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ProSecure Unified Threat Management (UTM) Appliance Reference Manual
LAN Configuration 4-5
v1.0, January 2010
The UTM delivers the following settings to any LAN device that requests DHCP:
• An IP address from the range that you have defined
• Subnet mask
• Gateway IP address (the UTM’s LAN IP address)
• Primary DNS server (the UTM’s LAN IP address)
• WINS server (if you entered a WINS server address in the DHCP Setup menu)
• Lease time (the date obtained and the duration of the lease).
DHCP Relay
DHCP relay options allow you to make the UTM a DHCP relay agent for a VLAN. The DHCP
Relay Agent makes it possible for DHCP broadcast messages to be sent over routers that do not
support forwarding of these types of messages. The DHCP Relay Agent is therefore the routing
protocol that enables DHCP clients to obtain IP addresses from a DHCP server on a remote subnet.
If you do not configure a DHCP Relay Agent for a VLAN, its clients can only obtain IP addresses
from a DHCP server that is on the same subnet. To enable clients to obtain IP addresses from a
DHCP server on a remote subnet, you must configure the DHCP Relay Agent on the subnet that
contains the remote clients, so that the DHCP Relay Agent can relay DHCP broadcast messages to
your DHCP server.
DNS Proxy
When the DNS Proxy option is enabled for a VLAN, the UTM acts as a proxy for all DNS requests
and communicates with the ISP’s DNS servers (as configured on the WAN ISP Settings screens).
All DHCP clients receive the primary and secondary DNS IP addresses along with the IP address
where the DNS proxy is located (that is, the UTM's LAN IP address). When the DNS Proxy option
is disabled for a VLAN, all DHCP clients receive the DNS IP addresses of the ISP but without the
DNS proxy IP address. A DNS proxy is particularly useful in auto-rollover mode. For example, if
the DNS servers for each WAN connection are different servers, then a link failure might render
the DNS servers inaccessible. However, when the DNS Proxy option is enabled, the DHCP clients
can make requests to the UTM, which, in turn, can send those requests to the DNS servers of the
active WAN connection. However, disable the DNS Proxy if you are using a dual-WAN
configuration in auto-rollover mode with route diversity (that is, with two different ISPs) and you
cannot ensure that the DNS server is available after a rollover has occurred.