doc/doc-txt/README.SIEVE

   1 $Cambridge: exim/doc/doc-txt/README.SIEVE,v 1.11 2007/03/21 15:15:12 ph10 Exp $
   2
   3               Notes on the Sieve implementation for Exim
   4
   5 Exim Filter Versus Sieve Filter
   6
   7 Exim supports two incompatible filters: The traditional Exim filter and
   8 the Sieve filter. Since Sieve is a extensible language, it is important
   9 to understand "Sieve" in this context as "the specific implementation
  10 of Sieve for Exim".
  11
  12 The Exim filter contains more features, such as variable expansion, and
  13 better integration with the host environment, like external processes
  14 and pipes.
  15
  16 Sieve is a standard for interoperable filters, defined in RFC 3028,
  17 with multiple implementations around. If interoperability is important,
  18 then there is no way around it.
  19
  20
  21 Exim Implementation
  22
  23 The Exim Sieve implementation offers the core as defined by
  24 draft-ietf-sieve-3028bis-10.txt (next version of RFC 3028 that
  25 fixes specification mistakes), the "envelope" test (3028bis), the
  26 "fileinto" action (3028bis), the "copy" parameter (RFC 3894), the
  27 "vacation" action (draft-ietf-sieve-vacation-06), the "notify" action
  28 (draft-ietf-sieve-notify-06.), the "i;ascii-numeric" comparator (RFC 2244)
  29 and the subaddress parameter (draft-ietf-sieve-rfc3598bis-05).
  30
  31 The Sieve filter is integrated in Exim and works very similar to the
  32 Exim filter: Sieve scripts are recognized by the first line containing
  33 "# sieve filter".  When using "keep" or "fileinto" to save a mail into a
  34 folder, the resulting string is available as the variable $address_file
  35 in the transport that stores it.  The following routers and transport
  36 show a typical use of Sieve:
  37
  38 begin routers
  39
  40 localuser_verify:
  41   driver = accept
  42   domains = +localdomains
  43   local_part_suffix = "-*"
  44   local_part_suffix_optional
  45   check_local_user
  46   require_files = $home/.forward
  47   verify_only = true
  48
  49 localuser_deliver:
  50   driver = redirect
  51   domains = +localdomains
  52   local_part_suffix = "-*"
  53   local_part_suffix_optional
  54   sieve_subaddress = "${sg{$local_part_suffix}{^-}{}}"
  55   sieve_useraddress = "$local_part"
  56   check_local_user
  57   require_files = $home/.forward
  58   file = $home/.forward
  59   check_ancestor
  60   allow_filter
  61   file_transport = localuser
  62   reply_transport = vacation
  63   sieve_vacation_directory = $home/mail/vacation
  64   verify = false
  65
  66 begin transports
  67
  68 localuser:
  69   driver = appendfile
  70   file = ${if eq{$address_file}{inbox} \
  71               {/var/mail/$local_part} \
  72               {${if eq{${substr_0_1:$address_file}}{/} \
  73                     {$address_file} \
  74                     {$home/mail/$address_file} \
  75               }} \
  76          }
  77   delivery_date_add
  78   envelope_to_add
  79   return_path_add
  80   mode = 0600
  81
  82 vacation:
  83   driver = autoreply
  84
  85 Absolute files are stored where specified, relative files are stored
  86 relative to $home/mail and "inbox" goes to the standard mailbox location.
  87 To enable "vacation", sieve_vacation_directory is set to the directory
  88 where vacation databases are held (don't put anything else in that
  89 directory) and point reply_transport to an autoreply transport.
  90 Setting the Sieve useraddress and subaddress allows to use the subaddress
  91 extension.
  92
  93
  94 RFC Compliance
  95
  96 Exim requires the first line to be "# sieve filter".  Of course the RFC
  97 does not enforce that line.  Don't expect examples to work without adding
  98 it, though.
  99
 100 RFC 3028 requires using CRLF to terminate the end of a line.
 101 The rationale was that CRLF is universally used in network protocols
 102 to mark the end of the line.  This implementation does not embed Sieve
 103 in a network protocol, but uses Sieve scripts as part of the Exim MTA.
 104 Since all parts of Exim use \n as newline character, this implementation
 105 does, too.  You can change this by defining the macro RFC_EOL at compile
 106 time to enforce CRLF being used.
 107
 108 Sieve scripts can not contain NUL characters in strings, but mail
 109 headers could contain MIME encoded NUL characters, which could never
 110 be matched by Sieve scripts using exact comparisons.  For that reason,
 111 this implementation extends the Sieve quoted string syntax with \0
 112 to describe a NUL character, violating \0 being the same as 0 in
 113 RFC 3028.
 114
 115 The folder specified by "fileinto" must not contain the character
 116 sequence ".." to avoid security problems.  RFC 3028 does not specify the
 117 syntax of folders apart from keep being equivalent to fileinto "INBOX".
 118 This implementation uses "inbox" instead.
 119
 120 Sieve script errors currently cause that messages are silently filed into
 121 "inbox".  RFC 3028 requires that the user is notified of that condition.
 122 This may be implemented in future by adding a header line to mails that
 123 are filed into "inbox" due to an error in the filter.
 124
 125 The automatic replies generated by "vacation" do not contain an updated
 126 "references" header field.
 127
 128
 129 Semantics Of Keep
 130
 131 The keep command is equivalent to fileinto "inbox": It saves the
 132 message and resets the implicit keep flag.  It does not set the
 133 implicit keep flag; there is no command to set it once it has
 134 been reset.
 135
 136
 137 Semantics Of Fileinto
 138
 139 RFC 3028 does not specify if "fileinto" tries to create a mail folder,
 140 in case it does not exist.  This implementation allows to configure
 141 that aspect using the appendfile transport options "create_directory",
 142 "create_file" and "file_must_exist".  See the appendfile transport in
 143 the Exim specification for details.
 144
 145
 146 Allof And Anyof Test
 147
 148 RFC 3028 does not specify if these tests use shortcut/lazy evaluation.
 149 Exim uses shortcut evaluation.
 150
 151
 152 Action Reordering
 153
 154 RFC 3028 does not specify if actions may be executed out of order.
 155 Exim may execute them out of order, e.g. messages may be filed to
 156 folders or forwarded in a different order than specified, because
 157 those actions only setup delivery, but do not execute it themselves.
 158
 159
 160 Wildcard Matching
 161
 162 RFC 3028 is not exactly clear if comparators act on unicode characters
 163 or on octets containing their UTF-8 representation.  As it turns out,
 164 many implementations go the second way.  This does not make a difference
 165 but for wildcard matching and octet-wise comparison.  Working on unicode
 166 means a dot matches a character.  Working on UTF-8 means the dot matches
 167 a single octet of a multi-octet sequence.  For octet-wise comparisons,
 168 working on UTF-8 means arbitrary byte sequences in headers can not be
 169 matches, as they are rarely correct UTF-8 sequences and can thus not be
 170 expressed as string literal.  This implementation works on unicode, but
 171 this may be changed in case RFC3028bis specifies this issue safe and sound.
 172
 173
 174 Sieve Syntax And Semantics
 175
 176 RFC 3028 confuses syntax and semantics sometimes.  It uses a generic
 177 grammar as syntax for commands and tests and performs many checks during
 178 semantic analysis.  Syntax is specified by grammar rules, semantics
 179 by natural language, despite the latter often talking about syntax.
 180 The intention was to provide a framework for the syntax that describes
 181 current commands as well as future extensions, and describing commands
 182 by semantics.
 183
 184 The following replacement for section 8.2 gives two grammars, one for
 185 the framework, and one for specific commands, thus removing most of the
 186 semantic analysis.  Since the parser can not parse unsupported extensions,
 187 the result is strict error checking of any executed and not executed code
 188 until "stop" is executed or the end of the script is reached.
 189
 190 8.2. Grammar
 191
 192 The atoms of the grammar are lexical tokens.  White space or comments may
 193 appear anywhere between lexical tokens, they are not part of the grammar.
 194 The grammar is specified in ABNF with two extensions to describe tagged
 195 arguments that can be reordered and grammar extensions: { } denotes a
 196 sequence of symbols that may appear in any order.  Example:
 197
 198   options = a b c
 199   start   = { options }
 200
 201 is equivalent to:
 202
 203   start   =  ( a b c ) / ( a c b ) / ( b a c ) / ( b c a ) / ( c a b ) / ( c b a )
 204
 205 The symbol =) is used to append to a rule:
 206
 207   start =  a
 208   start =) b
 209
 210 is equivalent to
 211
 212   start =  a b
 213
 214 All Sieve commands, including extensions, MUST be words of the following
 215 generic grammar with the start symbol "start".  They SHOULD be specified
 216 using a specific grammar, though.
 217
 218    argument        = string-list / number / tag
 219    arguments       = *argument [test / test-list]
 220    block           = "{" commands "}"
 221    commands        = *command
 222    string          = quoted-string / multi-line
 223    string-list     = "[" string *("," string) "]" / string
 224    test            = identifier arguments
 225    test-list       = "(" test *("," test) ")"
 226    command         = identifier arguments ( ";" / block )
 227    start           = command
 228
 229 The basic Sieve commands are specified using the following grammar, which
 230 language is a subset of the generic grammar above.  The start symbol is
 231 "start".
 232
 233   address-part     =  ":localpart" / ":domain" / ":all"
 234   comparator       =  ":comparator" string
 235   match-type       =  ":is" / ":contains" / ":matches"
 236   string           =  quoted-string / multi-line
 237   string-list      =  "[" string *("," string) "]" / string
 238   address-test     =  "address" { [address-part] [comparator] [match-type] }
 239                       string-list string-list
 240   test-list        =  "(" test *("," test) ")"
 241   allof-test       =  "allof" test-list
 242   anyof-test       =  "anyof" test-list
 243   exists-test      =  "exists" string-list
 244   false-test       =  "false"
 245   true=test        =  "true"
 246   header-test      =  "header" { [comparator] [match-type] }
 247                       string-list string-list
 248   not-test         =  "not" test
 249   relop            =  ":over" / ":under"
 250   size-test        =  "size" relop number
 251   block            =  "{" commands "}"
 252   if-command       =  "if" test block *( "elsif" test block ) [ "else" block ]
 253   stop-command     =  "stop" { stop-options } ";"
 254   stop-options     =
 255   keep-command     =  "keep" { keep-options } ";"
 256   keep-options     =
 257   discard-command  =  "discard" { discard-options } ";"
 258   discard-options  =
 259   redirect-command =  "redirect" { redirect-options } string ";"
 260   redirect-options =
 261   require-command  =  "require" { require-options } string-list ";"
 262   require-options  =
 263   test             =  address-test / allof-test / anyof-test / exists-test
 264                       / false-test / true-test / header-test / not-test
 265                       / size-test
 266   command          =  if-command / stop-command / keep-command
 267                       / discard-command / redirect-command
 268   commands         =  *command
 269   start            =  *require-command commands
 270
 271 The extensions "envelope" and "fileinto" are specified using the following
 272 grammar extension.
 273
 274   envelope-test    =  "envelope" { [comparator] [address-part] [match-type] }
 275                       string-list string-list
 276   test             =/ envelope-test
 277
 278   fileinto-command =  "fileinto" { fileinto-options } string ";"
 279   fileinto-options =
 280   command          =/ fileinto-command
 281
 282 The extension "copy" is specified as:
 283
 284   fileinto-options =) ":copy"
 285   redirect-options =) ":copy"
 286
 287
 288 The i;ascii-numeric Comparator
 289
 290 RFC 2244 describes this comparator and specifies that non-numeric strings
 291 are considered equal with an ordinal value higher than any numeric string.
 292 Although not stated explicitly, this includes the empty string.  A range
 293 of at least 2^31 is required.  This implementation does not limit the
 294 range, because it does not convert numbers to binary representation
 295 before comparing them.
 296
 297
 298 The vacation extension
 299
 300 The extension "vacation" is specified using the following grammar
 301 extension.
 302
 303   vacation-command =  "vacation" { vacation-options } <reason: string>
 304   vacation-options =  [":days" number]
 305                       [":subject" string]
 306                       [":from" string]
 307                       [":addresses" string-list]
 308                       [":mime"]
 309                       [":handle" string]
 310   command          =/ vacation-command
 311
 312
 313 Semantics Of ":mime"
 314
 315 The draft does not specify how strings using MIME entities are used
 316 to compose messages.  As a result, different implementations generate
 317 different mails.  The Exim Sieve implementation splits the reason into
 318 header and body.  It adds the header to the mail header and uses the body
 319 as mail body.  Be aware, that other imlementations compose a multipart
 320 structure with the reason as only part.  Both conform to the specification
 321 (or lack thereof).
 322
 323
 324 Semantics Of Not Using ":mime"
 325
 326 Sieve scripts are written in UTF-8, so is the reason string in this
 327 case.  This implementation adds MIME headers to indicate that.  This
 328 is not required by the vacation draft, which does not specify how
 329 the UTF-8 reason is processed to compose the resulting message.
 330
 331
 332 Default Subject
 333
 334 The draft specifies that the default message subject is "Auto: " plus
 335 the old subject.  Using this subject is dangerous, because many mailing
 336 lists verify addresses by sending a secret key in the subject of a
 337 message, asking to reply to the message for confirmation.  Using the
 338 default vacation subject confirms any subscription request of this kind,
 339 allowing to subscribe a third party to any mailing list, either to annoy
 340 the user or to declare spam as legitimate mail by proving to use opt-in.
 341
 342
 343 Rate Limiting Responses
 344
 345 In absence of a handle, this implementation hashes the reason,
 346 ":subject" option, ":mime" option and ":from" option and uses the hex
 347 string representation as filename within the "sieve_vacation_directory"
 348 to store the recipient addresses for this vacation parameter set.
 349
 350 The draft specifies that sites may define a minimum ":days" value than 1.
 351 This implementation uses 1.  The maximum value MUST greater than 7,
 352 and SHOULD be greater than 30.  This implementation uses a maximum of 31.
 353
 354 Vacation recipient address databases older than 31 days are automatically
 355 removed.  Users do not have to remove them manually when modifying their
 356 scripts.  Don't put anything but vacation databases in that directory
 357 or you risk that it will be removed, too!
 358
 359
 360 Global Reply Address Blacklist
 361
 362 The draft requires that each implementation offers a global black list
 363 of addresses that will never be replied to.  Exim offers this as option
 364 "never_mail" in the autoreply transport.
 365
 366
 367 The enotify extension
 368
 369 The extension "enotify" is specified using the following grammar
 370 extension.
 371
 372   notify-command =  "notify" { notify-options } <method: string>
 373   notify-options =  [":from" string]
 374                     [":importance" <"1" / "2" / "3">]
 375                     [":options" 1*(string-list / number)]
 376                     [":message" string]
 377
 378   command          =/ notify-command
 379
 380   valid_notify_method = "valid_notify_method"
 381                         <notification-uris: string-list>
 382
 383   test             =/ valid_notify_method
 384
 385 Only the mailto URI scheme is implemented.