Transcript Firewalls and Firewall Testing Techniques Sonia Fahmy Department of Computer Sciences Purdue University [email protected] http://www.cs.purdue.edu/homes/fahmy/ Purdue University Sonia Fahmy.

Firewalls and Firewall Testing
Techniques
Sonia Fahmy
Department of Computer Sciences
Purdue University
[email protected]
http://www.cs.purdue.edu/homes/fahmy/
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Overview
 What is a firewall?
 Firewall types and architectures
 Firewall operations
 Firewall testing
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What is a
Firewall?
Gateway (DMZ)
A firewall is a method of achieving security between
trusted and untrusted networks
 The choice, configuration and operation of a firewall is
defined by policy, which determines the the services and
type of access permitted
 Firewall = policy+implementation
 Firewall = “zone of risk” for the trusted network
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Firewalls Should…
Support and not impose a security policy
 Use a “deny all services except those specifically
permitted” policy
 Accommodate new facilities and services
 Contain advanced authentication measures
 Employ filtering techniques to permit or deny services
to specific hosts and use flexible and user-friendly
filtering
 Use proxy services for applications
 Handle dial-in
 Log suspicious activity
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Firewalls Cannot…
Protect against malicious insiders
 Protect against connections that do not go through
them (e.g., dial-up)
 Protect against new threats or new viruses

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Firewalls in Relation to 7 Layers
Packet Level
Filter
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Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Link Layer
Physical Layer
Application
Level Filter
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Simple Packet Filters
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Example:
Interface
2
1
Source
*
128.5.*.*
Dest. Prot.
*
TCP
*
TCP
SPort Dport
*
21 (FTP)
*
25 (SMTP)
Internal network
Internet
1
Filter
2
Internal
Difficult to handle X-Windows, RPC (including NFS and
NIS), rlogin, rsh, rexec, rcp, and TFTP
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Stateful Inspection
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Also known as dynamic packet filtering (dynamic rule set)
Requires storing state for each stream, assuming:
 If there is one packet, there will be more
 If there is one packet, responses will be returned
Prime candidate for resource starvation attacks
What should be done when table is full?
 Least recently used
 Random early drop
 Time out entries
 Wait for FIN messages, etc.
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Bastion Host
Internet
R1
R2
Internal
Inside users log onto the bastion host to use outside
services
 Outside snoopers cannot see internal traffic even if they
break in the firewall (perimeter = stub network = DMZ)
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Firewall Architectures
Internet
R1
R2
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Internal
Screened host: Bastion host and exterior router
Screened subnet: Exterior and interior routers
Multiple bastion hosts, multiple interior routers, multiple
exterior routers, multiple internal networks (with/without
backbone), multiple perimeter networks
Merged interior and exterior routers, bastion host and exterior
router and bastion host and interior router (not recommended)
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Dual-homed Host
Internet
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Internal
The dual-homed host is the firewall in this case
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Application-Level Gateways
Internet
Server
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Dual-homed Host
and Proxy Server
Client
Specialized programs on bastion host relay requests and
responses, enforcing site policies (refusing some requests)
Sometimes referred to as “proxy servers”
Transparent with special “proxy client” programs
Full protocol decomposition, e.g, Raptor, or “plug mode” e.g.,
Firewall-1 and PIX
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Policies
Network service access policy
 Defines which services are to be explicitly
allowed or denied+ways in which these services
are to be used
 Firewall design policy
 Defines how the firewall implements restricted
access and service filtering specified by the
NSAP
 FDP must be continuously updated with new
vulnerabilities
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Packet Traversal in a Firewall
Packet flow
Packet receipt by firewall
Link layer filtering
Dynamic ruleset (state)
Bypass
Packet legality checks
Packet may be dropped
On
IP and port filtering
Packet may be dropped
Match
NAT/PAT (header rewrite)
Packet reassembly
Application level analysis Stream may be dropped
Routing decision
Dynamic ruleset (state)
Packet sanity checks
Optional outbound filtering
IP and port filtering
Packet release
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Firewall Testing
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Vulnerabilities = design or coding flaws = invalid
assumptions, e.g., insufficient verification, memory
available, user data, trusted network object, predictable
sequence, etc.
Develop a vulnerability-operation matrix
Place Common Vulnerabilities Exposure (CVE), and its
candidates (CAN), and other known and new firewall
problems in appropriate matrix cell
Find clusters in matrix
Predict problems
Automate firewall testing through focusing on common
problems
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Example
CVE-1999-0158
 Cisco PIX firewall manager (PFM) allows retrieval
of any file whose name and location is known
 PIX proxy’s verification failure
 Application level
 Insufficient verification
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Key Points
Firewalls can employ:
 Packet filters
 Stateful inspection
 Application-level gateways (many types)
 Also circuit-level
 Large variations, e.g., Raptor, PIX, Firewall-1, Gauntlet
 Several architectures to prevent all-or-nothing effect
 Importance of policy
 Firewall testing automation underway
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References
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RFC 2647, “Benchmarking” + drafts
Simonds, “Network Security”, 1996
Hunt, “Internet/Intranet firewall security”, Computer
Communications, 1998
Frantzen et al, “A framework for understanding
vulnerabilties in firewalls using a dataflow model of firewall
internals”, in preparation
Kamara et al, “Testing firewalls”, in preparation
Chapman, “Network (In)Security through IP packet
filtering”
http://cve.mitre.org/, www.sans.org, www.cert.org
Web pages and mailing lists
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Thank You!
Questions?
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