Firewalls are an important component of network security that serve to protect networks by regulating incoming and outgoing traffic. However, setting up firewalls correctly is a challenging task, which becomes more difficult with the growth of the network's size. Firewall configuration files consist of rule sets that might be hard to understand even for professionals who deal with them regularly. The main reason for this is that most firewall rule sets have a certain structure: the higher the position of a rule in the rule set, the higher priority it has. Challenging problems arise when a new rule is added to the set and a proper position for it needs to be found or the existing rules are removed due to a security policy change. This brings us to the usability problem associated with the configuration of firewalls.

The overall aim of this thesis is to help system administrators better manage firewalls. We explore three different aspects of firewall configuration: 1) the syntax of rules, 2) the organization of rules in a rule set, and 3) the way rule sets are presented to a user. Using this acquired knowledge, we offer system administrators more usable firewall solutions and approaches to the configuration process that can help facilitate their daily work.

Most companies have access to the Internet and their corporate networks connected to it. Many threats to computer systems, e.g. worms, trojans, and denial-of-service attacks, can be encountered online and they may entail, for example, confidential data theft, service disruption and financial losses. Every organization, regardless of its size, type of activity or infrastructure, requires network security solutions in place in order to protect it from the ever-increasing number of cyber threats. Firewalls are an important component of network security that protect networks by regulating incoming and outgoing traffic.

Simply having a firewall does not guarantee any protection against Internet threats, unless it is properly configured. However, setting up firewalls correctly is a challenging task, which becomes more difficult with the growth of the network's size. Firewall configuration files consist of rule sets that might be hard to understand even for professionals that deal with them regularly. The main reason for this is that most firewall rule sets have a certain structure: the higher the position of a rule in the rule set, the higher priority it has. Challenging problems arise when a new rule is added to the set and a proper position for it needs to be found, or when existing rules are removed due to a security policy change. This brings us to the usability problem associated with the configuration of firewalls.

The overall aim of this thesis is to help system administrators better manage firewalls. First, we conduct a series of semi-structured interviews with system administrators, in which we ask them about problems confronted when managing firewalls. After having ascertained that there are usability problems involved, we begin to address them. We compare two different firewall rule set representation approaches and identify that a preference for one or the other depends on the firewall expertise of the individual. We introduce and mathematically formalize a set of four usability metrics which are designed to evaluate the quality of firewall rule sets. Furthermore, we not only investigate which firewall interfaces are utilized and preferred by system administrators but also identify and classify the interfaces' strengths and limitations. Finally, we conduct a systematic literature review to gain an understanding of the state of the art in firewall usability. This review classifies the available solutions and identifies the open challenges that exist in the field.