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Firewall Technology
Firewall Technology - Outline
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Defining the types of firewalls.
Developing a firewall configuration.
Designing a firewall rule set.
Firewall: Origin of the Term
Since old days, brick walls were built between buildings
in apartment complexes so that if a fire broke out, it
would not spread from one building to another. Quite
naturally, the walls were called "
firewalls ".
Castle and Moat Analogy
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A network firewalls is more like the moat around
a castle than a firewall
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Restricts access from the outside
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Restricts outbound connections (i.e. filter out
undesirable activity from internal hosts)
Overview
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A firewall is a network access control device.
It can perform a centralized security
management function.
It denies all traffic except that which is
explicitly allowed.
It can be configured based on services, source
or destination IP address, User ID, … etc.
Why use Firewalls?
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Prevent denial of service attacks:
 SYN flooding: attacker establishes many bogus
TCP connections, no resources left for “real”
connections.
Prevent illegal modification/access of internal data.
 e.g., attacker replaces Company's homepage with
something else
Allow only authorized access to inside network (set of
authenticated users/hosts)
Prevent insider attacks on critical systems:
 Critical systems (human resources, payroll, etc.)
can be hidden behind their own firewalls to
prevent attacks from the inside.
The Unprotected Network
Web Server
Internet
Router
What could possibly be wrong with this setup?
Hackers paradise & administrators nightmare!
What Can We Do?
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Firewalls give good protection against attacks
from the Internet.
The problem is that there are various firewall
techogies and deployment strategies.
To choose the right solution, we need to
know a bit more about the underlying TCP/IP
protocols used on the Internet.
The TCP/IP-Protocol stack
The IP-protocol
Important fields: Source Address, Destination Address, Protocol
The TCP-protocol
Important fields: Source Port and Destination Port
ACK and SYN flags
The UDP-protocol
Important fields: Source Port and Destination Port
Common Ports and Services
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Windows:
%windir%\System32\drivers\etc\services
Linux:
/etc/services
Examples:
SMTP = port 25
HTTP = port 80
POP3 = port 110
PPTP = port 1723
Types of Firewalls
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Packet filtering firewall (network layer)
Statefull Inspection firewall (connection state
of the transport layer protocol)
Application layer firewall (known as Application
Gateway or Proxy)
Hybrid: combination of the above or a generic
service proxy (i.e. a Socks Proxy work for all
TCP-based apps).
Packet Filter Firewalls (Chokes)Should arriving
packet be allowed
in? Departing packet
let out?
Internet
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They are IP routers restricting flow of packets between
an internal network and external networks.
packet-by-packet decision to forward/drop based on:
 Source/Destination IP Addresses
 Protocol Value (i.e. drop ICMP packets)
 TCP/UDP Source Port/Destination Port
 TCP SYN and ACK bits
Packet Filtering Examples
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Example 1: Block incoming and outgoing datagrams
with IP protocol field = 17 or with either source or
destination port = 23.
 All incoming and outgoing UDP flows and telnet
connections are blocked.
Example 2: Block inbound TCP segments with (SYN=1
& ACK=0).
 Prevents external clients from making TCP
connections with internal clients, but allows internal
clients to connect to outside.
Packet Filter Firewall - Pros
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They are fast because they operate on IP
addresses and TCP/UDP port numbers alone,
ignoring the data part of packets.
Due to the fact that packet payload is ignored,
application independence exists.
Least expensive of the three types of firewalls.
Packet filtering rules are relatively easy to
configure.
There are no configuration changes necessary
to the protected workstations.
Packet Filter Firewall - Cons
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Allow a direct connection between endpoints through the firewall.
This leaves the potential for a vulnerability to be exploited.
There is no screening of packet payload available. It is impossible
to block users from visiting web sites deemed off limits, for
example.
Logging of network traffic includes only IP addresses and
TCP/UDP port numbers, no packet payload information is
available.
Complex firewall policies are difficult to implement using filtering
rules alone.
Rely on IP addresses for authentication rather than user
authentication.
Stateful packet inspection
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It is an advanced packet filter that keeps track of the
state of the network connections going through it.
Take into account the state of the connection; an
incoming packet is matched to its outbound request for
the packet to be allowed in.
Examine and filter based on packet content; for
example, drop incoming mail with certain type of
attachments
Stateful packet inspection - Pros
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Offers improved security over basic packet
filters due to packet examination.
Offers a degree of application independence,
based on level of stateful packet examination.
Better logging of activities over basic packet
filters.
Good performance.
Configuration changes to the protected
workstations are unnecessary.
Stateful packet inspection - Cons
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Allow a direct connection between endpoints
through the firewall. This leaves the potential
for a vulnerability to be exploited.
No hiding of your private systems.
Setting up stateful packet examination rules
is more complicated.
Only supported protocols at the application
layer.
No user authentication.
Application Gateway (Proxy) Firewalls
Typically, an application proxy is a program that
runs on a MULTI-HOMED HOST and emulates
both client and server ends of some application
protocol.
For example, an HTTP proxy (web proxy) acts as
an HTTP server for local users and an HTTP client
for web servers on the Internet.
Application Gateway (Proxy) Firewall
Application Gateway (Proxy) Firewall connections
Application Gateway creates an illusion
Characteristics of Proxy Firewalls
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Outbound connections terminate on the firewall.
A set of policy rules defines how traffic from
one network is transported to any other.
If no rule exists, firewalls deny or drop the data
packets.
Policy rules are enforced through the use of
proxies.
Each protocol on a firewall must have its own
proxy.
Proxy/Gateway Firewall Example
host-to-gateway
telnet session
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Example: allow select
internal users to telnet
outside.
application
gateway
router and filter
1. Require all telnet users to telnet through gateway.
2. For authorized users, gateway sets up telnet
connection to destination host. Gateway relays data
between the two connections.
3. Router packet filter blocks all telnet connections not
originating from gateway.
Application proxies - Pros
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end points do not communicate directly with one
another. A vulnerability in a protocol which could slip
by a packet filter or stateful packet inspection firewall
could be overcome by the proxy program.
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Provide content filtering capability.
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Hide internal addresses.
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Web Proxy often do caching to improve response and
reduce load on the Internet link
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Robust user authentication.
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Offers logging and auditing of user’s activities on the
Internet .
Policy rules are usually easier than packet filtering
rules. (i.e. amount and timing of communication)
Application proxies - Cons
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Performance problems; much slower than the other two
Must have a proxy for every protocol. Failure to have a
proxy may prevent a protocol from being handled
correctly by the firewall.
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TCP is the preferred transport. UDP may not be
supported.
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Limited transparency, clients may need to be modified.
Setting up the proxy server in a browser, for example.
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No protection from all protocol weaknesses.
Hybrids
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Hybrid firewalls provide a way for handling
protocols for which specific proxies do not
exist.
The generic services proxy (GSP) allows
application layer proxies to handle other
protocols.
In a hybrid system, the GSP behaves like
packet filtering firewalls.
Developing a Firewall Configuration
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Screened Host Configuration.
Screened Subnet Configuration.
Dual Firewall Configuration.
Screened Host Configuration
Gateway/Proxy firewall
internal
network
Internet
Packet filter firewall
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Built using a packet filter firewall and a gateway/proxy firewall
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The gateway is a specially chosen computer running network
servers and proxy servers.
Only external packets destined for the gateway are allowed
through the packet filter firewall.
All internal packets destined for outside must go through a
proxy server; otherwise, they are dropped by the packet
filter firewall.
Screened Subnet Configuration
SMTP Server
Proxy firewall
internal
network
Internet
Packet filter firewall
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Use a single packet filtering firewall with three
network interface cards
Place systems that need to be accessed from
outside on a network separate from the internal
network
Dual Firewall Configuration
gateway
FW 1
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DMZ
network
Internet
FW 2
For higher security, two chokes and a gateway are used
 The external choke and gateway are configured as in
a screened host firewall.
 The second choke is a failsafe – if an attacker gains
access to the gate, the internal choke prevents further
attacks against the internal network.
 Internet accessible servers are placed in the DMZ (DeMilitarized Zone) network between FW1 and FW2.
SOHO Configuration
Internet Service Provider
Ethernet Switch
UTP
UTP
User PC
UTP
Broadband
Modem
(DSL or
Cable)
SOHO
Router
--Router
DHCP Sever,
NAT Firewall, and
Limited Application Firewall
Many Access Routers Combine the Router
and Ethernet Switch in a Single Box
User PC
User PC
Developing a Firewall Configuration
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Organization’s Internet policy allows users to
use services such as HTTP, HTTPS, FTP,
Telnet, and SSH.
Based on the Internet policy, a set of policy
rules for various architectures can be
constructed.
Designing a Firewall Rule Set
When designing a firewall rule set, the first
match algorithm dictates:
 The most specific rules to be placed at the
top of the rule set.
 The least specific rules to be placed at the
bottom of the rule set.
Designing a Firewall Rule Set
To define a general rule set, examine:
 The expected traffic load of the firewall.
 Rank the traffic types in order. The Internet
service with the largest traffic at the top of
the rule set.
 Place any deny rules pertaining to the
protocol.
Firewall Rule Set: Special Rules
To define a general rule set, examine:
 Cleanup Rule – deny everything not explicitly
allowed:
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Cleanup rule is always the last (bottommost) rule
Stealth Rule : to protect firewall itself
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Fom any any To any any – Deny
From any any To firewall any – Deny
Stealth Rule is placed first (topmost) rule
Internet Accessible Systems
Firewall Rules for Internet Systems
Accessible Outside the Firewall.
Single Firewall
Architecture #2: Single firewall.
Single Firewall - Rule Set
Firewall Rules for the Single Firewall
Architecture.
Dual Firewalls
Architecture #3: Dual Firewalls.
Dual Firewalls
Firewall Rules for Firewall #1 in the Dual
Firewall Architecture.
Dual Firewalls
Firewall Rules for Firewall #2 in the Dual
Firewall Architecture.
NAT and PAT
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Because the firewall keeps track of all live
connections through it, the firewall is able to
make both NAT and PAT, or any combination
thereof.
NAT: Network Address Translation
PAT: Port Address Translation
A firewall performing NAT or PAT is often
referred to as a masquerading firewall.
NAT Explained
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A NAT firewall hides internal IP addresses by replacing
internal source IP address of an outgoing packet to that
of the firewall device (e.g. – see figure - 200.24.5.8).
Allow hosts with private IP addresses (not-Internetregistered) to access the Internet.
NAT Explained
The NAT device replaces the source IP address in an
outgoing packet by its own address and saves some
information to enable reverse translation for the reply
packet that come back from the Internet
Network Address Translation (NAT)
From 172.47.9.6,
Port 31789 From 192.168.34.2,
1
Port 13472
2
Internet
Client
NAT
Firewall
Translation Table
Server
Host
Internal
External
IP Addr
Port
IP Addr
Port
172.47.9.6 31789 192.168.34.2 13472
…
…
…
…
Network Address Translation (NAT)
Internet
Client
NAT
4
Firewall
To 172.47.9.6,
Port 31789
3
To 192.168.34.2,
Port 13472
Translation Table
Server
Host
Internal
External
IP Addr
Port
IP Addr
Port
172.47.9.6 31789 192.168.34.2 13472
…
…
…
…
NAT used by ISPs
A large group of internal clients share a small set of Internet
addresses for the purpose of hiding their identities or
expanding the internal network address space.
Loading Balancing Translation
A single IP address and port is translated to a pool of
identically configured servers so that a single public
address can be served by a number of servers.