
to respond to a single request. See [DNS:2] pages 43-
44 for specific recommendations.
(2) After a query has been retransmitted several times
without a response, an implementation MUST give up and
return a soft error to the application.
(3) All DNS name servers and resolvers SHOULD cache
temporary failures, with a timeout period of the order
of minutes.
DISCUSSION:
This will prevent applications that immediately
retry soft failures (in violation of Section 2.2
of this document) from generating excessive DNS
traffic.
(4) All DNS name servers and resolvers SHOULD cache
negative responses that indicate the specified name, or
data of the specified type, does not exist, as
described in [DNS:2].
(5) When a DNS server or resolver retries a UDP query, the
retry interval SHOULD be constrained by an exponential
backoff algorithm, and SHOULD also have upper and lower
bounds.
IMPLEMENTATION:
A measured RTT and variance (if available) should
be used to calculate an initial retransmission
interval. If this information is not available, a
default of no less than 5 seconds should be used.
Implementations may limit the retransmission
interval, but this limit must exceed twice the
Internet maximum segment lifetime plus service
delay at the name server.
(6) When a resolver or server receives a Source Quench for
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RFC1123 SUPPORT SERVICES -- DOMAINS October 1989
a query it has issued, it SHOULD take steps to reduce
the rate of querying that server in the near future. A
server MAY ignore a Source Quench that it receives as
the result of sending a response datagram.
IMPLEMENTATION:
One recommended action to reduce the rate is to
send the next query attempt to an alternate
server, if there is one available. Another is to
backoff the retry interval for the same server.
6.1.3.4 Multihomed Hosts
When the host name-to-address function encounters a host
with multiple addresses, it SHOULD rank or sort the
addresses using knowledge of the immediately connected
network number(s) and any other applicable performance or
history information.
DISCUSSION:
The different addresses of a multihomed host generally
imply different Internet paths, and some paths may be
preferable to others in performance, reliability, or
administrative restrictions. There is no general way
for the domain system to determine the best path. A
recommended approach is to base this decision on local
configuration information set by the system
administrator.
IMPLEMENTATION:
The following scheme has been used successfully:
(a) Incorporate into the host configuration data a
Network-Preference List, that is simply a list of
networks in preferred order. This list may be
empty if there is no preference.
(b) When a host name is mapped into a list of IP
addresses, these addresses should be sorted by
network number, into the same order as the
corresponding networks in the Network-Preference
List. IP addresses whose networks do not appear
in the Network-Preference List should be placed at
the end of the list.
Internet Engineering Task Force [Page 78]
RFC1123 SUPPORT SERVICES -- DOMAINS October 1989
6.1.3.5 Extensibility
DNS software MUST support all well-known, class-independent
formats [DNS:2], and SHOULD be written to minimize the
trauma associated with the introduction of new well-known
types and local experimentation with non-standard types.
DISCUSSION:
The data types and classes used by the DNS are
extensible, and thus new types will be added and old
types deleted or redefined. Introduction of new data
types ought to be dependent only upon the rules for
compression of domain names inside DNS messages, and
the translation between printable (i.e., master file)
and internal formats for Resource Records (RRs).
Compression relies on knowledge of the format of data
inside a particular RR. Hence compression must only be
used for the contents of well-known, class-independent
RRs, and must never be used for class-specific RRs or
RR types that are not well-known. The owner name of an
RR is always eligible for compression.
A name server may acquire, via zone transfer, RRs that
the server doesn't know how to convert to printable
format. A resolver can receive similar information as
the result of queries. For proper operation, this data
must be preserved, and hence the implication is that
DNS software cannot use textual formats for internal
storage.
The DNS defines domain name syntax very generally -- a
string of labels each containing up to 63 8-bit octets,
separated by dots, and with a maximum total of 255
octets. Particular applications of the DNS are
permitted to further constrain the syntax of the domain
names they use, although the DNS deployment has led to
some applications allowing more general names. In
particular, Section 2.1 of this document liberalizes
slightly the syntax of a legal Internet host name that
was defined in RFC-952 [DNS:4].
6.1.3.6 Status of RR Types
Name servers MUST be able to load all RR types except MD and
MF from configuration files. The MD and MF types are
obsolete and MUST NOT be implemented; in particular, name
servers MUST NOT load these types from configuration files.
Internet Engineering Task Force [Page 79]
RFC1123 SUPPORT SERVICES -- DOMAINS October 1989
DISCUSSION:
The RR types MB, MG, MR, NULL, MINFO and RP are
considered experimental, and applications that use the
DNS cannot expect these RR types to be supported by
most domains. Furthermore these types are subject to
redefinition.
The TXT and WKS RR types have not been widely used by
Internet sites; as a result, an application cannot rely
on the the existence of a TXT or WKS RR in most
domains.
6.1.3.7 Robustness
DNS software may need to operate in environments where the
root servers or other servers are unavailable due to network
connectivity or other problems. In this situation, DNS name
servers and resolvers MUST continue to provide service for
the reachable part of the name space, while giving temporary
failures for the rest.
DISCUSSION:
Although the DNS is meant to be used primarily in the
connected Internet, it should be possible to use the
system in networks which are unconnected to the
Internet. Hence implementations must not depend on
access to root servers before providing service for
local names.
6.1.3.8 Local Host Table
DISCUSSION:
A host may use a local host table as a backup or
supplement to the DNS. This raises the question of
which takes precedence, the DNS or the host table; the
most flexible approach would make this a configuration
option.
Typically, the contents of such a supplementary host
table will be determined locally by the site. However,
a publically-available table of Internet hosts is
maintained by the DDN Network Information Center (DDN
NIC), with a format documented in [DNS:4]. This table
can be retrieved from the DDN NIC using a protocol
described in [DNS:5]. It must be noted that this table
contains only a small fraction of all Internet hosts.
Hosts using this protocol to retrieve the DDN NIC host
table should use the VERSION command to check if the
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table has changed before requesting the entire table
with the ALL command. The VERSION identifier should be
treated as an arbitrary string and tested only for
equality; no numerical sequence may be assumed.
The DDN NIC host table includes administrative
information that is not needed for host operation and
is therefore not currently included in the DNS
database; examples include network and gateway entries.
However, much of this additional information will be
added to the DNS in the future. Conversely, the DNS
provides essential services (in particular, MX records)
that are not available from the DDN NIC host table.
6.1.4 DNS USER INTERFACE
6.1.4.1 DNS Administration
This document is concerned with design and implementation
issues in host software, not with administrative or
operational issues. However, administrative issues are of
particular importance in the DNS, since errors in particular
segments of this large distributed database can cause poor
or erroneous performance for many sites. These issues are
discussed in [DNS:6] and [DNS:7].
6.1.4.2 DNS User Interface
Hosts MUST provide an interface to the DNS for all
application programs running on the host. This interface
will typically direct requests to a system process to
perform the resolver function [DNS:1, 6.1:2].
At a minimum, the basic interface MUST support a request for
all information of a specific type and class associated with
a specific name, and it MUST return either all of the
requested information, a hard error code, or a soft error
indication. When there is no error, the basic interface
returns the complete response information without
modification, deletion, or ordering, so that the basic
interface will not need to be changed to accommodate new
data types.
DISCUSSION:
The soft error indication is an essential part of the
interface, since it may not always be possible to
access particular information from the DNS; see Section
6.1.3.3.
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A host MAY provide other DNS interfaces tailored to
particular functions, transforming the raw domain data into
formats more suited to these functions. In particular, a
host MUST provide a DNS interface to facilitate translation
between host addresses and host names.
6.1.4.3 Interface Abbreviation Facilities
User interfaces MAY provide a method for users to enter
abbreviations for commonly-used names. Although the
definition of such methods is outside of the scope of the
DNS specification, certain rules are necessary to insure
that these methods allow access to the entire DNS name space
and to prevent excessive use of Internet resources.
If an abbreviation method is provided, then:
(a) There MUST be some convention for denoting that a name
is already complete, so that the abbreviation method(s)
are suppressed. A trailing dot is the usual method.
(b) Abbreviation expansion MUST be done exactly once, and
MUST be done in the context in which the name was
entered.
DISCUSSION:
For example, if an abbreviation is used in a mail
program for a destination, the abbreviation should be
expanded into a full domain name and stored in the
queued message with an indication that it is already
complete. Otherwise, the abbreviation might be
expanded with a mail system search list, not the
user's, or a name could grow due to repeated
canonicalizations attempts interacting with wildcards.
The two most common abbreviation methods are:
(1) Interface-level aliases
Interface-level aliases are conceptually implemented as
a list of alias/domain name pairs. The list can be
per-user or per-host, and separate lists can be
associated with different functions, e.g. one list for
host name-to-address translation, and a different list
for mail domains. When the user enters a name, the
interface attempts to match the name to the alias
component of a list entry, and if a matching entry can
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be found, the name is replaced by the domain name found
in the pair.
Note that interface-level aliases and CNAMEs are
completely separate mechanisms; interface-level aliases
are a local matter while CNAMEs are an Internet-wide
aliasing mechanism which is a required part of any DNS
implementation.
(2) Search Lists
A search list is conceptually implemented as an ordered
list of domain names. When the user enters a name, the
domain names in the search list are used as suffixes to
the user-supplied name, one by one, until a domain name
with the desired associated data is found, or the
search list is exhausted. Search lists often contain
the name of the local host's parent domain or other
ancestor domains. Search lists are often per-user or
per-process.
It SHOULD be possible for an administrator to disable a
DNS search-list facility. Administrative denial may be
warranted in some cases, to prevent abuse of the DNS.
There is danger that a search-list mechanism will
generate excessive queries to the root servers while
testing whether user input is a complete domain name,
lacking a final period to mark it as complete. A
search-list mechanism MUST have one of, and SHOULD have
both of, the following two provisions to prevent this:
(a) The local resolver/name server can implement
caching of negative responses (see Section
6.1.3.3).
(b) The search list expander can require two or more
interior dots in a generated domain name before it
tries using the name in a query to non-local
domain servers, such as the root.
DISCUSSION:
The intent of this requirement is to avoid
excessive delay for the user as the search list is
tested, and more importantly to prevent excessive
traffic to the root and other high-level servers.
For example, if the user supplied a name "X" and
the search list contained the root as a component,
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a query would have to consult a root server before
the next search list alternative could be tried.
The resulting load seen by the root servers and
gateways near the root would be multiplied by the
number of hosts in the Internet.
The negative caching alternative limits the effect
to the first time a name is used. The interior
dot rule is simpler to implement but can prevent
easy use of some top-level names.
6.1.5 DOMAIN NAME SYSTEM REQUIREMENTS SUMMARY
| | | | |S| |
| | | | |H| |F
| | | | |O|M|o
| | |S| |U|U|o
| | |H| |L|S|t
| |M|O| |D|T|n
| |U|U|M| | |o
| |S|L|A|N|N|t
| |T|D|Y|O|O|t
FEATURE |SECTION | | | |T|T|e
-----------------------------------------------|-----------|-|-|-|-|-|--
GENERAL ISSUES | | | | | | |
| | | | | | |
Implement DNS name-to-address conversion |6.1.1 |x| | | | |
Implement DNS address-to-name conversion |6.1.1 |x| | | | |
Support conversions using host table |6.1.1 | | |x| | |
Properly handle RR with zero TTL |6.1.2.1 |x| | | | |
Use QCLASS=* unnecessarily |6.1.2.2 | |x| | | |
Use QCLASS=IN for Internet class |6.1.2.2 |x| | | | |
Unused fields zero |6.1.2.3 |x| | | | |
Use compression in responses |6.1.2.4 |x| | | | |
| | | | | | |
Include config info in responses |6.1.2.5 | | | | |x|
Support all well-known, class-indep. types |6.1.3.5 |x| | | | |
Easily expand type list |6.1.3.5 | |x| | | |
Load all RR types (except MD and MF) |6.1.3.6 |x| | | | |
Load MD or MF type |6.1.3.6 | | | | |x|
Operate when root servers, etc. unavailable |6.1.3.7 |x| | | | |
-----------------------------------------------|-----------|-|-|-|-|-|--
RESOLVER ISSUES: | | | | | | |
| | | | | | |
Resolver support multiple concurrent requests |6.1.3.1 | |x| | | |
Full-service resolver: |6.1.3.1 | | |x| | |
Local caching |6.1.3.1 |x| | | | |
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Information in local cache times out |6.1.3.1 |x| | | | |
Configurable with starting info |6.1.3.1 | |x| | | |
Stub resolver: |6.1.3.1 | | |x| | |
Use redundant recursive name servers |6.1.3.1 |x| | | | |
Local caching |6.1.3.1 | | |x| | |
Information in local cache times out |6.1.3.1 |x| | | | |
Support for remote multi-homed hosts: | | | | | | |
Sort multiple addresses by preference list |6.1.3.4 | |x| | | |
| | | | | | |
-----------------------------------------------|-----------|-|-|-|-|-|--
TRANSPORT PROTOCOLS: | | | | | | |
| | | | | | |
Support UDP queries |6.1.3.2 |x| | | | |
Support TCP queries |6.1.3.2 | |x| | | |
Send query using UDP first |6.1.3.2 |x| | | | |1
Try TCP if UDP answers are truncated |6.1.3.2 | |x| | | |
Name server limit TCP query resources |6.1.3.2 | | |x| | |
Punish unnecessary TCP query |6.1.3.2 | | | |x| |
Use truncated data as if it were not |6.1.3.2 | | | | |x|
Private agreement to use only TCP |6.1.3.2 | | |x| | |
Use TCP for zone transfers |6.1.3.2 |x| | | | |
TCP usage not block UDP queries |6.1.3.2 |x| | | | |
Support broadcast or multicast queries |6.1.3.2 | | |x| | |
RD bit set in query |6.1.3.2 | | | | |x|
RD bit ignored by server is b'cast/m'cast |6.1.3.2 |x| | | | |
Send only as occasional probe for addr's |6.1.3.2 | |x| | | |
-----------------------------------------------|-----------|-|-|-|-|-|--
RESOURCE USAGE: | | | | | | |
| | | | | | |
Transmission controls, per [DNS:2] |6.1.3.3 |x| | | | |
Finite bounds per request |6.1.3.3 |x| | | | |
Failure after retries => soft error |6.1.3.3 |x| | | | |
Cache temporary failures |6.1.3.3 | |x| | | |
Cache negative responses |6.1.3.3 | |x| | | |
Retries use exponential backoff |6.1.3.3 | |x| | | |
Upper, lower bounds |6.1.3.3 | |x| | | |
Client handle Source Quench |6.1.3.3 | |x| | | |
Server ignore Source Quench |6.1.3.3 | | |x| | |
-----------------------------------------------|-----------|-|-|-|-|-|--
USER INTERFACE: | | | | | | |
| | | | | | |
All programs have access to DNS interface |6.1.4.2 |x| | | | |
Able to request all info for given name |6.1.4.2 |x| | | | |
Returns complete info or error |6.1.4.2 |x| | | | |
Special interfaces |6.1.4.2 | | |x| | |
Name<->Address translation |6.1.4.2 |x| | | | |
| | | | | | |
Abbreviation Facilities: |6.1.4.3 | | |x| | |
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Convention for complete names |6.1.4.3 |x| | | | |
Conversion exactly once |6.1.4.3 |x| | | | |
Conversion in proper context |6.1.4.3 |x| | | | |
Search list: |6.1.4.3 | | |x| | |
Administrator can disable |6.1.4.3 | |x| | | |
Prevention of excessive root queries |6.1.4.3 |x| | | | |
Both methods |6.1.4.3 | |x| | | |
-----------------------------------------------|-----------|-|-|-|-|-|--
-----------------------------------------------|-----------|-|-|-|-|-|--
1. Unless there is private agreement between particular resolver and
particular server.
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6.2 HOST INITIALIZATION
6.2.1 INTRODUCTION
This section discusses the initialization of host software
across a connected network, or more generally across an
Internet path. This is necessary for a diskless host, and may
optionally be used for a host with disk drives. For a diskless
host, the initialization process is called "network booting"
and is controlled by a bootstrap program located in a boot ROM.
To initialize a diskless host across the network, there are two
distinct phases:
(1) Configure the IP layer.
Diskless machines often have no permanent storage in which
to store network configuration information, so that
sufficient configuration information must be obtained
dynamically to support the loading phase that follows.
This information must include at least the IP addresses of
the host and of the boot server. To support booting
across a gateway, the address mask and a list of default
gateways are also required.
(2) Load the host system code.
During the loading phase, an appropriate file transfer
protocol is used to copy the system code across the
network from the boot server.
A host with a disk may perform the first step, dynamic
configuration. This is important for microcomputers, whose
floppy disks allow network configuration information to be
mistakenly duplicated on more than one host. Also,
installation of new hosts is much simpler if they automatically
obtain their configuration information from a central server,
saving administrator time and decreasing the probability of
mistakes.
6.2.2 REQUIREMENTS
6.2.2.1 Dynamic Configuration
A number of protocol provisions have been made for dynamic
configuration.
o ICMP Information Request/Reply messages
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This obsolete message pair was designed to allow a host
to find the number of the network it is on.
Unfortunately, it was useful only if the host already
knew the host number part of its IP address,
information that hosts requiring dynamic configuration
seldom had.
o Reverse Address Resolution Protocol (RARP) [BOOT:4]
RARP is a link-layer protocol for a broadcast medium
that allows a host to find its IP address given its
link layer address. Unfortunately, RARP does not work
across IP gateways and therefore requires a RARP server
on every network. In addition, RARP does not provide
any other configuration information.
o ICMP Address Mask Request/Reply messages
These ICMP messages allow a host to learn the address
mask for a particular network interface.
o BOOTP Protocol [BOOT:2]
This protocol allows a host to determine the IP
addresses of the local host and the boot server, the
name of an appropriate boot file, and optionally the
address mask and list of default gateways. To locate a
BOOTP server, the host broadcasts a BOOTP request using
UDP. Ad hoc gateway extensions have been used to
transmit the BOOTP broadcast through gateways, and in
the future the IP Multicasting facility will provide a
standard mechanism for this purpose.
The suggested approach to dynamic configuration is to use
the BOOTP protocol with the extensions defined in "BOOTP
Vendor Information Extensions" RFC-1084 [BOOT:3]. RFC-1084
defines some important general (not vendor-specific)
extensions. In particular, these extensions allow the
address mask to be supplied in BOOTP; we RECOMMEND that the
address mask be supplied in this manner.
DISCUSSION:
Historically, subnetting was defined long after IP, and
so a separate mechanism (ICMP Address Mask messages)
was designed to supply the address mask to a host.
However, the IP address mask and the corresponding IP
address conceptually form a pair, and for operational
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simplicity they ought to be defined at the same time
and by the same mechanism, whether a configuration file
or a dynamic mechanism like BOOTP.
Note that BOOTP is not sufficiently general to specify
the configurations of all interfaces of a multihomed
host. A multihomed host must either use BOOTP
separately for each interface, or configure one
interface using BOOTP to perform the loading, and
perform the complete initialization from a file later.
Application layer configuration information is expected
to be obtained from files after loading of the system
code.
6.2.2.2 Loading Phase
A suggested approach for the loading phase is to use TFTP
[BOOT:1] between the IP addresses established by BOOTP.
TFTP to a broadcast address SHOULD NOT be used, for reasons
explained in Section 4.2.3.4.
Internet Engineering Task Force [Page 89]
RFC1123 SUPPORT SERVICES -- MANAGEMENT October 1989
6.3 REMOTE MANAGEMENT
6.3.1 INTRODUCTION
The Internet community has recently put considerable effort
into the development of network management protocols. The
result has been a two-pronged approach [MGT:1, MGT:6]: the
Simple Network Management Protocol (SNMP) [MGT:4] and the
Common Management Information Protocol over TCP (CMOT) [MGT:5].
In order to be managed using SNMP or CMOT, a host will need to
implement an appropriate management agent. An Internet host
SHOULD include an agent for either SNMP or CMOT.
Both SNMP and CMOT operate on a Management Information Base
(MIB) that defines a collection of management values. By
reading and setting these values, a remote application may
query and change the state of the managed system.
A standard MIB [MGT:3] has been defined for use by both
management protocols, using data types defined by the Structure
of Management Information (SMI) defined in [MGT:2]. Additional
MIB variables can be introduced under the "enterprises" and
"experimental" subtrees of the MIB naming space [MGT:2].
Every protocol module in the host SHOULD implement the relevant
MIB variables. A host SHOULD implement the MIB variables as
defined in the most recent standard MIB, and MAY implement
other MIB variables when appropriate and useful.
6.3.2 PROTOCOL WALK-THROUGH
The MIB is intended to cover both hosts and gateways, although
there may be detailed differences in MIB application to the two
cases. This section contains the appropriate interpretation of
the MIB for hosts. It is likely that later versions of the MIB
will include more entries for host management.
A managed host must implement the following groups of MIB
object definitions: System, Interfaces, Address Translation,
IP, ICMP, TCP, and UDP.
The following specific interpretations apply to hosts:
o ipInHdrErrors
Note that the error "time-to-live exceeded" can occur in a
host only when it is forwarding a source-routed datagram.
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o ipOutNoRoutes
This object counts datagrams discarded because no route
can be found. This may happen in a host if all the
default gateways in the host's configuration are down.
o ipFragOKs, ipFragFails, ipFragCreates
A host that does not implement intentional fragmentation
(see "Fragmentation" section of [INTRO:1]) MUST return the
value zero for these three objects.
o icmpOutRedirects
For a host, this object MUST always be zero, since hosts
do not send Redirects.
o icmpOutAddrMaskReps
For a host, this object MUST always be zero, unless the
host is an authoritative source of address mask
information.
o ipAddrTable
For a host, the "IP Address Table" object is effectively a
table of logical interfaces.
o ipRoutingTable
For a host, the "IP Routing Table" object is effectively a
combination of the host's Routing Cache and the static
route table described in "Routing Outbound Datagrams"
section of [INTRO:1].
Within each ipRouteEntry, ipRouteMetric1...4 normally will
have no meaning for a host and SHOULD always be -1, while
ipRouteType will normally have the value "remote".
If destinations on the connected network do not appear in
the Route Cache (see "Routing Outbound Datagrams section
of [INTRO:1]), there will be no entries with ipRouteType
of "direct".
DISCUSSION:
The current MIB does not include Type-of-Service in an
ipRouteEntry, but a future revision is expected to make
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this addition.
We also expect the MIB to be expanded to allow the remote
management of applications (e.g., the ability to partially
reconfigure mail systems). Network service applications
such as mail systems should therefore be written with the
"hooks" for remote management.
6.3.3 MANAGEMENT REQUIREMENTS SUMMARY
| | | | |S| |
| | | | |H| |F
| | | | |O|M|o
| | |S| |U|U|o
| | |H| |L|S|t
| |M|O| |D|T|n
| |U|U|M| | |o
| |S|L|A|N|N|t
| |T|D|Y|O|O|t
FEATURE |SECTION | | | |T|T|e
-----------------------------------------------|-----------|-|-|-|-|-|--
Support SNMP or CMOT agent |6.3.1 | |x| | | |
Implement specified objects in standard MIB |6.3.1 | |x| | | |
Internet Engineering Task Force [Page 92]
RFC1123 SUPPORT SERVICES -- MANAGEMENT October 1989
7. REFERENCES
This section lists the primary references with which every
implementer must be thoroughly familiar. It also lists some
secondary references that are suggested additional reading.
INTRODUCTORY REFERENCES:
[INTRO:1] "Requirements for Internet Hosts -- Communication Layers,"
IETF Host Requirements Working Group, R. Braden, Ed., RFC-1122,
October 1989.
[INTRO:2] "DDN Protocol Handbook," NIC-50004, NIC-50005, NIC-50006,
(three volumes), SRI International, December 1985.
[INTRO:3] "Official Internet Protocols," J. Reynolds and J. Postel,
RFC-1011, May 1987.
This document is republished periodically with new RFC numbers;
the latest version must be used.
[INTRO:4] "Protocol Document Order Information," O. Jacobsen and J.
Postel, RFC-980, March 1986.
[INTRO:5] "Assigned Numbers," J. Reynolds and J. Postel, RFC-1010,
May 1987.
This document is republished periodically with new RFC numbers;
the latest version must be used.
TELNET REFERENCES:
[TELNET:1] "Telnet Protocol Specification," J. Postel and J.
Reynolds, RFC-854, May 1983.
[TELNET:2] "Telnet Option Specification," J. Postel and J. Reynolds,
RFC-855, May 1983.
[TELNET:3] "Telnet Binary Transmission," J. Postel and J. Reynolds,
RFC-856, May 1983.
[TELNET:4] "Telnet Echo Option," J. Postel and J. Reynolds, RFC-857,
May 1983.
[TELNET:5] "Telnet Suppress Go Ahead Option," J. Postel and J.
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RFC1123 SUPPORT SERVICES -- MANAGEMENT October 1989
Reynolds, RFC-858, May 1983.
[TELNET:6] "Telnet Status Option," J. Postel and J. Reynolds, RFC-
859, May 1983.
[TELNET:7] "Telnet Timing Mark Option," J. Postel and J. Reynolds,
RFC-860, May 1983.
[TELNET:8] "Telnet Extended Options List," J. Postel and J.
Reynolds, RFC-861, May 1983.
[TELNET:9] "Telnet End-Of-Record Option," J. Postel, RFC-855,
December 1983.
[TELNET:10] "Telnet Terminal-Type Option," J. VanBokkelen, RFC-1091,
February 1989.
This document supercedes RFC-930.
[TELNET:11] "Telnet Window Size Option," D. Waitzman, RFC-1073,
October 1988.
[TELNET:12] "Telnet Linemode Option," D. Borman, RFC-1116, August
1989.
[TELNET:13] "Telnet Terminal Speed Option," C. Hedrick, RFC-1079,
December 1988.
[TELNET:14] "Telnet Remote Flow Control Option," C. Hedrick, RFC-
1080, November 1988.
SECONDARY TELNET REFERENCES:
[TELNET:15] "Telnet Protocol," MIL-STD-1782, U.S. Department of
Defense, May 1984.
This document is intended to describe the same protocol as RFC-
854. In case of conflict, RFC-854 takes precedence, and the
present document takes precedence over both.
[TELNET:16] "SUPDUP Protocol," M. Crispin, RFC-734, October 1977.
[TELNET:17] "Telnet SUPDUP Option," M. Crispin, RFC-736, October
1977.
[TELNET:18] "Data Entry Terminal Option," J. Day, RFC-732, June 1977.
Internet Engineering Task Force [Page 94]
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[TELNET:19] "TELNET Data Entry Terminal option -- DODIIS
Implementation," A. Yasuda and T. Thompson, RFC-1043, February
1988.
FTP REFERENCES:
[FTP:1] "File Transfer Protocol," J. Postel and J. Reynolds, RFC-
959, October 1985.
[FTP:2] "Document File Format Standards," J. Postel, RFC-678,
December 1974.
[FTP:3] "File Transfer Protocol," MIL-STD-1780, U.S. Department of
Defense, May 1984.
This document is based on an earlier version of the FTP
specification (RFC-765) and is obsolete.
TFTP REFERENCES:
[TFTP:1] "The TFTP Protocol Revision 2," K. Sollins, RFC-783, June
1981.
MAIL REFERENCES:
[SMTP:1] "Simple Mail Transfer Protocol," J. Postel, RFC-821, August
1982.
[SMTP:2] "Standard For The Format of ARPA Internet Text Messages,"
D. Crocker, RFC-822, August 1982.
This document obsoleted an earlier specification, RFC-733.
[SMTP:3] "Mail Routing and the Domain System," C. Partridge, RFC-
974, January 1986.
This RFC describes the use of MX records, a mandatory extension
to the mail delivery process.
[SMTP:4] "Duplicate Messages and SMTP," C. Partridge, RFC-1047,
February 1988.
Internet Engineering Task Force [Page 95]
RFC1123 SUPPORT SERVICES -- MANAGEMENT October 1989
[SMTP:5a] "Mapping between X.400 and RFC 822," S. Kille, RFC-987,
June 1986.
[SMTP:5b] "Addendum to RFC-987," S. Kille, RFC-???, September 1987.
The two preceding RFC's define a proposed standard for
gatewaying mail between the Internet and the X.400 environments.
[SMTP:6] "Simple Mail Transfer Protocol," MIL-STD-1781, U.S.
Department of Defense, May 1984.
This specification is intended to describe the same protocol as
does RFC-821. However, MIL-STD-1781 is incomplete; in
particular, it does not include MX records [SMTP:3].
[SMTP:7] "A Content-Type Field for Internet Messages," M. Sirbu,
RFC-1049, March 1988.
DOMAIN NAME SYSTEM REFERENCES:
[DNS:1] "Domain Names - Concepts and Facilities," P. Mockapetris,
RFC-1034, November 1987.
This document and the following one obsolete RFC-882, RFC-883,
and RFC-973.
[DNS:2] "Domain Names - Implementation and Specification," RFC-1035,
P. Mockapetris, November 1987.
[DNS:3] "Mail Routing and the Domain System," C. Partridge, RFC-974,
January 1986.
[DNS:4] "DoD Internet Host Table Specification," K. Harrenstein,
RFC-952, M. Stahl, E. Feinler, October 1985.
SECONDARY DNS REFERENCES:
[DNS:5] "Hostname Server," K. Harrenstein, M. Stahl, E. Feinler,
RFC-953, October 1985.
[DNS:6] "Domain Administrators Guide," M. Stahl, RFC-1032, November
1987.
Internet Engineering Task Force [Page 96]
RFC1123 SUPPORT SERVICES -- MANAGEMENT October 1989
[DNS:7] "Domain Administrators Operations Guide," M. Lottor, RFC-
1033, November 1987.
[DNS:8] "The Domain Name System Handbook," Vol. 4 of Internet
Protocol Handbook, NIC 50007, SRI Network Information Center,
August 1989.
SYSTEM INITIALIZATION REFERENCES:
[BOOT:1] "Bootstrap Loading Using TFTP," R. Finlayson, RFC-906, June
1984.
[BOOT:2] "Bootstrap Protocol (BOOTP)," W. Croft and J. Gilmore, RFC-
951, September 1985.
[BOOT:3] "BOOTP Vendor Information Extensions," J. Reynolds, RFC-
1084, December 1988.
Note: this RFC revised and obsoleted RFC-1048.
[BOOT:4] "A Reverse Address Resolution Protocol," R. Finlayson, T.
Mann, J. Mogul, and M. Theimer, RFC-903, June 1984.
MANAGEMENT REFERENCES:
[MGT:1] "IAB Recommendations for the Development of Internet Network
Management Standards," V. Cerf, RFC-1052, April 1988.
[MGT:2] "Structure and Identification of Management Information for
TCP/IP-based internets," M. Rose and K. McCloghrie, RFC-1065,
August 1988.
[MGT:3] "Management Information Base for Network Management of
TCP/IP-based internets," M. Rose and K. McCloghrie, RFC-1066,
August 1988.
[MGT:4] "A Simple Network Management Protocol," J. Case, M. Fedor,
M. Schoffstall, and C. Davin, RFC-1098, April 1989.
[MGT:5] "The Common Management Information Services and Protocol
over TCP/IP," U. Warrier and L. Besaw, RFC-1095, April 1989.
[MGT:6] "Report of the Second Ad Hoc Network Management Review
Group," V. Cerf, RFC-1109, August 1989.
Internet Engineering Task Force [Page 97]
RFC1123 SUPPORT SERVICES -- MANAGEMENT October 1989
Security Considerations
There are many security issues in the application and support
programs of host software, but a full discussion is beyond the scope
of this RFC. Security-related issues are mentioned in sections
concerning TFTP (Sections 4.2.1, 4.2.3.4, 4.2.3.5), the SMTP VRFY and
EXPN commands (Section 5.2.3), the SMTP HELO command (5.2.5), and the
SMTP DATA command (Section 5.2.8).
Author's Address
Robert Braden
USC/Information Sciences Institute
4676 Admiralty Way
Marina del Rey, CA 90292-6695
Phone: (213) 822 1511
EMail: Braden@ISI.EDU
Internet Engineering Task Force [Page 98]