Download William Stallings, Cryptography and Network Security 3/e

FIREWALLS
The function of a strong position is to make
the forces holding it practically unassailable
—On War, Carl Von Clausewitz
On the day that you take up your command,
block the frontier passes, destroy the official
tallies, and stop the passage of all emissaries
—The Art of War, Sun Tzu
What is a Firewall?
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A firewall is hardware or software (or a
combination of hardware and software) that
monitors the transmission of packets of digital
information that attempt to pass through the
perimeter of a network.
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A firewall is simply a program or hardware device
that filters the information coming through the
Internet connection into your private network or
computer system. If an incoming packet of
information is flagged by the filters, it is not
allowed through.
Perimeter Defense
A firewall is said to provide “perimeter security” because it sits on the
outer boundary, or perimeter, of a network. The network boundary is
the point at which one network connects to another.
What is a Firewall?
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a choke point that keeps unauthorized users
out of the protected network.
interconnects networks with differing trust
imposes restrictions on network services
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auditing and controlling access
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only authorized traffic is allowed
can implement alarms for abnormal behavior
is itself immune to penetration
provides perimeter defence
Firewall Limitations
cannot protect from attacks bypassing it
 cannot protect against internal threats
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e.g. disgruntled employee
cannot protect against transfer of all virus
infected programs or files
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because of huge range of O/S & file types
Types of Firewalls
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Packet Filters
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Application-Level Gateways
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Circuit-Level Gateways
Firewalls – Packet Filters
Firewalls – Packet Filters

A packet filtering router applies a set of
rules to each incoming IP packet and then
forwards or discards the packet.
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The router is typically configured to filter
packets going in both directions (from and
to the internal network).
Firewalls – Packet Filters
Filtering rules are based on information contained
in a network packet:
 Source IP address: The IP address of the
system that originated the IP packet (e.g.,
192.168.1.1)
 Destination IP address: The IP address of the
system the IP packet is trying to reach (e.g.
192.168.1.2)
 Source and destination transport-level address:
The transport level (e.g., TCP or UDP) port
number, which defines applications such as
SNMP or TELNET
Firewalls – Packet Filters:
Default Policies
Packet filtering is typically set up as a list of rules
based on matches to fields in the IP or TCP header.
When there is no match to any rule, a default action
is taken.
There are two possible default policies: discard or
forward.
Firewalls – Packet Filters:
Default Policies
Default = discard: that which is not expressly
permitted is prohibited.
It is very conservative. Initially, everything is
blocked—services must be
added on a caseby-case basic.
Default = forward: that which is not expressly
prohibited is permitted.
It increases ease of use for end users but
provides reduced security. The security
administrator must, in essence, react to each new
security threat as it becomes available
Firewalls – Packet Filters
Attacks on Packet Filters
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IP address spoofing
fake source address to be trusted
 add filters on router to block
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source routing attacks
attacker sets a route other than default
 block source routed packets
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tiny fragment attacks
split header info over several tiny packets
 either discard or reassemble before check
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Firewalls - Application Level
Gateway (or Proxy)
Firewalls - Application Level
Gateway (or Proxy)
Acts as relay of application-level traffic. The user
contacts the gateway using a TCP/IP
application, such as FTP, and the gateway asks
the user for the name of a remote host to be
accessed. When the user responds and
provides a valid user ID and authentication
information, the gateway contacts the application
on the remote host and relays TCP segments
containing the application data between the two
points.
Firewalls - Application Level
Gateway (or Proxy)
Tend to be more secure than packet filters.
 Need only scrutinize a few allowable
applications.
 It is easy to log and audit all incoming
traffic at the application level.
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Firewalls - Application Level
Gateway (or Proxy)
Main Disadvantage
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Additional Processing overhead on each
connection.
Firewalls - Circuit Level Gateway
Firewalls - Circuit Level Gateway
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relays two TCP connections (one between itself and a
TCP user on an inner host and one between itself and a TCP user
on an outside host)
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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 (a protocol) commonly used for this
Bastion Host
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highly secure host system that serves as a
platform for an application-level or circuitlevel gateway.
host hardware platform executes a secure
version of it’s operating system, making it a
trusted system.
only services that the network administrator
considers essential are installed on the
bastion host (e.g. Telnet, DNS, FTP, and user
authentication)
Firewall Configurations
Single-Homed Bastion: Advantages
Consists of two systems: a packet-filtering router and a
bastion host. The router is configured so that
For traffic from the Internet, only IP packets destined for the
bastion host are allowed in.
For the traffic from the internal network, only IP packets from
the bastion host are allowed to out.
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The bastion host performs authentication and proxy
functions.
Single-Homed Bastion
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Has greater security than simply a packet filtering router
or an application level gateway alone.
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Implements both packet-level and application-level filtering,
allowing for considerable flexibility in defining security policy.
An intruder must generally penetrate two separate systems before
the security of the internal network is compromised.
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Affords flexibility in providing direct Internet access.
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If the packet-filtering router is completely compromised,
traffic could flow directly through the router between the
Internet and other hosts on the private network.
Firewall Configurations
Firewall Configurations
Screened Subnet Firewall
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There are now three levels of defense to thwart
intruders.
The outside router advertises only the existence
of the screened subnet to the Internet; therefore,
the internal network is invisible to the Internet.
Similarly, the inside router advertises only the
existence of the screened subnet to the internal
network; therefore, the systems on the inside
network cannot construct direct routes to the
Internet.