Controlling access with packet filters and firewalls
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Controlling access with packet filters and firewalls. Security vulnarabilities of the TCP/IP protocols. IP packets are transmitted in the clear and without authentication facilities Can routers trust routing updates received from others?

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Controlling access with packet filters and firewalls

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Controlling access with packet filters and firewalls


Security vulnarabilities of the TCP/IP protocols

  • IP packets are transmitted in the clear and without authentication facilities

  • Can routers trust routing updates received from others?

  • TCP and UDP segments are transmitted in clear and without authentication facilities

  • Auxiliary protocols have similar problems (ICMP, DNS, ARP, BOOTP, TFTP)

  • Application protocols are without protection or use weak password protection (TELNET, FTP)

  • Specific protection applied as “add ons” (NFS, SNMP, X11)


Methods of access control

  • Physical protection of entities (devices, cables)

  • Packet Filter

  • Network Relay

  • Firewalls

    • visible

    • invisible

  • Security mechanisms of individual computers or applications („personal firewall“, „personal internet security“, e-mail security, telebanking)


Physical security

  • Protection against physical access to power distribution or network cables

  • Protection of internal or external access points (distributors, patch panels)

  • Protection of active devices (routers, bridges) against physical access (lock them up)

    Problems:

  • How to support mobile users

  • How to protect a wireless infrastructure

  • How to allow secure access to external resources


Access control using packet filters

  • Operates primarily on IP layer, however also peeking into transport layer information

  • Filtering based on

    • IP address of the source

    • IP address of the receiver

    • Port number of receiver

    • Sometimes port number of the source

    • Type of transport protocol used (TCP/UDP)

  • Uses set of filter rules

  • Pure packet filters do not have information on connection states


Filter rules

123.45.6.0

123.45.0.0

RuleSourceDestinationAction

A135.79.0.0/16123.45.6.0/24Permit

B135.79.99.0/24123.45.0.0/16Deny

C0.0.0.0/00.0.0.0/0Deny

PF

135.79.0.0

135.79.99.0


Access control using network relay

External connections

Monitoring and controlling host

Router

Configuration and logging database

Invisible private subnet

Internal connections


Access control by visible firewall

  • Users use the Internet exclusively from the firewall

  • All users need to have a user account on the firewall

  • The firewall terminates DNS, e-mail, http

  • User authentication must be secure (with cryptographic means)

  • Reduced user friendliness


Access control by invisible firewall

  • Termination of all store-and-forward services (DNS, e-mail) with servers on the firewall

  • Selective forwarding of connections (stateful)

  • Authentication of external and internal peers

  • Logging and intrusion detection

  • Network Address Translation

  • Proxy functions

Protectedinternal

network

Internet

Firewall 1

Firewall 2

D

N

S

D

N

S

publicservers

Variant 1

(DMZ – „de-militarized zone“)


Access control by invisible firewall(Variant 2)

  • Uses only one physical firewall unit

Ruleset 2

Protectedinternal

network

Firewall

Internet

Ruleset 1

D

N

S

D

N

S

publicservers

(DMZ – „de-militarized zone“)


User or application is “proxy aware”

Netscape Navigator

Internet Explorer


Proxy-based firewall services


Some applications are not “proxy aware”

  • talk, ping, …

  • Specific implementation of such applications

  • Offering replacement applications

  • Such appliations may also not be accessible to normal users at all


Literature

  • B. Chapman, E. Zwicky, “Building Internet Firewalls”, O’Reilly & Associates, 1995

  • W. Cheswick, S. Bellovin, „Firewalls and Internet Security“, Addison-Wesley, 1994


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