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Firewalls at Stanford:

May 14, 2004

Sunia Yangsunia@networkingThe Group Formerly Known as Networking

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Topics• Changing how we look at networking

– Security by protocol stack– Why protect the network– Specific pros & cons of firewalls– Specific pros & cons of VPNs

• Living with firewalls– Firewall topology– Firewall rules– User education, monitoring, documenting, auditing– Troubleshooting

• Building firewall exercise

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Networks: Past & Future• Past

– Just get the bits there!– Open highway system– Trust

• Future– Patriot act– Who are you? What are you doing?– Make up for other layer's security weaknesses by

centralizing security into network layer– More bureaucracy, process

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Security by Protocol Stack

• Firewalls and VPNs are just part of a total security approach– Firewall would not have caught bugbear-b virus– Firewall at Stanford border would not have

prevented Windows RPC exploits

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Physical Layer Security (Fences)

• "If you can touch it, you can hack it"– Lock up servers, network closets

• Wireless-– firewall defeated if wireless behind firewall– allowing unencrypted wireless session through

firewall defeats data security

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Data layer (bus vs star topology)

• Switches as security device– isolates conversations- sniffer protection

• may misbehave and "leak"– block by hardware address

• not possible in all switches– hardcode hw address to port- tedious,

unscalable

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Network/Transport Layers (Guardposts checking license plates)• Filter traffic by IP addresses and ports

– Router ACLs (may be leaky)– Firewalls– Host IP filters

• Require secure protocols or vpn– data encrypted (ssl, ssh) – encrypted data could still be virus or worm

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Application Layer (Stuff inside car)• Design in security

– good architecture- 3 tier– no clear text passwords– secure transports

• Proxy "firewalls"– screens traffic at app layer before passing to real

application• Good sys admins

– patch, antivirus-software– turnoff unused services

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Why implement security?

• Financial risks– loss of data and reputation– cost of cleaning hacked machines

• Legal risks– Hipaa (medical data), Ferpa (student records)– Lawsuits

• "Cuz they said so…"

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Why firewalls/vpns?

• Physical and data layer security is critical– mostly implemented already (except wireless)

• Too many badly architected apps on market• Often best return of security for given staff,

time and money

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Firewall Cons- #1

• Inconvenience to users– re-educate users– good rules > minor changes may break app– need good communication, docs and response– protective rules constrain traffic

• ex. protecting workstations by denying incoming connections may break peering apps

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Firewall Cons- #2

• Incomplete security– Firewall does not protect needed server ports

• e.g., if running IIS server, need to open hole for http. IIS vulnerability still must be patched. But may prevent hacker from reaching backdoor

– Does not protect against email viruses/worms– May lead to complacency– Hard to firewall if app uses random ports

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Firewall Costs- #1

• Software & Hardware costs– firewalls, maintenance, support, spares– network analyzer– management/log/monitoring tools– management/log/monitoring servers

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Firewall Costs- #2

• Staff costs– Training– Traffic analysis and rule development– Monitoring traffic, vulnerabilities, breakins– Rule changes- proactive or reactive?– Meetings and politics – Documentation, rule change processes

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Firewall Technical Issues• Manageable rule set vs. many exceptions• False positives

– ex. Monitoring pings might look like icmp attack• Hard to secure port-hopping apps- VPN?• Session timeout limits• Server initiates new session to client (AFS)• Reply to client from different IP (load balancers)

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VPN Specifics

• Common way to deal with application data transparency by encrypting packets

• Another layer of authentication and authorization

Note:Board Diagram

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VPN Pros

• With limited staff time and money, may get most application layer security

• Sometimes can be used to enforce patch level of client operating systems

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VPN Cons- #1

• Inconvenience– not all VPN clients compatible or can co-exist– VPN clients fiddle with host's tcp/ip stack

• may break some apps– may break IP dependent services– split tunnel issues- discussed later

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VPN Cons- #2

• Incomplete security– Does not protect if client machine hacked

• in fact, provides encrypted tunnel for hacker– May lead to complacency in users, sys admins,

app developers– Has its own security issues

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VPN Costs- #1

• Software & Hardware costs– VPN concentrator, maintenance/support, spares– VPN clients, maintenance, support– management/log/monitoring tools– management/log/monitoring servers

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VPN Costs- #2

• Staff costs– Training– Monitoring traffic, vulnerabilities, breakins– VPN client support/upgrades– VPN user administration– Meetings and politics – Documentation, rule change processes

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VPN Technical Issues- #1

• Scalability issues• Encryption overhead affects throughput• VPN client picks up new IP• Software vs hardware VPN clients

– cost vs convenience vs compatibility

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VPN Technical Issues- #2Split Tunnel• only traffic to specific servers is encrypted• pros- performance

– less encryption overhead– less traffic to central VPN concentrator

• cons- security– if client host is hacked, hacker can control VPN

session

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Living with Firewalls- Mantras

• "Know Thy Network Traffic"– If you don't know it now, you're going to learn

it the hard way• "Know Thy Servers"

– ditto

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Living with Firewalls- Steps

• Design topology• Firewall Rules• Enforce rules• Monitor, document, audit• Troubleshooting

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Laying out Firewall Topology

• Group servers by – Sensitivity and type of data– Security level (don't put petty cash in the safe)– Production vs development

• Especially as projects are out-sourced, don't want our data somewhere else in the world

• Sharing switches– Generally, databases or servers actually holding

data should be on separate switch (no VLANs)

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Basic Firewall Topology FW = firewallSW = switchS = server

FW1

S1 S2 S3

SW2

S7 S8 S9

Zone 2Ex. Web Servers

Zone 4Ex. Database Servers

Zone 1

Firewall can only filter between zones by IP address and portApplications often use a well-known port

S4 S5 S6

Zone 3Ex. App Servers

vlan 20 vlan 30SW1

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Firewall Rules- Part 1Rule requires the following pieces:Action: Permit, Deny, TunnelSource IPs: Client, VPN Client, Admin, HackerDestination IPs: ServersDestination Port: 80(web), 25(smtp), etc.Port Type: tcp, udp

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Firewall Rules- Part 2Examples:Allow 10.0.1.5 to 171.64.7.77 on udp port 53 (DNS)Allow 10.0.1.0/24 to 10.0.2.10 on tcp port 25 (SMTP)Deny 10.0.1.0/24 to any on tcp port 25 (SMTP)

Sources: servers, clients, vpn clients, hackers (remember the last one when you are writing rules!!!!)

Rules applied in order

To document or not to document- that is the question!

Note: Board Diagram

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Categories of Rules - Part 1• Host

DNS, NTP• Administration

Monitoring – snmp, email, icmpRemote session - telnet, ssh, rsh, citrixAuthentication - sident, kerberos, MS authMaintenance - upgrades, virus, rebuilds, backup,

file transferCentral systems –Microsoft domains/AD, afs, nfs

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Categories of Rules - Part 2

• ApplicationClient: Web servicesServer to server: db sharing, file transfer, app to db

• DevelopmentEnvironments- training, development, etcServer to server: db sharing, file transfer, app to dbApplication buildDeveloper access- in-house, remote

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Educating Users• Firewalls are inconvenient and bureaucratic• Can't ignore the network anymore• Develop process around requesting and

approving rules• Application owner owns security of

application? Security and firewall team may comment on quality of rules

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Enforcing Rules

• When developing rules, usually last rule is– permit any to any on port any– Catches any unknown traffic

• To enforce rules, disable or delete "permit any any" rule.

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Monitoring, Documentation, Auditing• Monitoring- alarm system is still on• Documentation- balance between usability

and security- policy decision• Security auditing- make sure rules are good

and rules still work!

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Troubleshooting• A can't reach B which is behind firewall

– Try ping first (allowed by default at Stanford on FWs)• If fails, check IP addr, physical connection

– Try telnet to desired port• If okay, then not a firewall issue- probably app layer

– Message like "Connected to B"• If fails, depends on message:

– "Connection closed by foreign host" or "Connection refused" means B rejects A

– Hangs with message "Trying B", finally getting message like "Unable to connect to remote host: timed out" means that port is not reachable- possibly firewall

– Run "netstat" on B to see if ports are open

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Common Problems

• ~80% requests to check firewall show that firewall is not the problem

• ~10% of time, previously unknown traffic ("know thy app") has no appropriate rule

• Typos, miscommunication• Host IP changes, thus breaking rule

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Team Exercise/Lab

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Questions and Feedback