Cryptography and Network Security Chapter 22 · Cryptography and Network Security Chapter 22 Fifth Edition by William Stallings Lecture slides by Lawrie Brown Chapter 20 – Firewalls

Cryptography and Cryptography and Network SecurityNetwork Security

Chapter 22Chapter 22

Fifth EditionFifth Edition

by William Stallingsby William Stallings

Lecture slides by Lawrie BrownLecture slides by Lawrie Brown

Chapter 20 – FirewallsChapter 20 – Firewalls

The function of a strong position is to make the The function of a strong position is to make the forces holding it practically unassailableforces holding it practically unassailable

——On War, On War, Carl Von ClausewitzCarl Von Clausewitz

IntroductionIntroduction

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

– can’t easily secure every system in orgcan’t easily secure every system in org● typically use a typically use a FirewallFirewall● to provide to provide perimeter defenceperimeter defence● as part of comprehensive security strategyas part of comprehensive security strategy

What is a Firewall?What is a Firewall?

● a a choke pointchoke point of control and monitoring of control and monitoring ● interconnects networks with differing trustinterconnects networks with differing trust● imposes restrictions on network servicesimposes restrictions on network services

– only authorized traffic is allowed only authorized traffic is allowed ● auditing and controlling accessauditing and controlling access

– can implement alarms for abnormal behaviorcan implement alarms for abnormal behavior● provide NAT & usage monitoringprovide NAT & usage monitoring● implement VPNs using IPSecimplement VPNs using IPSec● must be immune to penetrationmust be immune to penetration

What is a Firewall?What is a Firewall?

Firewall LimitationsFirewall Limitations

● cannot protect from attacks bypassing itcannot protect from attacks bypassing it– eg sneaker net, utility modems, trusted eg sneaker net, utility modems, trusted

organisations, trusted services (eg SSL/SSH)organisations, trusted services (eg SSL/SSH)● cannot protect against internal threatscannot protect against internal threats

– eg disgruntled or colluding employeeseg disgruntled or colluding employees● cannot protect against access via WLANcannot protect against access via WLAN

– if improperly secured against external useif improperly secured against external use● cannot protect against malware imported via cannot protect against malware imported via

laptop, PDA, storage infected outsidelaptop, PDA, storage infected outside

Firewalls – Packet FiltersFirewalls – Packet Filters

● simplest, fastest firewall component simplest, fastest firewall component ● foundation of any firewall system foundation of any firewall system ● examine each IP packet (no context) and examine each IP packet (no context) and

permit or deny according to rules permit or deny according to rules ● hence restrict access to services (ports)hence restrict access to services (ports)● possible default policiespossible default policies

– that not expressly permitted is prohibited that not expressly permitted is prohibited – that not expressly prohibited is permittedthat not expressly prohibited is permitted



Attacks on Packet FiltersAttacks on Packet Filters

● IP address spoofingIP address spoofing– fake source address to be trustedfake source address to be trusted– add filters on router to blockadd filters on router to block

● source routing attackssource routing attacks– attacker sets a route other than defaultattacker sets a route other than default– block source routed packetsblock source routed packets

● tiny fragment attackstiny fragment attacks– split header info over several tiny packetssplit header info over several tiny packets– either discard or reassemble before checkeither discard or reassemble before check

Firewalls – Stateful Packet FiltersFirewalls – Stateful Packet Filters● traditional packet filters do not examine higher traditional packet filters do not examine higher

layer contextlayer context– ie matching return packets with outgoing flowie matching return packets with outgoing flow

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

– keep track of client-server sessionskeep track of client-server sessions– check each packet validly belongs to onecheck each packet validly belongs to one

● hence are better able to detect bogus packets hence are better able to detect bogus packets out of contextout of context

● may even inspect limited application datamay even inspect limited application data

Firewalls - Firewalls - Application Level Application Level Gateway (or Proxy)Gateway (or Proxy)

● have application specific gateway / proxy have application specific gateway / proxy ● has full access to protocol has full access to protocol

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

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

Firewalls - Firewalls - Application Level Application Level Gateway (or Proxy)Gateway (or Proxy)

Firewalls - Firewalls - Circuit Level GatewayCircuit Level Gateway

● relays two TCP connectionsrelays two TCP connections● imposes security by limiting which such imposes security by limiting which such

connections are allowedconnections are allowed● once created usually relays traffic without once created usually relays traffic without

examining contentsexamining contents● typically used when trust internal users by typically used when trust internal users by

allowing general outbound connectionsallowing general outbound connections● SOCKS is commonly usedSOCKS is commonly used

Firewalls - Firewalls - Circuit Level GatewayCircuit Level Gateway

Bastion HostBastion Host

● highly secure host system highly secure host system ● runs circuit / application level gateways runs circuit / application level gateways ● or provides externally accessible servicesor provides externally accessible services● potentially exposed to "hostile" elements potentially exposed to "hostile" elements ● hence is secured to withstand thishence is secured to withstand this

– hardened O/S, essential services, extra authhardened O/S, essential services, extra auth– proxies small, secure, independent, non-privileged proxies small, secure, independent, non-privileged

● may support 2 or more net connectionsmay support 2 or more net connections● may be trusted to enforce policy of trusted may be trusted to enforce policy of trusted

separation between these net connectionsseparation between these net connections

Host-Based FirewallsHost-Based Firewalls

● s/w module used to secure individual hosts/w module used to secure individual host– available in many operating systems available in many operating systems – or can be provided as an add-on packageor can be provided as an add-on package

● often used on serversoften used on servers● advantages:advantages:

– can tailor filtering rules to host environmentcan tailor filtering rules to host environment– protection is provided independent of topologyprotection is provided independent of topology– provides an additional layer of protectionprovides an additional layer of protection

Personal FirewallsPersonal Firewalls

● controls traffic between PC/workstation and controls traffic between PC/workstation and Internet or enterprise networkInternet or enterprise network

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

typestypes● primary role to deny unauthorized remote primary role to deny unauthorized remote

access to the computeraccess to the computer● and monitor outgoing activity for malwareand monitor outgoing activity for malware

Personal FirewallsPersonal Firewalls

Firewall ConfigurationsFirewall Configurations



DMZ DMZ NetworksNetworks

Virtual Private NetworksVirtual Private Networks

Distributed Distributed FirewallsFirewalls

Summary of Firewall Locations and Summary of Firewall Locations and TopologiesTopologies

● host-resident firewallhost-resident firewall● screening routerscreening router● single bastion inlinesingle bastion inline● single bastion Tsingle bastion T● double bastion inlinedouble bastion inline● double bastion Tdouble bastion T● distributed firewall configurationdistributed firewall configuration

SummarySummary

● have considered:have considered:– firewallsfirewalls– types of firewallstypes of firewalls

● packet-filter, stateful inspection, application proxy, packet-filter, stateful inspection, application proxy, circuit-levelcircuit-level

– basingbasing● bastion, host, personalbastion, host, personal

– location and configurationslocation and configurations● DMZ, VPN, distributed, topologiesDMZ, VPN, distributed, topologies

Cryptography and Network Security Chapter 22 · Cryptography and Network Security Chapter 22 Fifth Edition by William Stallings Lecture slides by Lawrie Brown Chapter 20 – Firewalls

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