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Updated on July 1, 2015

What's a Firewall

  • Barrier between us and them.
  • Limits communication to the outside world.
  • The outside world can be another part of the same organization.
  • Only a very few machines exposed to attack.

Why Use Firewalls?

Most hosts have security holes.
Proof: Most software is buggy. Therefore, most security software has security bugs.
■ Firewalls run much less code, and hence have few bugs (and holes).
■ Firewalls can be professionally (and hence better) administered.
■ Firewalls run less software, with more logging and monitoring.
■ They enforce the partition of a network into separate security domains.
■ Without such a partition, a network acts as a giant virtual machine, with an unknown set of
privileged and ordinary users.

Tradttional Firewalls by Analogy

Passports are (generally) checked at the border.
My office doesn’t have a door direct to the outside.
My bedroom doesn’t have a real lock.
But a bank still has a vault. . .

Should We Fix the Network Protocols Instead?

Network security is not the problem.
■ Firewalls are not a solution to network problems. They are a network response to a host security problem.
■ More precisely, they are a response to the dismal state of software engineering; taken as
a whole, the profession does not know how to produce software that is secure, correct, and
easy to administer.
■ Consequently, better network protocols will not obviate the need for firewalls. The best
cryptography in the world will not guard against buggy code.

Firewall Advantages

If you don’t need it, get rid of it.
■ No ordinary users, and hence no passowrds for them
■ Run as few servers as possible
■ Install conservative software, don’t get the latest fancy servers, etc.)
■ Log everything, and monitor the log files.
■ Keep copious backups, including a “Day 0” backup.
Ordinary machines cannot be run that way.

Conceptual Pieces

An “inside” — everyone on the inside is presumed to be a good guy
■ An “outside” — bad guys live there
■ A “DMZ” (Demilitarized Zone) — put
necessary but potentially dangerous servers there


■ Good spot for things like mail and web servers
■ Outsiders can send email, retrieve web pages
■ Insiders can retrieve email, update web pages
■ Must monitor such machines very carefully!

Why Administrative Domains?

■ Firewalls enforce policy
■ Policy follows administrative boundaries, not physical ones
■ Example: separate protection domains for Legal, HR, Research, etc.

Firewall Philosophies

1. Block all dangerous destinations.
2. Block everything; unblock things known to be both safe and necessary.
Option 1 gets you into an arms race with the attackers; you have to know everything that is
dangerous, in all parts of your network. Option 2 is much safer.

Blocking Outbound Traffic?

■ Many sites permit arbitrary outbound traffic, but. . .
■ Internal bad guys?
■ Extrusion detection?
■ Regulatory requirements?
■ Other corporate policy?


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