university of washingtoncomputing & communications firewalls for open networks terry gray...

University of Washington Computing & Communications

Firewalls for Open Networks

Terry GrayDirector, Networks & Distributed Computing

University of Washington

08 May 2002


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.


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.


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


Credo

• Open networks*

• Closed servers

• Protected sessions

*With one exception: DDOS attacks require

network-level blocking


“Inverted Networks”

• 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? :)


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


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

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


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...


Good Uses for a Firewall Reducing exposure of vulnerable services on hosts you can't patch

because they are: 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). 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.


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


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– 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– Give a false sense of security– Encourage backdoors– Separate policy configuration from best policy makers– Increase tensions between security, network, and sys admins


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


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!


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 policy-based 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


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


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.


LFW Traffic Flow


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


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


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/


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.


Is it enough?

• Hard to find anyone who believes all end-systems 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?


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.

university of washingtoncomputing & communications firewalls for open networks terry gray...

Documents

border firewalls

networklevel blocking

policy slide

older network gear slide

number of systems

number of vulnerable

cluster firewalls

network little protection