Table Of Contents


PIX Firewall Adaptive Security

PIX Firewall Features

Understanding PIX Firewall

Access Modes

PIX Firewall Equipment

Before Installing PIX Firewall

Command Changes In this Release

Command Changes Summary

New Command Summary


Cisco Systems' PIX Firewall provides firewall and network translation services.

shows the Cisco PIX Firewall front view.

Figure 1-1 PIX Firewall Front View

PIX (Private Internet Exchange) Firewall provides full firewall protection that completely conceals the architecture of an internal network from the outside world. PIX Firewall allows secure access to the Internet from within existing private networks and the ability to expand and reconfigure TCP/IP networks without being concerned about a shortage of IP addresses. With PIX Firewall, users can take advantage of larger address classes than they may have been assigned by the Internet's Network Information Center (NIC). PIX Firewall provides this access through its Network Address Translation (NAT) facility as described by RFC 1631.

PIX Firewall Adaptive Security

The Adaptive Security (AS) feature applies to the dynamic translation slots and can be applied to static translation slots via the static command. The Adaptive Security algorithm is a very stateful approach to security. Every inbound packet is checked exhaustively against the Adaptive Security algorithm and against connection state information in memory. This stateful approach to security is regarded in the industry as being far more secure than a stateless packet screening approach.

Adaptive Security follows these rules:

Allow any TCP connections that originate from the inside network.

Ensure that if an FTP data connection is initiated to a translation slot, there is already an FTP control connection between that translation slot and the remote host. If not, drop and log the attempt to initiate an FTP data connection.

Drop and log attempts to initiate TCP connections to a translation slot from the outside.

Drop and log source routed IP packets sent to any translation slot on the PIX Firewall.

Allow ICMP of types 0, 3, 4, 8, 11, 12, 17 and 18. By implication, deny ICMP redirects (type 5).

Silently drop ping requests to dynamic translation slots.

Answer (by the PIX Firewall) ping requests directed to static translation slots.

You can protect static translation slots with Adaptive Security, and you can have exceptions (called conduits) to the previously described rules, which you create with the conduit command. Multiple exceptions may be applied to a single static translation slot (via multiple conduit commands). This lets you permit access from an arbitrary machine, network, or any host on the Internet to the inside host defined by the static translation slot. PIX Firewall handles UDP data transfers in a manner similar to TCP. Special handling allows DNS service, archie, and RealAudio to work securely. PIX Firewall creates UDP connection state information when a UDP packet is sent from the inside network. Response packets resulting from this traffic are accepted if they match the connection state information. The connection state information is deleted after a short period of inactivity.

PIX Firewall Features

With the firewall feature, you can eliminate the overhead and risks associated with UNIX-based firewall systems and have complete accounting and logging of all transactions, including attempted break-ins. Both NCSA and SRI certify that the PIX Firewall secures your network from outside intrusion.

PIX Firewall has the following features:

Firewall capability that keeps intruders out of your internal network while permitting regulated conduit access through the firewall for services such as electronic mail, Telnet, FTP, SNMP, and HTTP (World Wide Web) use.

Network translation services that let a site share one or more NIC-registered IP addresses among many users.

An Identity feature that lets NIC-registered IP addresses pass through the firewall without address translation, while still retaining Adaptive Security.

The PIX Firewall offers performance dramatically better than competing firewalls. It gains speed through a patent pending process called Cut-Through proxies, which is the fastest way for a firewall to authenticate a user. Unlike a proxy server that must analyze every packet at layer seven of the OSI model, a time- and processing-intensive function, the PIX Firewall first queries a TACACS+ or RADIUS server for authentication. Once approved, the PIX Firewall then establishes a data flow and all traffic thereafter flows directly and quickly between the two parties. This Cut-Through capability allows the PIX Firewall to perform dramatically faster than proxy-based servers while maintaining session state.

An IOS-like command set for simplified configuration and administration.

Support for SNMP MIB-II gets and traps.

Simplified configuration and system management with an HTML interface.

Support for Telnet, FTP, and HTTP access using RADIUS (Remote Authentication Dial-In User Service) and TACACS+ security systems. PIX Firewall authenticates users in conjunction with the security systems that Cisco routers support. The security clients run on Cisco routers and send authentication requests to a central security server, which contains all user authentication and network service access information.

For domestic sites, the Private Link encryption option that permits up to 64 PIX Firewall units to interact together across a WAN with completely secure data transfer. From the internal networks, the other networks connected through Private Link appear as one contiguous network. Private Link supports IETF IPSEC AH/ESP with DES.

Failover capability that permits a secondary PIX Firewall unit to take over firewall communications if the primary unit fails.

Support for 10BaseT and 100BaseTX networking.

Support for Token-Ring network cards that can be operated singly so that Ethernet networking transforms into Token-Ring or in pairs for a standard Token-Ring network.

Note   You can view information on the PIX Firewall and additional documentation over the World Wide Web at this URL:

Understanding PIX Firewall

The PIX Firewall contains two Ethernet interfaces, one for the inside, secure network and the other for the outside, unprotected network. Both the inside and outside Ethernet interfaces can listen to RIP routing updates, and the inside interface can broadcast a RIP default route.

When packets arrive at the inside Ethernet, the PIX Firewall checks to see if previous packets have come from the inside host. If not, the PIX Firewall creates a dynamic translation slot in its state table. The dynamic translation slot includes the inside IP address and the new globally unique IP address, which is drawn from the virtual network of up to 64K host addresses. PIX Firewall then changes the IP address, the checksums, and other aspects of the packet so they agree, and forwards the packet to the outside Ethernet interface on its way to the Internet.

When a packet arrives at the outside interface, it must first pass the PIX Firewall Adaptive Security criteria. If the packet passes the security tests, PIX Firewall removes the destination IP address, and the internal IP address is inserted in its place. The packet is forwarded to the inside interface.

Dynamic translation slots are useful for desktop machines that do not need constant addresses on the Internet. Inside network hosts with IP addresses not registered with the NIC (Network Information Center) can directly access the Internet with standard TCP/IP software on the desktop. No special client software is needed.

Another class of address translation on the PIX Firewall is static translation. Static translation effectively moves an internal unregistered host into the virtual network in the PIX Firewall. This is useful for internal machines that need to be addressed from the outside Internet gateways; for example, an SMTP server.

For more information on firewalls refer to Firewalls and Internet Security by William Cheswick and Steven Bellows, 1994, Addison-Wesley, ISBN 0-201-63357-4.

Access Modes

New to the version 3 release, the PIX Firewall command interpreter provides a new command set based on IOS technologies. This command set provides three administrator access modes:

Unprivileged mode displays the ">" prompt and lets you view current running settings.

Privileged mode displays the "#" prompt and lets you change current settings and write to flash memory. Any unprivileged command also works in privileged mode.

Configuration mode displays the "(config)#" prompt and lets you change system configurations. Only configuration mode commands work in this mode.

By default, the console is in unprivileged mode. You can access privileged mode by entering the enable command. PIX Firewall then prompts you for a password. Enter the default password cisco. When you are done configuring PIX Firewall, change the password with the enable password command. Exit privileged mode by entering the disable command.

You can access configuration mode by entering the config command. You can then write your settings to flash memory, diskette, or to your console computer. Exit configuration mode by entering the ^z command.

PIX Firewall Equipment

The PIX Firewall shipping carton contains the following:

Rack-mountable PIX Firewall unit

Keys for the front panel lock

Power cord

DB-9 to DB-25 null modem serial cable

DB-25 gender adapter

PIX Firewall system diskette

This guide

Regulatory Compliance and Safety Information for the Cisco PIX Firewall

Cisco PIX Firewall Release Notes

Cisco Connection Documentation CD

Before Installing PIX Firewall

Note   Read the Regulatory Compliance and Safety Information for the Cisco PIX Firewall before installing. Studying the safety material in this guide and the brief section that follows can help keep you safe and focused as you continue preparing your PIX Firewall for service.

Follow these guidelines to ensure general safety:

Keep the chassis area clear and dust-free during and after installation.

Put the removed chassis cover in a safe place.

Keep tools away from walk areas where you and others could fall over them.

Do not wear loose clothing that could get caught in the chassis. Fasten your tie or scarf and roll up your sleeves.

Wear safety glasses if you are working under any conditions that might be hazardous to your eyes.

Do not perform any action that creates a potential hazard to people or makes the equipment unsafe.


Ultimate disposal of this product should be handled according to all national laws and regulations. Refer to the Regulatory Compliance and Safety Information for the Cisco PIX Firewall for more information.


Do not work on the system or connect or disconnect cables during periods of lightning activity. Refer to the Regulatory Compliance and Safety Information for the Cisco PIX Firewall for more information.


This product relies on the building's installation for short-circuit (overcurrent) protection. Ensure that a fuse or circuit breaker no larger than 120 VAC, 15A U.S. (240 VAC, 10A international) is used on the phase conductors (all current-carrying conductors). Refer to the Regulatory Compliance and Safety Information for the Cisco PIX Firewall for more information.


The device is designed to work with TN power systems. Refer to the Regulatory Compliance and Safety Information for the Cisco PIX Firewall for more information.


The ports labeled "Ethernet," "10BaseT," "Token Ring," "Console," and "AUX" are safety extra-low voltage (SELV) circuits. SELV circuits should only be connected to other SELV circuits. Because the BRI circuits are treated like telephone-network voltage, avoid connecting the SELV circuit to the telephone network voltage (TNV) circuits.


Before working on equipment that is connected to power lines, remove jewelry (including rings, necklaces, and watches). Metal objects will heat up when connected to power and ground and can cause serious burns or can weld the metal object to the terminals. Refer to the Regulatory Compliance and Safety Information for the Cisco PIX Firewall for more information.


Before working on a system that has an on/off switch, turn OFF the power and unplug the power cord. Refer to the Regulatory Compliance and Safety Information for the Cisco PIX Firewall for more information.


Do not touch the power supply when the power cord is connected. For systems with a power switch, line voltages are present within the power supply even when the power switch is off and the power cord is connected. For systems without a power switch, line voltages are present within the power supply when the power cord is connected. Refer to the Regulatory Compliance and Safety Information for the Cisco PIX Firewall for more information.


This equipment is intended to be grounded. Ensure that the host is connected to earth ground during normal use. Refer to the Regulatory Compliance and Safety Information for the Cisco PIX Firewall for more information.


Read the installation instructions before you connect the system to its power source. Refer to the Regulatory Compliance and Safety Information for the Cisco PIX Firewall for more information.

Command Changes In this Release

New commands have been added for the following features:

Access modes — to mimic the IOS command set, access modes now group commands by security level. Unprivileged mode is the default, the enable command starts privileged mode, and config term starts configuration mode. Use ^z to exit configuration and privileged mode, and disable to return to unprivileged mode. The command prompt changes for each mode.

User authorization — users can now log into the protected network using security authentication systems supported by Cisco routers such as RADIUS and TACACS. The auth, auth-user, and auth-server commands enable authentication, define who can use it, and from which servers grant access. The show auth, show auth-user, and show auth-server verify status and let you monitor system access.

Administration command syntax — the new show commands give you improved system monitoring capability in all areas of PIX Firewall access and use. Preface a command with show to list its status. In addition, if provided, preface the command with no to disable the action, or clear to erase information. You can list command information with the expanded help facility by entering a question mark after a command, such as show ?, no ?, or clear ? to list which commands accept each preface. The "Command Reference" section at the end of this guide lists each prefacing command individually for further information.

Global lists — you can now have up to 16,384 separate groups of IP addresses available for outbound connections. The new outbound command replaces the V2 access_list command and the global command has changed to accomodate the address grouping feature.

Address translation — the new nat commands let you restrict access to network IP addresses.

HTTP configuration — you can now configure and monitor PIX Firewall status from your network browser. The http command, analogous to the telnet command, lets you define who has access to this configuration capability. The who command now displays both HTTP and Telnet access and the show http command lets you monitor progress. The no http and clear http commands let you disable access.

SNMP access — you can use SNMP to monitor PIX Firewall events, such as SYSLOG messages. The snmp-server command lets you specify which servers have access to these events and show snmp-server lets you monitor access status.

Failover connection (optional) — in the event of system failure, a second PIX Firewall can keep the protected network online. The failover and the show failover commands let you enable or disable this feature and monitor its status.

Private Link — new features extend encryption with up to 7 keys that you can change on timed intervals. The linkpath command replaces the V2 route link command, lnko and lnkopath provide backward compatibility to V2 systems, and age specifies the duration a key is valid.

Command Changes Summary

compares version 2 and version 3 commands.

Table 1-1 Command Changes Between PIX Firewall Versions 2 and 3

V2 Command
V3 Command
V2 Command
V3 Command




show memory, show blocks, show xlate










write erase and reboot


config memory and config floppy











route link







ip address and interface ethernet




show config




show interface


show tcp










show link




show rip


show version




who or show who



show xlate

New Command Summary

lists commands that are new to this release.

Table 1-2

V3 Command


Specify duration that a Private Link encryption key is active


Enable user authentication


Specify which users can login with authentication


Specify which servers authenticate users

clear cmd

Clear or disable command functionality


Enter configuration mode or download RAM from flash memory or floppy


Exit privileged mode


Start privileged mode


Enable failover access


Specify host name for PIX Firewall command prompt


Specify which users can use HTTP configuration


Identify network interface type and speed

ip address

Indicate network interface IP address and subnet mask


Identify Private Link remote system IP address and its network mask

lnko, lnkopath

Maintain compatibility with V2 PIX Firewall Private Link systems


Restrict IP addresses from network access

no cmd

Disable command functionality


Define access lists for outbound connections


Determine if IP address is available to PIX Firewall

show cmd

Provide status or additional information about command functionality

show blocks

View system buffer utilization

show hw

View hardware idenfication information

show memory

View memory utilization

show processes

View process status

show rif

View Token-Ring status

show xlate

View slot and translation information


Specfiy which servers have access to PIX Firewall events

New Commands for This Release