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495ae4b0 PH |
1 | Notes on the Sieve implementation for Exim |
2 | ||
3 | Exim Filter Versus Sieve Filter | |
4 | ||
5 | Exim supports two incompatible filters: The traditional Exim filter and | |
6 | the Sieve filter. Since Sieve is a extensible language, it is important | |
7 | to understand "Sieve" in this context as "the specific implementation | |
8 | of Sieve for Exim". | |
9 | ||
10 | The Exim filter contains more features, such as variable expansion, and | |
11 | better integration with the host environment, like external processes | |
12 | and pipes. | |
13 | ||
eed2e0b1 | 14 | Sieve is a standard for interoperable filters, defined in RFC 5228, |
495ae4b0 PH |
15 | with multiple implementations around. If interoperability is important, |
16 | then there is no way around it. | |
17 | ||
18 | ||
19 | Exim Implementation | |
20 | ||
eed2e0b1 MH |
21 | The Exim Sieve implementation offers the core as defined by RFC 5228, |
22 | the "encoded-character" extension (RFC 5228), the "envelope" test (RFC | |
23 | 5228), the "fileinto" action (5228), the "copy" parameter (RFC 3894), the | |
24 | "vacation" action (5230), the "notify" action (draft-ietf-sieve-notify-12) | |
25 | with mailto URIs (draft-ietf-sieve-notify-mailto-05), the | |
26 | "i;ascii-numeric" comparator (RFC 2244) and the subaddress parameter | |
27 | (RFC 5233). | |
495ae4b0 PH |
28 | |
29 | The Sieve filter is integrated in Exim and works very similar to the | |
30 | Exim filter: Sieve scripts are recognized by the first line containing | |
31 | "# sieve filter". When using "keep" or "fileinto" to save a mail into a | |
32 | folder, the resulting string is available as the variable $address_file | |
1c59d63b PH |
33 | in the transport that stores it. The following routers and transport |
34 | show a typical use of Sieve: | |
35 | ||
36 | begin routers | |
37 | ||
38 | localuser_verify: | |
39 | driver = accept | |
40 | domains = +localdomains | |
41 | local_part_suffix = "-*" | |
42 | local_part_suffix_optional | |
43 | check_local_user | |
44 | require_files = $home/.forward | |
45 | verify_only = true | |
46 | ||
47 | localuser_deliver: | |
48 | driver = redirect | |
49 | domains = +localdomains | |
50 | local_part_suffix = "-*" | |
51 | local_part_suffix_optional | |
52 | sieve_subaddress = "${sg{$local_part_suffix}{^-}{}}" | |
53 | sieve_useraddress = "$local_part" | |
54 | check_local_user | |
55 | require_files = $home/.forward | |
56 | file = $home/.forward | |
57 | check_ancestor | |
58 | allow_filter | |
59 | file_transport = localuser | |
60 | reply_transport = vacation | |
61 | sieve_vacation_directory = $home/mail/vacation | |
62 | verify = false | |
63 | ||
64 | begin transports | |
495ae4b0 PH |
65 | |
66 | localuser: | |
67 | driver = appendfile | |
68 | file = ${if eq{$address_file}{inbox} \ | |
69 | {/var/mail/$local_part} \ | |
70 | {${if eq{${substr_0_1:$address_file}}{/} \ | |
71 | {$address_file} \ | |
1c59d63b | 72 | {$home/mail/$address_file} \ |
495ae4b0 PH |
73 | }} \ |
74 | } | |
75 | delivery_date_add | |
76 | envelope_to_add | |
77 | return_path_add | |
78 | mode = 0600 | |
79 | ||
1c59d63b PH |
80 | vacation: |
81 | driver = autoreply | |
495ae4b0 | 82 | |
1c59d63b PH |
83 | Absolute files are stored where specified, relative files are stored |
84 | relative to $home/mail and "inbox" goes to the standard mailbox location. | |
85 | To enable "vacation", sieve_vacation_directory is set to the directory | |
86 | where vacation databases are held (don't put anything else in that | |
87 | directory) and point reply_transport to an autoreply transport. | |
88 | Setting the Sieve useraddress and subaddress allows to use the subaddress | |
89 | extension. | |
495ae4b0 PH |
90 | |
91 | ||
92 | RFC Compliance | |
93 | ||
94 | Exim requires the first line to be "# sieve filter". Of course the RFC | |
95 | does not enforce that line. Don't expect examples to work without adding | |
96 | it, though. | |
97 | ||
eed2e0b1 | 98 | RFC 5228 requires using CRLF to terminate the end of a line. |
495ae4b0 PH |
99 | The rationale was that CRLF is universally used in network protocols |
100 | to mark the end of the line. This implementation does not embed Sieve | |
101 | in a network protocol, but uses Sieve scripts as part of the Exim MTA. | |
102 | Since all parts of Exim use \n as newline character, this implementation | |
103 | does, too. You can change this by defining the macro RFC_EOL at compile | |
104 | time to enforce CRLF being used. | |
105 | ||
495ae4b0 | 106 | The folder specified by "fileinto" must not contain the character |
eed2e0b1 | 107 | sequence ".." to avoid security problems. RFC 5228 does not specify the |
495ae4b0 PH |
108 | syntax of folders apart from keep being equivalent to fileinto "INBOX". |
109 | This implementation uses "inbox" instead. | |
110 | ||
111 | Sieve script errors currently cause that messages are silently filed into | |
eed2e0b1 | 112 | "inbox". RFC 5228 requires that the user is notified of that condition. |
495ae4b0 PH |
113 | This may be implemented in future by adding a header line to mails that |
114 | are filed into "inbox" due to an error in the filter. | |
115 | ||
87fcc8b9 PH |
116 | The automatic replies generated by "vacation" do not contain an updated |
117 | "references" header field. | |
118 | ||
495ae4b0 | 119 | |
495ae4b0 PH |
120 | Semantics Of Keep |
121 | ||
122 | The keep command is equivalent to fileinto "inbox": It saves the | |
123 | message and resets the implicit keep flag. It does not set the | |
124 | implicit keep flag; there is no command to set it once it has | |
125 | been reset. | |
126 | ||
127 | ||
024bd3c2 | 128 | Semantics Of Fileinto |
495ae4b0 | 129 | |
eed2e0b1 | 130 | RFC 5228 does not specify if "fileinto" tries to create a mail folder, |
495ae4b0 PH |
131 | in case it does not exist. This implementation allows to configure |
132 | that aspect using the appendfile transport options "create_directory", | |
133 | "create_file" and "file_must_exist". See the appendfile transport in | |
134 | the Exim specification for details. | |
135 | ||
136 | ||
024bd3c2 PH |
137 | Allof And Anyof Test |
138 | ||
eed2e0b1 | 139 | RFC 5228 does not specify if these tests use shortcut/lazy evaluation. |
024bd3c2 PH |
140 | Exim uses shortcut evaluation. |
141 | ||
142 | ||
143 | Action Reordering | |
144 | ||
eed2e0b1 | 145 | RFC 5228 does not specify if actions may be executed out of order. |
024bd3c2 PH |
146 | Exim may execute them out of order, e.g. messages may be filed to |
147 | folders or forwarded in a different order than specified, because | |
148 | those actions only setup delivery, but do not execute it themselves. | |
149 | ||
150 | ||
024bd3c2 | 151 | Sieve Syntax And Semantics |
495ae4b0 | 152 | |
eed2e0b1 MH |
153 | RFC 5228 uses a generic grammar as syntax for commands and tests and |
154 | performs many checks during semantic analysis. Syntax is specified | |
155 | by grammar rules, semantics by natural language. The intention is to | |
156 | provide a framework for the syntax that describes current commands as | |
157 | well as future extensions, and describing commands by semantics. | |
158 | ||
159 | The following replacement for section 8.2 gives a grammar for specific | |
160 | commands of this implementation, thus removing most of the semantic | |
161 | analysis. Since the parser can not parse unsupported extensions, the | |
162 | result is strict error checking of any executed and not executed code | |
1c59d63b | 163 | until "stop" is executed or the end of the script is reached. |
495ae4b0 PH |
164 | |
165 | 8.2. Grammar | |
166 | ||
495ae4b0 PH |
167 | The grammar is specified in ABNF with two extensions to describe tagged |
168 | arguments that can be reordered and grammar extensions: { } denotes a | |
169 | sequence of symbols that may appear in any order. Example: | |
170 | ||
1c59d63b PH |
171 | options = a b c |
172 | start = { options } | |
495ae4b0 PH |
173 | |
174 | is equivalent to: | |
175 | ||
1c59d63b | 176 | start = ( a b c ) / ( a c b ) / ( b a c ) / ( b c a ) / ( c a b ) / ( c b a ) |
495ae4b0 PH |
177 | |
178 | The symbol =) is used to append to a rule: | |
179 | ||
180 | start = a | |
181 | start =) b | |
182 | ||
183 | is equivalent to | |
184 | ||
185 | start = a b | |
186 | ||
495ae4b0 PH |
187 | The basic Sieve commands are specified using the following grammar, which |
188 | language is a subset of the generic grammar above. The start symbol is | |
189 | "start". | |
190 | ||
191 | address-part = ":localpart" / ":domain" / ":all" | |
192 | comparator = ":comparator" string | |
193 | match-type = ":is" / ":contains" / ":matches" | |
194 | string = quoted-string / multi-line | |
195 | string-list = "[" string *("," string) "]" / string | |
196 | address-test = "address" { [address-part] [comparator] [match-type] } | |
197 | string-list string-list | |
198 | test-list = "(" test *("," test) ")" | |
199 | allof-test = "allof" test-list | |
200 | anyof-test = "anyof" test-list | |
201 | exists-test = "exists" string-list | |
202 | false-test = "false" | |
203 | true=test = "true" | |
204 | header-test = "header" { [comparator] [match-type] } | |
205 | string-list string-list | |
206 | not-test = "not" test | |
207 | relop = ":over" / ":under" | |
208 | size-test = "size" relop number | |
209 | block = "{" commands "}" | |
210 | if-command = "if" test block *( "elsif" test block ) [ "else" block ] | |
211 | stop-command = "stop" { stop-options } ";" | |
212 | stop-options = | |
213 | keep-command = "keep" { keep-options } ";" | |
214 | keep-options = | |
215 | discard-command = "discard" { discard-options } ";" | |
216 | discard-options = | |
217 | redirect-command = "redirect" { redirect-options } string ";" | |
218 | redirect-options = | |
219 | require-command = "require" { require-options } string-list ";" | |
220 | require-options = | |
221 | test = address-test / allof-test / anyof-test / exists-test | |
222 | / false-test / true-test / header-test / not-test | |
223 | / size-test | |
224 | command = if-command / stop-command / keep-command | |
225 | / discard-command / redirect-command | |
226 | commands = *command | |
227 | start = *require-command commands | |
228 | ||
229 | The extensions "envelope" and "fileinto" are specified using the following | |
230 | grammar extension. | |
231 | ||
232 | envelope-test = "envelope" { [comparator] [address-part] [match-type] } | |
233 | string-list string-list | |
234 | test =/ envelope-test | |
235 | ||
236 | fileinto-command = "fileinto" { fileinto-options } string ";" | |
237 | fileinto-options = | |
238 | command =/ fileinto-command | |
239 | ||
240 | The extension "copy" is specified as: | |
241 | ||
242 | fileinto-options =) ":copy" | |
243 | redirect-options =) ":copy" | |
244 | ||
245 | ||
246 | The i;ascii-numeric Comparator | |
247 | ||
248 | RFC 2244 describes this comparator and specifies that non-numeric strings | |
249 | are considered equal with an ordinal value higher than any numeric string. | |
250 | Although not stated explicitly, this includes the empty string. A range | |
251 | of at least 2^31 is required. This implementation does not limit the | |
252 | range, because it does not convert numbers to binary representation | |
253 | before comparing them. | |
254 | ||
255 | ||
256 | The vacation extension | |
257 | ||
258 | The extension "vacation" is specified using the following grammar | |
259 | extension. | |
260 | ||
261 | vacation-command = "vacation" { vacation-options } <reason: string> | |
262 | vacation-options = [":days" number] | |
495ae4b0 | 263 | [":subject" string] |
f656d135 PH |
264 | [":from" string] |
265 | [":addresses" string-list] | |
495ae4b0 | 266 | [":mime"] |
f656d135 | 267 | [":handle" string] |
495ae4b0 PH |
268 | command =/ vacation-command |
269 | ||
270 | ||
271 | Semantics Of ":mime" | |
272 | ||
f656d135 PH |
273 | The draft does not specify how strings using MIME entities are used |
274 | to compose messages. As a result, different implementations generate | |
275 | different mails. The Exim Sieve implementation splits the reason into | |
276 | header and body. It adds the header to the mail header and uses the body | |
277 | as mail body. Be aware, that other imlementations compose a multipart | |
278 | structure with the reason as only part. Both conform to the specification | |
279 | (or lack thereof). | |
495ae4b0 PH |
280 | |
281 | ||
282 | Semantics Of Not Using ":mime" | |
283 | ||
284 | Sieve scripts are written in UTF-8, so is the reason string in this | |
285 | case. This implementation adds MIME headers to indicate that. This | |
286 | is not required by the vacation draft, which does not specify how | |
287 | the UTF-8 reason is processed to compose the resulting message. | |
288 | ||
289 | ||
495ae4b0 PH |
290 | Default Subject |
291 | ||
eed2e0b1 | 292 | RFC 5230 specifies that the default message subject is "Auto: " plus |
5ea81592 PH |
293 | the old subject. Using this subject is dangerous, because many mailing |
294 | lists verify addresses by sending a secret key in the subject of a | |
295 | message, asking to reply to the message for confirmation. Using the | |
296 | default vacation subject confirms any subscription request of this kind, | |
297 | allowing to subscribe a third party to any mailing list, either to annoy | |
298 | the user or to declare spam as legitimate mail by proving to use opt-in. | |
495ae4b0 PH |
299 | |
300 | ||
301 | Rate Limiting Responses | |
302 | ||
f656d135 PH |
303 | In absence of a handle, this implementation hashes the reason, |
304 | ":subject" option, ":mime" option and ":from" option and uses the hex | |
305 | string representation as filename within the "sieve_vacation_directory" | |
306 | to store the recipient addresses for this vacation parameter set. | |
495ae4b0 PH |
307 | |
308 | The draft specifies that sites may define a minimum ":days" value than 1. | |
309 | This implementation uses 1. The maximum value MUST greater than 7, | |
310 | and SHOULD be greater than 30. This implementation uses a maximum of 31. | |
311 | ||
312 | Vacation recipient address databases older than 31 days are automatically | |
313 | removed. Users do not have to remove them manually when modifying their | |
314 | scripts. Don't put anything but vacation databases in that directory | |
315 | or you risk that it will be removed, too! | |
316 | ||
317 | ||
318 | Global Reply Address Blacklist | |
319 | ||
320 | The draft requires that each implementation offers a global black list | |
321 | of addresses that will never be replied to. Exim offers this as option | |
322 | "never_mail" in the autoreply transport. | |
84024b72 PH |
323 | |
324 | ||
325 | The enotify extension | |
326 | ||
327 | The extension "enotify" is specified using the following grammar | |
328 | extension. | |
329 | ||
330 | notify-command = "notify" { notify-options } <method: string> | |
331 | notify-options = [":from" string] | |
332 | [":importance" <"1" / "2" / "3">] | |
333 | [":options" 1*(string-list / number)] | |
334 | [":message" string] | |
335 | ||
336 | command =/ notify-command | |
337 | ||
338 | valid_notify_method = "valid_notify_method" | |
339 | <notification-uris: string-list> | |
340 | ||
341 | test =/ valid_notify_method | |
342 | ||
343 | Only the mailto URI scheme is implemented. |