doc/doc-txt/README.SIEVE

   1 $Cambridge: exim/doc/doc-txt/README.SIEVE,v 1.9 2005/11/21 10:09:13 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 draft
  24 3028bis-4 (next version of RFC 3028 that fixes specification mistakes),
  25 the "envelope" (3028bis), the "fileinto" (3028bis), the "copy" (RFC 3894)
  26 and the "vacation" (draft-ietf-sieve-vacation-04.txt) extension, the
  27 "i;ascii-numeric" comparator (RFC 2244).
  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 3028 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 Sieve scripts can not contain NUL characters in strings, but mail
 107 headers could contain MIME encoded NUL characters, which could never
 108 be matched by Sieve scripts using exact comparisons.  For that reason,
 109 this implementation extends the Sieve quoted string syntax with \0
 110 to describe a NUL character, violating \0 being the same as 0 in
 111 RFC 3028.
 112
 113 The folder specified by "fileinto" must not contain the character
 114 sequence ".." to avoid security problems.  RFC 3028 does not specify the
 115 syntax of folders apart from keep being equivalent to fileinto "INBOX".
 116 This implementation uses "inbox" instead.
 117
 118 Sieve script errors currently cause that messages are silently filed into
 119 "inbox".  RFC 3028 requires that the user is notified of that condition.
 120 This may be implemented in future by adding a header line to mails that
 121 are filed into "inbox" due to an error in the filter.
 122
 123 The automatic replies generated by "vacation" do not contain an updated
 124 "references" header field.
 125
 126
 127 Semantics Of Keep
 128
 129 The keep command is equivalent to fileinto "inbox": It saves the
 130 message and resets the implicit keep flag.  It does not set the
 131 implicit keep flag; there is no command to set it once it has
 132 been reset.
 133
 134
 135 Semantics Of Fileinto
 136
 137 RFC 3028 does not specify if "fileinto" tries to create a mail folder,
 138 in case it does not exist.  This implementation allows to configure
 139 that aspect using the appendfile transport options "create_directory",
 140 "create_file" and "file_must_exist".  See the appendfile transport in
 141 the Exim specification for details.
 142
 143
 144 Allof And Anyof Test
 145
 146 RFC 3028 does not specify if these tests use shortcut/lazy evaluation.
 147 Exim uses shortcut evaluation.
 148
 149
 150 Action Reordering
 151
 152 RFC 3028 does not specify if actions may be executed out of order.
 153 Exim may execute them out of order, e.g. messages may be filed to
 154 folders or forwarded in a different order than specified, because
 155 those actions only setup delivery, but do not execute it themselves.
 156
 157
 158 Wildcard Matching
 159
 160 RFC 3028 is not exactly clear if comparators act on unicode characters
 161 or on octets containing their UTF-8 representation.  As it turns out,
 162 many implementations go the second way.  This does not make a difference
 163 but for wildcard matching and octet-wise comparison.  Working on unicode
 164 means a dot matches a character.  Working on UTF-8 means the dot matches
 165 a single octet of a multi-octet sequence.  For octet-wise comparisons,
 166 working on UTF-8 means arbitrary byte sequences in headers can not be
 167 matches, as they are rarely correct UTF-8 sequences and can thus not be
 168 expressed as string literal.  This implementation works on unicode, but
 169 this may be changed in case RFC3028bis specifies this issue safe and sound.
 170
 171
 172 Sieve Syntax And Semantics
 173
 174 RFC 3028 confuses syntax and semantics sometimes.  It uses a generic
 175 grammar as syntax for commands and tests and performs many checks during
 176 semantic analysis.  Syntax is specified by grammar rules, semantics
 177 by natural language, despite the latter often talking about syntax.
 178 The intention was to provide a framework for the syntax that describes
 179 current commands as well as future extensions, and describing commands
 180 by semantics.
 181
 182 The following replacement for section 8.2 gives two grammars, one for
 183 the framework, and one for specific commands, thus removing most of the
 184 semantic analysis.  Since the parser can not parse unsupported extensions,
 185 the result is strict error checking of any executed and not executed code
 186 until "stop" is executed or the end of the script is reached.
 187
 188 8.2. Grammar
 189
 190 The atoms of the grammar are lexical tokens.  White space or comments may
 191 appear anywhere between lexical tokens, they are not part of the grammar.
 192 The grammar is specified in ABNF with two extensions to describe tagged
 193 arguments that can be reordered and grammar extensions: { } denotes a
 194 sequence of symbols that may appear in any order.  Example:
 195
 196   options = a b c
 197   start   = { options }
 198
 199 is equivalent to:
 200
 201   start   =  ( a b c ) / ( a c b ) / ( b a c ) / ( b c a ) / ( c a b ) / ( c b a )
 202
 203 The symbol =) is used to append to a rule:
 204
 205   start =  a
 206   start =) b
 207
 208 is equivalent to
 209
 210   start =  a b
 211
 212 All Sieve commands, including extensions, MUST be words of the following
 213 generic grammar with the start symbol "start".  They SHOULD be specified
 214 using a specific grammar, though.
 215
 216    argument        = string-list / number / tag
 217    arguments       = *argument [test / test-list]
 218    block           = "{" commands "}"
 219    commands        = *command
 220    string          = quoted-string / multi-line
 221    string-list     = "[" string *("," string) "]" / string
 222    test            = identifier arguments
 223    test-list       = "(" test *("," test) ")"
 224    command         = identifier arguments ( ";" / block )
 225    start           = command
 226
 227 The basic Sieve commands are specified using the following grammar, which
 228 language is a subset of the generic grammar above.  The start symbol is
 229 "start".
 230
 231   address-part     =  ":localpart" / ":domain" / ":all"
 232   comparator       =  ":comparator" string
 233   match-type       =  ":is" / ":contains" / ":matches"
 234   string           =  quoted-string / multi-line
 235   string-list      =  "[" string *("," string) "]" / string
 236   address-test     =  "address" { [address-part] [comparator] [match-type] }
 237                       string-list string-list
 238   test-list        =  "(" test *("," test) ")"
 239   allof-test       =  "allof" test-list
 240   anyof-test       =  "anyof" test-list
 241   exists-test      =  "exists" string-list
 242   false-test       =  "false"
 243   true=test        =  "true"
 244   header-test      =  "header" { [comparator] [match-type] }
 245                       string-list string-list
 246   not-test         =  "not" test
 247   relop            =  ":over" / ":under"
 248   size-test        =  "size" relop number
 249   block            =  "{" commands "}"
 250   if-command       =  "if" test block *( "elsif" test block ) [ "else" block ]
 251   stop-command     =  "stop" { stop-options } ";"
 252   stop-options     =
 253   keep-command     =  "keep" { keep-options } ";"
 254   keep-options     =
 255   discard-command  =  "discard" { discard-options } ";"
 256   discard-options  =
 257   redirect-command =  "redirect" { redirect-options } string ";"
 258   redirect-options =
 259   require-command  =  "require" { require-options } string-list ";"
 260   require-options  =
 261   test             =  address-test / allof-test / anyof-test / exists-test
 262                       / false-test / true-test / header-test / not-test
 263                       / size-test
 264   command          =  if-command / stop-command / keep-command
 265                       / discard-command / redirect-command
 266   commands         =  *command
 267   start            =  *require-command commands
 268
 269 The extensions "envelope" and "fileinto" are specified using the following
 270 grammar extension.
 271
 272   envelope-test    =  "envelope" { [comparator] [address-part] [match-type] }
 273                       string-list string-list
 274   test             =/ envelope-test
 275
 276   fileinto-command =  "fileinto" { fileinto-options } string ";"
 277   fileinto-options =
 278   command          =/ fileinto-command
 279
 280 The extension "copy" is specified as:
 281
 282   fileinto-options =) ":copy"
 283   redirect-options =) ":copy"
 284
 285
 286 The i;ascii-numeric Comparator
 287
 288 RFC 2244 describes this comparator and specifies that non-numeric strings
 289 are considered equal with an ordinal value higher than any numeric string.
 290 Although not stated explicitly, this includes the empty string.  A range
 291 of at least 2^31 is required.  This implementation does not limit the
 292 range, because it does not convert numbers to binary representation
 293 before comparing them.
 294
 295
 296 The vacation extension
 297
 298 The extension "vacation" is specified using the following grammar
 299 extension.
 300
 301   vacation-command =  "vacation" { vacation-options } <reason: string>
 302   vacation-options =  [":days" number]
 303                       [":subject" string]
 304                       [":from" string]
 305                       [":addresses" string-list]
 306                       [":mime"]
 307                       [":handle" string]
 308   command          =/ vacation-command
 309
 310
 311 Semantics Of ":mime"
 312
 313 The draft does not specify how strings using MIME entities are used
 314 to compose messages.  As a result, different implementations generate
 315 different mails.  The Exim Sieve implementation splits the reason into
 316 header and body.  It adds the header to the mail header and uses the body
 317 as mail body.  Be aware, that other imlementations compose a multipart
 318 structure with the reason as only part.  Both conform to the specification
 319 (or lack thereof).
 320
 321
 322 Semantics Of Not Using ":mime"
 323
 324 Sieve scripts are written in UTF-8, so is the reason string in this
 325 case.  This implementation adds MIME headers to indicate that.  This
 326 is not required by the vacation draft, which does not specify how
 327 the UTF-8 reason is processed to compose the resulting message.
 328
 329
 330 Default Subject
 331
 332 The draft specifies that the default message subject is "Auto: " plus
 333 the old subject.  Using this subject is dangerous, because many mailing
 334 lists verify addresses by sending a secret key in the subject of a
 335 message, asking to reply to the message for confirmation.  Using the
 336 default vacation subject confirms any subscription request of this kind,
 337 allowing to subscribe a third party to any mailing list, either to annoy
 338 the user or to declare spam as legitimate mail by proving to use opt-in.
 339
 340
 341 Rate Limiting Responses
 342
 343 In absence of a handle, this implementation hashes the reason,
 344 ":subject" option, ":mime" option and ":from" option and uses the hex
 345 string representation as filename within the "sieve_vacation_directory"
 346 to store the recipient addresses for this vacation parameter set.
 347
 348 The draft specifies that sites may define a minimum ":days" value than 1.
 349 This implementation uses 1.  The maximum value MUST greater than 7,
 350 and SHOULD be greater than 30.  This implementation uses a maximum of 31.
 351
 352 Vacation recipient address databases older than 31 days are automatically
 353 removed.  Users do not have to remove them manually when modifying their
 354 scripts.  Don't put anything but vacation databases in that directory
 355 or you risk that it will be removed, too!
 356
 357
 358 Global Reply Address Blacklist
 359
 360 The draft requires that each implementation offers a global black list
 361 of addresses that will never be replied to.  Exim offers this as option
 362 "never_mail" in the autoreply transport.