NAME
scanmail, testscan – spam filters

SYNOPSIS
upas/scanmail [ options ] [ qer–args ] root mail sender system rcpt–list

upas/testscan [ –avd ] [ –p patfile ] [ filename ]

DESCRIPTION
Scanmail accepts a mail message supplied on standard input, applies a file of patterns to a portion of it, and dispatches the message based on the results. It exactly replaces the generic queuing command qer(8) that is executed from the rc(1) script /mail/lib/qmail in the mail processing pipeline. Associated with each pattern is an action in order of decreasing priority:
dump    the message is deleted and a log entry is written to /sys/log/smtpd
hold    
the message is placed in a queue for human inspection
log     a line containing the matching portion of the message is written to a log

If no pattern matches or only patterns with an action of log match, the message is accepted and scanmail queues the message for delivery. Scanmail meshes with the blocking facilities of smtpd(6) to provide several layers of filtering on gateway systems. In all cases the sender is notified that the message has been successfully delivered, leaving the sender unaware that the message has been potentially delayed or deleted.

Scanmail accepts the arguments of qer(8) as well as the following:
c         Save a copy of each message in a randomly–named file in directory /mail/copy.
d         Write debugging information to standard error.
h         Queue held messages by sending domain name. The –q option must specify a root directory; messages are queued in subdirectories of this directory. If the –h option is not specified, messages are accumulated in a subdirectory of /mail/queue.hold named for the contents of /dev/user, usually
none.
n         Messages are never held for inspection, but are delivered. Also known as vacation mode.
p filename   Read the patterns from filename rather than /mail/lib/patterns.
q holdroot   Queue deliverable messages in subdirectories of holdroot. This option is the same as the –q option of qer(8) and must be present if the –h option is given.
s         Save deleted messages. Messages are stored, one per randomly–named file, in subdirectories of /mail/queue.dump named with the date.
t         Test mode. The pattern matcher is applied but the message is discarded and the result is not logged.
v         Print the highest priority match. This is useful with the –t option for testing the pattern matcher without actually sending a message.

Testscan is the command line version of scanmail. If filename is missing, it applies the pattern set to the message on standard input. Unlike scanmail, which finds the highest priority match, testscan prints all matches in the portion of the message under test. It is useful for testing a pattern set or implementing a personal filter using the pipeto file in a user's mail directory. Testscan accepts the following options:
a    Print matches in the complete input message
d    Enable debug mode
v    Print the message after conversion to canonical form (q.v.).
p filename
Read the patterns from filename rather than /mail/lib/patterns.

Canonicalization
Before pattern matching, both programs convert a portion of the message header and the beginning of the message to a canonical form. The amount of the header and message body processed are set by compile–time parameters in the source files. The canonicalization process converts letters to lower–case and replaces consecutive spaces, tabs and newline characters with a single space. HTML commands are deleted except for the parameters following A HREF, IMG SRC, and IMG BORDER directives. Additionally, the following MIME escape sequences are replaced by their ASCII equivalents:

Escape Seq     ASCII
––––––––––     –––––
=2e         .
=2f         /
=20      <space>
=3d         =
and the sequence =<newline> is elided. Scanmail assembles the sender, destination domain and recipient fields of the command line into a string that is subjected to the same canonical processing. Following canonicalization, the command line and the two long strings containing the header and the message body are passed to the matching engine for analysis.

Pattern Syntax
The matching engine compiles the pattern set and matches it to each canonicalized input string. Patterns are specified one per line as follows:

{*}action: pattern–spec {~~override...~~override}

On all lines, a # introduces a comment; there is no way to escape this character.

Lines beginning with * contain a pattern–spec that is a string; otherwise, the the pattern–spec is a regular expression in the style of regexp(6). Regular expression matching is many times less efficient than string matching, so it is wiser to enumerate several similar strings than to combine them into a regular expression. The action is a keyword terminated by a : and separated from the pattern by optional white–space. It must be one of the following:
dump    if the pattern matches, the message is deleted. If the –s command line option is set, the message is saved.
hold    if the pattern matches, the message is queued in a subdirectory of /mail/queue.hold for manual inspection. After inspection, the queue can be swept manually using runq (see qer(8)) to deliver messages that were inadvertently matched.
headerthis is the same as the hold action, except the pattern is only applied to the message header. This optimization is useful for patterns that match header fields that are unlikely to be present in the body of the message.
line    the sender and a section of the message around the match are written to the file /sys/log/lines. The message is always delivered.
loff    patterns of this type are applied only to the canonicalized command line. When a match occurs, all patterns with line actions are disabled. This is useful for limiting the size of the log file by excluding repetitive messages, such as those from mailing lists.

Patterns are accumulated into pattern sets sharing the same action. The matching engine applies the dump pattern set first, then the header and hold pattern sets, and finally the line pattern set. Each pattern set is applied three times: to the canonicalized command line, to the message header, and finally to the message body. The ordering of patterns in the pattern file is insignificant.

The pattern–spec is a string of characters terminated by a newline, # or override indicator, ~~. Trailing white–space is deleted but patterns containing leading or trailing white–space can be enclosed in double–quote characters. A pattern containing a double–quote must be enclosed in double–quote characters and preceded by a backslash. For example, the pattern

"
this is not \"spam\""

matches the string this is not "spam". The pattern–spec is followed by zero or more override strings. When the specific pattern matches, each override is applied and if one matches, it cancels the effect of the pattern. Overrides must be strings; regular expressions are not supported. Each override is introduced by the string ~~ and continues until a subsequent ~~, # or newline, white–space included. A ~~ immediately followed by a newline indicates a line continuation and further overrides continue on the following line. Leading white–space on the continuation line is ignored. For example,

*hold:     sex.com~~essex.com~~sussex.com~~sysex.com~~
lasex.com~~cse.psu.edu!owner–9fans

matches all input containing the string sex.com except for messages that also contain the strings in the override list. Often it is desirable to override a pattern based on the name of the sender or recipient. For this reason, each override pattern is applied to the header and the command line as well as the section of the canonicalized input containing the matching data. Thus a pattern matching the command line or the header searches both the command line and the header for overrides while a match in the body searches the body, header and command line for overrides.

The structure of the pattern file and the matching algorithm define the strategy for detecting and filtering unwanted messages. Ideally, a hold pattern selects a message for inspection and if it is determined to be undesirable, a specific dump pattern is added to delete further instances of the message. Additionally, it is often useful to block the sender by updating the smtpd control file.

In this regime, patterns with a dump action, generally match phrases that are likely to be unique. Patterns that hold a message for inspection match phrases commonly found in undesirable material and occasionally in legitimate messages. Patterns that log matches are less specific yet. In all cases the ability to override a pattern by matching another string, allows repetitive messages that trigger the pattern, such as mailing lists, to pass the filter after the first one is processed manually. The –s option allows deleted messages to be salvaged by either manual or semi–automatic review, supporting the specification of more aggressive patterns. Finally, the utility of the pattern matcher is not confined to filtering spam; it is a generally useful administrative tool for deleting inadvertently harmful messages, for example, mail loops, stuck senders or viruses. It is also useful for collecting or counting messages matching certain criteria.

FILES
/mail/lib/patterns   default pattern file
/sys/log/smtpd       log of deleted messages
/mail/log/lines      file where log matches are logged
/mail/queue/*        directories where legitimate messages are queued for delivery
/mail/queue.hold     directory where held messages are queued for inspection
/mail/queue.dump/*   directory where dumped messages are stored when the –s command line option is specified.
/mail/copy/*         directory where copies of all incoming messages are stored.

SOURCE
/sys/src/cmd/upas/scanmail

SEE ALSO
mail(1), qer(8), smtpd(6)

BUGS
Testscan does not report a match when the body of a message contains exactly one line.
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