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  1. Firewalling Techniques Prabhaker Mateti

  2. ACK • Not linux specific • Some figures are from 3com

  3. Components of the Firewall System • Bastion Host • Packet-filtering router • Application-level gateway (or proxy server) • Circuit-level gateway

  4. Dual Homed Gateway • A system that has • two or more network interfaces, each of which is connected to a different network. • Acts to block or filter some or all of the traffic trying to pass between the networks.

  5. Bastion Host • Runs general purpose operating system • hardened to resist attack

  6. Proxy services • Proxy servers on a bastion host can prohibit direct connections from the outside and reduce data-driven attacks.

  7. Circuit Relay • Determines if the connection is valid according to rules • opens a session and permits traffic • only from the allowed source and • possibly only for a limited period of time. • Whether a connection is valid is based upon: • destination IP address and/or port • source IP address and/or port • time of day • protocol • user • password

  8. Demilitarized Zone (DMZ) • a neutral zone between the private LAN and the public Internet. • FTP servers, Web servers and the like are located in DMZ.

  9. UntrustedNetwork External LAN www DMZ Firewall SMTP Internal LAN Location of a Firewall

  10. An Application Gateway: Problem • Allow select internal users to telnet outside. • Users authenticate themselves to create telnet connection • A “gateway” used in this sense is different from a standard gateway.

  11. gateway-to-remote host telnet session host-to-gateway telnet session application gateway router and filter An Application Gateway: Solution • Router filter blocks all telnet connections not originating from gateway. • For authorized users, gateway sets up telnet connection to dest host. Gateway relays data between 2 connections

  12. Packet Filtering Router

  13. Packet Filtering Router • Decide not only how, but should a packet be forwarded • Not best when detail protocol knowledge required for decision • Proxy may be a better choice • Lots of leverage as all hosts behind are protected • Can provide unique capabilities • Rejecting forged internal or external packets (address spoofing) • Recognition of malformed packets

  14. Packet-Filtering Router • Service-Dependent Filtering • Some typical filtering rules include: • Permit incoming Telnet sessions only to a • specific list of internal hosts • Permit incoming FTP sessions only to • specific internal hosts • Permit all outbound Telnet sessions • Permit all outbound FTP sessions • Deny all incoming traffic from specific • external networks • Service-Independent Filtering • Source IP Address Spoofing Attacks. Source Routing Attacks. In Tiny • Fragment Attacks. Tiny fragment attacks are designed to circumvent • userdefined filtering rules; the hacker hopes that a filtering router • will examine only the first fragment and allows all other fragments to • pass. A tiny fragment attack can be defeated by discarding all packets • where the protocol type is TCP and the IP FragmentOffset is equal to • 1. • Defining packet filters can be a complex task • Generally, the packet throughput of a router decreases as the number • of filters increases.

  15. Filtering by Service • Characteristics of internal to external telnet connection • Source is inside, • destination is outside, • is TCP, destination port 23, • source port > 1023, • first packet an outbound SYN • Characteristics of ext to int ‘opposite’ • Risk: trusting the port implies trusting the server on that port • Any service can be run from any port by root • Can telnet from port 23, for example

  16. Security Policy • “It is important to note that an Internet firewall is not just a router, a bastion host, or a combination of devices that provides security for a network. • “The firewall is part of an overall security policy that creates a perimeter defense designed to protect the information resources of the organization. • “This security policy must include published security guidelines to inform users of their responsibilities; corporate policies defining network access, service access, local and remote user authentication, dial-in and dialout, disk and data encryption, and virus protection measures; and employee training. All potential points of network attack must be protected with the same level of network security. Setting up an Internet firewall without a comprehensive security policy is like placing a steel door on a tent.” • [From a web based article]

  17. Security Policy Describes a Perimeter Defense

  18. A Connection Circumventing an Internet Firewall

  19. Benefits of an Internet Firewall • Without a firewall, each host system on the private network is exposed to attacks from other hosts on the Internet. • Firewalls offer a convenient point where Internet security can be monitored and alarms generated. • An Internet firewall is a logical place to deploy a Network Address Translator (NAT) that can help alleviate the address space shortage and eliminate the need to renumber when an organization changes Internet service providers (ISPs). • An Internet firewall is the perfect point to audit or log Internet usage. • An Internet firewall can also offer a central point of contact for information delivery service to customers.

  20. Limitations of an Internet Firewall • Creates a single point of failure. • Cannot protect against attacks that do not go through the firewall. • Cannot protect against the types of threats posed by traitors or unwitting users. • Cannot protect against the transfer of virus-infected software or files. • Cannot protect against data-driven attacks. A data-driven attack occurs when seemingly harmless data is mailed or copied to an internal host and is executed to launch an attack.

  21. Limitations of firewalls and gateways • IP spoofing • router can’t know if data “really” comes from claimed source • If multiple app’s. need special treatment, each has own app. gateway. • client software must know how to contact gateway. • e.g., must set IP address of proxy in Web browser • Tradeoff • degree of communication with outside world, level of security • Performance problem

  22. Three Myths of Firewalls • Firewalls make the assumption that the only way in or out of a corporate network is through the firewalls; that there are no "back doors" to your network. In practice, this is rarely the case, especially for a network which spans a large enterprise. Users may setup their own backdoors, using modems, terminal servers, or use such programs as "PC Anywhere" so that they can work from home. The more inconvenient a firewall is to your user community, the more likely someone will set up their own "back door" channel to their machine, thus bypassing your firewall. • Firewalls make the assumption that all of the bad guys are on the outside of the firewall, and everyone on the inside of the can be considered trustworthy. This neglects the large number of computer crimes which are committed by insiders. • Newly evolving systems are blurring the lines between data and executables more and more. With  macros, JavaScript, Java, and other forms executable fragments which can be embedded inside data, a security model which neglects this will leave you wide open to a wide range of attacks.