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Network Firewalls

Learn how network firewalls establish trusted boundaries, inspect traffic, and shield hosts, illustrated with key ideas and common features. Explore why a holistic approach is crucial in network security.

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Network Firewalls

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  1. Network Firewalls • John Kristoff • jtk@depaul.edu • +1 312 362-5878 • DePaul University • Chicago, IL 60604

  2. The network is just a highway • How do you secure the highway • Police patrol • Toll booths • Licensed drivers • Vehicle inspections and standards • Rules of the road • Are the highways completely safe now?

  3. What network firewalls do • Define untrusted and trusted boundaries • Inspect traffic traversing firewall boundary • Limit communication traversing boundary • Help shield insecure hosts

  4. Network firewalls illustrated

  5. Key ideas • Firewalls should be unnecessary • They're a network solution to a host problem • They don't solve the real problem and... • ..make it hard/impossible to do certain things • Ultimate control of hosts is out of our hands • Securing a LOT of hosts is hard! • But.. network solutions are *sigh* necessary

  6. Packet filtering firewalls • Filter everything - not very useful • Filter by IP address • Filter by application type (TCP, UDP) • Filter on field/flag settings (source route) • Filter invalid packets (SYN/FIN packets) • Other pattern match

  7. Screened subnet implementation

  8. Application Layer Gateway (ALG) • Also commonly called a proxy firewall • These permit no direct communication • Firewall intercepts all traffic in each direction • Very intelligent device... • ...must understand what a user is doing • Difficult to install if it doesn't currently exist

  9. Proxy/ALG illustrated

  10. Other common firewall features • Stateful inspection • Network address translation (NAT) • Authenticaton (VPNs) • Dynamic triggers • Reporting, logging and IDS support

  11. What can't a network firewall stop? • Bad packets that look good • Denial of service (DoS) attacks • Well, they can stop them at the firewall • But then the firewall has just been DoS'd • Stupid user tricks • Things that go around the firewall • Things that don't cross the firewall boundary

  12. So you're saying...? • It would be nice if all hosts could be secured • Network solutions can help • Malicious insiders can get by anything you got • A holistic approach is needed. Including: • Audits, detection and response • Education • Standards and best practices

  13. What does DePaul do? • We stop some obvious stuff in various places • We're beginning to do more at the edges • Note: the network will be very fast soon... • ...big firewalls get in the way big time • Regardless of what you may have heard... • We're better off than we were 2 years ago • Of course so are the attackers

  14. Final thoughts • Overly secure systems are not at all useful • Big border firewalls are obsolescent • Distributed firewalls are getting a lot of talk • Firewall vendors of course like this approach • You should demand open AND secure access • We can do it, but it ain't gonna easy • If we fail, the Internet will become very boring

  15. References http://networks.depaul.edu/security/ http://condor.depaul.edu/~jkristof/ news://news.depaul.edu/dpu.security http://www.cert.org http://www.sans.org http://www.cerias.purdue.edu http://www.neohapsis.com http://www.lists.gnac.net/firewalls/ http://www.interhack.net/pubs/fwfaq/

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