Dell Force10 E300, Configuration Manual

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312 | Dynamic Host Configuration Protocol
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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 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 13-1 .
Figure 13-1. DHCP Packet Format
Table 13-1. Common DHCP Options
Option Code Description
Subnet Mask 1 Specifies the clients subnet mask.
Router 3 Specifies the router IP addresses that may serve as the client’s default
gateway.
Domain Name Server 6 Specifies the the DNS servers that are available to the client.
Domain Name 15 Specifies the domain name that client 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 octects 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