312 | Dynamic Host Configuration Protocolw w w . d e l l . c o m | s u p p o r t . d e l l . c o m DHCP Packet Format and OptionsDHCP uses UDP as its transport protocol. The server listens on port 67 and transmits to port 68; the clientlistens on port 68 and transmits to port 67. The configuration parameters are carried as options in theDHCP packet in Type, Length, Value (TLV) format; many options are specified in RFC 2132. To limit thenumber parameters that servers must provide, hosts specify the parameters that they require, and the serversends only those; some common options are given in Table 13-1.Figure 13-1. DHCP Packet FormatTable 13-1. Common DHCP OptionsOption Code DescriptionSubnet Mask 1 Specifies the clients subnet mask.Router 3 Specifies the router IP addresses that may serve as the client’s defaultgateway.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 hostnamesvia DNS.IP Address Lease Time 51 Specifies the amount of time that the client is allowed to use an assigned IPaddress.DHCP Message Type 53 1: DHCPDISCOVER2: DHCPOFFER3: DHCPREQUEST4: DHCPDECLINE5:DHCPACK6:DHCPNACK7:DHCPRELEASE8:DHCPINFORMParameter Request List 55 Clients use this option to tell the server which parameters it requires. It is aseries 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 clientattempts to renew its lease with the original server.Rebinding Time 59 Specifies the amount of time after the IP address is granted that the clientattempts to renew its lease with any server, if the original server does notrespond.End 255 Signals the last option in the DHCP packet.op htype hlen hops xid secs yiaddr siaddrflags ciaddr giaddr chaddr sname optionsfileCode Length Value
