Reference Guide

342 | Dynamic Host Configuration Protocol (DHCP)
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DHCP Packet Format and Options
DHCP uses UDP as its transport protocol. The server listens on port 67 and transmits to port 68; the client
listens on port 68 and transmits to port 67. The configuration parameters are carried as options in the
DHCP packet in Type, Length, Value (TLV) format; many options are specified in RFC 2132. To limit the
number of parameters that servers must provide, hosts specify the parameters that they require, and the
server sends only those; some common options are given in Table 14-1.
Figure 14-1. DHCP Packet Format
Table 14-1. Common DHCP Options
Option Code Description
Subnet Mask 1 Specifies the client’s subnet mask.
Router 3 Specifies the router IP addresses that may serve as the client’s default
gateway.
Domain Name Server 6 Specifies the DNS servers that are available to the client.
Domain Name 15 Specifies the domain name that clients should use when resolving hostnames
via DNS.
IP Address Lease Time 51 Specifies the amount of time that the client is allowed to use an assigned IP
address.
DHCP Message Type 53 1: DHCPDISCOVER
2: DHCPOFFER
3: DHCPREQUEST
4: DHCPDECLINE
5: DHCPACK
6: DHCPNACK
7: DHCPRELEASE
8: DHCPINFORM
Parameter Request List 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 58 Specifies the amount of time after the IP address is granted that the client
attempts to renew its lease with the original server.
Rebinding Time 59 Specifies 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 255 Signals the last option in the DHCP packet.
op
htype
hlen
hops
xid
secs
yiaddr
siaddr
flags
ciaddr
giaddr
chaddr sname
options
file
Code
Length
Value