Download Computer Security: Principles and Practice, 1/e

Chapter 9 – Firewalls and Intrusion
Prevention Systems
Second Edition
by William Stallings and Lawrie Brown
Lecture slides by Susan Lincke & Lawrie
Brown
Chapter Objectives
Objectives:
 The student should be able to:
 Describe the different types of firewalls: Circuit,
Application Proxy, Packet, Stateful, Personal including how
they differ in an example attack that each can fend off.
 Describe 3 firewall vulnerabilities
 Draw configurations for network types: double inline, T,
multihomed, distributed, load balanced.
 Describe what a firewall policy is, give an example policy,
and indicate how a policy may be used
 Describe each of the following: border router, static and
dynamic NAT and PAT.
Security: Defense in Depth
Border Router
Perimeter firewall
Internal firewall
Intrusion Detection System
Policies & Procedures & Audits
Authentication
Access Controls
Firewall Required Functions
Required Functions:
 Serve as an entry point into a network
 Screens all packets entering the network
 Service control
 Direction control
 User control
 Behavior control (access only part of Web service)
 Log and alarm events
 Perform Network Address Translation functions (optional)
 Perform Virtual Private Network functions (optional)
 Support no other functions (that can be compromised)
Firewall
Locations
Network Defense Configuration:
Double Inline Firewalls
Internet
Screening
Router
IDS
Firewall A
External
DNS
VPN
Server
IDS
Web
Server
Firewall B
IDS
Protected
Internal
Network
Database/File
Servers
E-Commerce
Network Defense Configuration:
Load Balanced Firewalls
Internet
Screening
Router
Router
Firewall A
Firewall B
External
DNS
Protected
Internal
Network
IDS
Database/File
Servers
IDS
Web
Server
E-Commerce
VPN
Server
Network Defense Configuration:
Single T or Multihomed (Separate Zones)
Internet
Screening
Router
Router
Firewall
Demilitarized Zone
External
DNS
Protected
Internal
Network
Zone
IPS
Database/File
Servers
IPS
Web
Server
E-Commerce
VPN
Server
Network Defense Configuration:
Distributed Firewall
Internet
Screening
Router
IDS
Firewall A
Firewall
External
DNS
Firewal
lVPN
Server
IDS
Firewall
Web
Server
Firewall B
IDS
Protected
Internal
Network
Database/File
Servers
Firewall
E-Commerce
Bastion Hosts
 critical strongpoint in network
 hosts application/circuit-level gateways
 common characteristics:
 runs secure O/S, only essential services
 may require user auth to access proxy or host
 each proxy can restrict services & hosts accessed
 each proxy small, simple, checked for security
 each proxy is independent, non-privileged
 proxy disk use is boot only; hence read-only code
Firewall Topologies
 host-resident firewall
 screening router
 single bastion inline: One firewall
 single bastion T: with 3 zones
 double bastion inline: Serial firewalls
 double bastion T: with 3 zones
 distributed firewall configuration: double with host
firewalls
Network Defenses – with Logs
Internet
Router
Security
Mgmt
IDS
Firewall
Vendor A
External
DNS
VPN
Server
IPS
Web
Server
Firewall
Vendor B
IPS
Protected
Internal
Network
Database/File
Servers
E-Commerce
Syslog
Types of
Firewalls
Firewall
Configurations
terminal
host
firewall
A
A
terminal
host
firewall
A
A
A
Router Packet Filtering:
Packet header is inspected
Single packet attacks caught
Very little overhead in firewall: very quick
High volume filter
Stateful Inspection
State retained in firewall memory
Most multi-packet attacks caught
More fields in packet header inspected
Little overhead in firewall: quick
Packet Filtering Firewall
 applies rules to packets in/out of firewall
 based on information in packet header
 src/dest IP addr & port, IP protocol, interface
 typically a list of rules of matches on fields
 if match rule says if forward or discard packet
 two default policies:
 discard - prohibit unless expressly permitted
 forward - permit unless expressly prohibited
Packet Filter Problems
 In heavy load may forward all packets without logging
 Cannot catch application-level errors
 ICMP can have invalid contents
 FTP, RPC use ports > 1023, dynamically allocated
 Cannot recognized spoofed IP or port addresses
 Do not support advanced user authentication
 Tiny fragments can hide attacks
 Improper configuration can lead to breaches
 Routers can do packet filtering, most firewalls do more
Stateful Inspection Firewall
 reviews packet header information but also keeps
info on TCP connections
 typically have low, “known” port no for server
 and high, dynamically assigned client port no
 simple packet filter must allow all return high port
numbered packets back in
 stateful inspection packet firewall tightens rules for TCP
traffic using a directory of TCP connections
 only allow incoming traffic to high-numbered ports for
packets matching an entry in this directory
 may also track TCP seq numbers as well
Packet
Filter Rules
Stateful Packet Filters







Tracks TCP/UDP connection status
Can configure outbound-only connections
Packets are allowed in if connection is established
Records source/destination IP and port addresses, protocol (TCP/UDP), timer expiration
TCP: Also supports TCP state, Sequence numbers
UDP: Supports expiration timer, pseudo state
May prevent fragmentation attacks
Advantage: Supports higher loads than Circuit-Level Firewalls at same memory/processor
speed levels
Problems
 ICMP: Messages may come from intermediate node, not destination. Must accept/reject
all ICMP messages of type N
 DOS Attack: Establish connections to fill table
 Applications change ports or use multiple ports: e.g., ftp
 Application attacks not detected since application protocols not scanned
 Some routers support Stateful packet filtering; nearly all firewalls do
Stateful Firewall Connection
State Table
Source
Address
Source Port
Destination
Address
Destination
Port
Connection
State
215.34.55.143
2011
188.55.43.59
80
Established
84.22.428.143
1027
188.55.43.59
80
Established
188.55.42.34
1022
89.42.33.143
23
Established
184.56.23.123
88
188.55.43.49
80
Established
Firewall
Configurations
terminal
host
firewall
A
B
A B
terminal
host
firewall
A
B
A B
Circuit-Level Firewall:
Packet session terminated and recreated
via a Proxy Server
All multi-packet attacks caught
Packet header completely inspected
High overhead in firewall: slow
Application-Proxy Firewall
Packet session terminated and recreated
via a Proxy Server
Packet header completely inspected
Most or all of application inspected
Highest overhead: slow & low volume
Firewall & Router Protection Levels
Packet Filter
IP Header
…
Pr
ot
Src
Addr
TCP Header
Dest
Addr
… … Src
Port
Application Header & Data
Dest
Port
Stateful Packet Filter – Circuit Level Filter – Proxy Server
IP Header
Prot
…
Src
Addr
TCP Header
Dest
Addr
… … Src
Port
Dest
Port
Application Header & Data
Fl
ag
Seq
No
Application Proxy Firewall
IP Header
Prot
…
Src
Addr
TCP Header
Dest
Addr
… … Src
Port
Dest
Port
Application Header & Data
Fl
ag
Seq
No
Appl
Hdr
Guard Firewall – Sophisticated Application Proxy Firewall – IDS/IPS
IP Header
Prot
…
Src
Addr
TCP Header
Dest
Addr
… … Src
Port
Dest
Port
Application Header & Data
Fl
ag
Seq
No
Appl
Hdr
Appl
Data
Fields shown are monitored by these types of Firewalls
Circuit-Level Firewalls or Proxy
Server
 Establishes a TCP connection with remote end before passing information
through.
 Creates two sessions: one with sender & one with receiver
 Does not filter based on packet contents (other than state)
 Also known as Pass-Through Proxy or Generic Proxy
 Advantages: If firewall failure, no packets are forwarded through firewall
 Catches fragmentation errors
Problems:
 Does not detect invalid application data
 Moves security issues from service to firewall: e.g., DOS attacks
 Less able to handle high loads since each connection becomes two
 Requires much greater memory and processor at application level (Web page is
> 1 connection)
 Slower interfaces can result in poor performance for streaming applications
Application Proxy Firewall
 Examines packets and their contents at the Application
Layer
 Can cause delay due to additional processing
 May strip info on internal servers, server version on
outgoing messages (e.g., email)
 May allow only certain types of sessions through:
 FTP: May permit receives, no sends. Or sends of specific files
only.
 Email: Encrypts email between all of company’s offices
 HTTP: May filter PUT commands, URL names. Can cache
replies.
 Authentication: Perform extra authentication for external
access (via dialup or internet)
Application-Level Gateway
 acts as a relay of application-level traffic
 user contacts gateway with remote host name
 authenticates themselves
 gateway contacts application on remote host and relays
TCP segments between server and user
 must have proxy code for each application
 may restrict application features supported
 more secure than packet filters
 but have higher overheads
SOCKS Circuit-Level Gateway
 SOCKS v5 defined as RFC1928 to allow TCP/UDP
client-server applications to use firewall
 components:
 SOCKS server on firewall
 SOCKS client library on all internal hosts
 SOCKS-ified client applications
 client app contacts SOCKS server, authenticates,
sends relay request
 server evaluates & establishes relay connection
 UDP handled with parallel TCP control channel
Distributed
Firewalls
Host-Based Firewalls
 often used on servers
 used to secure individual host
 available in/add-on for many O/S
 filter packet flows
 advantages:
 taylored filter rules for specific host needs
 protection from both internal / external attacks
 additional layer of protection to org firewall
Personal Firewall
 controls traffic flow to/from PC/workstation
 for both home or corporate use
 may be software module on PC
 or in home cable/DSL router/gateway
 typically much less complex
 primary role to deny unauthorized access
 may also monitor outgoing traffic to detect/block
worm/malware activity
Virtual Private Networks
Network Address Translation
External
Organization IP:
201.25.44.0/24
x
Internal Addresses:
10.0.0.0/8
N
A
T
Static NAT: External IP address
translates into Internal IP address.
N
A
T
Dynamic NAT: Single external IP
address may translate into many IP
addresses.
N
A
T
Hide NAT or PAT: IP/Port translates
to IP/Port
x
x
y
Network Address Translation
Static NAT: One external IP address translates into one
fixed internal IP address
Dynamic NAT: Internal IP addresses are assigned an
external IP address on a FCFS basis.
Port Address Translation (PAT) or Hide NAT: Translates
one incoming IP address/port into an internal IP
address/port. Multiple internal IP addresses can map
to one external IP address
Firewall Capabilities & Limits
 capabilities:
 defines a single choke point
 provides a location for monitoring security events
 convenient platform for some Internet functions such as
NAT, usage monitoring, IPSEC VPNs
 limitations:
 cannot protect against attacks bypassing firewall
 may not protect fully against internal threats
 improperly secure wireless LAN
 laptop, PDA, portable storage device infected outside
then used inside
Firewall Vulnerabilities
 Firewalls can be bypassed via other means (e.g., modem,

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
CDs)
Data transmitted to the outside may be vulnerable
Firewalls may lie: in heavy loads attack packets may get
through without logging.
Extra software on the firewall device increase vulnerability
Firewalls are vulnerable if installed above a generalpurpose OS
Firewalls do not prevent malicious acts within the network
Layers of defense are safer than a single firewall
Auditing: Scan weekly or at every change
Retain a baseline of perimeter device configurations
Designing Firewalls/Routers
 Before creating a firewall configuration, create firewall


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

policies.
Firewall policy: An Access Control List (ACL) item in
English
Policies can be reviewed, turned into ACLs, and tested
Example Policy: IP addresses with internal source
addresses shall not be allowed into the internal network
from the outside.
Often ports > 1000 cannot be closed due to applications
like FTP
Other policies may deal with failover protection, detecting
malicious code, …
Configuring Firewalls/Routers
 Put specific rules first, then general rules
 When a rule matches, no further testing is done.
 Minimize tests & speed processing by placing common
rules first
Auditing Firewalls
 If there is no security policy, speak with mgmt about their
expectations of the firewall
 After configuring the firewall, test the firewall by launching
an attack
 Use a sniffer to determine which attack packets get through
 Other required operations include:
 Log Monitoring and Notification
 User Mgmt and Password policy
 Patch Update and Backup
 Change Control
 Secure build for firewall platforms
Audit Testing
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Scan all TCP and UDP ports 0-65,535 on the firewall
Ping devices to see if Echo Requests pass
Scan using ‘TCP Connect Scan’ (Full SYN-ACK)
Do a slow SYN scan (with 15 second delay) to se if port scans are
detected (by IDS)
Scan with FINs, ACKs, and fragmented ACKs , Xmas Tree scans (URG,
PUSH, FIN flags) to see how all perform
Scan the subnet using UDP ports to look for open applications
Check routing capability, including NAT
Test other blocked source IP addresses: Spoofed, private, loopback,
undefined
Test other protocols: ICMP, IP fragmentation, all policies, all directions.
Verify logging occurs for illegal probes
Always get signed-off permission first!!!
Intrusion Prevention Systems (IPS)
 enhanced security product which
 inline net/host-based IDS that can block traffic
 functional addition to firewall that adds IDS capabilities
 can block traffic like a firewall
 alternatively send commands to firewall
 uses IDS algorithms
 may be network or host based
Host-Based IPS
 addresses:
 modification of system resources
 privilege-escalation exploits
 buffer overflow exploits
 access to email contact list
 directory traversal
 identifies attacks using:
 sandbox applets to monitor behavior
 signature techniques
 anomaly detection techniques
 can be tailored to the specific platform
 e.g. general purpose, web/database
 may protect file access, system registry, I/O, system calls
Network-Based IPS
 inline NIDS that can discard packets or terminate TCP
connections
 can provide flow data protection
 reassembling whole packets
 monitoring full application flow content
 can identify malicious packets using:
 pattern matching, stateful matching, protocol anomaly,
traffic anomaly, statistical anomaly
 cf. SNORT inline can drop/modify packets
Unified Threat
Management
Products
Summary
 Firewall filters packets
 Types of networks
 double inline, T or multihomed, distributed, load balanced,
screening router
 Types of firewalls
 packet filter, stateful inspection, application and circuit gateways
 Other capabilities
 Virtual Private Network (VPN), Network Address Translation
(NAT)
 Advanced configurations:
 IPS, Unified Threat Management