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Cisco PIX Firewall and VPN Configuration Guide

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Chapter 9 Accessing and Monitoring PIX Firewall

Using Network Time Protocol

Overview

The Network Time Protocol (NTP) is used to implement a hierarchical system of servers that provide a

source for precisely synchronized time among network systems. This kind of accuracy is required for

time-sensitive operations such as validating a certificate revocation lists (CRL), which includes a precise

time stamp.

PIX Firewall Version 6.2 and higher provides an NTP client that allows the PIX Firewall to obtain its

system time from NTP version 3 servers, like those provided with Cisco IOS routers.

Enabling NTP

To enable the PIX Firewall NTP client, enter the following command:

[no] ntp server ip_address [key number] source if_name [prefer]

This command causes the PIX Firewall to synchronize with the time server identified by ip_address. The

key option requires a authentication key when sending packets to this server. When using this option,

replace number with the authentication key. The interface specified by if_name is used to send packets

to the time server. If the source keyword is not specified, the routing table will be used to determine the

interface. The prefer option makes the specified server the preferred server to provide synchronization,

which reduces switching back and forth between servers.

To enable authentication for NTP messages, enter the following command:

[no] ntp authenticate

[no] ntp authentication-key number md5 value

[no] ntp trusted-key number

The ntp authenticate command enables NTP authentication. If you enter this command, the

PIX

Firewall will not synchronize to an NTP server unless the server is configured with one of the

authentication keys specified using the ntp trusted-key command.

The ntp authentication-key command is used to define authentication keys for use with other NTP

commands to provide a higher degree of security. The number parameter is the key number (1 to

4294967295). The value parameter is the key value (an arbitrary string of up to 32 characters). The key

value will be replaced with ‘********’ when the configuration is viewed with either the write terminal,

show configuration, or show tech-support commands.

Use the ntp trusted-key command to define one or more key numbers corresponding to the keys defined

with the ntp authentication-key command. The PIX

Firewall will require the NTP server to provide this

key number in its NTP packets. This provides protection against synchronizing the PIX

Firewall system

clock with an NTP server that is not trusted.

To remove NTP configuration, enter the following command:

clear ntp

This command removes the NTP configuration, disables authentication, and removes all the

authentication keys.

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