Firewall
Raghunathan Srinivasan
October 30, 2007
CSE 466/598
Computer Systems Security
Before we start
 Something Interesting I found about
XEN
 And something more:
 http://kerneltrap.org/OpenBSD/Virtualiz
ation_Security
 A little bit on HW 2, problem 1 & 2
 Not discussing problem 3 & 4 as they are
fairly simple
What are we protecting
 Data
 Private Data
 Secret
 Integrity
 Availability
 Resources
 Network resources
 Other computer resources
 Reputation
 Your reputation
Means for Protection
 Anti-Virus
 Why doesn’t it work?
 Rather why is it ineffective
 Firewall
 Does it suffer from same problems as
above
What is a firewall
 Is it just a wall that we are burning?
 No, I guess bad joke
 Ok, it is a barrier between your computer
and the outside world
 Rather protects the boundary of an intranet
against the Internet
 Computer networks are designed to
exchange data
 So why do we want to restrict data flow?
Ideal World
Everyone is good
No attacker
No one can compromise data
No one will try to steal data
No one will try to install backdoor
No one …. (basically a really good
world)
 Unfortunately, this can never exist






Working World
 There are attackers
 People will try and steal data
 People will try opening ports on your
machine for remote exploitation
 Individual users are not smart enough to
configure network connections
 So we need some service that can at least
differentiate between good & bad connections
 In practice may not be the case
Firewall
Your
Network
Outside
Network
Tasks of a Firewall
 Access control based on
sender/receiver address or on
addressed services
 Hiding Internal network
 Logging of traffic
 Implements Packet Filter & Proxy
server
7 Layered OSI
 Application Layer
 Supports end – user processes, Telnet, FTP
 Presentation Layer
 Session Layer
 Transport Layer
 Flow Control
 Network Layer
 Switching, routing
 Data Link Layer
 Data encoded and coded into bits
 Physical Layer
Packet Filter
 Analyzes network traffic and filters
based on rules in layers 3 & 4
 Typically can be Source / Dest Addr
 If firewall is combined with a router, it
is called screening router
 Simple, Cheap
Packet Filter
 Possible Principles
 Everything that is not explicitly allowed
is denied
 Everything that is not explicitly denied is
allowed
Example
 Lame Example 1: Let your SMTP
server be 149.169.0.1, and port be
40
 Rule1
 From (IP *), (port *) TO (149.169.0.1)
(40) : DENY
 From (149.169.0.1), (40) TO (*) (*):
Allow
 Rules are applied in order listed
Proxy Server
 Controls access to a service
 Proxy is the only known computer to
outside Internet
 Access control can be done based on
user identity, content, used protocol
Packet Filter vs Proxy Server
 PF
 Simple, Cheap
 Correctly specifying filters is error prone
 If you re-order rules, then policy may change
 Proxy
 User authentication possible
 Application Protocol control can be integrated
 Logging
 Circuit level proxies/Application level proxies
 AL proxies more expensive, but versatile
 Need one ALP for each application
 Circuit level Proxies hide network info apart from
providing packet filter functionalities
Firewall Generations
 First – Packet Filter
 Second – Stateful Filters
 Third – Application Layer
First generation
 Just checks for the individual packets
 Which means most filtering is done
based on a strict set of rules
 Lame example: Drop packets coming from
a specific IP address
 The filter does not care whether the
incoming/outgoing packet is part of an
existing connection
2nd Gen - Stateful Filters
 Also called circuit level firewalls
 Do not examine each packet
 It maintains records of all connections
passing through the firewall
 Can determine whether a packet is
part of an existing connection or a
new connection
 There are static rules that configure
firewall behaviour
3rd generation
 Application layer firewall
 it can "understand" certain
applications and protocols
 can detect whether an unwanted
protocol is being sneaked through on
a non-standard port
 whether a protocol is being abused in
a known harmful way.
Firewall Architectures




Single Box Architecture
Screened Host Architecture
Screened Subnet Architectures
Other Variations
Single Box Architecture
 Screening Router
 Dual Homed Host
Screening router
Internet
Screener
Internal Network
PC 1
PC n
Features
 You can configure connections at one
place
 So the firewall is installed in the
router
 Can deny by port numbers/IP addr
 Not flexible
 Useful where network inside is
considered secure
Dual-Homed Host
Internet
eth1
eth0
DualHomedHost
Internal Network
PC 1
PC n
Features
 The protected network cannot directly
communicate to the Internet
 Applications should not be real time
or business critical
 Traffic to Internet is small
 Users do not perform only Internet
based jobs
 Packet filter & Proxy server together
Bastion Host
 special purpose computer on a network
 specifically designed and configured to
withstand attack
 Contains very few applications
 proxy server
 services the requests of its clients by forwarding
requests to other servers
 Why?
 To reduce threats and vulnerabilities
Screened Host Architecture
Internet
Screener
Internal Network
Bastion
Host
PC 1
PC n
Features
 Bastion Host provides proxy
 Screening router provides packet
filtering of incoming traffic
Personal Firewall
 A software installed on a PC
 Part of OS to protect user machines
 Learning filter
 Annoying at times
Honeypot
 Show a machine with weak security
to outside world
 Monitor all the attacks that it
experiences
NAT - Network address translation
 Technique for transmitting/receiving
network traffic through a router
 Re-writing of source/destination addresses
 Re-writing of TCP port number
 NAT is a popular way of dealing with IPv4
address shortage
 NAT enables multiple hosts on a private
network to use a single public IP address
NAT







A host typically uses 192.168.x.x
10.x.x.x
172.16-31.x.x
The router has a public address
Example
My router’s add 75-167-48-xxx
My PC address 192.168.1.100
NAT
 When traffic moves from local
network to Internet
 Router performs address change on
source IP
 Router stores data about outgoing
connection
 When reply returns to router, it uses
stored data to forward packets to
corresponding machine
Drawbacks
 True end to end connectivity not
there
 Cannot participate in some network
protocols
 Services that require initiation from
outside network cannot function
Benefits
 NAT helps prevent many malicious
attacks
 External network cannot initiate a
connection
 I wont receive any malicious data unless
my machine initiated it
 Can my machine initiate it?
 Practical solution to exhaustion of
IPv4 address
Can a firewall inside a computer be
bypassed
 Yes
 It is just a service
 A program can disable it
 Bagle
 Bagz
 So it all boils down to
 Is my PC secure
 I believe that this problem is not in P
A little refresher
 Digital signature
 Challenge Response – midterm
 The mid term problem 1: