Filtering Traffic Using Firewall

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If you’re planning to take the SY0-501 exam, you should understand firewalls and how to filter traffic using a firewall. A firewall filters incoming and outgoing traffic for a single host or between networks. In other words, a firewall can ensure only specific types of traffic are allowed into a network or host, and only specific types of traffic are allowed out of a network or host.

For example, can you answer this practice test question?

Q. An organization has recently had several attacks against servers within a DMZ. Security administrators discovered that many of these attacks are using TCP, but they did not start with a three-way handshake. Which of the following devices provides the BEST solution?

A. Stateless firewall

B. Stateful firewall

C. Network firewall

D. Application-based firewall

More, do you know why the correct answer is correct and the incorrect answers are incorrect? The answer and explanation are available at the end of this post.

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Host-Based Firewalls

A host-based firewall monitors traffic going in and out of a single host, such as a server or a workstation. It monitors traffic passing through the NIC and can prevent intrusions into the computer via the NIC. Many operating systems include software-based firewalls used as host- based firewalls. For example, Microsoft has included a host-based firewall on operating systems since Windows XP. Additionally, many third-party host-based firewalls are available.

The figure shows the host-based Windows Firewall on Windows 10. Notice that you can configure inbound rules to allow or restrict inbound traffic and outbound rules to allow or restrict outbound traffic. The connection security rules provide additional capabilities, such as configuring an IPsec connection in Tunnel or Transport mode to encrypt the traffic.

Filtering Traffic Using Firewall

Linux systems support iptables and many additions to iptables, such as ipv6tables, arptables, and so on. Generically, administrators commonly refer to these as xtables. You can configure rules within different tables. Combined, these rules work just like an ACL.

Personal firewalls provide valuable protection for systems against unwanted intrusions. Many organizations use personal firewalls on each system in addition to network firewalls as part of an overall defense-in-depth strategy.

It’s especially important to use personal firewalls when accessing the Internet in a public place. Free Wi-Fi Internet access is often available in public places, such as airports, hotels, and many fast-food establishments, such as Starbucks and McDonald’s. However, connecting to a public Wi-Fi hot spot without the personal firewall enabled is risky, and never recommended.

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Application-Based Versus Network-Based Firewalls

An application-based firewall is typically software running on a system. For example, host- based firewalls are commonly application-based. A network-based firewall is usually a dedicated system with additional software installed to monitor, filter, and log traffic. For example, Cisco makes a variety of different network-based firewalls. Many of them are dedicated servers with proprietary firewall software installed.

A network-based firewall would have two or more network interface cards (NICs) and all traffic passes through the firewall. The firewall controls traffic going in and out of a network. It does this by filtering traffic based on firewall rules and allows only authorized traffic to pass through it. Most organizations include at least one network-based firewall at the border, between their intranet (or internal network) and the Internet.

Stateless Firewall Rules

Stateless firewalls use rules implemented as ACLs to identify allowed and blocked traffic. This is similar to how a router uses rules. Firewalls use an implicit deny strategy to block all traffic that is not explicitly allowed. Although rules within ACLs look a little different depending on what hardware you’re using, they generally take the following format:

Permission Protocol Source Destination Port

  • Permission. You’ll typically see this as PERMIT or ALLOW allowing the traffic. Most systems use DENY to block the traffic.
  • Protocol. Typically, you’ll see TCP or UDP here, especially when blocking specific TCP or UDP ports. If you want to block both TCP and UDP traffic using the same port, you can use IP instead. Using ICMP here blocks ICMP traffic, effectively blocking ping and some other diagnostics that use ICMP.
  • Source. Traffic comes from a source IP address. You identify an IP address to allow or block traffic from a single computer, or from a range of IP addresses, such as from a single subnet. Wildcards such as any or all include all IP addresses.
  • Destination. Traffic is addressed to a destination IP address. You identify an IP address to allow or block traffic to a single computer, or to a range of IP addresses, such as to an entire subnet. Wildcards such as any or all include all IP addresses.
  • Port or protocol. Typically, you’ll see the well-known port such as port 80 for HTTP. However, some devices support codes such as www for HTTP traffic. Some systems support the use of keywords such as eq for equal, lt for less than, and gt for greater than. For example, instead of just using port 80, it might indicate eq 80.

Some firewalls require you to include a subnet mask in the rule. For example, if you want to block all SMTP traffic to the 192.168.1.0/24 network, you would use an IP address of 192.168.1.0 and a subnet mask of 255.255.255.0. However, if you only wanted to allow SMTP traffic to a single computer with the IP address of 192.168.1.20/24, you would use an IP address of 192.168.1.20 and a subnet mask of 255.255.255.255.

Stateful Versus Stateless

A stateful firewall inspects traffic and makes decisions based on the context, or state, of the traffic. It keeps track of established sessions and inspects traffic based on its state within a session. It blocks traffic that isn’t part of an established session. As an example, a TCP session starts with a three-way handshake. If a stateful firewall detects TCP traffic without a corresponding three-way handshake, it recognizes this as suspicious traffic and can block it.

A common security issue with stateless firewalls is misconfigured ACLs. For example, if the ACL doesn’t include an implicit deny rule, it can allow almost all traffic into the network.


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Q. An organization has recently had several attacks against servers within a DMZ. Security administrators discovered that many of these attacks are using TCP, but they did not start with a three-way handshake. Which of the following devices provides the BEST solution?

A. Stateless firewall

B. Stateful firewall

C. Network firewall

D. Application-based firewall

Answer is B. A stateful firewall filters traffic based on the state of the packet within a session. It would filter a packet that isn’t part of a TCP three-way handshake.

A stateless firewall filters traffic based on the IP address, port, or protocol ID.

While it’s appropriate to place a network firewall in a demilitarized zone (DMZ), a network firewall could be either a stateless firewall or a stateful firewall.

An application-based firewall is typically only protecting a host, not a network.

See Chapter 3 of the CompTIA Security+: Get Certified Get Ahead: SY0-501 Study Guide for more information on basic network devices.

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