Book Excerpt: Cisco Secure Internet Security Solutions, Part 1 - Page 3

 By Cisco Press
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PIX Features
The PIX Firewalls, regardless of model number, all provide the same security features. The PIX is a stateful firewall that delivers full protection to the corporate network by completely concealing the nature of the internal network to those outside. The main operating features of the PIX follow:

  • Sequence random numbering -- IP spoofing generally relies on the ability to guess a sequence number. The PIX randomizes the IP sequence numbers for each session. This makes IP spoofing much more difficult to accomplish.
  • Stateful filtering -- This is a secure method of analyzing data packets that is also known as the Adaptive Security Algorithm (ASA). When data traverses from the trusted interface on the PIX to a less trusted interface, information about this packet is entered into a table. When the PIX receives a data packet with the SYN bit set, the PIX checks the table to see if, in fact, the destination host has previously sent data out to the responding host. If the table does not contain an entry showing that the local host has requested data, the packet is dropped. This technique virtually eliminates all SYN-based DoS attacks.
  • Network Address Translation (NAT) -- NAT is the process of changing the source IP address on all packets sent out by a host and changing the destination IP address of all incoming packets for that host. This prevents hosts outside of the LAN from knowing the true IP address of a local host. NAT uses a pool of IP addresses for all local hosts. The IP address a local host will receive changes as addresses are used and returned to the pool.
  • Port Address Translation (PAT) -- PAT is similar to NAT except that all local hosts receive the same IP address. Using different ports for each session differentiates local host sessions. The IP address of the local host is still changed using PAT, but the ports associated with the session are also changed. Both PAT and NAT can be used concurrently on a PIX Firewall.
  • Embedded operating system -- A UNIX, Linux, or Windows NT machine can be used as a proxy server. However, the throughput of such a machine is slower by design than that available through the PIX. A proxy server receives an Ethernet packet, strips off the header, extracts the IP packet, and then moves that packet up through the OSI model until it reaches the application layer (Layer 7), where the proxy server software changes the address. The new IP packet is rebuilt and sent down to Layer 1 of the OSI model, where it is transmitted. This uses a large number of CPU cycles and introduces delay. Because the PIX is a proprietary system, the OSI model constraints can be bypassed and made to allow cut-through proxy to operate.
  • Cut-through proxy and ASA -- The combination of cut-through proxy and ASA allows the PIX to process more than 500,000 connections simultaneously with virtually no packet delay. Cut-through proxy is the process where the first packet in a session is checked as in any proxy server, but all subsequent packets are passed through. This technique allows the PIX to transfer packets extremely efficiently.
  • DNS guard -- By default, all outgoing DNS requests are allowed. Only the first response is allowed to enter the LAN.
  • Mail guard -- Only RFC 821-specific commands are allowed to a Simple Mail Transfer Protocol (SMTP) server on an inside interface. These commands are HELLO, MAIL, RCPT, DATA, RSET, NOOP, and QUIT. The PIX responds with an OK to all other mail requests to confuse attackers. This is configured with the fixup command.
  • Flood defender -- This limits the total number of connections and the number of halfopen connections. User Datagram Protocol (UDP) response packets that either have not been requested or arrive after a timeout period are also dropped.
  • ICMP deny -- By default, all Internet Control Message Protocol (ICMP) traffic does not get sent over the inside interface. The administrator must specifically allow ICMP traffic to enter if needed.
  • IP Frag Guard -- This limits the number of IP full-fragment packets per second per internal host to 100. This prevents DoS attacks such as LAND.c and teardrop. Additionally, this ensures that all responsive IP packets are let through only after an initial IP packet requesting the response has traversed the PIX.
  • Flood guard -- This feature is designed to prevent DoS attacks that continuously request an authentication of a user. The repetitive requests for authentication in this type of DoS attack are designed to use memory resources on a network device. The PIX relies on a subroutine that uses its own section of memory. When an excessive number of authentication requests are received, the PIX starts dropping these requests and reclaiming memory, thus defeating this form of attack.
  • Automatic Telnet denial -- By default, the PIX Firewall will not respond to any Telnet request except through the console port. When enabling Telnet, set it to allow only those connections that are actually necessary.
  • Dynamic Host Configuration Protocol (DHCP) client and server support -- The PIX can rely on a DHCP server to gain an IP address for an interface. As a DHCP server, the PIX provides IP addresses for hosts attached to one of the interfaces.
  • Secure Shell (SSH) support -- The PIX supports the SSH remote shell functionality available in SSH version 1. SSH is an application that runs on top of a connection-oriented Layer 3 protocol such as TCP. SSH provides encryption and authentication services for Telnet sessions. Support for SSH requires third-party software, which may be obtained at the following sites:
    -- Windows client:
    -- Linux, Solaris, OpenBSD, AIX, IRIX, HP/UX, FreeBSD, and NetBSD client:
    www.openssh.com -- Macintosh client:
  • Intrusion Detection System (IDS) -- The PIX integrates the same IDS features that are available on routers through the Cisco Secure IOS. The IDS detects 53 specific types of intrusion. See Chapter 6, "Intrusion Detection Systems," for more details on IDS.
  • TCP intercept -- The PIX can act like a TCP intercept device, isolating protected hosts from direct contact through TCP connections. TCP intercept is discussed in Chapter 2, "Basic Cisco Router Security."

Part 2 of this chapter will cover Cisco Secure Private Internet Exchange (PIX) Firewall configuration.

This article was originally published on Sep 12, 2001
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