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Lesson 1
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Recognizing Quick Response Code In C#
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Introduction to ISA Server as a Firewall
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ISA Server 2004 and Packet Filtering
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ISA Server 2004 does not have an option for directly configuring packet filtering. However, ISA Server does operate as a packet filter firewall, inspecting traffic at the network and transport layers. For example, if you define a firewall access rule that enables all protocol traffic from an IP address on one network to an IP address on another network, ISA Server uses a packet filter to allow that traffic. Or, if you configure a firewall access rule that denies the use of the default Telnet port (TCP Port 23), ISA Server will use a packet filter to block that port. ISA Server also enables ingress and egress filtering by default. ISA Server 2000 supports direct configuration of packet filters. If you upgrade to ISA Server 2004 from ISA Server 2000, the packet filter definitions are replaced by access rules.
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Exam Tip
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You may get an exam question that states a company requirement to block all connection attempts from a specific host, or to allow a specific protocol from a particular host. Remember that, with ISA Server 2004, you can only configure this type of access using an access rule.
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What Is Stateful Filtering
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When a firewall uses stateful filtering, the firewall examines not only the packet header information, but the status of the packet as well. For example, the firewall can inspect a packet at its external interface and determine whether the packet is a response to a request from the internal network. This check can be performed at both the transport and the application layers. Stateful filtering uses information about the TCP session to determine whether a packet should be blocked or allowed through the firewall. TCP sessions are established using the TCP three-way handshake, the purpose of which is to synchronize the sequence number and acknowledgment numbers and to exchange other information defining how the two hosts will exchange packets. The following steps outline the process: 1. The initiator of the TCP session, typically a client, sends a TCP segment to the server. The client sends its sequence number and requested that the server provide its sequence numbers (by setting the SYN bit to 1). 2. The responder of the TCP session, typically a server, sends back a TCP segment containing its initial sequence number and an acknowledgment (ACK) of the client s sequence number. The server sets both the SYN bit and ACK bit to 1. The ACK bit indicates that the server has received the client s sequence number.
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7
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Configuring ISA Server as a Firewall
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3. The initiator sends the server a TCP segment containing an acknowledgment of the server s sequence number. Once the client and server have agreed on the sequence numbers, they will use the sequence numbers to track all packets. TCP uses the information to recover from errors, such as packets arriving out of order or packets not arriving. TCP uses a similar handshake process to end a connection. This guarantees that both hosts have finished transmitting and that all data was received. A firewall uses this TCP information to perform stateful filtering. When a client on the internal network sends the first packet in the three-way handshake, the server forwards the packet and records that the packet has been sent. When the response comes back from the server, the firewall accepts the packet because it is in response to an internal request. If a packet arrives with only the SYN bit set, or with the SYN and ACK bits set, and the firewall does not have a record of a client request, the firewall blocks the packet. The firewall can also use other characteristics of TCP session to control traffic. For example, when the client initiates the session, the firewall can set a timer and keep the session open only as long as specified by the timer. The firewall can also analyze application-level data to perform stateful filtering. For example, when a client sends a GET command for a specific Uniform Resource Locator (URL) on a Web server, the firewall can track the request and allow a response. An HTTP packet that arrives without a corresponding client request is dropped.
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