Exchange Server 2010 : Managing Transport Rules (part 1) - Using Transport Rules

Moderated transport is a new feature in Exchange Server 2010 that enables a moderator to intercept and check mail to a specified recipient (typically a distribution group) and allow or block delivery depending on the acceptability of the message. This lesson discusses how moderated transport works, how you configure a moderated recipient and specify a moderator, and how you configure an additional arbitration mailbox.

1. Using Transport Rules

Your organization may be required by law, regulatory requirements, or company policies to apply messaging policies that limit interaction between recipients and senders (both individual senders and departmental groups). Such limitations can apply both inside and outside the organization. In addition to limiting interactions inside the organization, you also need to prevent inappropriate content from entering or leaving the organization, filter confidential information, track or archive specified messages, redirect inbound and outbound messages so that they can be inspected, and apply disclaimers to messages as they pass through the organization. The mechanism that enables you to accomplish all these aims is the transport rule.

You can use transport rules to apply messaging policies to email messages that flow through the transport pipeline on Hub Transport and Edge Transport servers. These rules permit you to comply with messaging policies, secure messages, prevent information leakage, and protect messaging systems.

You create a transport rule by specifying rule conditions, exceptions, and actions. The transport rule agent (on Hub Transport servers) or the edge rules agent (on edge servers) processes the transport rule. If the condition is satisfied and none of the exceptions apply, the action is performed.

1.1. Transport Rule Conditions

You use transport rule conditions to identify messages to which a transport rule action is applied. A condition consists of one or more predicates that specify which parts of a message should be examined. Predicates can examine message fields or headers, such as To, From, or Cc. They can also examine message characteristics, such as message subject, message size, message body, attachments, and message classification. If appropriate, you can specify a comparison operator, such as equals, does not equal, or contains, and a matching value.

For example a predicate could be MessageSize, From, FromMemberOf, FromScope, SubjectContains, FromAddressContains, SubjectMatches, and so on. Some predicates can be used only on Hub Transport servers, whereas others can be used on both Hub and Edge Transport servers. You can obtain list of transport rule predicates by entering the following Exchange Management Shell (EMS) command:

Get-TransportRulePredicate | FT

Figure 1 shows some of the output from this command run on the Hub Transport server VAN-EX1. The output you obtain from the command depends on whether you run it on a Hub Transport or an Edge Transport server. If you want to save this list in a convenient format, you can redirect the output of the command to a text file.

Transport rule exceptions are based on the transport rule predicates that you use to build transport rule conditions. However, unlike conditions, exceptions identify messages to which transport rule actions should not be applied. If an exception is met, this prevents the actions specified in the transport rule from being applied to an email message, even if that message matches all configured conditions.

Exceptions include, for example, ExceptIfFrom, ExceptIfFromMemberOf, ExceptIfFromScope, ExceptIfAttachmentContainsWords, ExceptIfAttachmentSizeOver, ExceptIfSCLOver, and so on. As with predicates, the name of the exception is typically self-explanatory.

A transport rule action defines the action that is applied to messages that match the transport rule conditions and do not match any exceptions. You can use a transport rule to reject, delete, or redirect a message; to add recipients; to add prefixes in the message subject; to insert disclaimers and personalized signatures in the message body; and to apply a message classification . You can obtain a list of transport rule actions by entering the following EMS command:

Get-TransportRuleAction | FL

Figure 2 shows some of the output from this command run on a Hub Transport server. As with transport rule predicates, the output you obtain from the command depends on whether you run it on a Hub Transport or an Edge Transport server. If you want to save the list in a convenient format, you can redirect the output of the command to a text file.

                                                                                                   Figure 2. Listing transport rule actions

You can use a command based on the New-TransportRule EMS cmdlet to create a transport rule and specify conditions, exceptions, and actions. For example, the following command creates the transport rule TransportRuleExample, which adds Kim Akers to the recipients of any email messages sent to Mark Harrington except for messages that are sent by the external user [email protected]:

New-TransportRule -Name TransportRuleExample -SentTo "Mark Harrington" -AddToRecipients
"Kim Akers" -ExceptIfFrom [email protected]

					  

The output from this command is shown in Figure 3. If you want to try out this command, you need to first create the Mark Harrington mailbox.

Transport rules allow you to apply messaging policies to messages in the transport pipeline. Actions such as redirecting a message or adding recipients, rights-protecting a message, and rejecting or silently deleting a message can be taken on messages that match the conditions and none of the exceptions defined in the rule.

The transport rules agent applies transport rules on a Hub Transport server and fires on the OnRoutedMessage transport event. All messages in an Exchange Server 2010 organization pass though at least one Hub Transport server before they are delivered, whether they are internal messages or messages to and from external users.

Active Directory stores transport rules that are configured on Hub Transport servers so that these transport rules are accessible to all Hub Transport servers in the organization through Active Directory replication. This lets you apply a single set of rules across an entire organization. Hub Transport servers query Active Directory to retrieve an organization’s current transport rule configuration and then apply the rules to messages.

The scope of transport rules applied to Hub Transport servers is the entire exchange organization, and they can be applied to all message types except system messages. These transport rules can expand distribution group membership and access Active Directory attributes, and they can inspect or modify IRM-protected message content. IRM, RMS templates, and transport protection rules are discussed later in this lesson.

Note:

Bear in mind that a transport rule can block delivery of email messages to an Exchange Server 2010 organization. However, it cannot prevent users from communicating through networked file shares, newsgroups, and forums.

Note:

ACTIVE DIRECTORY REPLICATION

For more information about Active Directory replication, see http://go.microsoft.com/fwlink/?LinkId=129505.

The edge rules agent processes transport rules on Edge Transport servers and fires on the EndOfData transport event. You should, as much as possible, apply messaging hygiene and policy to inbound Internet email on Edge Transport servers so that unwanted messages are not sent to your internal servers. The edge rules agent can also remove or block messages that contain harmful or objectionable content and can help block messages that contain viruses, worms, and other types of malicious code. This is particularly important during the interval between the creation of malicious code and updates to your organization’s antivirus software. In addition, the edge rules agent can mitigate the impact of denial of service attacks by blocking traffic from a source that is sending an excessive number of messages.

Outbound Internet email can also be subjected to policy-based scrutiny at Edge Transport servers, and you can prevent harmful or objectionable content from leaving your organization. Message content can be checked to prevent sensitive information from being leaked to external recipients.

Transport rules that are configured on Edge Transport servers are stored in Active Directory Lightweight Directory Services (AD LDS), formerly known as Active Directory Application Mode (ADAM), on each server. Rules configured on one Edge Transport server do not automatically replicate to other Edge Transport servers in an Exchange organization. You may decide to configure each Edge Transport server with identical transport rules, and you can use the EMS commands based on the Export-TransportRuleCollection and Import-TransportRuleCollection cmdlets to do so.

You also have the option of configuring different transport rules on each of your Edge Transport servers to address the email message traffic patterns of each server. The scope of a transport rule configured on an Edge Transport server is the local server. Edge server transport rules apply to all types of message, cannot expand distribution group membership, cannot access Active Directory attributes, and cannot inspect or modify IRM-protected message content.

When you are matching text patterns in different parts of a message (such as message headers, sender, recipients, message subject, and body) as specified in a transport rule, you can use expressions in transport rule predicates to determine whether a configured action should be applied to an email message.

You can use simple expressions or regular expressions. A simple expression is a specific value that you want to match exactly in a message. For example, a simple expression could be the title of a document such as Sales_Forecast.doc. Data in an email message identified by a simple expression must exactly match that simple expression to satisfy either a condition or an exception in a transport rule.

A regular expression contains flexible notation that you can use to find a text pattern in a message. The notation consists of literal characters and metacharacters. Literal characters must exist in the target string. They are normal characters, as typed. Metacharacters are special characters that indicate how the text can vary in the target string. For example the \d character matches any single numeric digit (note that metacharacters are case sensitive), the \D pattern string matches any nonnumeric digit, the \s pattern string matches any single white-space character, the \S pattern string matches any single character that is not a space, and so on.

For example, the following EMS command creates a transport rule named “Check For Number Pattern” that redirects any email message containing a number in the format xx-xxx-xx-xxxx in its subject or body to the Kim Akers mailbox:

New-TransportRule -Name "Check For Number Pattern" -SubjectOrBodyMatchesPatterns
'\d\d-\d\d\d-\d\d-\d\d\d\d' -RedirectMessageTo "Kim Akers"

					  

Because regular expressions can appear to be complex and lead to lengthy EMS commands being written to interpret such expressions, administrators often write code in the EMS to implement such rules. This code is not complex programming but consists mainly of defining variables that simplify the final statement of the rule.

A typical example detects that a number pattern is in the format of a U.S. Social Security number. For the benefit of those not based in the United States, Social Security numbers take the form xxx-xx-xxxx (for example, 123-45-6789). The transmission of such numbers in email messages is typically prohibited. The following code, entered into the EMS, creates a transport rule that prohibits the transmission of a U.S. Social Security number:

$Condition = Get-TransportRulePredicate SubjectMatches
$Condition.Patterns = @("\d\d\d-\d\d-\d\d\d\d")
$Action = Get-TransportRuleAction RejectMessage
$Action.RejectReason = "You are not permitted to transmit Social Security Numbers."
New-TransportRule -Name "Block Social Security Numbers" -Condition $Condition -Action
$Action

					  

Note that this code, given as an example, blocks email messages that contain any number that takes the form xxx-xx-xxxx. Code that can specifically identify Social Security numbers by detecting their valid prefixes would be much more complex.