Policy Script Tutorial. Sophos Unix
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Sophos | Appendices | 233
•
/opt/pmx/sendmail/doc/op.txt
: Sendmail Installation and Operation Guide
•
/opt/pmx/sendmail/doc/op.pdf
: Sendmail Installation and Operation Guide
•
/opt/pmx/sendmail/doc/FAQ
: Sendmail FAQ
•
/opt/pmx/sendmail/doc/KNOWNBUGS
: Known Bugs in Sendmail
•
/opt/pmx/sendmail/doc/LICENSE
: Sendmail License
•
/opt/pmx/sendmail/doc/README
: Sendmail Readme
•
/opt/pmx/sendmail/doc/RELEASE_NOTES
: Sendmail Release Notes
Some sendmail documentation is available on the sendmail website.
Postfix Documentation
If you are using Postfix as your mail transfer agent, refer to the documentation included in your installation. If you have installed the version of Postfix distributed with PureMessage, the Postfix documentation is located in
/opt/pmx/postfix/doc
.
•
/opt/pmx/postfix/doc/index.html
: Postfix Introduction
•
/opt/pmx/postfix/doc/basic.html
: Postfix Configuration
•
/opt/pmx/postfix/doc/motivation.html
: Postfix Overview
•
/opt/pmx/postfix/doc/receiving.html
: Postfix Anatomy
•
/opt/pmx/postfix/doc/faq.html
: Postfix FAQ
Postfix documentation is also available on the
Postfix
website.
Java System Messaging Server (JSMS) Documentation
If you are using JSMS version 5.2 or 6.0 as your mail transfer agent (MTA), PureMessage offers basic configuration options
configuration file.
If you are using JSMS version 6.3 or 7.0, you can configure a direct connection with the PureMessage milter as described in “Configuring PureMessage for Java Systems Messaging Server (JSMS)” in the Knowledgebase on the Sophos website.
For advanced configuration options, refer to the
Java System Messaging Server Administration Guide
.
PostgreSQL Documentation
If you are using PostgreSQL to consolidate quarantined messages, refer to the documentation on the PostgreSQL website:
http://www.postgresql.org/docs/
.
Policy Script Tutorial
PureMessage filters email according to the configuration contained in the policy script ( policy.siv
, located by default in the directory
/opt/pmx/etc
). The policy script can be modified via the PureMessage Manager, or can be edited with standard text editing programs such as vi
. This tutorial describes the syntax used in the policy script, analyzes the default
PureMessage policy script, and shows examples of common policy script modifications.
Refer to the pmx-policy
manpage for the policy command reference. Refer to “Policy Configuration” in the PureMessage
User Guide for a general overview of the PureMessage policy, and links to the PureMessage Manager interface for modifying policies.
For information about specific PureMessage policy tests and actions, see the pmx-policy
man page.
Sieve is a language used for filtering email messages. It is a multi-vendor effort, and has been proposed as a standard to the
Internet Engineering Task Force. PureMessage makes use of Sieve for filtering email via policies. Filter parameters are stored in the file policy.siv
, located by default in the
/opt/pmx/etc
directory. For information about manually modifying
234 | Sophos | Appendices the Sieve code in the policy.siv
file, see the pmx-policy
man page and the web pages http://www.gnu.org/software/mailutils/manual/html_node/mailutils_93.html and http://www.ietf.org/rfc/rfc3028.txt
Policy Script Syntax Overview
This section introduces the PureMessage policy syntax. Examples in this overview use pseudocode to best describe policy syntax structure and generically demonstrate various PureMessage mail filtering scenarios. Specific PureMessage policy syntax is discussed later in PureMessage Default Policy Overview.
The PureMessage policy script, called policy.siv
, is written using a variation of the Sieve mail filtering language. Sieve is a simple command-based scripting language; its syntax resembles C or Perl, but it has no variables or looping constructs.
In its simplest form, a Sieve script is a set of commands executed in a sequence. The commands in the PureMessage policy.siv
script can be actions, action modifiers, tests, controls, blocks or comments.
The following example identifies the commands in a policy filter:
# This script performs a virus test.
# Messages containing viruses are filed as SPAM and then quarantined.
if pmx_virus {
pmx_file "SPAM";
pmx_quarantine "Virus";
stop;
}
Description:
• The '
#
' characters are Sieve comments.
• The if
and stop
commands are controls.
• The pmx_virus
command is one example of many possible tests in PureMessage.
• The '
{
' and '
}
' characters delimit blocks.
• The pmx_quarantine
and pmx_file
commands are PureMessage action
• The "Virus" and "SPAM" strings are action modifiers.
• Actions are delimited with the '
;
' character.
Actions
Policy actions can modify the routing or content of a message. Possible actions include: keep;
(deliver message), reject;
(reject message), and pmx_quarantine;
(quarantine message). Actions always end with a semicolon. Semicolons delimit when an action ends.
Generic Example:
action;
Policy Script Example:
keep; reject; pmx_quarantine;
Action Modifiers
Action modifiers are optional parameters that alter the outcome of a particular policy action. For example, the pmx_quarantine action modifier is a string argument that specifies the reason that PureMessage quarantined a message. Reasons could include
'Virus', 'Spam', or 'Attachment Over 100K'.
Sophos | Appendices | 235
Generic Example:
Policy Script Example:
action <action-modifier>; pmx_quarantine "Spam";
Note: Not all PureMessage action modifiers are of type 'string'. See the pmx-policy
manpage for a definitive list of all syntax parameters and action modifiers.
Blocks
PureMessage policy actions are grouped together in blocks. Blocks act like containers; they hold actions together. A block begins with an opening brace “{” and ends with a closing brace “}”. Every action in a block must end with a semicolon.
The block itself ends with a closing brace.
Generic Example:
{
action;
}
Policy Script Example:
Policy Script Example:
{
keep;
}
In this example, the policy block contains the single action keep
. The keep
command delivers the message to all envelope recipients. Blocks can, however, contain one or many actions. When multiple actions are listed inside a block, each action is executed in a sequence from top to bottom.
{
pmx_quarantine "Blacklisted";
stop;
}
In this example, the policy block contains a sequence of two mail filtering actions. The message is first quarantined with the reason “Blacklisted”, and then the script stops. Each action is delimited with a semicolon to indicate to the interpreter when each action ends.
Controls
Control commands determine whether a block of actions are executed, or if no further actions should execute. Control keywords are positioned with an associated test command before the opening brace in a block to help control the order of execution through the policy script. Note that tests are covered further in the next section of the tutorial. For now, understand that a test command evaluates to either “true” or “false”. The PureMessage policy.siv
script uses three control keywords to filter messages: if
, else
, and stop
.
if
The most basic control command is the if
keyword. When the interpreter sees an if
command, it evaluates the associated test. If the test is “true”, the actions within the block execute.
236 | Sophos | Appendices
Generic Example:
Policy Script Example:
if test {
action;
next_action;
} if pmx_virus {
reject "Virus Found in Message";
stop;
}
This example tests if a message contains a virus. If the test is “true” and the message contains a virus, the actions within the block execute from top to bottom. The message is rejected with reason “Virus Found in Message”, and message filtering stops. But what happens if the test is “false” and the message is legitimate?
else
The else
command can execute when an if
command fails. The if
and else
control statements work together in a policy script to filter messages.
Generic Example:
if test {
action;
next_action;
} else {
action;
next_action;
}
In this example, if
the test
is “true”, execute the first set of statements. else
, execute the second.
Policy Script Example:
if pmx_virus {
reject "Virus Found in Message";
stop;
} else {
keep;
}
In this policy filter, the if
and else
control statements work together with tests and actions to either reject a virus-laden message or to deliver virus-free mail to envelope recipients. The else
command executes when the if
statement fails.
stop
The stop
control halts execution of the policy script. Unlike the if
and else
control commands, the stop
command is used within a block to end message filtering when the required actions have executed and message filtering is no longer required.
Generic Example:
if test {
action;
next_action;
stop;
}
Policy Script Example:
if pmx_relay :memberof "whitelisted-hosts" {
keep;
stop;
}
In this example, the filter delivers the message if the host is on the PureMessage “whitelisted-hosts” list. The stop
control command ends the policy script after message delivery.
Sophos | Appendices | 237
Tests
Tests act as block gatekeepers. They check whether a certain condition in the policy script is met. A test is positioned with a control command before the opening brace in a block to determine whether the policy actions should execute. If a condition is met, the test is “true” and the actions within the block execute. If the test fails, the condition is “false” and no actions will execute for that particular policy.
Generic Example:
if test {
action;
next_action;
}
Policy Script Example:
if pmx_virus {
reject "Virus Found in Message";
stop;
}
In this example, a message must pass the pmx-virus
test before the actions in the block can execute. If the test is “true” and the message contains a virus, the message is rejected and the policy script stops filtering the message. If the message does not contain a virus, the test fails and no actions from the block will execute.
Match-Types
A match-type performs a matching operation on a particular feature of a message. In the policy script, the match-type modifies a test to check for specific characteristics.
For example, the
:is
match-type is used here to modify the envelope test to determine if a message is “From”
if envelope :is "From" "[email protected]" {
pmx_quarantine "SPAM";
}
PureMessage uses several match-types to filter messages. These include:
:memberof
for matching content in lists ,
:contains
and
:is
for matching strings,
:over
or
:under
for numeric comparison, and
:re
for searching messages with regular expressions. Depending on the specific match-type used, additional parameters will have to be specified. See
“MATCH-TYPE” in the pmx-policy
manpage guide for more information.
Generic Example:
if test :match_type {
action;
next_action;
}
Lists
Use the
:memberof
match-type tag to determine if a sender or host is on a particular PureMessage list. Lists are enclosed in quotes when passed as arguments with the
:memberof
match-type tag.
Policy Script Example:
if envelope :memberof "From" "whitelisted-senders" {
keep;
stop;
}
In this example, the match-type
:memberof
takes a PureMessage “whitelisted-senders” list as an argument. If the message sender is matched with a sender on this PureMessage list, the message is delivered to the envelope recipient.
Strings
Use the
:contains
or
:is
match-type to determine if a text string can be matched against content in a message. Strings are enclosed in quotes when passed as arguments with the
:memberof
and
:is
match-type tags.
238 | Sophos | Appendices
Policy Script Example:
if header :contains "Buy Now!" {
pmx_quarantine "SPAM";
stop;
}
In this example, the match-type
:contains
takes a text string as an argument. If the string “Buy Now!” is matched with content in any message header, the message is quarantined with the reason “SPAM”. The stop command ends filter execution.
Numeric Comparison
Use either the
:over
or
:under
match-type tags to specify a numeric comparison. Both tags take a number as an argument to determine whether a test is “true” or “false”. For example, the
:over
tag can compare a message's spam probability to determine if a particular message should be delivered.
Policy Script Example:
if pmx_spam_prob :over 50 {
pmx_quarantine;
stop;
}
In this example, the
:over
match-type is used to perform a numeric comparison. The
:over match-type takes a number argument. The test is “true” if the message has a spam probability of over 50%.
Policy Script Example:
if size :over 100K {
pmx_file;
discard;
}
This example tests to see if the total message size is over 100K. If the message is over 100K, a copy of the message is filed in the quarantine. The message is then discarded.
Regular Expressions
Regular expressions can be used as match-type operators for many types of policy rule tests. For example, a regular expression can be used to test the contents of a message's “Envelope To” field. Certain policy rule actions also support the use of regular expressions. For example, the “Deliver immediately for” action supports the use of a regular expression as a match-type for exceptions.
Policy Script Example:
if pmx_relay :re ".*\.com$" {
keep;
stop;
}
In this example, the match-type
:re
is used to perform a regular expression comparison. If the sender's address ends in the string “.com”, the message is delivered and the filter stops evaluating the message.
Note: The
:re
match-type is a PureMessage extension to the Sieve language. When regular expressions are used in
PureMessage policy rule tests or actions, they are not prefixed or suffixed with slashes or braces. However, when manually editing the policy script on the command-line, you must “escape” backslashes, quotes and periods within regular expressions by preceding them with a backslash. For example, to search for the string “and\or”, enter “ and\\or
” as the regular expression. The PureMessage Manager, a web-based interface for managing PureMessage, will automatically escape these characters for you.
For information about using regular expressions with the policy script via the PureMessage Manager, see “Policy Rule
Tests” and “Policy Rule Actions” in the Policy section of the PureMessage Administrator's Reference. If you are manually editing the policy script, see the pmx-policy
manpage (the documentation for the command-line interface to the PureMessage policy engine). See the “Regular Expressions Primer” in the PureMessage Administrator's Reference for general information about regular expressions.
Sophos | Appendices | 239
Compound Tests, allof
and anyof
Some tests in the PureMessage policy script are used as logical operators. These tests are called compound tests as they take other tests as arguments. Use the allof
or anyof
commands to build a compound test. The allof
command functions as logical AND. (All compound tests in the allof
list must be “true” for the corresponding actions in a block to execute.) The anyof
command functions as a logical OR. (Only one of the compound tests in the anyof
list needs to be “true” for the actions in the block to execute).
In the policy script, the allof
and anyof
commands take a test and a list as arguments. To build a compound test, group individual tests and associated arguments in parentheses before the beginning of a block statement. Each compound test in the list must be separated with a single comma “,” to delimit the end of each test.
Generic Example:
if anyof (test :match_type "list", test :match_type "list") {
action;
next_action;
}
In this example, the anyof
command takes two tests. Each test takes a match-type with a list argument. The compound test is enclosed in parentheses. Each test is separated with a comma character.
Policy Script Example:
if anyof (host :memberof "whitelisted-hosts",
sender :memberof "whitelisted-senders") {
keep;
stop;
}
This policy filter evaluates both the host and sender for membership on a PureMessage “whitelist”.
If either the host or sender are found on a “whitelist”, the message is delivered to all envelope recipients. The script then halts with the stop
command.
Comments
Comments are used in the policy script to document how a particular test, action, or control statement contributes to a filter.
For each line of commented text, use the “#” character followed by a single space to denote information the Sieve interpreter should ignore.
Note: Comments should not be placed within multi-line strings.
Generic Example:
# Comments
# More comments if test { # This is a valid comment!
action;
next_action;
}
Policy Script Example:
# This filter scans all message headers for the words "Buy Now!"
# If this string is found, PureMessage rejects the message
# and the filter stops evaluating the message for spam.
if header :contains "Buy Now!" {
reject;
stop;
}
PureMessage Default Policy Overview
This section reviews the structure, syntax, and execution of each filter in the default policy.siv
script. The policy script is executed for each message processed by PureMessage. It can test various characteristics of the message, and perform a variety of actions based on the results of those tests.
240 | Sophos | Appendices
Note: The default policy script is installed and enabled during PureMessage installation. The default policy varies according to your PureMessage license; for example, if you do not have a license for the PureMessage Virus component, virus-checking rules are not configured.
PureMessage Sieve Extensions
The PureMessage policy.siv
script uses extensions to the Sieve language to filter mail. Each PureMessage specific extension is prefixed with pmx_
to identify the action, test, or command as unique to the program. To use these extensions include the command “ require PureMessage;
” at the beginning of the policy script. PureMessage also provides the standard extensions reject
and envelope
as specified in RFC 3028. Note that both reject
and envelope
are included implicitly with require "PureMessage";
. See the pmx-policy
manpage for more on specific PureMessage Sieve extensions.
Lastly, PureMessage has an extension for using regular expressions in match-type tests. See Regular Expressions.
policy.siv
In general, the default policy.siv
file is comprised of eight if
control statements and one else
statement. Each control statement is comprised of a type of test, a block and an associated sequence of actions. Action sequences can either modify or route particular messages in PureMessage. Actions, tests, and commands specific to PureMessage are prefixed with pmx_ to identify the command as an extension to Sieve.
For the purpose of discussion, the policy.siv
file is broken into six parts. Each script piece is followed by a detailed description of the commands and syntax used to filter mail in PureMessage.
Policy Script 1: Scan and Deliver Internal Messages
pmx_test_mark;
# attr NAME=Mail from internal hosts if pmx_relay :memberof "internal-hosts" {
# The 'pmx-mlog-watch' depends on this to know which messages
# are outgoing and which are not.
pmx_mark1 "i";
# attr NAME=Reject mail containing viruses
if pmx_virus {
reject "One or more viruses were detected in the message.";
stop;
}
}
Description:
• The pmx_test_mark
command is used to recognize sample messages sent by the “ pmx-test
” program.
Note: Consider disabling this action if running a site with a high-mail volume as this command will delay relay tests. See the pmx-policy
manpage for further details.
• If the pmx_relay
test finds the message originated from a relay defined in the “internal-hosts” list:
• The pmx_mark1
command adds a “i” mark to the message log to enable “Perimeter Protection” to distinguish outgoing messages from internal hosts. Messages accumulate “marks” throughout the filtering process. These marks can be used later to generate custom statistical reports.
• The pmx_virus
command scans the message for virus threats. If the message contains a virus:
• The message is rejected with the reason “One or more viruses were detected in the message”.
• The stop control ends message processing.
Policy Script 2: Scan External Mail for Viruses
# attr NAME=Mail from external hosts else {
pmx_add_header "X-PMX-Version" "%%PMX_VERSION%%";
pmx_mark "Size" "%%MESSAGE_SIZE%%";
Sophos | Appendices | 241
# attr NAME=Clean mail containing viruses
if pmx_virus {
pmx_file "Virus";
pmx_virus_clean "cantclean.tmpl";
pmx_replace_header "Subject" "[PMX:VIRUS] %%SUBJECT%%";
pmx_replace_header "X-PerlMx-Virus-Detected" "%%VIRUS_IDS%%";
}
Description:
• If the if pmx-relay
test is “false”, and the else control is executed:
• Mail is assumed to be from an external host.
• The pmx_add_header
command adds a header to the message. The header includes the string “
X-PMX-Version
” and the PureMessage version (which is added using the
%%PMX_VERSION%%
template variable.)
• The size of the message is then written to the message log. The
%%MESSAGE_SIZE%%
template variable is substituted for the actual size of the message, in bytes.
• The pmx_virus
command tests the message for virus threats. If the test is “true”, and the message contains a virus:
• pmx_file
copies the message to the quarantine with the reason “Virus”. This action does not affect the delivery of the message.
• The pmx_virus_clean
action attempts to clean the virus from the message. If cleaning fails, the message is quarantined, and a message is sent to the recipient based on the specified failure template file, cantclean.tmpl
.
• The pmx_replace_header
command prefixes the “Subject” header with “
[PMX:VIRUS]
”. The original “Subject” is added to the end of the header with the
%%SUBJECT%%
template variable.
• The pmx_replace_header
command then adds, or alters, a “X-PerlMx-Virus-Detected” header. The
%%VIRUS_IDS%%
template variable adds the found virus IDs to the message header.
Policy Script 3: Deliver External Whitelisted Messages
# attr NAME=Deliver mail from whitelisted hosts and senders if anyof(pmx_relay :memberof "whitelisted-hosts",
envelope :memberof "From" "whitelisted-senders") {
keep;
stop;
}
Description:
• If the message originates from a relay defined in the “whitelisted-hosts” list OR the message has an “Envelope From” address defined in the “whitelisted-senders” list:
• The keep
action accepts the message for delivery.
• The stop
control ends message processing.
Policy Script 4: Deliver External Messages to Anti-Spam Opt-Outs
# attr NAME=Deliver mail to anti-spam opt-outs if true {
pmx_deliver_for :memberof "anti-spam-optouts";
}
• If the message recipient address is defined in the “Anti-Spam Opt-Outs” list:
• The pmx_deliver_for
“opt-in/opt-out” command delivers the message. This command matches against the message envelope recipient and opts them out of further filter processing.
Policy Script 5: Quarantine External Blacklisted Messages
# attr NAME=Quarantine mail from blacklisted hosts and senders if anyof(pmx_relay :memberof "blacklisted-hosts",
envelope :memberof "From" "blacklisted-senders") {
pmx_quarantine "Blacklisted";
242 | Sophos | Appendices
stop;
}
Description:
• If the message originates from a relay defined in the “blacklisted-hosts” list, or the message has an “Envelope From” address defined in the “blacklisted-senders” list:
• The pmx_quarantine
action sends the message to the PureMessage quarantine with the reason “Blacklisted”.
• The stop control ends message processing.
Policy Script 6: Calculate Spam Probability, Modify and Deliver
Modify Message
# import levels here
# attr NAME=Copy to quarantine and deliver if spam probability is 50%
# or more.
if not pmx_spam_prob :under 50 {
pmx_replace_header "X-PerlMx-Spam" "Gauge=%%XGAUGE%%%%IGAUGE%%,
Probability=%%PROB%%, Report='%%HITS%%'";
pmx_file "Spam";
pmx_replace_header "Subject" "[PMX:%%GAUGE%%] %%SUBJECT%%";
stop;
}
Description:
• If the pmx_spam_prob
test finds the message has a spam probability of 50% or more:
• pmx_replace_header
adds, or alters, an “X-PerlMx-Spam” header. The
%%XGAUGE%%%%IGAUGE%%
variables add an “X” symbol to the header for every 10% of spam probability identified through the
%%PROB%%
variable. The
%%HITS%%
variable then adds a list of spam features found by the engine.
• The pmx_file
command then copies the message to the quarantine with the reason “Spam”.
• The pmx_replace_header
command prefixes the message “Subject” header with “PMX:” and a “#” symbol for every 10% the message's spam probability exceeds 50%. At least one '#' is always appended to each message, indicating that the message contains 0-50% spam. The original message subject is added to the end of the header with the
%%SUBJECT%%
template variable.
• The stop
control ends message processing.
Deliver Message
# attr NAME=Add X-Header and deliver messages else {
pmx_replace_header "X-PerlMx-Spam" "Gauge=%%XGAUGE%%%%IGAUGE%%,
Probability=%%PROB%%, Report='%%HITS%%'";
stop;
}
Description:
• If the pmx_spam_prob
test finds the message has a spam probability of less than 50%:
• pmx_replace_header
adds, or alters, an “X-PerlMx-Spam” header. The
%%XGAUGE%%%%IGAUGE%%
variables add an “X” symbol to the header for every 10% of spam probability identified through the
%%PROB%%
variable. The
%%HITS%%
variable then adds a list of spam features found by the engine.
• The stop
control then ends message processing.
Sophos | Appendices | 243
Customizing the PureMessage Default Policy
This section describes how to customize the PureMessage default policy to build mail filters suitable for your environment.
Various filtering scenarios are provided to illustrate different approaches to policy modification. These scenarios are:
Adding a Recipient
Occasionally, a site may require certain messages to be automatically sent to a known address. For example, all messages containing the keyword “bug” might be sent to an bug tracking system. This policy adds a new recipient to a message
) when the “Subject” header has the string “bug” in it, the recipient is “[email protected]”, or the header “X-Bug-Id” exists.
# attr NAME=add a recipient
# Adds a "[email protected]" recipient to the envelope.
if anyof (header :matches "Subject" "*bug*",
envelope "to" "[email protected]",
header :matches "X-Bug-Id" "*") {
pmx_add_recipient "[email protected]";
}
Description:
• The first command is an if anyof
statement comprised of three compound tests. The header test returns “true” if the
“Subject” header matches the string “bug”. The envelope
test returns “true” if anyof
the message's recipient “To” headers match the string “[email protected]”. The second header test returns “true” if the “X-Bug-Id” header matches itself. If any of these three tests return “true”:
• The pmx_add_recipient
command delivers the message to [email protected]
. The message is also delivered to all original envelope recipients.
Where does this filter go?
The “add a recipient” filter should be placed before the filter in “Policy Script 1: Scan and Deliver Internal Messages” and directly after the pmx_test_mark
action. Positioning the “add a recipient” filter at this point ensures that:
• The “add a recipient” filter always executes. (A stop
command from another filter will not prevent it from executing.)
• Subsequent spam and virus filters will always execute. (The “add recipient” filter does not contain a stop
command that would otherwise end the policy script before spam and virus filters execute.)
Adding a Header
Adding a new header to all messages can be useful for tracking purposes. This filter adds an “X-Seen-By” header to all messages.
# attr NAME=add a header
# Adds an 'X-Seen-By' header to all messages.
pmx_add_header "X-Seen-By" "%%HOSTNAME%%";
Description:
• The pmx_add_header
action adds an “X-Seen-By” header to the message. The value of the header is the hostname of the PureMessage machine (for example mail.example.com
). The
%%HOSTNAME%%
template variable is called. For a list of supported template variables, see the pmx-policy
manpage.
Where does this filter go?
The “add a header” filter should be placed before the filter in “Policy Script 1: Scan and Deliver Internal Messages” and directly after the pmx_test_mark
action. Positioning the “add a header” filter at this point ensures that:
• The “add a header” filter always executes. (A stop
command from another filter will not prevent it from executing.)
244 | Sophos | Appendices
• Subsequent spam and virus filters will always execute. (The “add a header” filter does not contain a stop
command that would otherwise end the policy script before spam and virus filters execute.)
Detecting Spam
In some instances, it may be preferable to deliver all spam messages to envelope recipients. For example, site administrators would consider this action when initially testing and optimizing PureMessage. In this situation, a spam detection filter is beneficial for identifying spam and delivering all messages to recipients with an associated spam probability. During the optimization process, mail recipients could then comment on the accuracy of a particular filter.
Note: The optional “PureMessage-Policy-Spam” component is required to use the “spam detection” filter.
# attr NAME=spam detection
# Detects spam probability over 50%.
# Prefixes 'Subject' header with '[SPAM:]'.
# Adds an 'X-PMX-Spam' header.
if pmx_spam_prob :over 50 {
pmx_replace_header "Subject" "[SPAM:%%GAUGE%%] %%SUBJECT%%";
pmx_add_header "X-PMX-Spam" "Probability=%%PROB%%";
stop;
}
Description:
• If the pmx_spam_prob
test finds the message has a spam probability of 50% or more:
• The pmx_replace_header
action prefixes the “Subject” of the message with a string similar to: “
[SPAM:###]
”, where each additional “#” character denotes 10% above the argument to pmx_spam_prob
(at least one '#' is always appended, indicating that the message contains 0-50% spam).
Example: Using this filter, a message with a spam probability of 60% would have its “Subject” header prefixed with
“
[SPAM:##]
”. A message with only 50% probability would have its “Subject” header prefixed with “
[SPAM:#]
”.
• The pmx_add_header
action then adds an “X-PMX-Spam” header to indicate the message's numerical spam probability. The message header displays as follows:
X-PMX-Spam: Probability=63%
• The stop
command then ends message processing.
Where does this filter go?
The “spam detection” filter replaces the “Copy to quarantine and deliver if spam probability is 50% or more” filter in Policy
Script 6: Calculate Spam Probability, Modify and Deliver. Replacing the first part of this default policy filter with the “spam detection” filter ensures that:
• The “spam detection” filter executes when messages from external hosts contain over 50% spam.
• Messages with a spam probability of 50% or more are not quarantined. Headers are added with an associated spam probability.
Quarantining Spam Messages
Use the following “quarantine spam” filter to decrease the number of spam messages directed to a recipient's mailbox. The
“quarantine spam” filter is a modification to the default policy script filter found in “Policy Script 6: Calculate Spam
Probability, Modify and Deliver”. Unlike the default policy script filter, the “quarantine spam” filter quarantines messages when the spam probability is over 80%. (The default policy filter only “files” spam messages when the spam probability is over 50%. See the pmx_file
command in the pmx-policy
manpage.
# attr NAME=quarantine spam
# Quarantines spam if probability over 80%.
# Else, delivers message if probability over 50%.
# Prefixes "Subject" header with "[SPAM:]".
Sophos | Appendices | 245
# Adds an "X-PMX-Spam" header.
if pmx_spam_prob :over 80 {
pmx_quarantine "Spam";
stop;
} elsif pmx_spam_prob :over 50 {
pmx_replace_header "Subject" "[SPAM:%%GAUGE%%] %%SUBJECT%%";
pmx_add_header "X-PMX-Spam" "Probability=%%PROB%%";
}
Description:
• If the pmx_spam_prob
test finds the message has a spam probability of 80% or more:
• The pmx_quarantine
action copies the message to the PureMessage quarantine with the reason “Spam”. This string should be a single word. If multiple words are used the spaces between the words are silently changed to underscores.
For example, the string “message is spam” becomes “message_is_spam”. Quarantined messages can be viewed, released and digested using the PureMessage Manager and the quarantine tools.
• The stop
command then ends message processing.
• If the pmx_spam_prob
test finds the message has a spam probability of 50% or more:
• The pmx_replace_header
action prefixes the “Subject” of the message with a string similar to: “
[SPAM:###]
”, where each additional “#” character denotes 10% above the argument to pmx_spam_prob
(at least one '#' is always appended, indicating that the message contains 0-50% spam).
• The pmx_add_header
action then adds an “X-PMX-Spam” header to indicate the message's complete spam probability.
Where does this filter go?
The “quarantine spam” filter replaces the filter found in “Policy Script 6: Calculate Spam Probability, Modify and Deliver”.
Replacing this default policy filter with the “quarantine spam” filter ensures that:
• The “quarantine spam” filter executes when messages from external hosts have spam probabilities of either over 80% or over 50%.
• Messages with a spam probability of 80% or more are quarantined.
• Messages are delivered, with additional headers, to envelope recipients if the spam probability is between 50% and 79%.
Catching Viruses
Use the following three “virus” filters to modify how PureMessage handles virus-laden messages.
Note: The optional “PureMessage-Policy-Virus” package is required when using any of the following virus filters.
Example 1: Quarantine all external messages containing virus variants.
Use the “virus 1” filter to quarantine all external messages containing virus variants. The “virus 1” filter is a modification to the default policy filter found in Policy Script 2: Scan External Mail for Viruses. Unlike the default filter, the “virus 1” filter quarantines all messages containing virus variants. No attempt is made to clean infected messages. See the pmx_virus command in the pmx-policy
manpage.
# attr NAME=virus 1
# Quarantines all infected messages.
if pmx_virus {
pmx_quarantine "Virus";
}
Description:
• If the pmx_virus
test detects a virus in the message:
246 | Sophos | Appendices
• The pmx_quarantine
action sends the message to the PureMessage quarantine with the reason “Virus”.
Where does this filter go?
The “virus 1” filter replaces the Policy Script 2: Scan External Mail for Viruses filter. Replacing this default policy filter with the “virus 1” filter ensures that:
• The “virus 1” filter executes when messages from external hosts containing virus variants are detected.
• All external messages containing virus variants are quarantined.
• Subsequent PureMessage policy filters will always execute. (The “virus 1” filter does not contain a stop
command that would otherwise end the policy script before other PureMessage filters execute.)
Example 2: Attempt to clean all internal messages containing virus variants.
Use the “virus 2” filter to clean all internal messages containing virus variants. The “virus 2” filter is a modification to the default policy filter found in Policy Script 1: Scan and Deliver Internal Messages. Unlike the default filter, the “virus 2” filter attempts to clean virus variants from all messages sent through internal hosts. The default policy rejects all internal mail containing viruses.
# attr NAME=virus 2
# if pmx_virus {
pmx_file "Virus";
pmx_virus_clean "cantclean.tmpl";
pmx_replace_header "Subject" "[PMX:VIRUS] %%SUBJECT%%";
stop;
}
Description:
This filter attempts to clean the virus-laden message. If the message is successfully cleaned, it is sent to its original recipients.
If the virus is not successfully cleaned, the infected part is replaced with the error template cantclean.tmpl
. The “Subject” is marked with “
[PMX:VIRUS]
” to inform recipients that PureMessage found a virus.
Where does this filter go?
The “virus 2” filter replaces the Policy Script 1: Scan and Deliver Internal Messages filter. Replacing this default policy filter with the “virus 2” filter ensures that:
• The “virus 2” filter executes when messages from internal hosts containing virus variants are detected.
• All messages sent through internal hosts are cleaned if they contain virus variants.
• Subsequent PureMessage policy filters never execute. (The “virus 2” filter contains a stop
command which ends the policy script and prevents other PureMessage filters from executing.)
Example 3: Discard external messages containing specific viruses.
Use the “virus 3” filter to evaluate mail sent through external hosts and to discard messages containing either the “Klez” or
“Sobig” variants. The “virus 3” filter is a modification to the default policy filter found in Policy Script 2: Scan External
Mail for Viruses. Unlike the default filter, the custom “virus 3” filter searches for specific viruses using the pmx_virus_id command.
# attr NAME=virus 3
# Discards messages infected with Klez or Sobig variants.
# Attempts to clean messages infected with other variants.
if pmx_virus {
if pmx_virus_id :matches ["*Klez*", "*Sobig*"] {
discard;
stop;
}
pmx_file "Virus";
pmx_virus_clean "cantclean.tmpl";
Sophos | Appendices | 247
pmx_replace_header "Subject" "[PMX:VIRUS] %%SUBJECT%%";
}
Description:
• The pmx_virus
command tests the message for virus threats. If the test is “true”, and the message contains a virus:
• The pmx_virus_id
test checks if the message contains either the “Klez” or “Sobig” variants. If either virus is found:
• The message is discarded.
• The stop
command ends message processing.
• The pmx_file
action then copies the message to the quarantine with the reason “Virus”.
• The pmx_virus_clean
action attempts to clean the virus from the message. If cleaning fails, the message is quarantined, and a message is sent to the recipient based on the specified failure template file, cantclean.tmpl
.
• The pmx_replace_header
command prefixes the “Subject” header with
[PMX:VIRUS]
. The original “Subject” is added to the end of the header with the
%%SUBJECT%%
template variable.
Where does this filter go?
The “virus 3” filter replaces the “Policy Script 2: Scan External Mail for Viruses filter”. Replacing this default policy filter with the custom “virus 3” filter ensures that:
• The “virus 3” filter always executes when messages from external hosts containing specific virus variants are detected.
• All external messages containing specific virus variants are discarded.
• Subsequent PureMessage policy filters will always execute. (The “virus 3” filter does not contain a stop command that would otherwise end the policy script before other PureMessage filters execute.)
Discarding Messages Based on Specific Characteristics
# attr NAME=discard
# Discards messages containing key phrases.
if header :matches "Subject" ["Re: Approved", "Re: Details", "Re: Movie",
"Re: My details", "Re: Thank you!"] {
discard;
stop;
}
Description:
• If the message “Subject” header matches any of the following strings in the list: “Re: Approved”, “Re: Details”, “Re:
Movie”, “Re: My details”, “Re: Thank you!”
• The message is discarded.
• The stop
command ends message processing.
Where does this filter go?
The “discard” filter should be added directly after the “Policy Script 5: Quarantine External Blacklisted Messages” policy filter. Positioning the “discard” filter at this point ensures that:
• The “discard” filter only executes after other PureMessage policies have filtered out spam, viruses, whitelists, blacklists, and opt-out lists.
(A stop
command from another filter will not prevent it from executing.)
• Subsequent filters will never execute. (The “discard” filter uses a stop
command to halt filtering on messages containing specific characteristics.)
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