Lecture-4-Firewall Basics, Lecture notes of Information Security and Markup Languages

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Cryptography and

Network Security

FIFTH EDITION BY WILLIAM STALLINGS

INFORMATION SECURITY

Firewalls

The function of a strong position is to make the forces holding it practically unassailable — On War, Carl Von Clausewitz

What is a Firewall? a choke point of control and monitoring interconnects networks with differing trust imposes restrictions on network services ◦ only authorized traffic is allowed

auditing and controlling access ◦ can implement alarms for abnormal behavior

provide NAT & usage monitoring implement VPNs using IPSec must be immune to penetration

What is a Firewall?

Firewalls – Packet Filters

simplest, fastest firewall component foundation of any firewall system examine each IP packet (no context) and permit or deny according to rules hence restrict access to services (ports) possible default policies ● that not expressly permitted is prohibited ● that not expressly prohibited is permitted

Firewalls – Packet Filters

Attacks on Packet Filters

IP address spoofing ◦ fake source address to be trusted ◦ add filters on router to block source routing attacks ◦ attacker sets a route other than default ◦ block source routed packets

tiny fragment attacks ◦ split header info over several tiny packets ◦ either discard or reassemble before check

Firewalls – Stateful Packet Filters

traditional packet filters do not examine higher layer context ◦ ie matching return packets with outgoing flow stateful packet filters address this need they examine each IP packet in context ◦ keep track of client-server sessions ◦ check each packet validly belongs to one hence are better able to detect bogus packets out of context may even inspect limited application data

Firewalls - Application Level Gateway

(or Proxy)

Firewalls - Circuit Level Gateway

relays two TCP connections imposes security by limiting which such connections are allowed once created usually relays traffic without examining contents typically used when trust internal users by allowing general outbound connections SOCKS is commonly used

Bastion Host highly secure host system

runs circuit / application level gateways

or provides externally accessible services

potentially exposed to "hostile" elements

hence is secured to withstand this

●hardened O/S, essential services, extra auth

●proxies small, secure, independent, non-privileged

may support 2 or more net connections

may be trusted to enforce policy of trusted separation between these net connections

Host-Based Firewalls

s/w module used to secure individual host ◦ available in many operating systems ◦ or can be provided as an add-on package often used on servers advantages: ◦ can tailor filtering rules to host environment ◦ protection is provided independent of topology ◦ provides an additional layer of protection

Personal Firewalls

Firewall Configurations