Firewalls

2014.04.07

Uyanga Tserengombo

Email: uyanga_0956@yahoo.com

Firewall topics

• The Need for Firewalls

• Firewall Characteristics

• Types of Firewalls

• Firewall Basing

• Firewall Location and Configurations

What is a Firewall?

• Effective means of protecting local network of systems from network-based security threats from outer world– while providing (limited) access to the outside

world (the Internet)

What is a Firewall?

Figure 11.1a General Model

Why do we need Firewalls?

• seen evolution of information systems• now everyone want to be on the Internet • and to interconnect networks • has persistent security concerns

– can’t easily secure every system in org

• typically use a Firewall• to provide perimeter defence• as part of comprehensive security strategy

Firewall Basics

• 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

Firewall Characteristics 1/2

• Design goals:– All traffic from inside from/to outside must pass

through the firewall

– Only authorized traffic (defined by the local security policy) will be allowed to pass

– The firewall itself should be immune to penetration (use of trusted system with a secure operating system)

Firewall Characteristics 2/2• General techniques for access control

– Service control• Determines the types of Internet services that can be accessed

– Mostly using TCP/UDP port numbers– Direction control

• Determines the direction in which particular service requests may be initiated and allowed to flow through the firewall.

– User control• Controls access to a service according to which user is attempting to

access it• need to authenticate users. This is easy for internal users, but what

can be done for external ones?– Behavior control

• Controls how particular services are used (e.g. filter e-mail for spam control)

Firewall Limitations

• cannot protect from attacks bypassing it– eg sneaker net, utility modems, trusted organisations,

trusted services (eg SSL/SSH)

• cannot protect against internal threats– eg disgruntled or colluding employees

• cannot protect against access via WLAN– if improperly secured against external use

• cannot protect against malware imported via laptop, PDA, storage infected outside

Types of Firewalls

• Packet-filtering routers

• Stateful Inspection

• Application-level gateways

• Circuit-level gateways (not common, so skipped)

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 policiesthat not expressly permitted is prohibited that not expressly prohibited is permitted

Firewalls – Packet Filters

Figure 11.1b Packet filtering firewall

Packet Filtering Examples

Table 11.1 Packet-Filtering Examples

Attacks on Packet Filters

IP address spoofingfake source address to be trustedadd filters on router to block

source routing attacksattacker sets a route other than defaultblock source routed packets

tiny fragment attackssplit header info over several tiny packetseither discard or reassemble before check

Stateful Inspection

traditional packet filters do not examine higher layer contextie matching return packets with outgoing flow

stateful packet filters address this need they examine each IP packet in context

keep track of client-server sessionscheck each packet validly belongs to one

hence are better able to detect bogus packets out of context

may even inspect limited application data

Stateful Inspection

Table 11.2 Stateful Inspection

Firewalls - Application Level Gateway (or Proxy)

have application specific gateway / proxy has full access to protocol

user requests service from proxy proxy validates request as legal then actions request and returns result to usercan log / audit traffic at application level

need separate proxies for each service some services naturally support proxying others are more problematic

Application-level Gateway

Figure 11.1c Application proxy firewall

Firewalls - Circuit Level Gateway

relays two TCP connectionsimposes security by limiting which such

connections are allowedonce created usually relays traffic without

examining contentstypically used when trust internal users by

allowing general outbound connections

Firewalls - Circuit Level Gateway

Figure 11.1d Circuit-level proxy firewall

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 authproxies 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 hostavailable in many operating systems or can be provided as an add-on package

often used on serversadvantages:

can tailor filtering rules to host environmentprotection is provided independent of topologyprovides an additional layer of protection

Personal Firewalls

controls traffic between PC/workstation and Internet or enterprise network

a software module on personal computer or in home/office DSL/cable/ISP router typically much less complex than other firewall

types primary role to deny unauthorized remote access to

the computer and monitor outgoing activity for malware

Firewall Configurations

Figure 11.3a Screened host firewall system

Firewall Configurations

Figure 11.3b Screened host firewall system

Firewall Configurations

Figure 11.3c Screened host firewall system

DMZ Networks

Figure 11.4 Example Firewall Configuration

Virtual Private Networks

Figure 11.5 A VPN Security Scenario

Distributed Firewalls

Figure 11.6 Example Distributed Firewall Configuration

Summary of Firewall Locations and Topologies

host-resident firewallscreening routersingle bastion inlinesingle bastion Tdouble bastion inlinedouble bastion Tdistributed firewall configuration

References• William Stallings, “Network Security Essential –

4th Edition”• http://mn.wikipedia.org/wiki/Firewall• http://forum.asuultserver.com/viewtopic.php?f=1

&t=44687&start=26• http://www.manageengine.com/products/firewall/

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