The differences between inbound and outbound firewall rules

Inbound traffic versus outbound traffic

Enterprise networks have both inbound traffic and outbound traffic:

• Inbound traffic requests. They originate from outside the network, such as an external user with a web browser, email client, server or application making requests -- like FTP and SSH -- or API calls to web services.
• Outbound traffic requests. They originate from inside the network, destined for services on the internet or outside networks, such as a user visiting an external website or an internal mail server connecting to an external one.

Firewalls are designed and deployed to prevent inbound traffic from entering a network and to stop outbound traffic from connecting to external resources that are noncompliant with an organization's security policies.

Inbound vs. outbound firewall rules

Firewall rules, which are either inbound or outbound, can be customized to allow traffic on specific ports, services and IP addresses to enter or leave the network:

• Inbound firewall rules. They protect a network by blocking traffic known to be from malicious sources. This stops various attacks, such as malware and DDoS, from affecting internal resources.
• Outbound firewall rules. They define the traffic allowed to leave a network and reach legitimate destinations. These rules also block requests sent to malicious websites and untrusted domains. They can also prevent data exfiltration by analyzing the contents of emails and files sent from a network.

The firewall policy that governs the configuration of inbound and outbound rules is based on a risk assessment of the assets it is protecting and the business needs for users and services inside the network. For example, the HR department might be allowed access to the internet and the company's accounting department's network but not vice versa.

Any changes to inbound and outbound firewall rules should be carefully planned, implemented and monitored to avoid unforeseen consequences, among them blocking valid requests, which can throttle legitimate business activities and frustrate users.

Using inbound firewall rules

The goal of inbound firewall rules is to keep malicious traffic out of internal network systems and protect the resources located within them.

Network segmentation enables teams to place firewalls at various points within a network, including at the perimeter and internally to divide a network into individual subnetworks. Each firewall's inbound rules can be configured to protect specific resources in each segment.

For example, the firewall protecting the HR segment of the network only permits inbound requests from HR employees with the necessary privileges. A firewall protecting the network perimeter, meanwhile, has less restrictive rules. These rules, however, are based on threat intelligence and block traffic from known bad IP addresses or locations.

Examples of inbound firewall rules include the following:

• Filtering traffic from a variety of sources, such as specific IP addresses.
• Restricting or permitting traffic to internal network ports.
• Allowing email and other communication from TCP (Transmission Control Protocol), UDP (User Datagram Protocol) or ICMP (Internet Control Message Protocol).

Using outbound firewall rules

Outbound firewall rules protect internal network resources by preventing the following:

• Internal users from accessing malicious content.
• Sensitive data from leaving the network in violation of security policy rules.
• Data exfiltration from malware or insider threats.

Teams can use an off-site cloud service, such as a secure web gateway, to control outbound traffic if specialized filtering technologies are necessary. Such systems perform targeted functions, such as content filtering for email or web browsing. They often tie into the business's directory service -- Active Directory and Lightweight Directory Access Protocol -- so they can provide access, filtering and reporting based on each user's network account.

Other firewall systems look for outbound malware and security-related threats, including DNS lookups to hosts known to be threatening or blocklisted.

Outbound firewall rules in locked-down environments can control network behavior down to the host, application and protocol levels.

Examples of outbound firewall rules include the following:

• Restricting users from accessing external malicious or inappropriate websites.
• Managing outbound communication formats, which can interrupt the ability for malware to connect to command-and-control servers.
• Generating logs to enable security teams or network admins to monitor outgoing traffic.

Firewall rules now and in the future

Firewalls are constantly evolving and will always be a key security control in any network. Modern firewalls use threat intelligence feeds, AI and machine learning to update inbound and outbound rules on the fly, enabling them to combat new and emerging threats as they develop.

Remember that inbound and outbound firewall rules require careful configuration, as well as monitoring for system anomalies. Even the most secure firewalls can only do so much. Those enterprises without the necessary internal resources -- among them product training and security knowledge -- might consider outsourcing the management of their firewall environments to an outsourced managed security service provider (MSSP). A dedicated, 24/7 MSSP network security monitoring service is often the best way to minimize associated risks.

Editor's note: This article was updated in July 2025 to improve the reader experience.

Michael Cobb, CISSP-ISSAP, is a renowned security author with more than 20 years of experience in the IT industry.

Kevin Beaver is an independent information security consultant, writer and professional speaker with Atlanta-based Principle Logic LLC. With more than 30 years of experience in the industry, he specializes in performing vulnerability and penetration tests, as well as virtual CISO consulting work.