What Traffic Would an Implicit Deny Firewall Rule Block?

Table of Contents

 

Introduction

  - What is an implicit deny firewall rule?

Types of traffic blocked

  - Data traffic

    - Common internet traffic

    - Lateral traffic

    - East-west traffic

  - Control and management traffic

    - Routing protocols

    - Management access

    - Monitoring protocols

Why implicit deny is essential

  - Limits exposure

    - Stops unknown threats

    - Reduces attack surface

  - Enforces positive security model

    - Enumerates allowed traffic

    - Blocks everything else by default

How implicit deny works

  - Default drop behavior

    - H2: Inline blocking

    - Black hole discarding

  - Allow rules

    - Permitting critical traffic

    - Overriding default drop

  - Rule order and evaluation

    - Allow first, then deny

    - Match criteria processing 

Conclusion

Frequently Asked Questions

 

What Traffic Would an Implicit Deny Firewall Rule Block?

 

Introduction

 

An implicit deny firewall rule is foundational to strong network security, blocking all traffic by default unless explicitly allowed. This default-drop behavior limits exposure from unsolicited or malicious traffic attempting ingress or egress. Without configured allow rules to permit specific traffic, an implicit deny rule will block any and all packets trying to traverse the firewall interface. Understanding what traffic an implicit deny rule affects is critical for proper network security design. 🚫

 

Types of Traffic Blocked

 

-         Data Traffic

 

The implicit deny rule will block any data plane traffic attempting to flow through the firewall without an associated allow rule permitting it. This encompasses all types of user data, application, and system traffic.

 

Common Internet Traffic

 

Common Internet and web application traffic will be blocked by an implicit deny rule. This includes HTTP, HTTPS, DNS, SMTP, POP3, FTP, and myriad other protocols used to access resources, transfer files, and enable communication over the public Internet or private networks. Basic business activities like web browsing, email, video conferencing, cloud apps, etc. will all be stopped without configuring firewall rules to specifically allow this traffic.

 

Lateral Traffic

 

Lateral traffic moving east-west between endpoints within the internal network will also face an implicit deny blocking it. Resources across network segments and zones will become unreachable due to blocking of traffic like Windows file shares, database protocols, application APIs, virtual desktop traffic and more. Granular rules are required to allow approved lateral connections.

 

East-West Traffic

 

East-west traffic traversing laterally across server farms, application tiers, and virtualized infrastructure will also face denial. Valuable traffic like load balancing, caching, redundancy protocols, storage replication and virtual machine migrations can be disrupted without allow rules in place. Micro segmentation rules may be required.

 

-         Control and Management Traffic

 

In addition to data traffic, control plane and management plane traffic will also be blocked by the default implicit deny behavior.

 

Routing Protocols

 

Traffic from network routing protocols like OSPF, EIGRP, BGP that is essential for dynamic routing functions and topology discovery will be dropped. Devices will be unable to share routes or converge without allowance for bidirectional routing protocol flows.

 

Management Access

 

Management traffic like SSH, Telnet, RDP, VNC and other protocols enabling configuration access to network devices and servers will be blocked as well. Centralized management and automation systems will lose connectivity without allows for management traffic.

 

Monitoring Protocols

 

Monitoring protocols like SNMP, Syslog for traps and events, Netflow for statistics and visibility will also be denied and unable to function across the firewall. Network monitoring and observability will suffer without allowing bidirectional flows for monitoring.

 

Why Implicit Deny is Essential

 

Limits Exposure

 

Implicit deny rules drastically limit exposure to threats by only allowing traffic that is explicitly permitted. This "default-deny" approach prevents anything unsolicited from successfully traversing the firewall.

 

Stops Unknown Threats

 

The implicit deny effect provides proactive protection against unknown threats by blocking traffic if it is not specifically allowed. Malicious scans, brute force attacks, worms, vulnerabilities and more are mitigated by simply not responding. What you don't allow in, can't hurt you.

 

Reduces Attack Surface

 

Implicit deny shrinks the attack surface down to only the protocols and flows permitted in allow rules. Even if an allowed service is vulnerable, it limits damage potential by blocking everything else. The denied traffic has no way to reach the asset.

 

-         Enforces Positive Security Model

 

Implicit deny also enforces a "positive" security model requiring enumeration of allowed traffic rather than trying to identify and block all potential malicious traffic reactively.

 

Enumerates Allowed Traffic

 

The positive model defining allowed traffic is simpler and more secure than trying to enumerate all possible malicious actions to block. Allow required business traffic, deny everything else by default.

 

Blocks Everything Else by Default

 

There is no need to figure out the infinite varieties of unauthorized traffic to block when you simply deny by default. This massively reduces the logic and management overhead.

 

How Implicit Deny Works

 

-         Default Drop Behavior

 

The default drop behavior is achieved by implicit deny rules added at the end of the firewall ruleset that block any traffic not matching allow rules earlier in the sequence.

 

Inline Blocking

 

Physical and virtual firewall appliances utilize inline blocking, where implicit deny actively discards traffic flows not specifically permitted. Packets have no way past the firewall to the destination.

 

Black Hole Discarding

 

Hosts can also enforce implicit deny via black hole discarding where routing drops denied traffic into a null route that simply discards it silently. Rejected packets don't progress.

 

-         Allow Rules

 

Allow rules added before the implicit deny override the default drop behavior and permit required traffic.

 

Permitting Critical Traffic

 

Allow rules utilized important criteria like IP addresses, ports, protocols and direction to permit critical business connectivity for users, systems and applications through the firewall.

 

Overriding Default Drop

 

By enumerating allows for characterized connectivity needs, resource access and system functions before the implicit deny, this traffic is able to flow where it would normally face default blocking.

 

-         Rule Order and Evaluation

 

Rule order is critical, with allow permissions above implicit deny blocking in the ruleset sequence. Rule criteria is evaluated top down until the first match.

 

Allow First, Then Deny

 

Allow rules must precede any deny rules in the ruleset to override the default behavior. If deny was first, allow rules would never be reached.

 

Match Criteria Processing

 

The firewall processes rules sequentially, stopping after the first match. So criteria must be carefully sequenced from most specific allows to general deny.

 

Conclusion

 

Implicit deny rules are foundational for robust network security by enforcing default-drop behavior to limit exposure to threats. All traffic is blocked by default until explicitly allowed based on protocol, IP, port and direction by rules permitting only necessary connectivity. This positive security model shrinks attack surface by cutting off unsolicited traffic, enforcing least privilege for traffic forwarding. Understanding what traffic implicit deny affects is key for designing effective security policies and configuring firewall rules allowing only authorized connectivity. 🔒

 

Frequently Asked Questions

 

What is the main benefit of an implicit deny rule?

 

The main benefit is greatly improved security posture by defaulting to drop all traffic, forcing allows enumerating only necessary traffic. Exposure is limited.

 

Does an implicit deny rule block outbound traffic too?

 

Yes, egress traffic is also denied by default without allows permitting outbound connections, limiting exposure in both directions.

 

Can critical business traffic still flow with implicit deny in place?

 

Yes, required traffic is permitted by allow rules overriding the default drop behavior based on protocol, port, source, destination, etc.

 

What risks does implicit deny mitigate?

 

Mainly external threats trying to enter the network via unsolicited attack traffic, malicious payloads, recon, exploits and infections are denied by default. 

 

How do you determine traffic needs for allow rules?

 

Methodically inventory business systems, resources, users and map required traffic flows. Allow rules permit characterized needs. Deny the rest.

 

Does an implicit deny rule log all dropped traffic?

 

No logs unless a rule is added to log denied connections. Useful for identifying needed traffic to allow.

 

Is implicit deny more work than firewall blacklists?

 

Yes, carefully auditing and mapping existing traffic to build allow rules has more overhead than simply blocking some high-risk traffic.

 

What are disadvantages of implicit deny rules?

 

Mainly complexity - must fully understand existing connectivity needs. Can break undocumented access.

 

How is implicit deny different than traditional firewalls?

 

Traditional firewalls allow all, then deny specific threats reactively. Implicit deny proactively denies all first, then allows specifically.

 

When should implicit deny rules be considered?

 

When strong security is critical, and existing traffic patterns are well defined. Use cases like DMZ segments lend well.