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<title>Exim's interfaces to mail filtering: 2. Sieve filter files</title>
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<h1 class="chapter"> 2. Sieve filter files </h1>
<p>The code for Sieve filtering in Exim was contributed by Michael Haardt, and
most of the content of this chapter is taken from the notes he provided. Since
Sieve is an extensible language, it is important to understand "Sieve" in
this context as "the specific implementation of Sieve for Exim".
</p>
<p>This chapter does not contain a description of Sieve, since that can be found
in RFC 3028, which should be read in conjunction with these notes.
</p>
<p>The Exim Sieve implementation offers the core as defined by RFC 3028,
comparison tests, the subaddress parameter, the <strong>copy</strong>, <strong>envelope</strong>,
<strong>fileinto</strong>, <strong>notify</strong>, and <strong>vacation</strong> extensions, but not the <strong>reject</strong>
extension. Exim does not support message delivery notifications (MDNs), so
adding it just to the Sieve filter (as required for <strong>reject</strong>) makes little
sense.
</p>
<p>In order for Sieve to work properly in Exim, the system administrator needs to
make some adjustments to the Exim configuration. These are described in the
chapter on the <code>redirect</code> router in the full Exim specification.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="#SEC9">2.1 Recognition of Sieve filters</a></td><td> </td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top"><a href="#SEC10">2.2 Saving to specified folders</a></td><td> </td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top"><a href="#SEC11">2.3 Strings containing header names</a></td><td> </td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top"><a href="#SEC12">2.4 Exists test with empty list of headers</a></td><td> </td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top"><a href="#SEC13">2.5 Header test with invalid MIME encoding in header</a></td><td> </td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top"><a href="#SEC14">2.6 Address test for multiple addresses per header</a></td><td> </td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top"><a href="#SEC15">2.7 Semantics of keep</a></td><td> </td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top"><a href="#SEC16">2.8 Semantics of fileinto</a></td><td> </td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top"><a href="#SEC17">2.9 Semantics of redirect</a></td><td> </td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top"><a href="#SEC18">2.10 String arguments</a></td><td> </td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top"><a href="#SEC19">2.11 Number units</a></td><td> </td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top"><a href="#SEC20">2.12 RFC compliance</a></td><td> </td><td align="left" valign="top">
</td></tr>
</table>
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<h2 class="section"> 2.1 Recognition of Sieve filters </h2>
<p>A filter file is interpreted as a Sieve filter if its first line is
</p>
<table><tr><td> </td><td><pre class="example"># Sieve filter
</pre></td></tr></table>
<p>This is what distinguishes it from a conventional ‘<tt>.forward</tt>’ file or an Exim
filter file.
</p>
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<h2 class="section"> 2.2 Saving to specified folders </h2>
<p>If the system administrator has set things up as suggested in the Exim
specification, and you use <code>keep</code> or <code>fileinto</code> to save a mail into a
folder, absolute files are stored where specified, relative files are stored
relative to <code>$home</code>, and ‘<tt>inbox</tt>’ goes to the standard mailbox location.
</p>
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<h2 class="section"> 2.3 Strings containing header names </h2>
<p>RFC 3028 does not specify what happens if a string denoting a header field does
not contain a valid header name, for example, it contains a colon. This
implementation generates an error instead of ignoring the header field in order
to ease script debugging, which fits in with the common picture of Sieve.
</p>
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<h2 class="section"> 2.4 Exists test with empty list of headers </h2>
<p>The <strong>exists</strong> test succeeds only if all the specified headers exist. RFC 3028
does not explicitly specify what happens on an empty list of headers. This
implementation evaluates that condition as true, interpreting the RFC in a
strict sense.
</p>
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<h2 class="section"> 2.5 Header test with invalid MIME encoding in header </h2>
<p>Some MUAs process invalid base64 encoded data, generating junk. Others ignore
junk after seeing an equal sign in base64 encoded data. RFC 2047 does not
specify how to react in this case, other than stating that a client must not
forbid to process a message for that reason. RFC 2045 specifies that invalid
data should be ignored (apparently looking at end of line characters). It also
specifies that invalid data may lead to rejecting messages containing them (and
there it appears to talk about true encoding violations), which is a clear
contradiction to ignoring them.
</p>
<p>RFC 3028 does not specify how to process incorrect MIME words. This
implementation treats them literally, as it does if the word is correct but its
character set cannot be converted to UTF-8.
</p>
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<h2 class="section"> 2.6 Address test for multiple addresses per header </h2>
<p>A header may contain multiple addresses. RFC 3028 does not explicitly specify
how to deal with them, but since the address test checks if anything matches
anything else, matching one address suffices to satisfy the condition. That
makes it impossible to test if a header contains a certain set of addresses and
no more, but it is more logical than letting the test fail if the header
contains an additional address besides the one the test checks for.
</p>
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<h2 class="section"> 2.7 Semantics of keep </h2>
<p>The <code>keep</code> command is equivalent to
</p>
<table><tr><td> </td><td><pre class="example">fileinto "inbox";
</pre></td></tr></table>
<p>It saves the message and resets the implicit keep flag. It does not set the
implicit keep flag; there is no command to set it once it has been reset.
</p>
<hr size="6">
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<h2 class="section"> 2.8 Semantics of fileinto </h2>
<p>RFC 3028 does not specify whether <code>fileinto</code> should try to create a mail
folder if it does not exist. This implementation allows the sysadmin to
configure that aspect using the <code>appendfile</code> transport options
<code>create_directory</code>, <code>create_file</code>, and <code>file_must_exist</code>. See the
<code>appendfile</code> transport in the Exim specification for details.
</p>
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<h2 class="section"> 2.9 Semantics of redirect </h2>
<p>Sieve scripts are supposed to be interoperable between servers, so this
implementation does not allow mail to be redirected to unqualified addresses,
because the domain would depend on the system being used. On systems with
virtual mail domains, the default domain is probably not what the user expects
it to be.
</p>
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<h2 class="section"> 2.10 String arguments </h2>
<p>There has been confusion if the string arguments to <code>require</code> are to be
matched case-sensitively or not. This implementation matches them with the
match type <code>:is</code> (default, see section 2.7.1 of the RFC) and the comparator
<code>i;ascii-casemap</code> (default, see section 2.7.3 of the RFC). The RFC defines
the command defaults clearly, so any different implementations violate RFC
3028. The same is valid for comparator names, also specified as strings.
</p>
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<h2 class="section"> 2.11 Number units </h2>
<p>There is a mistake in RFC 3028: the suffix G denotes gibi-, not tebibyte.
The mistake is obvious, because RFC 3028 specifies G to denote 2^30
(which is gibi, not tebi), and that is what this implementation uses as
the scaling factor for the suffix G.
</p>
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<h2 class="section"> 2.12 RFC compliance </h2>
<p>Exim requires the first line of a Sieve filter to be
</p>
<table><tr><td> </td><td><pre class="example"># Sieve filter
</pre></td></tr></table>
<p>Of course the RFC does not specify that line. Do not expect examples to work
without adding it, though.
</p>
<p>RFC 3028 requires the use of CRLF to terminate a line. The rationale was that
CRLF is universally used in network protocols to mark the end of the line. This
implementation does not embed Sieve in a network protocol, but uses Sieve
scripts as part of the Exim MTA. Since all parts of Exim use LF as the newline
character, this implementation does, too, by default, though the system
administrator may choose (at Exim compile time) to use CRLF instead.
</p>
<p>Exim violates RFC 2822, section 3.6.8, by accepting 8-bit header names, so this
implementation repeats this violation to stay consistent with Exim. This is in
preparation for UTF-8 data.
</p>
<p>Sieve scripts cannot contain NUL characters in strings, but mail headers could
contain MIME encoded NUL characters, which could never be matched by Sieve
scripts using exact comparisons. For that reason, this implementation extends
the Sieve quoted string syntax with \0 to describe a NUL character, violating
\0 being the same as 0 in RFC 3028. Even without using \0, the following tests
are all true in this implementation. Implementations that use C-style strings
will only evaluate the first test as true.
</p>
<table><tr><td> </td><td><pre class="example">Subject: =?iso-8859-1?q?abc=00def
header :contains "Subject" ["abc"]
header :contains "Subject" ["def"]
header :matches "Subject" ["abc?def"]
</pre></td></tr></table>
<p>Note that by considering Sieve to be an MUA, RFC 2047 can be interpreted in a
way that NUL characters truncating strings is allowed for Sieve
implementations, although not recommended. It is further allowed to use encoded
NUL characters in headers, but that's not recommended either. The above example
shows why.
</p>
<p>RFC 3028 states that if an implementation fails to convert a character set to
UTF-8, two strings cannot be equal if one contains octets greater than 127.
Assuming that all unknown character sets are one-byte character sets with the
lower 128 octets being US-ASCII is not sound, so this implementation violates
RFC 3028 and treats such MIME words literally. That way at least something
could be matched.
</p>
<p>The folder specified by <code>fileinto</code> must not contain the character sequence
".." to avoid security problems. RFC 3028 does not specify the syntax of
folders apart from <code>keep</code> being equivalent to
</p>
<table><tr><td> </td><td><pre class="example">fileinto "INBOX";
</pre></td></tr></table>
<p>This implementation uses ‘<tt>inbox</tt>’ instead.
</p>
<p>Sieve script errors currently cause messages to be silently filed into
‘<tt>inbox</tt>’. RFC 3028 requires that the user is notified of that condition.
This may be implemented in the future by adding a header line to mails that
are filed into ‘<tt>inbox</tt>’ due to an error in the filter.
</p>
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