How does a network firewall distinguish between trusted and untrusted traffic?

A network firewall distinguishes between trusted and untrusted traffic by analysing the data packets based on predefined security rules.

A network firewall is a security system designed to prevent unauthorised access to or from a private network. It acts as a barrier between a trusted network and an untrusted network, such as the internet. The firewall's primary function is to control the incoming and outgoing network traffic by analysing the data packets and determining whether they should be allowed through or not, based on a predetermined rule set.

A firewall's rule set is essentially a list of instructions that guide how it should handle incoming and outgoing network traffic. These rules are based on several factors, including IP addresses, domain names, protocols, programs, ports and keywords. For instance, a rule might block all incoming traffic from a specific IP address or allow outgoing traffic to a particular domain name.

When a data packet arrives at a firewall, the firewall examines the packet's header information, which includes the source and destination IP addresses, the protocol used (such as TCP or UDP), and the source and destination ports. The firewall then compares this information with its rule set. If the packet matches a rule that says it should be allowed, the firewall lets the packet through. If the packet matches a rule that says it should be blocked, the firewall discards the packet. If the packet doesn't match any rule, the firewall applies a default rule, which usually involves discarding the packet.

In addition to filtering traffic based on rules, some advanced firewalls also use other techniques to distinguish between trusted and untrusted traffic. For example, stateful inspection firewalls keep track of the state of network connections, such as TCP streams or UDP communication, and use this information to assess the legitimacy of packet traffic. Intrusion detection systems (IDS) and intrusion prevention systems (IPS) can identify and block traffic that exhibits signs of malicious activity, such as attempts to exploit vulnerabilities or perform denial-of-service attacks.

In conclusion, a network firewall distinguishes between trusted and untrusted traffic by analysing the data packets based on predefined security rules and using advanced techniques to detect and block malicious activity.