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NOTE You can specify individual IP addresses as well as a range of IP addresses under the Allowed Addresses tab on the IP Allow List Properties page (see Figure 7.31). Figure 7.31 The IP Allow List If the IP address is listed here, the SMTP server will be allowed to connect and transmit e-mail messages to the Exchange 2007 organization, but the e-mail messages will be sent to the Sender Filtering agent for further processing. If the IP address of the SMTP server isn’t listed on the IP Allow list, the Connection Filtering agent will check to see whether the server is listed on the IP Block list shown in Figure 7.32. If the IP address of the SMTP server is listed on the IP Block list, connections from the server will be refused. www.syngress.com 350 Chapter 7 • Managing the Edge Transport Server 429_HTC_2007_07.qxd 2/8/07 11:34 AM Page 350 Figure 7.32 The IP Block List NOTE A neat little improvement to the IP Address Block list is that you now can set an expiration date and time for an individual IP address or a range of IP addresses. This was not possible with Exchange Server 2003 SP2. If the IP address of the SMTP server isn’t listed on either the IP Allow list or the IP Block list, the Connection Filtering agent will check to see whether the IP address is allowed by any IP Allow list provider you have specified (see Figure 7.33). Figure 7.33 IP Allow List Providers Managing the Edge Transport Server • Chapter 7 351 www.syngress.com 429_HTC_2007_07.qxd 2/8/07 11:34 AM Page 351 An IP Allow list provider is a provider that maintains a list of sender domains/IP addresses that you can rely on for sending legitimate e-mail messages and not spam.You can specify multiple IP Allow list providers and even specify how the providers’ features should interpret the returned status. If the SMTP server isn’t listed on any of these lists, the Connection Filtering agent will do one last check before it allows the SMTP connection. It will check whether the server is listed on any real-time block lists (RBLs) you have specified under the Providers tab on the IP Block List Providers Properties page (see Figure 7.34). Figure 7.34 Adding an IP Block List Provider An RBL is an Internet-based service that tracks systems (and then adds those systems’ IP addresses to a public list) that are known to send or suspected of sending out spam. NOTE You can read more about what RBLs are as well as how they work at http://en.wikipedia.org/wiki/DNSBL. In addition, you can find a list of the most popular RBLs at www.email-policy.com/Spam-black-lists.htm. In addition to specifying IP Block list providers, you can also enter a custom error mes- sage that should be returned to the blocked SMTP server. Last but not least, there’s an www.syngress.com 352 Chapter 7 • Managing the Edge Transport Server 429_HTC_2007_07.qxd 2/8/07 11:34 AM Page 352 Exceptions tab where you can specify IP addresses to which e-mail messages shouldn’t be blocked, regardless of the feedback from the RBL. Sender Filtering When the Connection Filtering agent has processed the SMTP connection, the next fil- tering agent involved is Sender Filtering, which will check the e-mail address of the sender against the list of e-mail addresses or domains you have specified under the Sender Filtering Properties page (see Figure 7.35). Figure 7.35 Blocked Sender List on the Sender Filtering Properties Page The Sender Filtering agent lets you reject individual e-mail addresses, single domains, or whole blocks of domains (that is, a domain and any subdomains). When the Sender Filtering agent rejects an e-mail message, a “554 5.1.0 Sender Denied” message is returned to the sending server.The agent also lets you reject any e-mail messages that don’t contain a sender. In addition to rejecting e-mail address and/or domains specified on the Blocked Senders list on the Sender Filtering Properties page, you can also choose to stamp messages instead of rejecting them (done under the Action tab). When you choose this action, the metadata of the message will be updated to indicate that the message was sent by a blocked sender.The stamp will then be used when the Content Filtering agent calculates the spam confidence level (SCL) of the message. Bear in mind that the Sender Filtering agent overrides the Outlook Safe Senders list (which we will talk about later in this section), which means that senders specified on the Block Senders list will be rejected even though they are included on a Outlook Safe Senders list. www.syngress.com Managing the Edge Transport Server • Chapter 7 353 429_HTC_2007_07.qxd 2/8/07 11:34 AM Page 353 Recipient Filtering When a message has been processed by the Sender Filtering agent and hasn’t been rejected, it will be handed over to the Recipient Filtering agent. (Well, this isn’t exactly true; the Connection Filtering agent will run once more, before doing so.) This will check the recip- ient of a given e-mail message against the Recipient Block list.As you can see in Figure 7.36, you can block recipients based on their e-mail addresses (that is, the SMTP address in the RCPT TO: field) as well as messages sent to recipients not listed in the Global Address List (GAL).The Edge Transport server can only check whether a recipient is in the GAL if you use EdgeSync subscription; otherwise, recipient data will not be replicated from Active Directory to ADAM. NOTE Any SMTP addresses entered on the Blocked Recipients list will only be blocked for senders located on the Internet. Internal users will still be able to send messages to these recipients. Figure 7.36 The Blocked Recipients List on the Recipient Filtering Properties Page If an external sender sends an e-mail message to a recipient that is either listed on the Blocked Recipient list or not present in the GAL, a “550 5.1.1 User unknown SMTP” ses- sion error will be returned to the sending server. www.syngress.com 354 Chapter 7 • Managing the Edge Transport Server 429_HTC_2007_07.qxd 2/8/07 11:34 AM Page 354 It worth noting that the Recipient Filtering agent works for only domains for which the Edge Transport server is authoritative.This means that any domains for which the Edge Transport server is configured as a relay server won’t be able to take advantage of Recipient Filtering. Diagrams of the Edge Transport Server with the Recipient Filtering Agent disabled and enabled are shown in Figures 7.37 and 7.38, respectively. SOME INDEPENDENT ADVICE As mentioned earlier in this chapter, the EdgeSync service will replicate recip- ient data from Active Directory to ADAM every fourth hour. With this in mind, be aware that any new recipients created on your mailbox server on the internal network won’t be able to receive e-mail messages from external senders before the EdgeSync service has taken place hereafter. The Recipient Lookup feature also includes a SMTP Tarpitting feature that helps combat directory harvest attacks (DHAs). A DHA is a technique spammers use in an attempt to find valid SMTP addresses within an organiza- tion. This is typically done with the help of a special program that is capable of generating random SMTP addresses for one or more domains. For each generated SMTP address, the program also sends out a spam message to the specific address. Because the program will try to deliver a message to each generated SMTP address, an SMTP session is, of course, also established to the respective Edge Transport server (or whatever SMTP gateway is used in the organization). The program can therefore collect a list of valid SMTP addresses, since the SMTP session will either respond with “250 2.1.5 Recipient OK” or “550 5.1.1 User unknown,” depending on whether the SMTP address is valid or not. This is where the SMTP Tarpitting feature comes into the picture. This feature basically delays the “250 2.1.5 Recipient OK” or “550 5.1.1 User unknown” SMTP response codes during an SMTP session. By default, the SMTP Tarpitting feature on an Edge Transport server is configured to a delay of 5 seconds (but the value can be changed for each Receive connector), which should help make it more difficult for a spammer to harvest valid SMTP addresses from your domain. SOME INDEPENDENT ADVICE The SMTP Tarpitting feature was originally introduced in Exchange Server 2003. In Exchange 2003 the administrator had the option of specifying a tarpit value in which he or she could define the number of seconds to delay a response to the RCPT TO command during an SMTP session. The problem in Exchange 2003 was that this value was fixed, which enabled spammers to detect this behavior so they could work around it. A common practice was to www.syngress.com Managing the Edge Transport Server • Chapter 7 355 429_HTC_2007_07.qxd 2/8/07 11:34 AM Page 355 have the spam application establish a new SMTP session, if it detected it was being tarpitted. To solve this problem, the Edge Transport server uses a random number of seconds, making predictions much harder. Even if the spam application reconnects, it won’t be in better shape; the Edge Transport server will know it’s the same sending server, so it will retain the tarpit state. Figure 7.37 The Edge Transport Server with the Recipient Filtering Agent Disabled Figure 7.38 The Edge Transport Server with the Recipient Filtering Agent Enabled Sender ID Filtering When an e-mail message has been processed by the Recipient Filtering agent and still hasn’t been rejected, it will be handed over to the Sender ID Filtering agent. The Sender ID is an e-mail industry initiative invented by Microsoft and a few other industry leaders.The purpose of Sender ID is to help counter spoofing (at least to make it more difficult to spoof messages), which is the number-one deceptive practice used by spam- mers. Sender ID works by verifying that every e-mail message indeed originates from the www.syngress.com 356 Chapter 7 • Managing the Edge Transport Server Spammer Perimeter Network Edge Transport Firewall Spammer Performs a Directory Harvest Attack Edge Transport Server Responds as Fast as it Can Firewall Perimeter Network Spammer Spammer Performs a Directory Harvest Attack Edge Transport Server Responds with a Delay (Default 5 Seconds) Edge Transport 429_HTC_2007_07.qxd 2/8/07 11:34 AM Page 356 Internet domain from which it was sent.This is accomplished by checking the address of the server sending the mail against a registered list of servers that the domain owner has autho- rized to send e-mail. If you don’t have any experience with Sender ID, it can be a bit difficult to understand, so let’s take a closer look at how it works. An organization can publish a Sender Policy Framework (SPF) record on the public DNS server(s) hosting their domain.The published SPF record contains a list of the IP addresses that should be or are allowed to send out messages for a particular domain. If a par- ticular organization has published a SPF record and someone at that organization sends a message to a recipient behind an Edge Transport server in another organization, the Edge Transport server will examine the SPF record to see whether the SMTP server that sent the message is listed there (see Figure 7.39). Figure 7.39 How Sender ID Works Behind the Scenes Sender ID can provide several different results and stamp them appropriately.Table 7.2 lists each of the results as well a short description and the action taken. www.syngress.com Managing the Edge Transport Server • Chapter 7 357 Firewall Perimeter Network Firewall Internal Network SPF Lookup SPF result Sender Public DNS Server with SPF record Inbound message Inbound message Inbound message Delivery Recipient Mailbox Server Edge Transport Hub Transport 429_HTC_2007_07.qxd 2/8/07 11:34 AM Page 357 Table 7.2 Sender ID Results Sender ID Result Description Action Taken Neutral Domain is neutral (makes no decision Stamp and about IP address) Accept Pass (+) IP address for PRA permitted set Stamp and Accept Fail (-) - Domain doesn’t exist IP address for PRA not permitted set Stamp and - Sender isn’t permitted Accept then - Malformed domain either Delete or - No Purported Reject Responsible Address (PRA) in header Soft Fail (~) IP address for PRA not permitted set Stamp and Accept None No SPF record published for Stamp and the domain Accept Temp Error Transient error (could be unreachable Stamp and DNS server) Accept Perm Error Possible error in record so couldn’t Stamp and be read correctly Accept No matter what the result of the SPF check, the result will be used in the calculation process when an SCL rating is generated for a message. TIP In you want to check which IP addresses are allowed to send e-mail messages for a given domain, you can use a wizard such as the one at www.dnsstuff.com/pages/spf.htm, or open a command prompt and type nslookup –q=TXT domain.com. You should then be able to see the SPF record, including the list of the IP addresses allowed to send e-mail mes- sages for this domain. For additional information about Sender ID, visit http://en.wikipedia.org/ wiki/Sender_id. When the Sender ID Filtering agent checks whether a sending SMTP server has an appropriate purported responsible address (PRA), you can specify what action it should take for a given e-mail message that doesn’t have an appropriate PRA. www.syngress.com 358 Chapter 7 • Managing the Edge Transport Server 429_HTC_2007_07.qxd 2/8/07 11:34 AM Page 358 You can configure it to Reject message, Delete message or Stamp message with Sender ID result and continue processing; the last one is the option selected by default (see Figure 7.40). Figure 7.40 The Action Tab on the Sender ID Properties Page If you set Sender ID to reject the message, the message will be rejected by the Edge Transport server and an SMTP error response will be returned to the sender. If you configure Sender ID to delete message, the message will be deleted without sending an SMTP error response to the sender. Since the message is deleted without informing the sending SMTP server, you would think that the sending SMTP server would retry sending the message, but this is not the case.The Sender ID filter has been made so cleverly that the Edge Transport server will send a fake OK SMTP command before deleting the message. When you configure Sender ID to stamp messages with the Sender ID result and con- tinue processing, the e-mail message will be stamped with information that will be used when the message is evaluated by the Content Filtering agent (which we will look at in a moment) to calculate the SCL. SOME INDEPENDENT ADVICE If you haven’t already done so, we highly recommend that you create an SPF record for your domain. This will make it much more difficult for spammers to forge your domain so that they can spam domains in other organizations. Creating your own SPF record is a relatively simple process; Microsoft even provides a Web-based GUI wizard that will help you do it (see Figure 7.41). www.syngress.com Managing the Edge Transport Server • Chapter 7 359 429_HTC_2007_07.qxd 2/8/07 11:34 AM Page 359 [...]... the View Configuration Database button If the Exchange Server 20 07 SCW extension file has been properly registered, you should see an entry for the Edge Transport server role as well as the other Exchange 20 07 server roles in the SCW Viewer, as shown in Figure 7. 53 www.syngress.com 377 429_HTC _20 07_ 07. qxd 378 2/8/ 07 11:34 AM Page 378 Chapter 7 • Managing the Edge Transport Server Figure 7. 53 SCW Viewer... similar to 0xac 0xbd 0x03 0xca, the user objects have been updated www.syngress.com 429_HTC _20 07_ 07. qxd 2/8/ 07 11:34 AM Page 365 Managing the Edge Transport Server • Chapter 7 NOTE To use ADSI Edit, you need to install the Windows Server 2003 Support Tools on the respective Exchange 20 07 server To see whether safelist aggregation works as expected on the Edge Transport server, try to add a custom word... connector, type GetReceiveConnector | FL If you want to see a list of the current settings for AttachmentFilterListConfig, type GetAttachmentFilterListConfig and press Enter (see Figure 7. 47) Figure 7. 47 The Attachment Filter List Configuration Settings For any additional information on how to configure the attachment filtering behavior using the Set-AttachmentFilterListConfig CMDlet, see the Exchange Server 20 07. .. Edge Transport server with the Security Configuration Wizard (SCW), a tool for reducing the attack surface of computers running Windows Server 2003 R2 or Windows 2003 server with Service Pack 1 (SP1) or higher applied.The SCW tool www.syngress.com 375 429_HTC _20 07_ 07. qxd 376 2/8/ 07 11:34 AM Page 376 Chapter 7 • Managing the Edge Transport Server makes it a relatively easy and simple process to lock down... directory under C:\Program Files\Microsoft \Exchange (or whatever the path to your Exchange installation is) Since you need to do so using the scwcmd register command, open a command prompt window and type the following: scwcmd register /kbname:MSExchangeEdge /kbfile: “C:\program files\Microsoft \Exchange Server\ scripts \Exchange2 0 07. xml.” Next, press Enter See Figure 7. 51 www.syngress.com 429_HTC _20 07_ 07. qxd... 429_HTC _20 07_ 07. qxd 2/8/ 07 11:34 AM Page 377 Managing the Edge Transport Server • Chapter 7 Figure 7. 51 Registering the Exchange 20 07 SCW Extension File Now that the Exchange 20 07 SCW extension file has been properly registered, you can launch the SCW Wizard.This is done by clicking Start | Administrative Tools | Security Configuration Wizard.Then follow these steps: 1 On the Welcome to Security Configuration Wizard... list of connectors from attachment filtering, which means that attachment filtering won’t be applied to messages flowing through the specified connectors.You can exclude one or more connectors using Set-AttachmentFilterListConfig –Action Reject –ExceptionConnectors www.syngress.com 429_HTC _20 07_ 07. qxd 2/8/ 07 11:34 AM Page 371 Managing the Edge Transport Server • Chapter 7 To get the GUID... need to add both these ports on this page .To do so, click the Add button shown in Figure 7. 56 Figure 7. 56 Adding the Respective Ports 15 On the Add Port or Application page, enter 50389 in the port number field, check TCP, and click OK (see Figure 7. 57) Figure 7. 57 Adding the LDAP Port www.syngress.com 429_HTC _20 07_ 07. qxd 2/8/ 07 11:34 AM Page 381 Managing the Edge Transport Server • Chapter 7 16 Repeat... organization) so that the rule points to the external IP address of the Edge Transport server In Chapter 12, which covers how you publish the different Exchange 20 07 services and protocols through an ISA 2006 Server, we’ll go through step-by-step instructions on how to publish your Exchange 20 07 Server SMTP protocol, which is the same procedure for both an Edge Transport and a Hub transport server. .. series to ForeFront Security, which means that the old Exchange AntiGen product now is known as www.syngress.com 429_HTC _20 07_ 07. qxd 2/8/ 07 11:34 AM Page 375 Managing the Edge Transport Server • Chapter 7 ForeFront Security for Exchange Server Not only has the product name changed, but Microsoft has also been busy improving the product as well as integrating it more tightly with Exchange Server 20 07; now . recipient data, be replicated from Active Directory to the ADAM store on the Edge Transport server. www.syngress.com Managing the Edge Transport Server • Chapter 7 363 429_HTC _20 07_ 07. qxd 2/8/ 07 11:34. need to install the Windows Server 2003 Support Tools on the respective Exchange 20 07 server. To see whether safelist aggregation works as expected on the Edge Transport server, try to add a custom. still need to enable and configure the safelist aggregation feature before you can use it .To do so, perform the fol- lowing steps: 1. Log on to the Exchange 20 07 server that has the Mailbox server