The term “Firewall.”

The term “firewall” comes from a building and automotive construction concept of a wall built to prevent the spread of fire from one area into another. This concept was then taken into the world of network security. The firewall’s assignment is to set all restrictions and boundaries described in the security policy on all network traffic that passes the firewall interfaces. Then, we have the concept of firewall filtering that compares each packet received to a set of rules that the firewall administration configures.

These exception rules are derived from the organization’s security policy. The firewall filtering rules state that the contents of the packet are either allowed or denied. Therefore, based on firewall traffic flow, the packet continues to its destination if it matches an allowed rule. If it matches a deny rule, the packet is dropped. The firewall is the barrier between a trusted and untrusted network, often used between your LAN and WAN. It’s typically placed in the forwarding path so that all packets have to be checked by the firewall, where we can drop or permit them.

Apply a multi-layer approach to security. 

When it comes to network security, organizations must adopt a multi-layered approach. While Stateful Inspection Firewalls provide essential protection, they should be used in conjunction with other security technologies, such as intrusion detection systems (IDS), intrusion prevention systems (IPS), and virtual private networks (VPNs). This combination of security measures ensures comprehensive protection against various cyber threats.

Stateful Inspection Firewalls are integral to network security infrastructure. By inspecting packets in the context of the entire communication session, these firewalls offer enhanced security and greater control over network traffic. By leveraging advanced inspection techniques, deep packet inspection, and a stateful approach, Stateful Inspection Firewalls provide a robust defense against evolving cyber threats. Organizations prioritizing network security should consider implementing Stateful Inspection Firewalls as part of their security strategy.

Guide on Cisco ASA firewall

In the following lab guide, you can see we have an ASA working in routed mode. In routed mode, the ASA is considered a router hop in the network. Each interface that you want to route between is on a different subnet. You can share Layer 3 interfaces between contexts.

Traditionally, a firewall is a routed hop and acts as a default gateway for hosts that connect to one of its screened subnets. On the other hand, a transparent firewall is a Layer 2 firewall that acts like a “bump in the wire” or a “stealth firewall” and is not seen as a router hop to connected devices. However, like any other firewall, access control between interfaces is controlled, and the usual firewall checks are in place.

The Adaptive Security Algorithm considers the state of a packet when deciding to permit or deny the traffic. One enforced parameter for the flow is that traffic enters and exits the same interface. The ASA drops any traffic for an existing flow that enters a different interface. Traffic zones let you group multiple interfaces so that traffic entering or exiting any interface in the zone fulfills the Adaptive Security Algorithm security checks.

The command:  show asp table routing displays the accelerated security path tables for debugging purposes and the zone associated with each route. See the following output for the show asp table routing command:

**Firewall filtering rules**

Firewall filtering rules help secure a network from unauthorized access and malicious activity. These rules protect the network by controlling traffic flow in and out of the network. Firewall filtering rules can allow or deny traffic based on source and destination IP addresses, ports, and protocols.

Firewall filtering rules should be tailored to the specific needs of a given network. Generally, it is recommended to implement a “deny all” rule and then add rules to allow only the necessary traffic. This helps block any malicious activity while allowing legitimate traffic. When creating firewall filtering rules, it is essential to consider the following:

• Make sure to use the most up-to-date protocols and ports.
• Be aware of any potential risks associated with the traffic being allowed.
• Use logging to monitor traffic and ensure that expected behavior is occurring.
• Ensure that the rules are implemented consistently across all firewalls.
• Ensure that the rules are regularly reviewed and updated as needed.

Guide on default firewall inspection

The Cisco ASA Firewall uses so-called “security levels” that indicate how trusted an interface is compared to another. The higher the security level, the more trusted the interface is. Each interface on the ASA is a security zone, so using these security levels gives us different trust levels for our security zones. Therefore, we have the default firewall inspection. We will discuss this more later.

Below, we have three routers and subnets with 1 ASA firewall.

• Interface G0/0 as the INSIDE.
• Interface G0/1 as the OUTSIDE.
• Interface G0/2 as our DMZ.

The name command is used to specify a name for the interface. As you can see, the ASA recognizes INSIDE, OUTSIDE, and DMZ names. And sets the security level for that interface to a default level. Therefore, restriction of traffic flow.

Remember that the ASA can reach any device in each security zone. This doesn’t work since we are trying to go from a security level of 0 (outside) to 100 (inside) or 50 (DMZ). We will have to use an access list if you want to allow this traffic.

What Is a Stateful Firewall?

The stateful firewall examines Layer 4 headers and above, analyzing firewall traffic flow and enabling support for Application-aware inspections. Stateful inspection keeps track of every connection passing through their interfaces by analyzing packet headers and additional payload information.

Stateful Firewall Operation

You can see how filtering occurs at layers 3 and 4 and that the packets are examined as a part of the TCP session.

The topmost part of the diagram shows the three-way handshake, which takes place before the commencement of the session and is explained as follows.

1. Syn refers to the initial synchronization packet sent from one host to another; in this case, the client to the server.
2. The server sends an acknowledgment of the syn, and this is known as syn-ack
3. The client again acknowledges this syn-ack, completing the process and initiating the TCP session.
4. Both parties can end the connection anytime by sending a FIN to the other side. This is similar to a telephone call where the caller or the receiver could hang up.

State and Context.

The two important terms to understand are state and context information. Filtering is based on the state and context information the firewall derives from a session’s packets. The firewall will store state information in its state table, which is updated regularly. For example, in TCP, this state is reflected in specific flags such as SYN, ACK, and FIN. Then, we have the context. This includes source and destination port, IP address, and sequence numbers of any metadata. The firewall also stores this information and updates regularly based on traffic flowing through the firewall.

Firewall state table

A firewall state table is a data structure that stores information about a network firewall’s connection state. It determines which packets are allowed to pass through the firewall and which are blocked. The table contains entries for each connection, including source and destination IP addresses, port numbers, and other related information.

The firewall state table is typically organized into columns, with each row representing an individual connection. Each row contains the source and destination IP address, the port numbers, and other related information.

For example, the source IP address and port number indicate the origin of the connection, while the destination IP address and port number indicate the destination of the connection. Additionally, the connection’s state is stored in the table, such as whether the connection is established, closed, or in transit.

The state table also includes other fields that help the firewall understand how to handle the connection, such as the connection duration, the type of connection being established, and the protocol used.

So whenever a packet arrives at a firewall to seek permission to pass through it, the firewall checks from its state table if there is an active connection between the two points of source and destination of that packet. The endpoints are identified by something known as sockets. A socket is similar to an electrical socket at your home, which you use to plug your appliances into the wall.

Similarly, a network socket consists of a unique IP address and a port number and is used to plug one network device into the other. The packet flags are matched against the state of the connection to which it belongs, which is allowed or denied based on that. For example, if a connection already exists and the packet is a Syn packet, it must be rejected since Syn is only required initially.

CBAC Firewalling on Cisco IOS

Understanding CBAC Firewall

CBAC firewall, also known as stateful firewall, is a robust security mechanism developed by Cisco Systems. Unlike traditional packet-filtering firewalls, the CBAC firewall adds layer of intelligence by examining the context of network connections. It analyzes individual packets and the entire session, providing enhanced security against advanced threats.

CBAC firewall offers a range of powerful features, making it a preferred choice for network administrators. First, it provides application-layer gateway functionality, which allows it to inspect and control traffic at the application layer. Second, the CBAC firewall can dynamically create temporary access rules based on a connection’s state. This adaptability ensures that only valid and authorized traffic is allowed through the firewall.

Compared to simple access lists, CBAC (Context-Based Access Control) offers some more features. CBAC can inspect up to layer 7 of the OSI model, and dynamic rules can be created to allow return traffic. Reflexive access lists are similar to this, but the reflexive ACL inspects only layers up to 4.

CBAC will be demonstrated in this lab, and you’ll see why this firewall feature is helpful. For this, I will use three routers: In the example above, we have three routers. Please assume that the router on the left (R1) is a device on the Internet, while the host on the right (R3) is a device on our local area network (LAN). We will configure CBAC on R2, the router that protects us from Internet traffic.

These pings are failing, as you can see on the console. The inbound ACL drops these packets on R2. To solve this problem, we must add a permit statement to the access list so the ping makes it through. That’s not a scalable solution since we don’t know what kind of traffic we have on our LAN, and we don’t want a big access list with hundreds of permit statements. What we are going to do is configure CBAC so it will inspect the traffic and automatically allow the return traffic through

Stateful Firewall and Interface Configuration

It would be best to consider the interfaces in firewall terms when considering a stateful inspection firewall. For example, some interfaces are connected to protected networks, where data or services must be secured. Others connect to public or unprotected networks, where untrusted users and resources are located.

The top portion of the diagram below shows a stateful firewall with only two interfaces connecting to the inside (more secure) and outside (less secure) networks. The bottom portion shows the stateful inspection firewall with three interfaces connecting to the inside (most secure), DMZ (less secure), and outside (least secure) networks. The firewall has no concept of these interface designations or security levels; these concepts are put into play by the inspection processes and policies configured.

So you need to explain to the firewall which interface is at what security level. And this will effect the firewall traffic flow. Some traffic will be denied by default between specific interfaces with default security levels.

Interface configuration specific to ASA

Since version 7.0 of the ASA code, configuring interfaces in the firewall appliance is very similar to configuring interfaces in IOS-based platforms. If the firewall connection to the switch is an 802.1q trunk (the ASA supports 802.1q only, not ISL), you can create sub-interfaces corresponding to the VLANs carried over the trunk. Do not forget to assign a VLAN number to the sub-interface. The native (untagged) VLAN of the trunk connection maps to the physical interface and cannot be assigned to a sub-interface.

Full state of active network connections

So, we know that the stateful firewall monitors the entire state of active network connections and constantly analyses the complete context of traffic and data packets. Then, we have the payload to consider. The payload is part of transmitted data, the intended message, headers, and metadata sent only to enable payload delivery.

Payloads offer transaction information, which can protect against some of the most advanced network attacks. For example, deep packet inspection configures the stateful firewall to deny specific Hypertext Transfer Protocol ( HTTP ) content types or specific File Transfer Protocol ( FTP ) commands, which may be used to penetrate networks. 

Stateful inspection and Deep Packet Inspection (DPI)

The following diagram shows the OSI layers involved in the stateful inspection. As you can see, Stateful inspection operates primarily at the transport and network layers of the Open Systems Interconnection (OSI) model for how applications communicate over a network. However, it can also examine application layer traffic, if only to a limited degree. Deep Packet Inspection (DPI) is higher up in the OSI layers.

DPI is considered to be more advanced than stateful packet filtering. It is a form of packet filtering that locates, identifies, classifies, and reroutes or blocks packets with specific data or code payloads that conventional packet filtering, which examines only packet headers, cannot detect. Many firewall vendors will have the stateful inspection and DPI on the same appliance. However, a required design may require a separate appliance for compliance or performance reasons.

Stateful Inspection Firewall

What is a stateful firewall?

A stateful firewall tracks and monitors the state of active network connections while analyzing incoming traffic and looking for potential traffic and data risks. The state is a process or application’s most recent or immediate status. In a firewall, the state of connections is stored, providing a list of connections against which to compare the connection a user is attempting to make.

Stateful packet inspection is a technology that stateful firewalls use to determine which packets are allowed through the firewall. It works by examining the contents of a data packet and then comparing them against data about packets that have previously passed through the firewall.

Stateful firewall and packet filters

The stateful firewall contrasts packet filters that match individual packets based on their source/destination network addresses and transport-layer port numbers. Packet filters have no state or check the validity of transport layer sessions such as sequence numbers, Transmission Control Protocol ( TCP ) control flags, TCP acknowledgment, or fragmented packets. The critical advantage of packet filters is that they are fast and processed in hardware.

Reflexive access lists are closer to a stateful tool than packet filters. Whenever a TCP or User Datagram Protocol ( UDP ) session permits, matching return traffic is automatically added. The disadvantage of reflexive access lists is they cannot detect/drop malicious fragments or overlapping TCP segments. Transport layer session inspection goes beyond reflexive access lists and addresses fragment reassembly and transport-layer validation.

Application-level gateways ( ALG ) add additional awareness. They can deal with FTP or Session Initiation Protocol ( SIP ) applications that exchange IP addresses and port numbers in the application payload. These protocols operate by opening additional data sessions and multiple ports.

**Simple packet filters for a perfect world*8

In a perfect world where most traffic exits the data center, servers are managed with regular patching, servers listen on standard TCP or UDP ports, and designers could get away with simple packet filters. However, in the real world, each server is a distinct client, has multiple traffic flows to and from the data center and back-end systems, and unpredictable source TCP or UDP port number makes using packet filters impractical.

Instead, additional control should be implemented with deep packet inspection for unpredictable scenarios and poorly managed servers. Stateful firewalls keep state connections and allow traffic to return dynamically. Return traffic is permitted if the state of that flow is already in the connection table. The traffic needs to be part of a return flow. If not, it’s dropped.

**A stateless firewall – predefined rule sets**

A stateless firewall uses a predefined set of rules. If the arriving data packet conforms to the rules, it is considered “safe.” The data packet is allowed to pass through. With this approach to firewalling, traffic is classified instead of inspected. The process is less rigorous compared to what a stateful firewall does.

Remember that a stateless firewall does not differentiate between certain kinds of traffic, such as Secure Shell (SSH) versus File Transfer Protocol (FTP). A stateless firewall may classify these as “safe” and allow them to pass through, which can result in potential vulnerabilities.

A stateful firewall holds context across all its current sessions rather than treating each packet as an isolated entity, as with a stateless firewall. With stateless inspection, lookup functions impact the processor and memory resources much less, resulting in faster performance even if traffic is heavy.

**The Stateful Firewall and Security Levels**

Regardless of the type of firewall mode or single or multiple contexts, the Adaptive Security Appliance ( ASA ) permits traffic based on a concept of security levels configured per interface. This is a crucial point to note for ASA failover and how you design your failover firewall strategy. The configurable range is from level 0 to 100. Every interface on ASA must have a security level.

The security level allows configured interface trust-ability and can range from 0, which is the lowest, to 100, which is the highest—offering ways to control traffic flow based on security level numbering. The default security level is “0”, configuring the name on the interface “inside” without explicitly entering a security level; then, the ASA automatically sets the security level to 100 ( highest ).

By default, based on the configured nameif, ASA assigns the following implicit security levels to interfaces:

• 100 to a nameif of inside.
• 0 to a nameif of outside.
• 0 to all other nameifs.

Without any configured access lists, ASA implicitly allows or restricts traffic flows based on the security levels:

Firewall traffic flow between security levels

By default, traffic can flow from highest to lowest without explicit configuration. Also, interfaces on the same security level cannot directly communicate, and packets cannot enter and exit the same interface. Override the defaults, permit traffic by allowing high to low; explicitly configure ACLs on the interface or newer version use-global ACL. Global ACL affects all interfaces in all directions.

Firewall traffic flows

Inter-interface communication ( Routed Mode only ): Enter the command “same-security-traffic permit inter-interface” or permit traffic explicitly with an ACL. This will give design granularity and allow the configuration of more communicating interfaces. Intra-interface communication: This is configured for traffic hair-pining (traffic leaves on the outside interface and goes back out the outside interface ).

This is useful for Hub and Spoke VPN deployments; traffic enters an interface and routes back out the same interface—Spoke-to-Spoke communication. To enable Intra-Interface communication, enter the command “same-security-traffic permit intra-interface.”

Default inspection and Modular Policy Framework ( MPF )

ASA implements what is known as the Modular Policy Framework ( MPF ). MPF controls WHAT traffic is inspected, such as Layer 3 or Layer 4 inspection of TCP, UDP, ICMP, an application-aware inspection of HTTP, or DNS. It also controls HOW traffic is inspected based on connection limits and QoS parameters.

ASA inspects TCP / UDP from the inside (higher-security level ) to the outside ( lower-security level ). This cannot be disabled. No traffic inspection from outside to inside unless it is from an original flow.

An entry is created in the state table, so when flows return, it checks the state table before it goes to implicit deny ACL. The state is created during traffic leaves, so it checks the specific connection and application data when the return flows come back. It does more than Layer 3 or 4 inspections and depends on the application.

It does not, by default, inspect ICMP traffic. Enable ICMP inspection with a global inspection policy or explicitly allow with an interface or Global ACLs. ASA global policy affects all interfaces in all directions. The state table is checked before any ACL. A good troubleshooting tool, Packet Tracer, goes through all inspections and displays the order the ASA is processing.

Key Stateful Inspection Firewall Summary Points:

Main Checklist Points To Consider Firewalls carry out specific actions based on policy. The default policy can exist. Different firewall types exist for different parts of the network. The stateful firewall monitors the full state of the connections. The state is held in a state table. Standard packet filters don’t state or check the valid nature of the transport layer sessions. They do not do a stateful inspection. Firewalls will have default rules based on interface configurations. Default firewall traffic flow is based on an interface security level. The Cisco ASA operates with a Modular Policy Framework (MPF) technology. ASA is a popular stateful firewall.

Firewalls and secure web gateways (SWGs) play similar and overlapping roles in securing your network. Both analyze incoming information and seek to identify threats before they enter your system. Despite sharing a similar function, they have some key differences, such as the “classical” distinction between secure web gateways and firewalls.

The basic distinctions:

• Firewalls inspect data packets
• Secure web gateways inspect applications
• Secure web gateways set and enforce rules for users

Guide on traffic flows and NAT

I have the Cisco ASA configured with Dynamic NAT in the following guide. This is the same setup as before. In the middle, we have our ASA; its G0/0 interface belongs to the inside, and the G0/1 interface belongs to the outside.  I have not configured anything on the DMZ interfaces.

You need to configure object groups for this ASA version. I have configured a network object that defines the pool with public IP addresses we want to use for translation. The IP address that has been translated is marked in the red box below.

The show nat command shows us that some traffic has been translated from the inside to the outside.

The show xlate command shows that the IP address 192.168.1.1 has been translated to 192.168.2.196. It also tells us what kind of NAT we are doing here (dynamic NAT in our example) and how long this entry has been idle.