EE579T/6GD #1 Summer 2003 © 2000-2003, Richard A. Stanley EE579T Network Security 6: Firewalls and Trusted Networks Prof. Richard A. Stanley

Summer 2003© 2000-2003, Richard A. Stanley

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EE579TNetwork Security

6: Firewalls and Trusted Networks

Prof. Richard A. Stanley


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Overview of Today’s Class

• Review last week’s lesson

• Security in the news

• Firewalls

• Trusted networks


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Summary

• TCP/IP was not intended as a secure protocol; as a result, it has vulnerabilities that can be exploited

• There are many ways to get access to info

• There are many types of attacks that can be mounted over network connections in order to gain unauthorized access to resources

• Never forget, the best access is hands-on


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Interesting Security Facts• The average number of formidable Internet-

based attacks currently are twice as likely toaffect power utilities in the United States than financial firms

• Overall number of these attacks is growing “very rapidly”

• Data shows steady increase in attacks against electronic infrastructure

Source: Securities Industry News


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More Statistics

• Most threats still from inside the firm

• Outside attacks still dominated by hackers

• Government or group sponsored attacks on the rise

• “...for the first time, empirical evidence has led to profiles of attacks that appear to be sponsored by governments or other organizations...” [Tim Belcher, CEO, Riptech]



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And more...• Origin of attacks:

– USA 30%

– South Korea 9%

– China 8%

• Based on number of Internet users, Israel leads the list as a source of attacks

• Beware jumping to conclusions--an attack from Country X may just be using their servers as a jumping-off point



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Outline

• Firewall Design Principles– Firewall Characteristics– Types of Firewalls– Firewall Configurations

• Trusted Systems– Data Access Control– The Concept of Trusted systems– Trojan Horse Defense


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Firewall is to Networkas

User privilege is to Operating system


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What Is a Firewall?

• A router with attitude?

• A device to implement an access control policy?

• A physical device?

• A logical device?

• The preferred solution for network protection?


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Firewalls

• Effective means of protection a local system or network of systems from network-based security threats while affording access to the outside world via WAN`s or the Internet

• Despite common opinion, not a panacea or an “out-of-the-box” security solution


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Where Does This Term Come From?

Firewall means a fire separation of noncombustible construction that subdivides a building or separates adjoining buildings to resist the spread of fire that has a fire-resistance

rating as prescribed in the Building Code and that has structural stability to remain intact under fire conditions for the

required fire-rated time.

Source: The Ontario Fire Code, § 1.2.1.2


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Firewall DesignPrinciples

• Information systems undergo a steady evolution (from small LAN`s to Internet connectivity)

• Strong security features for all workstations and servers not established


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Firewall DesignPrinciples

• The firewall is inserted between the premises network and the Internet or another external network

• Aims:– Establish a controlled link– Protect the premises network from Internet-

based or “outside” attacks– Provide a single choke point (good or bad?)


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Firewall Characteristics

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

through the firewall (physically blocking all access to the local network except via the firewall)

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


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Firewall Characteristics

• Design goals:– The firewall itself is immune to penetration

(use of trusted system with a secure operating system)

– Although this is a noble goal, it is virtually impossible to achieve!


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Firewall Characteristics - 1

• Service control– Determines the types of external services that

can be accessed, inbound or outbound

• Direction control– Determines the direction in which particular

service requests are allowed to flow


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Firewall Characteristics - 2

• User control– Controls access to a service according to which

user is attempting to access it

• Behavior control– Controls how particular services can be used

(e.g. filter e-mail)


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Types of Firewalls

• Three common types of Firewalls:– Packet-filtering routers– Application-level gateways– Circuit-level gateways


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Types of Firewalls

• Packet-filtering Router


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Packet-Filtering Firewall

• Applies a set of rules to each incoming IP packet and then forwards or discards the packet

• Filters packets going in both directions

• The packet filter is typically set up as a list of rules based on matches to fields in the IP or TCP header

• Two default policies (discard or forward)


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Packet Filtering Firewall

• Advantages:– Simplicity– Transparency to users– High speed

• Disadvantages:– Difficult to set up packet filter rules– Lack of authentication


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Packet Filtering Firewall

• Possible attacks and appropriate countermeasures– IP address spoofing– Source routing attacks– Tiny fragment attacks


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Types of Firewalls

• Application-level Gateway


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Application-level Gateway

• Also called proxy server

• Acts as a relay of application-level traffic


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Application-level Gateway

• Advantages:– Higher security than packet filters– Only need to scrutinize a few allowable applications– Easy to log and audit all incoming traffic

• Disadvantages:– Additional processing overhead on each connection

(gateway as splice point)– Speed


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Types of Firewalls

• Circuit-level Gateway


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Circuit-level Gateway

• Stand-alone system, or

• Specialized function performed by an application-level gateway

• Sets up two TCP connections

• The gateway typically relays TCP segments from one connection to the other without examining the contents


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Circuit-level Gateway

• Security function consists of determining which connections will be allowed

• Typically used where the system administrator trusts the internal users

• An example is the SOCKS package


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Types of Firewalls

• Bastion Host– Sometimes called a DMZ– A system identified by the firewall

administrator as a critical strong point in the network´s security

– The bastion host serves as a platform for an application-level or circuit-level gateway


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Firewall Configurations

• In addition to using simple configuration of a single system (single packet filtering router or single gateway), more complex configurations are possible

• Three common configurations


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• Screened host firewall system (single-homed bastion host)


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Screened Host Firewall

• Firewall consists of two systems:– A packet-filtering router– A bastion host

• Configuration for the packet-filtering router:– Only packets from and to the bastion host are

allowed to pass through the router

• The bastion host performs authentication and proxy functions


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Screened Host Firewall

• Greater security than single configurations :– Implements both packet-level and application-level

filtering (allowing for flexibility in defining security policy)

– An intruder must generally penetrate two separate systems (but if outside router compromised, what then?)

• Affords flexibility in providing direct Internet access (public information server, e.g. Web server)


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• Screened host firewall system (dual-homed bastion host)


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Dual-homed Bastion Host

• Even if the packet-filtering router is completely compromised– Traffic between the Internet and other hosts on

the private network has to flow through the bastion host

– Provides two layers of security


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• Screened-subnet firewall system


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Screened-Subnet Firewall

• Most secure configuration of the three

• Two packet-filtering routers are used– One between bastion host and external network– One between bastion host and internal network

• Creates an isolated sub-network


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Screened-Subnet Firewall

• Advantages:– Three levels of defense to thwart intruders– Outside router advertises only the existence of

the screened subnet to the Internet (internal network is invisible to the Internet)

– Inside router advertises only the existence of the screened subnet to the internal network (systems on the inside network cannot construct direct routes to the Internet)


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Firewalls

• Useful to enforce security policy at the network edges

• Popularly believed to provide “hardened” security as they come out of the box

• If not properly configured, can introduce more problems than they solve

• Come in both hardware and software flavors, but all have software inside


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Trusted Systems

• One way to enhance the ability of a system to defend against intruders and malicious programs is to implement trusted system technology

• Be careful whom you trust!


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Data Access Control

• Through the user access control procedure (log on), user is identified to the system

• Associated with each user, there is a profile that specifies permissible operations and file accesses

• The operating system can enforce rules based on the user profile– This is why Win9x cannot be used here


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Data Access Control

• General models of access control:– Access matrix– Access control list– Capability list

• We saw all these in Computer Security, in the implementation of security models like Bell-LaPadula


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Data Access Control

• Access Matrix


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Data Access Control

• Access Matrix: Basic elements of the model– Subject: An entity capable of accessing objects,

the concept of subject equates with that of process

– Object: Anything to which access is controlled (e.g. files, programs)

– Access right: The way in which an object is accessed by a subject (e.g. read, write, execute)


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Data Access Control

• Access Control List: Decomposition of the matrix by columns


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Data Access Control

• Access Control List– An access control list lists users and their

permitted access right– The list may contain a default or public entry– This is how Unix handles security, and is the

only mechanism available in Unix• Everything in Unix looks like a text file

• All files have 9-bit permissions in the inode pointer


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Data Access Control

• Capability list: Decomposition of the matrix by rows


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Data Access Control

• Capability list– A capability ticket specifies authorized objects

and operations for a user– Each user has a number of tickets


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Trusted Systems Concept

• Trusted Systems– Protect data and resources on the basis of levels

of security (e.g. military)– Users can be granted clearances to access

certain categories of data– Trusted systems need not discern levels of

permissions; they can operate “system high”• cf. Telephone systems


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Security Levels

• Multilevel security: multiple categories or levels of data

• Multilevel secure system must enforce:– No read up: A subject can only read an object of lower

or equal security level (BLP Simple Security Property)

– No write down: A subject can only write into an object of greater or equal security level (BLP *-Property)

– May enforce discretionary security (BLP DS property)

• Security levels may be linear or latticed


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Trusted Systems Implementation

• Reference Monitor provides multilevel security for a data processing system– Reference Monitor is a concept, not a thing


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Reference Monitor Up Close and Personal


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Reference Monitor

• Controlling element in the security kernel of a computer that regulates access of subjects to objects on basis of security parameters

• The monitor has access to a file (security kernel database)

• The monitor enforces the security rules (no read up, no write down)


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Reference Monitor Properties

• Complete mediation: Security rules are enforced on every access

• Isolation: Reference monitor and database protected from unauthorized modification

• Verifiability: reference monitor’s correctness must be mathematically provable – this may be where we bend the rules!


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Trusted Systems

• A system that can provide such verifications (properties) is referred to as a trusted system


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Trojan Horse Defense

• Secure, trusted operating systems are one way to secure against Trojan Horse attacks


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Summary

• Firewalls are useful tools to mediate access from internal networks to external networks

• Firewalls are not a single-point security solution

• Firewalls cannot protect against a malicious user on the internal network

• Trusted computing systems are needed to enforce security policy


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Homework

• Read text, Chapters 11 and 8• Choose a firewall product, and describe in detail

how you would implement the following security policy:– Anyone on the inside network may establish any

connection they desire from outside– No one on the outside network may initiate a

connection to the inside– Outsiders may not ping in, but insiders can ping out

Documents

EE579T/6GD #1 Summer 2003 © 2000-2003, Richard A. Stanley EE579T Network Security 6: Firewalls and Trusted Networks Prof. Richard A. Stanley