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Cisco PIX Firewall and VPN Configuration Guide

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Chapter 4 Using PIX Firewall in SOHO Networks

Using DHCP Relay

Note With PIX Firewall Version 6.2 and lower, the DHCP server can only be enabled on the inside interface

and therefore can only respond to DHCP option 150 and 66 requests from Cisco IP Phones or other

network devices on the internal network. With PIX

Firewall Version 6.3 and higher, the DHCP server

can be enabled on any interface and with as many instances as required.

Using DHCP Relay

PIX Firewall Version 6.3 provides a DHCP relay agent. This allows the PIX Firewall to assist in dynamic

configuration of IP device hosts on any Ethernet interface. Acting as a DHCP relay agent, when the

PIX

Firewall receives a request from a host on an interface, it forwards the request to a user-configured

DHCP server on another interface.

With previous versions of PIX Firewall, hosts on the inside interfaces must be statically configured or

use addresses provided by the PIX

Firewall DHCP Server.

The following restrictions apply to the use of the DHCP relay agent:

• The relay agent accepts and responds to client requests on any interface.

• The relay agent cannot be enabled if the PIX Firewall DHCP server is enabled.

• The relay agent will forward requests if IPSec is configured. VPN negotiations will be initiated if a

tunnel does not exist.

• Clients must be directly connected to the PIX Firewall and cannot send requests through another

relay agent or a router.

• DHCP relay will not work in client mode.

Note Some type of NAT must be specified to allows forwarding of a DHCP release message from a client to

a DHCP server.

Use the following command to enable the DHCP relay agent:

[no] dhcprelay enable interface

Replace interface with the name of the interface connected to the DHCP clients.

Use the following command to configure a DHCP server address for the relay agent:

[no] dhcprelay server dhcp_server_ip server_ifc

Replace dhcp_server_ip with the IP address of the DHCP server. Replace server_ifc with the interface

connected to the DHCP server. You can use this command to identify up to four servers.

By default, the default gateway used by the DHCP server is configured on the DHCP server. To specify

the default gateway to be used by the DHCP server in the PIX Firewall configuration, enter the following

command:

[no] dhcprelay setroute client_ifc

Replace client_ifc with the PIX Firewall interface to be used as the default gateway by DHCP clients for

reaching the DHCP server.

To set the timeout, use the following command:

[no] dhcprelay timeout seconds

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Cisco PIX 525 Spécifications Page 173

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