Administrator Guide

Option Number and Description
Species the amount of time that the client is allowed to use an assigned IP address.
DHCP Message
Type
Option 53
• 1: DHCPDISCOVER
• 2: DHCPOFFER
• 3: DHCPREQUEST
• 4: DHCPDECLINE
• 5: DHCPACK
• 6: DHCPNACK
• 7: DHCPRELEASE
• 8: DHCPINFORM
Parameter Request
List
Option 55
Clients use this option to tell the server which parameters it requires. It is a series of octets where each
octet is DHCP option code.
Renewal Time Option 58
Species the amount of time after the IP address is granted that the client attempts to renew its lease with
the original server.
Rebinding Time Option 59
Species the amount of time after the IP address is granted that the client attempts to renew its lease with
any server, if the original server does not respond.
End Option 255
Signals the last option in the DHCP packet.
Option 82
RFC 3046 (the relay agent information option, or Option 82) is used for class-based IP address assignment.
The code for the relay agent information option is 82, and is comprised of two sub-options, circuit ID and remote ID.
Circuit ID
This is the interface on which the client-originated message is received.
Remote ID This identies the host from which the message is received. The value of this sub-option is the MAC
address of the relay agent that adds Option 82.
The DHCP relay agent inserts Option 82 before forwarding DHCP packets to the server. The server can use this information to:
• track the number of address requests per relay agent. Restricting the number of addresses available per relay agent can harden a
server against address exhaustion attacks.
• associate client MAC addresses with a relay agent to prevent oering an IP address to a client spoong the same MAC address
on a dierent relay agent.
• assign IP addresses according to the relay agent. This prevents generating DHCP oers in response to requests from an
unauthorized relay agent.
The server echoes the option back to the relay agent in its response, and the relay agent can use the information in the option to
forward a reply out the interface on which the request was received rather than ooding it on the entire VLAN.
The relay agent strips Option 82 from DHCP responses before forwarding them to the client.
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Dynamic Host Conguration Protocol (DHCP)