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[exim.git] / doc / doc-misc / RFC.conform
1 Conformance with RFCs
2 ---------------------
4 Exim is written to follow the rules laid down in the RFCs. However, there are
5 some circumstances where it either extends what is specified, or chooses not to
6 follow them strictly, for various reasons. Sometimes variations are controlled
7 by an option, which may default on or off. This document lists the variations
8 from the latest email RFCs, and discusses their background and implications.
10 Last Updated: 25 January 1999
13 1. RFC 822
14 ----------
16 The original specification of the format of Internet mail messages is RFC 822,
17 later clarified and modified by RFC 1123. At the time of writing (January 1999)
18 a new RFC (currently known as draft-ietf-drums-msg-fmt-07) which updates and
19 consolidates all the material related to the message format is at a late stage
20 of drafting, and is expected to become an Internet Standard in due course.
22 The following is (I hope) a complete list of major variations from the draft
23 RFC. References in square brackets are to the -07 draft.
26 1.1 Line termination [2.1, 2.3]
27 -------------------------------
29 [Lines are terminated by CRLF; isolated CR and LF are not permitted.]
31 The CRLF requirement has to be interpreted carefully, because the RFC also says
32 that it does not cover the internal format "used by sites". Exim keeps messages
33 on its spool in Unix format, using only LF as the line terminator, and also
34 does local deliveries using only LF. I believe this is compliant with the RFC,
35 as these are both "internal formats".
37 Messages sent out by SMTP have CRLF line terminators. However, isolated CR
38 characters are treated as any other data characters, because Exim is eight-bit
39 clean (see 1.2 below).
41 See 2.1 below for a discussion of line terminators in incoming messages.
44 1.2 Eight-bit characters [2.1]
45 ------------------------------
47 [Messages consist of 7-bit characters.]
49 Exim is eight-bit clean. It does not do any processing of the characters in the
50 body of a message.
53 1.3 Maximum line length [2.1, 2.3]
54 ----------------------------------
56 [The maximum length of a line is 998 characters.]
58 Exim does not enforce any limit on line length.
61 1.4 The "phrase" part of an address [3.4]
62 -----------------------------------------
64 [The phrase is a sequence of "words"; a word is an "atom" or a quoted string.]
66 The characters that can be used in an "atom" do not include the full stop
67 (dot, period). Thus a header line such as
69 To: John Q. Public <>
71 is syntactically invalid under a strict interpretation of the RFC because the
72 dot in the phrase part is not quoted. However, many MTAs do not enforce this
73 restriction, so Exim was changed to be relaxed about it as well. In fact, the
74 draft RFC is moving towards allowing this. In section [4.1], which is defining
75 "obsolete" syntax that programs must accept (but not generate), it says this:
77 The period character is added to obs-phrase.
79 Note: The period character in obs-phrase is not a form that was allowed
80 in earlier versions of this or any other standard. Period (nor any other
81 character from specials) was not allowed in phrase because it introduced
82 a parsing difficulty distinguishing between phrases and portions of an
83 addr-spec (see section 4.4). It appears here because the period
84 character is currently used in many messages in the display-name portion
85 of addresses, especially for initials in names, and therefore must be
86 interpreted properly. In the future, period may appear in the regular
87 syntax of phrase.
90 1.5 Source routed addresses [4.4]
91 ---------------------------------
93 [Source routed addresses are always enclosed in <>.]
95 Source routed addresses are declared obsolete in the draft RFC, but MTAs are
96 still required to handle them. Strictly, a source-routed address must be
97 enclosed in <> characters, so a header such as
99 From: @a,@b:c@d
101 is syntactally invalid. Exim does not enforce this restriction.
104 1.6 Local parts [3.4.1]
105 -----------------------
107 [Dots in unquoted local parts may not be consecutive or at either end.]
109 Exim allows unquoted local parts to begin or end with a dot (period, full
110 stop), and it also permits two consecutive dots in a local part.
114 2. RFC 821
115 ----------
117 The original specification of SMTP is RFC 821, later clarified and modified by
118 RFC 1123. Domain name system requirements and their implications for mail are
119 covered in RFCs 1035 and 974. A scheme for extending the SMTP protocol is
120 described in RFC 1869, and there are subsequent RFCs specifying particular
121 extensions.
123 At the time of writing (January 1999) a new RFC (currently known as
124 draft-ietf-drums-smtpupd-09) which updates and consolidates all the material
125 connected with SMTP message transmission is at a late stage of drafting, and is
126 expected to become an Internet Standard in due course.
128 The new draft is written using the terms MUST, SHOULD, and MAY, which, when
129 written in capital letters, have precise meanings. To quote from the draft:
131 "MUST" or "MUST NOT" identify absolute requirements for conformance to
132 this specification. Implementations that do not conform to them lie
133 outside the scope of this specification and often will not
134 interoperate properly with SMTP implementations that do conform.
135 Implementations that are fully conforming also adhere to all "SHOULD"
136 and "SHOULD NOT" requirements. Implementations that adhere to all
137 "MUST" ("MUST NOT") but not to all of these are considered to be
138 partially conforming. Such implementations may interoperate properly
139 with fully conforming ones and with each other, but this will
140 typically be the case only if great care is taken. Consequently, an
141 implementation should violate "SHOULD" ("SHOULD NOT") requirements
142 only under exceptional and well-understood circumstances.
144 The implementation of Exim is intended to conform to the spirit of this
145 paragraph. The following is (I hope) a complete list of major variations
146 from the draft RFC. In addition to the items listed here, there are other minor
147 extensions such as the tolerance of white space in places where it is not
148 strictly permitted by the RFC. References in square brackets are to the -09
149 draft sections, and brief summaries of the RFC requirement are also given in
150 square brackets.
153 2.1 Line termination [2.3.7,]
154 -------------------------------------
156 [SMTP lines are terminated by CRLF.]
158 Exim recognizes LF without CR as a line terminator in all forms of input. For
159 SMTP input, any preceding CR is discarded. An early version of Exim followed
160 the RFC strictly, and did not recognize LF without CR in SMTP input. However,
161 it seems that sites on the net send out messages with just LF terminators,
162 despite the warnings in the RFCs, and other MTAs handle this, so Exim was
163 changed. However, there is a compile time macro called STRICT_CRLF which can be
164 set to restore the strict behaviour, though this is undocumented.
167 2.2 Eight-bit characters [2.4.1]
168 --------------------------------
170 [SMTP transmits only 7-bit characters.]
172 Exim is eight-bit clean, and makes no attempt to modify the data in a message
173 in any way. In particular, for messages containing characters with the top bit
174 set, it neither tries to negotiate 8-bit transmission, nor converts such
175 characters into an encoded form. In other words, it adopts the "just send 8"
176 strategy. It can be configured to send out 8BITMIME in its response to EHLO
177 (which it does not do by default), and it recognizes the 8BITMIME keyword on
178 incoming messages, but neither of these affect its handling of message data.
179 "Just send 8" is the strategy of a number of MTAs; it is argued that it
180 achieves what the user wants more often than other strategies.
183 2.3 Closing the connection []
184 -------------------------------------
186 [Client must wait for response to QUIT before closing the connection.]
188 Exim closes the connection immediately after sending QUIT, without waiting for
189 the reply. There was a lot of discussion about this on one of the mailing
190 lists. The conclusion was that this behaviour is fine on Unix systems, which
191 have TCP/IP implementations that close down the underlying channel tidily even
192 when the associated process has terminated. Indeed, not waiting may be
193 beneficial, as it moves the TIME_WAIT state (waiting to ensure there's no more
194 data in transit) from the server to the client system. On some other operating
195 systems (I understand) it is a disaster to terminate the sending process
196 without waiting for the QUIT response, because all the data about the
197 connection lives in the client's process space, and is therefore thrown away
198 before the response arrives. The subsequent arrival of the response then causes
199 bad behaviour.
202 2.4 IPv6 address literals [4.1.2]
203 ---------------------------------
205 [IPv6 address literals are introduced by "IPv6".]
207 Exim recognizes IPv6 literals as just the colon-separated hexadecimal form of
208 an IPv6 address, for example 1080:0:0:0:8:800:200C:417A, without the need for a
209 prefix. At present, it does not even recognize the prefix. When IPv6 becomes
210 more widespread, Exim will follow whatever the common usage is.
213 2.5 Underscores in domain names [4.1.2]
214 ---------------------------------------
216 [Underscores are not legal in domain names.]
218 RFC 822 allows all characters except specials, space, and controls in domain
219 names, but the SMTP RFCs are stricter, allowing only letters, digits, and
220 hyphen. Exim is compliant when checking incoming addresses in SMTP commands,
221 but it is more relaxed by default when checking domain names that are supplied
222 by EHLO or HELO commands, because many client workstations get set up with
223 underscores in their names. There is an option that can be set to cause Exim to
224 refuse underscores. (There are also options to specify certain hosts from which
225 it will accept any old junk after EHLO or HELO. Such is the woeful state of
226 some SMTP clients.)
229 2.6 Removal of return-path headers [4.4]
230 ----------------------------------------
232 [Relaying MTAs should not remove return-path.]
234 Exim removes Return-Path: headers from all messages, if return_path_remove is
235 set (the default). It does not attempt to determine if it is being a relay or
236 not. Indeed, for some messages it might be both a relay and a final destination
237 MTA for the same message.
240 2.7 Randomizing the order of addresses of multihomed hosts [5]
241 --------------------------------------------------------------
243 [Multihomed host addresses should not be randomized.]
245 Exim does randomize a list of several addresses for a single host, because
246 caching in resolvers will defeat the round-robinning that many namerservers
247 use. (Note: this is not the same as randomizing equal-valued MX records. That
248 is required by the RFC.)
251 2.8 Handling "MX points to self" [5]
252 ------------------------------------
254 [MX points to self must be treated as an error.]
256 The RFC doesn't allow for the possibility of special-purpose routing in the
257 case when the lowest numbered MX record points to the local host. The default
258 Exim configuration is compliant, but it is possible to configure Exim to behave
259 differently, and there are several situations where this can be useful.
262 2.9 Source routing [6.1]
263 -------------------------
265 [Source routes should be stripped.]
267 The new RFC has moved forward in deprecating source-routed email addresses.
268 Exim does not strip them down by default, but can be made to do so by setting
269 collapse_source_routes. However, even when it is not stripping them down, it
270 does not add host routing to reverse-paths when processing a source-routed
271 forward-path.
274 2.10 Loop detection [6.2]
275 -------------------------
277 [Loop count for Received: headers should be at least 100.]
279 Exim's default setting of the received_headers_max option is 30. Most messages
280 these days seem to accumulate less than half a dozen Received: headers, and
281 even a couple of forwardings don't bring this anywhere near 30.
284 2.11 Addition of missing headers [6.3]
285 --------------------------------------
287 [Missing headers may be added, and domains qualified, only if client is
288 identified.]
290 Exim always adds Message-Id: and Date: headers if these are missing, whatever
291 the source of the message, and likewise when it expands non-fully-qualified
292 domains, it does so independently of the message's source.
295 2.12 Syntax of MAIL and RCPT commands [,]
296 --------------------------------------------------------
298 Exim is more relaxed than the RFC requires:
300 (1) Trailing white space is ignored.
302 (2) It permits white space after the "FROM" and "TO" keywords.
304 (3) It does not insist on the address being enclosed in <> characters. In fact,
305 it recognizes addresses in RFC 822 format here, except that domain
306 components are restricted to containing only letters, digits, and hyphens.
308 (4) Local parts are permitted to contain null components, that is, may start or
309 end with an unquoted full stop (period) or contain two consecutive
310 unquoted full stops.
313 2.13 Non-fully-qualified domains [2.3.5]
314 ----------------------------------------
316 [All domains must be fully qualified.]
318 A domain that is not fully qualified has some of its trailing components
319 missing, and is normally a local alias of some sort, for example, just a
320 single-component host name.
322 Exim can be configured to "widen" non-fully-qualified domains, either by using
323 the facilities of the DNS resolver, or by an explicit list of widening strings.
324 When this is done, it applies to addresses received by SMTP from other hosts,
325 as well as to locally-originated addresses. Address re-writing could also be
326 used for this purpose.
329 2.14 Unqualified addresses [4.1.2]
330 ----------------------------------
332 [Addresses in SMTP commands must include domains.]
334 An unqualified address consists of a local part without a domain. Do not
335 confuse "qualified address" and "qualified domain". A qualified address may
336 include a non-fully-qualified domain.
338 There is one exception to the RFC rule: it is required that the unqualified
339 address "<postmaster>" always be accepted. Apart from this, Exim rejects
340 domainless addresses in SMTP commands by default, but it can be configured with
341 a list of hosts and/or networks that are permitted to send addresses without
342 domains in SMTP commands. Any such address that is accepted (including
343 <postmaster>) is qualified by adding the value of the qualify_domain option.
346 2.15 VRFY and EXPN [3.5.1, 3.5.2, 3.5.3, 7.3]
347 ---------------------------------------------
349 [VRFY and EXPN should be supported.]
351 Exim does not support VRFY and EXPN by default, but a list of hosts and
352 networks for which they are permitted can be given.
355 2.16 Checking of EHLO/HELO commands [4.1.4]
356 -------------------------------------------
358 [Client must send EHLO. Server must not refuse message if EHLO/HELO check
359 fails.]
361 Exim, as a client, always sends EHLO or HELO (see 2.3 above). As a server, it
362 does not insist on there having been a valid EHLO or HELO command before the
363 start of a message transaction. Any EHLO or HELO command that is received is
364 rejected only if it contains a syntax error. That is, it is never rejected on
365 the basis of any validation checking that may be performed on the data it
366 contains.
368 However, Exim can be configured to insist that (a) there is valid EHLO/HELO
369 command before any message transaction and (b) the domain in that command
370 matches the domain obtained by looking up the IP address of the sending host.
371 It is possible to specify exception lists of hosts and/or networks for which
372 this check does not apply.
375 2.17 Format of delivery error messages [3.7]
376 --------------------------------------------
378 [Standard report formats should be used if possible.]
380 Exim's delivery failure reports are MIME format, and might be RFC1894
381 conformant, but this has not been verified.
384 ## End ##