| 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 | |
| 14 | Sieve is a standard for interoperable filters, defined in RFC 5228, |
| 15 | with multiple implementations around. If interoperability is important, |
| 16 | then there is no way around it. |
| 17 | |
| 18 | |
| 19 | Exim Implementation |
| 20 | |
| 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). |
| 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 |
| 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 |
| 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 | {$address_file} \ |
| 72 | {$home/mail/$address_file} \ |
| 73 | }} \ |
| 74 | } |
| 75 | delivery_date_add |
| 76 | envelope_to_add |
| 77 | return_path_add |
| 78 | mode = 0600 |
| 79 | |
| 80 | vacation: |
| 81 | driver = autoreply |
| 82 | |
| 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. |
| 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 | |
| 98 | RFC 5228 requires using CRLF to terminate the end of a line. |
| 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 | |
| 106 | The folder specified by "fileinto" must not contain the character |
| 107 | sequence ".." to avoid security problems. RFC 5228 does not specify the |
| 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 |
| 112 | "inbox". RFC 5228 requires that the user is notified of that condition. |
| 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 | |
| 116 | The automatic replies generated by "vacation" do not contain an updated |
| 117 | "references" header field. |
| 118 | |
| 119 | |
| 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 | |
| 128 | Semantics Of Fileinto |
| 129 | |
| 130 | RFC 5228 does not specify if "fileinto" tries to create a mail folder, |
| 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 | |
| 137 | Allof And Anyof Test |
| 138 | |
| 139 | RFC 5228 does not specify if these tests use shortcut/lazy evaluation. |
| 140 | Exim uses shortcut evaluation. |
| 141 | |
| 142 | |
| 143 | Action Reordering |
| 144 | |
| 145 | RFC 5228 does not specify if actions may be executed out of order. |
| 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 | |
| 151 | Sieve Syntax And Semantics |
| 152 | |
| 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 |
| 163 | until "stop" is executed or the end of the script is reached. |
| 164 | |
| 165 | 8.2. Grammar |
| 166 | |
| 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 | |
| 171 | options = a b c |
| 172 | start = { options } |
| 173 | |
| 174 | is equivalent to: |
| 175 | |
| 176 | start = ( a b c ) / ( a c b ) / ( b a c ) / ( b c a ) / ( c a b ) / ( c b a ) |
| 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 | |
| 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] |
| 263 | [":subject" string] |
| 264 | [":from" string] |
| 265 | [":addresses" string-list] |
| 266 | [":mime"] |
| 267 | [":handle" string] |
| 268 | command =/ vacation-command |
| 269 | |
| 270 | |
| 271 | Semantics Of ":mime" |
| 272 | |
| 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 implementations compose a multipart |
| 278 | structure with the reason as only part. Both conform to the specification |
| 279 | (or lack thereof). |
| 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 | |
| 290 | Default Subject |
| 291 | |
| 292 | RFC 5230 specifies that the default message subject is "Auto: " plus |
| 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. |
| 299 | |
| 300 | |
| 301 | Rate Limiting Responses |
| 302 | |
| 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. |
| 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. |
| 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. |