Transcript Firewalls & VPNs - Stone Soup Fresno

University of Washington
Computing & Communications
Firewalls for Open Networks
Terry Gray
Director, Networks & Distributed Computing
University of Washington
08 May 2002
University of Washington
Computing & Communications
Conventional Security Wisdom
• Popular Myth:
“The network” caused the problem,
so “the network” should solve it:
– Border firewalls and border VPNs will save us!
• Unpopular Reality:
In a large, diverse enterprise such as UW,
security is not achieved by either one.
University of Washington
Computing & Communications
Gray’s Network Security Axioms
• Network security is maximized…
when we assume there is no such thing.
• Firewalls are such a good idea…
every host should have one. Seriously.
• Remote access is fraught with peril…
just like local access.
University of Washington
Computing & Communications
Perimeter Protection Paradox
• Firewall value is proportional to number of
systems protected.
• Firewall effectiveness is inversely
proportional to number of systems
protected.
– Probability of compromised systems existing inside
– Lowest-common-denominator blocking policy
University of Washington
Computing & Communications
Credo
• Open networks*
• Closed servers
• Protected sessions
*With one exception: DDOS attacks require
network-level blocking
University of Washington
Computing & Communications
“Inverted Networks”
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New trend in big companies (e.g. DuPont)
Ditch the border firewall
Assume LANs are “dirty”
Use VPNs from each workstation to servers
• Hey, an open network, with closed servers
and E2E encryption!
• Why didn’t we think of that? :)
University of Washington
Computing & Communications
Heroic (but futile) Endeavors
• Getting anyone to focus on policies first
• Getting any consensus on border blocking
• Patching old end-systems
• Pretending that clients are only clients
• Securing access to older network gear
University of Washington
Computing & Communications
Properties of ALL Firewalls
 Inserted between UN-trusted (outside) and trusted (inside) nets
 "All" traffic between inside and outside flows through them
 The more restrictive the rules, the more protection offered
 If rules are too restrictive, users may bypass them
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Increase complexity, complicate debugging
No protection between hosts on trusted (inside) network
Little protection from attacks against permitted services
Your vulnerability is proportional to both the number of hostile
hosts able to connect and the number of vulnerable servers to
connect to.
 Firewalls improve security primarily by reducing the number of
hosts able to connect. You still need to reduce the number of
vulnerable servers by applying patches
University of Washington
Computing & Communications
Where do firewalls make sense?
• Pervasively: (But of course we have a firewall…:)
– For blocking spoofed source addresses
• Small perimeter/edge:
– Cluster firewalls, e.g. server sanctuaries, labs
– OS-based and Personal firewalls
• Large perimeter/border:
– Maybe to block an immediate attack?
– Maybe if there is widespread consensus to block
certain ports? (Aye, and there’s the rub…)
– And then again, maybe not...
University of Washington
Computing & Communications
Good Uses for a Firewall
 Reducing exposure of vulnerable services on hosts you can't patch
because they are:
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Certified by the FDA for only one particular revision of software;
Old and no longer supported by the vendor;
Devices with code in ROM, such as a printer or terminal server;
Embedded in a device with a service contract where the service technician
routinely wipes out any custom configuration
 Protecting a new computer or service while you bring it up (even if you
don't intend it to be firewalled in production).
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Preventing the spread of worms and exploitation of back-doors.
As insurance against misconfigured hosts (defense in depth).
Explicitly blocking specific troublesome traffic.
Meeting due-diligence security requirements.
 Limiting access to network-attached printers and devices.
University of Washington
Computing & Communications
Fundamental Firewall Truths...
• Bad guys aren’t always "outside" the moat
• One person’s security perimeter is another’s
broken network
• Organization boundaries and filtering
requirements constantly change
• Perimeter defenses always have holes
University of Washington
Computing & Communications
The Dark Side of Border Firewalls
It’s not just that they don’t solve the problem very well;
large-perimeter firewalls have serious unintended consequences
• Operational consequences
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Force artificial mapping between biz and net perimeters
Catch 22: more port blocking -> more port 80 tunneling
Cost more than you think to manage; MTTR goes up
May inhibit legitimate activities
May be a performance bottleneck
• Organizational consequences
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Give a false sense of security
Encourage backdoors
Separate policy configuration from best policy makers
Increase tensions between security, network, and sys admins
University of Washington
Computing & Communications
Mitnick’s Perspective
"It's naive to assume that just installing a
firewall is going to protect you from all
potential security threats. That assumption
creates a false sense of security, and having
a false sense of security is worse than
having no security at all."
Kevin Mitnick
eWeek 28 Sep 00
University of Washington
Computing & Communications
Do You Feel Lucky?
• QUESTION:
If a restrictive border firewall surrounds
your --and 50,000 other-- computers, should
you feel safe?
• ANSWER:
Only if you regularly win the lottery!
University of Washington
Computing & Communications
Distributed Firewall Management
• Given the credo of:
– Open networks
– Closed servers
– Protected sessions
• What about all the desktops?
– Organizations that can tolerate a restrictive border
firewall usually centrally manage desktops
– Thus, they can also centrally configure policybased packet filters on each desktop and don’t
need to suffer the problems of border firewalls
– Centrally managing desktop firewalls possible
even if desktops generally unmanaged
University of Washington
Computing & Communications
UW’s Logical Firewall
• A response to pressure for dept’l firewalls in
our communication closets
• Plugs into any network port
• Departmentally managed
• Opt-in deployment
• Doesn’t interfere with network management
• Uses Network Address Translation (NAT)
• Intended for servers; can be used for clients
• Web-based rules generator
• Gibraltar Linux foundation
University of Washington
Computing & Communications
UW Logical Firewall - How it Works
 Ethernet allows two completely separate subnets to share a single wire.
 As per RFC 1918, our campus routers block all 10.x.y.z traffic.
 LFW clients are given 10.x.y.z unroutable network addresses.
 By changing just the first octet to 10, address allocation becomes trivial.
 Firewalled hosts can talk directly only to each other or their LFW.
 LFW does Network Address Translation (NAT) for every packet in/out.
• Note that the LFW is not physically between the outside network
and protected hosts but all traffic between the outside network
and protected hosts must go through it.
University of Washington
Computing & Communications
LFW Traffic Flow
University of Washington
Computing & Communications
LFW Advantages
• No re-wiring necessary
• Opt-in (easy to add/remove clients)
• Firewalls (plural) can live anywhere on the subnet
• Can have different administrators or policies, etc.
• Does not interfere with managing network infrastructure
• Software is available for free
• Requires only a PC with floppy, NIC and CDROM (no hard drive, keyboard,
mouse, monitor)
• Use your favorite linux or use "Gibraltar" (boots & runs from CDROM)
• Web-based firewall rule-generator supports hand-crafting rules too
• Stateful firewall rules (more expressive and simpler to write)
• Remotely and securely manageable (via SSH login)
• Supports IPSEC tunneling between subnets
University of Washington
Computing & Communications
LFW Disadvantages
• Potentially more vulnerable from hacked un-firewalled box on subnet
• A hacked box might be able to sniff traffic from the 10.x.y.z net
• A skillful intruder might be able to configure a 10.x.y.z virtual interface
• But this added threat is only from hosts on your own subnet
• You're always more vulnerable to arp-spoofing, IP spoofing and hijacking
attacks from your subnet anyway.
• Traffic through firewall (off subnet) travels your switch twice
--unless you use a second NIC and rewire (which _is_ supported)
• With a full-duplex switched network connection, this may not reduce
throughput significantly
• Clients must be re-configured with a new IP address
• A few protocols don't NAT well (or at all)
• Public and private IP addrs on one wire makes DHCP difficult
University of Washington
Computing & Communications
LFW - Setup Overview
• Download the "Gibraltar" CDROM image and burn it onto a CDROM
• Boot the Gibraltar CDROM
• Copy "uw-setup" script to a floppy, run it on Gibraltar, answer questions
• Visit LFW "Rule Generator" webpage to specify firewall rules and clients
• SSH into Gibraltar, copy/paste output of "Rule Generator" into Gibraltar
• Save configuration to floppy
• Once you have the CDROM, the remaining steps take
under 5 minutes
• More detail at the LFW homepage:
http://staff.washington.edu/corey/fw/
University of Washington
Computing & Communications
LFW Results
• Largest installation: Appled Physics Lab
– 5 LFWs on 5 subnets
– 219 protected clients
– IPSEC tunnels between them
• Publication Svcs: LFW protects hi-end printers
• FTP performance: 7.1MB/s vs. 8.6MB/s without
• Local policy-making a big win:
minimizes admin distance between policy
definition and policy enforcement.
University of Washington
Computing & Communications
Is it enough?
• Hard to find anyone who believes all endsystems can be properly managed/secured
• Server sanctuaries, centrally-managed
personal firewalls, logical-firewalls…
are they enough?
• Do we need a dual-policy network?
• What about DDOS attacks?
University of Washington
Computing & Communications
Resources
• http://staff.washington.edu/gray/papers/credo.html
• http://staff.washington.edu/corey/fw/
• http://staff.washington.edu/dittrich
• http://www.sans.org/
Thanks to Corey Satten for several of the LFW slides used
in this presentation.