Network Security Architecture

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Network Security Architecture
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Additional Reading

• Firewalls and Internet Security Repelling the
  Wily Hacker, Cheswick, Bellovin, and Rubin.
• New second edition
• Firewall and Internet Security, the Second
  Hundred (Internet) Years http//www.cisco.com/war
  p/public/759/ipj_2-2/ipj_2-2_fis1.html

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Overview

• Network Security Architecture
• Wireless
• Security Domains
• VPN
• Firewall Technology
• Address Translation
• Denial of Service attacks
• Intrusion Detection
• Both firewalls and IDS are introductions.

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802.11 or Wi-Fi

• IEEE standard for wireless communication
• Operates at the physical/data link layer
• Operates at the 2.4 or 5 GHz radio bands
• Wireless Access Point is the radio base station
• The access point acts as a gateway to a wired
  network e.g., ethernet
• Can advertise Service Set Identifier (SSID) or
  not
• Doesn't really matter, watcher will learn active
  SSIDs
• Laptop with wireless card uses 802.11 to
  communicate with the Access Point

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WEP

• Wired Equivalency Privacy -- early technique
  for encrypting wireless communication
• Authenticated devices use a key and
  initialization vector to seed RC4---a stream
  cipher
• V (initialization vector) is changed every frame
• Dangers of repeated encryption using the same key
  stream--XOR of ciphertexts gives XOR of
  plaintexts
• And if some of the plaintext is known, the other
  is recovered

v
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Frame transmission

• RC4(v,k) is stream generated by long-lived key k
  and initialization vector v
• v transmitted in the clear
• v is only 24 bits long---since k is long-lived
  (and used by all devices)---you are assured of
  getting repeated key sequences
• And knowing when you have them! Because v is in
  the clear

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Security Mechanisms

• MAC restrictions at the access point
• white list Protects servers from unexpected
  clients
• Unacceptable in a dynamic environment
• No identity integrity. You can reprogram your
  card to pose as an accepted MAC.
• IPSec
• To access point or some IPSec gateway beyond
• Protects clients from wireless sniffers
• Used by UIUC wireless networks
• 802.11i
• Authentication and integrity integral to the
  802.11 framework
• WEP, WPA, WPA2

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Network Security Protocols

• SSL/TLS
• Secure sockets layer / Transport layer security
• Used mainly to secure Web traffic
• SSH
• Secure Shell
• Remote login
• IPsec
• IP-level security suite

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SSL

• Mid 90s introduced concerns over credit card
  transactions over the Internet
• SSL designed to respond to thse concerns, develop
  e-commerce
• Initially designed by Netscape, moved to IETF
  standard later

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SSL model

• A client and a server
• Implements a socket interface
• Any socket-based application can be made to run
  on top of SSL
• Protect against
• Eavesdroppers
• MITM attacks
• Server has X.509 certificate
• Client may have a certificate, too
• Provides encryption, and authentication of server

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SSL Handshake, (1)

• Client requests https connection with server
• Passes information to server in message
  describing available protocols
• Key exchange method (e.g., RSA, Diffie-Hellman,
  DSA)
• Cipher (e.g., Triple DES, AES)
• Hash (e.g., HMAC-MD5, HMAC-SHA)
• Compression algorithms
• Client nonce
• Server responds with messages that
• Selects (key xchg, cipher, hash, compression)
• Provide servers certificate
• Server nonce

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SSL Handshake, (2)

• Client verifies server cert
• Likely that cert was signed by a CA whose cert is
  in the browser already
• generates pre_master_secret, encrypts using
  servers public key, sends it
• Client and server separately compute session key
  and MAC keys (these from prior random numbers
  passed)
• Client sends MAC of all messages it sent to
  server in this handshake
• Server sends MAC of all messages it sent to
  client in this exchange

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SSL certificates

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SSL history

• SSLv2 1994
• SSLv3 1996
• Fixed security problems
• TLS v1.0 1999
• TLS v1.1 2006

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SSL key lengths

• Earlier versions used 40-bit keys for export
  reasons
• Later versions switched to 128-bit keys, with an
  option to use 40-bit ones with legacy
  servers/clients
• Rollback attack
• MITM

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SSL sequence

• Negotiate parameters
• Key exchange
• Authentication
• Session

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SSL negotiation

• Choice of cipher suites, key exchange algorithms,
  protocol versions
• E.g. choice of 40- or 128-bit keys for export
  reasons
• Rollback attack MITM chooses least secure
  parameters

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SSL key exchange

• Diffie-Hellman key exchange
• RSA-based key exchange
• Encrypt secret s with public key of server

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SSL session

• Use ChangeCipherSpec message to start encrypting
  data
• Encryption RC4, also DES, 3DES, AES, ...
• Authentication HMAC, using MD5 or SHA1

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SSL sessionpushing the bits
Blocks, sized up to 18K
Algorithm agreed-up on in handshake
MAC added for authentication
Algorithm, key, agreed-up on in handshake
Passed on to TCP
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SSL pitfalls

• Hard to set up
• Expensive certificates
• Resource-intensive
• Insufficient verification
• Do people notice the lock icon?
• Do people check the URL?
• Improper use

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IPsec

• Designed as part of IPv6 suite
• One of the key features v6 was supposed to bring
• Backported to IPv4
• Two options AH (authentication) and ESP
  (encapsulated security)
• Two modes transport and tunnel
• Readable resource http//www.unixwiz.net/techtips/
  iguide-ipsec.html

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Transport vs. Tunnel Mode

• Grand vision eventually, all IP packets will be
  encrypted and authenticated
• Transport mode add headers to IP to do so
• May include encryption, authentication, or both
• Reality Most computers dont support IPsec (more
  on why later)
• Tunnel mode use IPsec between two gateways to
  relay IP packets through untrusted cloud

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Tunnel Mode
H1
H2
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AH - Authentication

• Simple design add header with authentication
  data
• Security parameters
• Authentication data just an HMAC with
• shared key to compute Integrity Check Value (ICV)

Different of the HMAC architecture picture
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AH Header

• Next hdr is protocol type of the following header
• AH Length gives size of AH header
• SPI -- sort of a switch code indicating which set
  of security parameters apply
• Sequence number --- basically a nonce to prevent
  replay attacks
• HMAC field

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AH diagram
HMAC applied only to fields in yellow
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Piggybacking AH on IPv4

• The structure allows IPSec logic to
• peel off the AH header, do verification and/or
  decoding,
• Modify length and next protocol fields to be
  that of an AH-free IP packet
• Push the packet up the stack with higher levels
  none the wiser that IPSec was present

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Tunneling in IPSec

• Change the source and destination addresses to be
  the tunnel endpoints
• IPSec tunnel endpoints strip off AH header, to
  authentication and endcoding
• Original IP packet is part of the payload, just
  released into the local network

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AH in Tunnel Mode

• How to detect
• tunnel mode

Original IP header
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ESP - Encapsulated Security Payload

• Encapsulate data
• Encapsulate datagram rather than add a header
• Encrypt authenticate
• Authentication header based only on
  encapsulation---
• not Iaddresses---hold that thought---

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ESP diagram
SPI describes encryption
Protocol using TCP is Completely hidden
Padding and pad len support block encryption
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Key management

• ESP and AH use session keys
• Sessions are called Security Associations
• Indexed by protocol, IP address, SPI
• ISAKMP Internet Security Association Key
  Management Protocol
• Authenticates parties
• Establishes session keys
• Authentication
• Big global PKI (DNSSEC??)
• Manual configuration

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IPsec redux

• Deployment of IPsec limited
• Some reasons
• Global PKI infrastructure hard to set up
• Fixes a solved problem
• SSL SSH work well
• IPsec success VPNs
• Use tunnel mode of IPsec

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Perimeter Defense

• Is it adequate?
• Locating and securing all perimeter points is
  quite difficult
• Less effective for large border
• Inspecting/ensuring that remote connections are
  adequately protected is difficult
• Insiders attack is often the most damaging

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Virtual Private Networks

• A private network that is configured within a
  public network
• A VPN appears to be dedicated network to
  customer
• The customer is actually sharing trunks and
  other physical infrastructure with other
  customers
• Security?
• Depends on implementing protocol

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Multiple VPN Technologies

• IPSec
• Confidentiality? Yes
• Data Integrity? Yes
• User Authentication? Yes
• Network access control? Yes
• Client configuration required.
• VLAN Layer 2 tunnelling technology
• Confidentiality? No
• Data Integrity? No
• User authentication? Yes
• Network access control? Yes
• Not viable over non-VLAN internetworks

• SSL
• Confidentiality? Yes
• Data integrity? Yes
• User authentication? Yes
• Network access control? No
• In addition, limited traffic

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Security Domains with VPNs
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Typical corporate network
Firewall
Demilitarized Zone (DMZ)
Intranet
Mail forwarding
DNS (DMZ)
Web Server
File Server
Web Server
Firewall
Mail server
DNS (internal)
User machines
User machines
User machines
Internet
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VPN using IPSec

• ESP does the encryption
• Difficulty with NAT means ESPAuth in tunnel mode
• Requires VPN gateway---view is a tunnel between
  two trusted networks

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VPN using IPSec
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Firewall Goal

• Insert after the fact security by wrapping or
  interposing a filter on network traffic

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Application Proxy Firewall

• Firewall software runs in application space on
  the firewall
• The traffic source must be aware of the proxy and
  add an additional header
• Leverage basic network stack functionality to
  sanitize application level traffic
• Block java or active X
• Filter out bad URLs
• Ensure well formed protocols or block suspect
  aspects of protocol

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Packet Filter Firewall

• Operates at Layer 3 in router or HW firewall
• Has access to the Layer 3 header and Layer 4
  header
• Can block traffic based on source and destination
  address, ports, and protocol
• Does not reconstruct Layer 4 payload, so cannot
  do reliable analysis of layer 4 or higher content

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Stateful Packet Filters

• Evolved as packet filters aimed for proxy
  functionality
• In addition to Layer 3 reassembly, it can
  reconstruct layer 4 traffic
• Some application layer analysis exists, e.g., for
  HTTP, FTP, H.323
• Called context-based access control (CBAC) on IOS
• Configured by fixup command on PIX
• Some of this analysis is necessary to enable
  address translation and dynamic access for
  negotiated data channels
• Reconstruction and analysis can be expensive.
• Must be configured on specified traffic streams
• At a minimum the user must tell the Firewall what
  kind of traffic to expect on a port
• Degree of reconstruction varies per platform,
  e.g. IOS does not do IP reassembly

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Traffic reconstruction
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Access Control Lists (ACLs)

• Used to define traffic streams
• Bind ACLs to interface and action
• Access Control Entry (ACE) contains
• Source address
• Destination Address
• Protocol, e.g., IP, TCP, UDP, ICMP, GRE
• Source Port
• Destination Port
• ACL runtime lookup
• Linear
• N-dimensional tree lookup (PIX Turbo ACL)
• Object Groups
• HW classification assists

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Ingress and Egress Filtering

• Ingress filtering
• Filter out packets from invalid addresses before
  entering your network
• Egress filtering
• Filter out packets from invalid addresses before
  leaving your network

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Denial of Service

• Example attacks
• Smurf Attack
• TCP SYN Attack
• Teardrop
• DoS general exploits resource limitations
• Denial by Consumption
• Denial by Disruption
• Denial by Reservation

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TCP SYN Attack

• Exploits the three-way handshake

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TCP SYN Attack Solutions

• Intermediate Firewall/Router
• Limit number of half open connections
• Ingress and egress filtering to reduce spoofed
  addresses
• Does not help against DDoS bot networks
• Reactively block attacking addresses
• Generally expensive to acquire technology to do
  fast enough
• Fix Protocol - IPv6

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Teardrop Attack

• Send series of fragments that don't fit together
• Poor stack implementations would crash
• Early windows stacks

Offset 0, len 60
Offset 30, len 90
Offset 41, len 173
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Address Translation

• Traditional NAT RFC 3022 Reference RFC
• Map real address to alias address
• Real address associated with physical device,
  generally an unroutable address
• Alias address generally a routeable associated
  with the translation device
• Originally motivated by limited access to
  publicly routable IP addresses
• Folks didnt want to pay for addresses and/or
  hassle with getting official addresses

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Address Translation

• Later folks said this also added security
• By hiding structure of internal network
• Obscuring access to internal machines
• Adds complexity to firewall technology
• Must dig around in data stream to rewrite
  references to IP addresses and ports
• Limits how quickly new protocols can be
  firewalled

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Address Hiding (NAPT)

• NAPT Network Address Port Translation
• Many to few dynamic mapping
• Packets from a large pool of private addresses
  are mapped to a small pool of public addresses at
  runtime
• Port remapping makes this sharing more scalable
• Two real addresses can be rewritten to the same
  alias address
• Rewrite the source port to differentiate the
  streams
• Traffic must be initiated from inside, e.g. the
  private address

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NAT example
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Static Mapping

• One-to-one fixed mapping
• One real address is mapped to one alias address
  at configuration time
• Traffic can be initiated from either side
• Used to statically map out small set of servers
  from a network that is otherwise hidden
• Static port remapping is also available

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NAT example
192.168.1.5
128.274.15
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NAT and IPSec AH dont mix

• Recall the diagram illustrating the fields
  covered by AH
• AH header created at the sender, src/dest IP
  addresses changed by NAT

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FW Runtime Characteristics

• Firewalls track streams of traffic
• TCP streams are obvious
• Creates pseudo UDP streams for UCP packets
  between the same addresses and ports that arrive
  near enough to each other
• Processing first packet in stream is more
  expensive
• Must evaluate ACLs and calculate address
  translations
• Subsequent packets get session data from a table

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Multi-legged Firewalls

• Historically firewalls have protected inside from
  outside
• Still true for the most part with personal and
  home firewalls
• No longer sufficient for larger enterprises
• PIX security level solution
• Outbound traffic from low security level
  interface to high security level interface
• Inbound traffic from high security level
  interface to low security level interface
• Different requirements for inbound and outbound
  traffic
• IOS divides interfaces into inside and outside
  groups
• Address translation can only be defined between
  inside and outside groups
• Routing conflicts with address translation
• Address translation specifies both interfaces
• Must be evaluated before the routing, better be
  consistent

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Four Legged FW

• Static translation from DMZ to Customer
• 10.10.10.10.1 to 128.1.1.1
• But routing table wants to route 128.1.1.1 from
  DMZ to outside interface
• Static translation interface selection will win

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Identity Aware Firewall

• Use TACACS or Radius to authenticate, authorize,
  account for user with respect to FW
• For administration of FW
• For traffic passing through FW
• PIX cut-through proxy allows authentication on
  one protocol to cover other protocols from same
  source
• Authorization for executing commands on the
  device
• Download or enable ACLs
• XAuth to integrate AAA with VPN authentication
  and other security mechanisms

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AAA Scenario
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Is the Firewall Dead?

• End-to-end security (encryption) renders
  firewalls useless
• Tunnels hide information that firewalls would
  filter or sanitize
• With IPSec decrypting and re-encrypting is viable
• Blurring security domain perimeters
• Who are you protecting from whom
• Dynamic entities due to DHCP and laptops
• More dynamic business arrangements, short term
  partnerships, outsourcing
• Total Cost of Ownership (TCO) is too high
• Managing firewalls for a large network is
  expensive
• Perhaps personal or distributed firewalls are the
  answer?
• Implementing a Distributed Firewall
  http//www1.cs.columbia.edu/angelos/Papers/df.pdf
  

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Intrusion Detection

• Holy Grail Detect and correct bad system
  behavior
• Detection can be viewed in two parts
• Anomaly detection Use statistical techniques to
  determine unusual behavior
• Mis-use detection Use signatures to determine
  occurrence of known attacks
• Detection can be performed on host data (HIDS),
  network data (NIDS), or a hybrid of both

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Intrusion Handling

• Preparation for attack
• Identification of the attack
• Containment of the attack
• Gather information about the attacker
• Honeypots
• Eradication
• Broadly quarantine the system so it can do no
  more harm
• BGP blackholing
• Tighten firewalls
• Cleanse the corrupted system
• Followup phase
• Gather evidence and take action against the
  attacker

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Honey Pots

• Reconnaissance for the good guys
• Deploy a fake system
• Observe it being attacked
• Resource management
• Cannot be completely passive
• Must provide enough information to keep attacker
  interested
• Must ensure that bait does not run away
• Scale
• Host, network, dark address space

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IDS Architecture

• Agents run at the lowest level gathering data.
  Perform some basic processing.
• Agents send data to a Director that performs more
  significant processing of the data. Potentially
  there is a hierarchy of agents and directors
• Director has information from multiple sources
  and can perform a time-based correlation to
  derive more significant actions
• Directors invoke Notifiers to perform some action
  in response to a detected attack
• Popup a window on a screen
• Send an email or a page
• Send a new syslog message elsewhere.
• Adjust a firewall or some other policy to block
  future action from the attacker

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Data Sources

• Direct data
• Network packets
• System calls
• Indirect data
• Syslog data, Windows event logs
• Events from other intrusion detection systems
• Netflow information generated by routers about
  network traffic

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Mis-use/Signature Detection

• Fixed signatures are used in most deployed IDS
  products
• E.g., Cisco, ISS, Snort
• Like virus scanners, part of the value of the
  product is the team of people producing new
  signatures for newly observed malevolent behavior
• The static signature mechanism has obvious
  problems in that a dedicated attacker can adjust
  his behaviour to avoid matching the signature.
• The volume of signatures can result in many false
  positives
• Must tune the IDS to match the characteristics of
  your network
• E.g., what might be unusual in a network of Unix
  systems might be normal in a network of Windows
  Systems (or visa versa)
• Can result in IDS tuned too low to miss real
  events
• Can hide real attacks in the mass of false
  positives

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Example Signature

• Signature for port sweep
• A set of TCP packets attempting to connect to a
  sequence of ports on the same device in a fixed
  amount of time
• In some environments, the admin might run nmap
  periodically to get an inventory of what is on
  the network
• You would not want to activate this signature in
  that case

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Anomaly/statistical detection

• Seems like using statistics will result in a more
  adaptable and self-tuning system
• Statistics, neural networks, data mining, etc.
• How do you characterize normal?
• Create training data from observing good runs
• E.g., Forrests program system call analysis
• Use visualization to rely on your eyes
• How do you adjust to real changes in behaviour?
• Gradual changes can be easily addressed.
  Gradually adjust expected changes over time
• Rapid changes can occur. E.g., different
  behaviour after work hours or changing to a work
  on the next project

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Host Based IDS

• Tripwire Very basic detection of changes to
  installed binaries
• More recent HIDS. Look at patterns of actions of
  system calls, file activity, etc. to permit,
  deny, or query operations
• Cisco Security Agent
• Symantec
• McAfee Entercept

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Classical NIDS deployment
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NIDS Remediation Options

• Log the event
• Drop the connection
• Reset the connection
• Change the configuration of a nearby router or
  firewall to block future connections

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Intrusion Protection Systems (IPS)

• Another name for inline NIDS
• Latest buzz among the current NIDS vendors
• Requires very fast signature handling
• Slow signature handling will not only miss
  attacks but it will also cause the delay of valid
  traffic
• Specialized hardware required for high volume
  gateways
• When IDS is inline, the intrusion detector can
  take direct steps to remediate.
• If you move IDS into the network processing path,
  how is this different from really clever
  firewalling?

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Summary

• Identification of security domains basis of
  perimeter security control
• Firewall is the main enforcer
• Intrusion detection introduces deeper analysis
  and potential for more dynamic enforcement
• Intermediate enforcement can handle some Denial
  of Service attacks