DHCP (Dynamic Host Configuration Protocol) BasicsLast reviewed: December 29, 1997Article ID: Q169289 |
The information in this article applies to:
SUMMARYDynamic Host Configuration Protocol (DHCP) is a standard protocol defined by RFC 1541 (which is superceded by RFC 2131) that allows a server to dynamically distribute IP addressing and configuration information to clients. Normally the DHCP server provides the client with at least this basic information:
MORE INFORMATIONThe following Microsoft products provide DHCP client functionality:
ARTICLE ID: Q121005 TITLE : DHCP Options Supported by ClientsThe following Microsoft server operating systems provide DHCP server functionality:
Below is a summary table of the conversation between client and server, which is followed by a packet-level description of the process:
Source Dest Source Dest Packet MAC addr MAC addr IP addr IP addr Description ----------------------------------------------------------------- Client Broadcast 0.0.0.0 255.255.255.255 DHCP Discover DHCPsrvr Broadcast DHCPsrvr 255.255.255.255 DHCP Offer Client Broadcast 0.0.0.0 255.255.255.255 DHCP Request DHCPsrvr Broadcast DHCPsrvr 255.255.255.255 DHCP ACKThe detailed conversation between DHCP client and DHCP server is as follows: DHCPDISCOVER The client sends a DHCPDISCOVER packet. The following is an excerpt from a network monitor capture showing the IP and DHCP potions of a DHCPDISCOVER packet. In the IP section, you can see the Destination address is 255.255.255.255 and the Source address is 0.0.0.0. The DHCP section identifies the packet as a Discover packet and identifies the client in two places using the physical address of the network card. Note the values in the CHADDR field and the DHCP: Client Identifier field are identical. IP: ID = 0x0; Proto = UDP; Len: 328 IP: Version = 4 (0x4)
IP: Header Length = 20 (0x14)
IP: Service Type = 0 (0x0)
IP: Precedence = Routine
IP: ...0.... = Normal Delay
IP: ....0... = Normal Throughput
IP: .....0.. = Normal Reliability
IP: Total Length = 328 (0x148)
IP: Identification = 0 (0x0)
IP: Flags Summary = 0 (0x0)
IP: .......0 = Last fragment in datagram
IP: ......0. = May fragment datagram if necessary
IP: Fragment Offset = 0 (0x0) bytes
IP: Time to Live = 128 (0x80)
IP: Protocol = UDP - User Datagram
IP: Checksum = 0x39A6
IP: Source Address = 0.0.0.0
IP: Destination Address = 255.255.255.255
IP: Data: Number of data bytes remaining = 308 (0x0134)
DHCP: Discover (xid=21274A1D)
DHCP: Op Code (op) = 1 (0x1)
DHCP: Hardware Type (htype) = 1 (0x1) 10Mb Ethernet
DHCP: Hardware Address Length (hlen) = 6 (0x6)
DHCP: Hops (hops) = 0 (0x0)
DHCP: Transaction ID (xid) = 556223005 (0x21274A1D)
DHCP: Seconds (secs) = 0 (0x0)
DHCP: Flags (flags) = 0 (0x0)
DHCP: 0............... = No Broadcast
DHCP: Client IP Address (ciaddr) = 0.0.0.0
DHCP: Your IP Address (yiaddr) = 0.0.0.0
DHCP: Server IP Address (siaddr) = 0.0.0.0
DHCP: Relay IP Address (giaddr) = 0.0.0.0
DHCP: Client Ethernet Address (chaddr) = 08002B2ED85E
DHCP: Server Host Name (sname) = <Blank>
DHCP: Boot File Name (file) = <Blank>
DHCP: Magic Cookie = [OK]
DHCP: Option Field (options)
DHCP: DHCP Message Type = DHCP Discover
DHCP: Client-identifier = (Type: 1) 08 00 2b 2e d8 5e
DHCP: Host Name = JUMBO-WS
DHCP: Parameter Request List = (Length: 7) 01 0f 03 2c 2e 2f 06
DHCP: End of this option field
DHCPOFFER
The DHCP server responds by sending a DHCPOFFER packet. In the IP section of the capture excerpt below, the Source address is now the DHCP server IP address, and the Destination address is the broadcast address 255.255.255.255. The DHCP section identifies the packet as an Offer. The YIADDR field is populated with the IP address the server is offering the client. Note the CHADDR field still contains the physical address of the requesting client. Also, we see in the DHCP Option Field section the various options being sent by the server along with the IP address. In this case the server is sending the Subnet Mask, Default Gateway (Router), Lease Time, WINS server address (NetBIOS Name Service), and the NetBIOS Node Type. IP: ID = 0x3C30; Proto = UDP; Len: 328 IP: Version = 4 (0x4)
IP: Header Length = 20 (0x14)
IP: Service Type = 0 (0x0)
IP: Precedence = Routine
IP: ...0.... = Normal Delay
IP: ....0... = Normal Throughput
IP: .....0.. = Normal Reliability
IP: Total Length = 328 (0x148)
IP: Identification = 15408 (0x3C30)
IP: Flags Summary = 0 (0x0)
IP: .......0 = Last fragment in datagram
IP: ......0. = May fragment datagram if necessary
IP: Fragment Offset = 0 (0x0) bytes
IP: Time to Live = 128 (0x80)
IP: Protocol = UDP - User Datagram
IP: Checksum = 0x2FA8
IP: Source Address = 157.54.48.151
IP: Destination Address = 255.255.255.255
IP: Data: Number of data bytes remaining = 308 (0x0134)
DHCP: Offer (xid=21274A1D)
DHCP: Op Code (op) = 2 (0x2)
DHCP: Hardware Type (htype) = 1 (0x1) 10Mb Ethernet
DHCP: Hardware Address Length (hlen) = 6 (0x6)
DHCP: Hops (hops) = 0 (0x0)
DHCP: Transaction ID (xid) = 556223005 (0x21274A1D)
DHCP: Seconds (secs) = 0 (0x0)
DHCP: Flags (flags) = 0 (0x0)
DHCP: 0............... = No Broadcast
DHCP: Client IP Address (ciaddr) = 0.0.0.0
DHCP: Your IP Address (yiaddr) = 157.54.50.5
DHCP: Server IP Address (siaddr) = 0.0.0.0
DHCP: Relay IP Address (giaddr) = 0.0.0.0
DHCP: Client Ethernet Address (chaddr) = 08002B2ED85E
DHCP: Server Host Name (sname) = <Blank>
DHCP: Boot File Name (file) = <Blank>
DHCP: Magic Cookie = [OK]
DHCP: Option Field (options)
DHCP: DHCP Message Type = DHCP Offer
DHCP: Subnet Mask = 255.255.240.0
DHCP: Renewal Time Value (T1) = 8 Days, 0:00:00
DHCP: Rebinding Time Value (T2) = 14 Days, 0:00:00
DHCP: IP Address Lease Time = 16 Days, 0:00:00
DHCP: Server Identifier = 157.54.48.151
DHCP: Router = 157.54.48.1
DHCP: NetBIOS Name Service = 157.54.16.154
DHCP: NetBIOS Node Type = (Length: 1) 04
DHCP: End of this option field
DHCPREQUEST
The client responds to the DHCPOFFER by sending a DHCPREQUEST. In the IP section of the capture below, the Source address of the client is still 0.0.0.0 and the Destination for the packet is still 255.255.255.255. The client retains 0.0.0.0 because the client hasn't received verification from the server that it's okay to start using the address offered. The Destination is still broadcast, because more than one DHCP server may have responded and may be holding a reservation for an Offer made to the client. This lets those other DHCP servers know they can release their offered addresses and return them to their available pools. The DHCP section identifies the packet as a Request and verifies the offered address using the DHCP: Requested Address field. The DHCP: Server Identifier field shows the IP address of the DHCP server offering the lease. IP: ID = 0x100; Proto = UDP; Len: 328 IP: Version = 4 (0x4)
IP: Header Length = 20 (0x14)
IP: Service Type = 0 (0x0)
IP: Precedence = Routine
IP: ...0.... = Normal Delay
IP: ....0... = Normal Throughput
IP: .....0.. = Normal Reliability
IP: Total Length = 328 (0x148)
IP: Identification = 256 (0x100)
IP: Flags Summary = 0 (0x0)
IP: .......0 = Last fragment in datagram
IP: ......0. = May fragment datagram if necessary
IP: Fragment Offset = 0 (0x0) bytes
IP: Time to Live = 128 (0x80)
IP: Protocol = UDP - User Datagram
IP: Checksum = 0x38A6
IP: Source Address = 0.0.0.0
IP: Destination Address = 255.255.255.255
IP: Data: Number of data bytes remaining = 308 (0x0134)
DHCP: Request (xid=21274A1D)
DHCP: Op Code (op) = 1 (0x1)
DHCP: Hardware Type (htype) = 1 (0x1) 10Mb Ethernet
DHCP: Hardware Address Length (hlen) = 6 (0x6)
DHCP: Hops (hops) = 0 (0x0)
DHCP: Transaction ID (xid) = 556223005 (0x21274A1D)
DHCP: Seconds (secs) = 0 (0x0)
DHCP: Flags (flags) = 0 (0x0)
DHCP: 0............... = No Broadcast
DHCP: Client IP Address (ciaddr) = 0.0.0.0
DHCP: Your IP Address (yiaddr) = 0.0.0.0
DHCP: Server IP Address (siaddr) = 0.0.0.0
DHCP: Relay IP Address (giaddr) = 0.0.0.0
DHCP: Client Ethernet Address (chaddr) = 08002B2ED85E
DHCP: Server Host Name (sname) = <Blank>
DHCP: Boot File Name (file) = <Blank>
DHCP: Magic Cookie = [OK]
DHCP: Option Field (options)
DHCP: DHCP Message Type = DHCP Request
DHCP: Client-identifier = (Type: 1) 08 00 2b 2e d8 5e
DHCP: Requested Address = 157.54.50.5
DHCP: Server Identifier = 157.54.48.151
DHCP: Host Name = JUMBO-WS
DHCP: Parameter Request List = (Length: 7) 01 0f 03 2c 2e 2f 06
DHCP: End of this option field
DHCPACK
The DHCP server responds to the DHCPREQUEST with a DHCPACK, thus completing the initialization cycle. The Source address is the DHCP server IP address, and the Destination address is still 255.255.255.255. The YIADDR field contains the client's address, and the CHADDR and DHCP: Client Identifier fields are the physical address of the network card in the requesting client. The DHCP Option section identifies the packet as an ACK. IP: ID = 0x3D30; Proto = UDP; Len: 328 IP: Version = 4 (0x4)
IP: Header Length = 20 (0x14)
IP: Service Type = 0 (0x0)
IP: Precedence = Routine
IP: ...0.... = Normal Delay
IP: ....0... = Normal Throughput
IP: .....0.. = Normal Reliability
IP: Total Length = 328 (0x148)
IP: Identification = 15664 (0x3D30)
IP: Flags Summary = 0 (0x0)
IP: .......0 = Last fragment in datagram
IP: ......0. = May fragment datagram if necessary
IP: Fragment Offset = 0 (0x0) bytes
IP: Time to Live = 128 (0x80)
IP: Protocol = UDP - User Datagram
IP: Checksum = 0x2EA8
IP: Source Address = 157.54.48.151
IP: Destination Address = 255.255.255.255
IP: Data: Number of data bytes remaining = 308 (0x0134)
DHCP: ACK (xid=21274A1D)
DHCP: Op Code (op) = 2 (0x2)
DHCP: Hardware Type (htype) = 1 (0x1) 10Mb Ethernet
DHCP: Hardware Address Length (hlen) = 6 (0x6)
DHCP: Hops (hops) = 0 (0x0)
DHCP: Transaction ID (xid) = 556223005 (0x21274A1D)
DHCP: Seconds (secs) = 0 (0x0)
DHCP: Flags (flags) = 0 (0x0)
DHCP: 0............... = No Broadcast
DHCP: Client IP Address (ciaddr) = 0.0.0.0
DHCP: Your IP Address (yiaddr) = 157.54.50.5
DHCP: Server IP Address (siaddr) = 0.0.0.0
DHCP: Relay IP Address (giaddr) = 0.0.0.0
DHCP: Client Ethernet Address (chaddr) = 08002B2ED85E
DHCP: Server Host Name (sname) = <Blank>
DHCP: Boot File Name (file) = <Blank>
DHCP: Magic Cookie = [OK]
DHCP: Option Field (options)
DHCP: DHCP Message Type = DHCP ACK
DHCP: Renewal Time Value (T1) = 8 Days, 0:00:00
DHCP: Rebinding Time Value (T2) = 14 Days, 0:00:00
DHCP: IP Address Lease Time = 16 Days, 0:00:00
DHCP: Server Identifier = 157.54.48.151
DHCP: Subnet Mask = 255.255.240.0
DHCP: Router = 157.54.48.1
DHCP: NetBIOS Name Service = 157.54.16.154
DHCP: NetBIOS Node Type = (Length: 1) 04
DHCP: End of this option field
If the client has previously had a DHCP assigned IP address and it is
restarted, the client will specifically request the previously leased IP
address in a special DHCPREQUEST packet. The Source address is 0.0.0.0 and
the Destination is the broadcast address 255.255.255.255. Microsoft
clients will populate the DHCP Option Field DHCP: Requested Address with
the previously assigned address. Strictly RFC compliant clients will
populate the CIADDR Field with the address requested. The Microsoft DHCP
server will accept either.
IP: ID = 0x0; Proto = UDP; Len: 328 IP: Version = 4 (0x4)
IP: Header Length = 20 (0x14)
IP: Service Type = 0 (0x0)
IP: Precedence = Routine
IP: ...0.... = Normal Delay
IP: ....0... = Normal Throughput
IP: .....0.. = Normal Reliability
IP: Total Length = 328 (0x148)
IP: Identification = 0 (0x0)
IP: Flags Summary = 0 (0x0)
IP: .......0 = Last fragment in datagram
IP: ......0. = May fragment datagram if necessary
IP: Fragment Offset = 0 (0x0) bytes
IP: Time to Live = 128 (0x80)
IP: Protocol = UDP - User Datagram
IP: Checksum = 0x39A6
IP: Source Address = 0.0.0.0
IP: Destination Address = 255.255.255.255
IP: Data: Number of data bytes remaining = 308 (0x0134)
DHCP: Request (xid=2757554E)
DHCP: Op Code (op) = 1 (0x1)
DHCP: Hardware Type (htype) = 1 (0x1) 10Mb Ethernet
DHCP: Hardware Address Length (hlen) = 6 (0x6)
DHCP: Hops (hops) = 0 (0x0)
DHCP: Transaction ID (xid) = 660034894 (0x2757554E)
DHCP: Seconds (secs) = 0 (0x0)
DHCP: Flags (flags) = 0 (0x0)
DHCP: 0............... = No Broadcast
DHCP: Client IP Address (ciaddr) = 0.0.0.0
DHCP: Your IP Address (yiaddr) = 0.0.0.0
DHCP: Server IP Address (siaddr) = 0.0.0.0
DHCP: Relay IP Address (giaddr) = 0.0.0.0
DHCP: Client Ethernet Address (chaddr) = 08002B2ED85E
DHCP: Server Host Name (sname) = <Blank>
DHCP: Boot File Name (file) = <Blank>
DHCP: Magic Cookie = [OK]
DHCP: Option Field (options)
DHCP: DHCP Message Type = DHCP Request
DHCP: Client-identifier = (Type: 1) 08 00 2b 2e d8 5e
DHCP: Requested Address = 157.54.50.5
DHCP: Host Name = JUMBO-WS
DHCP: Parameter Request List = (Length: 7) 01 0f 03 2c 2e 2f 06
DHCP: End of this option field
At this point, the server may or may not respond. The behavior of the
Windows NT DHCP server depends on the version of the operating system
being used as well as other factors such as superscoping. If the server
determines that the client can still use the address, it will either
remain silent or ACK the DHCPREQUEST. If the server determines that the
client cannot have the address, it will send a NACK.
IP: ID = 0x3F1A; Proto = UDP; Len: 328 IP: Version = 4 (0x4)
IP: Header Length = 20 (0x14)
IP: Service Type = 0 (0x0)
IP: Precedence = Routine
IP: ...0.... = Normal Delay
IP: ....0... = Normal Throughput
IP: .....0.. = Normal Reliability
IP: Total Length = 328 (0x148)
IP: Identification = 16154 (0x3F1A)
IP: Flags Summary = 0 (0x0)
IP: .......0 = Last fragment in datagram
IP: ......0. = May fragment datagram if necessary
IP: Fragment Offset = 0 (0x0) bytes
IP: Time to Live = 128 (0x80)
IP: Protocol = UDP - User Datagram
IP: Checksum = 0x2CBE
IP: Source Address = 157.54.48.151
IP: Destination Address = 255.255.255.255
IP: Data: Number of data bytes remaining = 308 (0x0134)
DHCP: NACK (xid=74A005CE)
DHCP: Op Code (op) = 2 (0x2)
DHCP: Hardware Type (htype) = 1 (0x1) 10Mb Ethernet
DHCP: Hardware Address Length (hlen) = 6 (0x6)
DHCP: Hops (hops) = 0 (0x0)
DHCP: Transaction ID (xid) = 1956644302 (0x74A005CE)
DHCP: Seconds (secs) = 0 (0x0)
DHCP: Flags (flags) = 0 (0x0)
DHCP: 0............... = No Broadcast
DHCP: Client IP Address (ciaddr) = 0.0.0.0
DHCP: Your IP Address (yiaddr) = 0.0.0.0
DHCP: Server IP Address (siaddr) = 0.0.0.0
DHCP: Relay IP Address (giaddr) = 0.0.0.0
DHCP: Client Ethernet Address (chaddr) = 08002B2ED85E
DHCP: Server Host Name (sname) = <Blank>
DHCP: Boot File Name (file) = <Blank>
DHCP: Magic Cookie = [OK]
DHCP: Option Field (options)
DHCP: DHCP Message Type = DHCP NACK
DHCP: Server Identifier = 157.54.48.151
DHCP: End of this option field
The client will then begin the discover process, but the DHCPDISCOVER
packet will still attempt to lease the same address. In many instances,
tth client will get the same address but may not.
IP: ID = 0x100; Proto = UDP; Len: 328 IP: Version = 4 (0x4)
IP: Header Length = 20 (0x14)
IP: Service Type = 0 (0x0)
IP: Precedence = Routine
IP: ...0.... = Normal Delay
IP: ....0... = Normal Throughput
IP: .....0.. = Normal Reliability
IP: Total Length = 328 (0x148)
IP: Identification = 256 (0x100)
IP: Flags Summary = 0 (0x0)
IP: .......0 = Last fragment in datagram
IP: ......0. = May fragment datagram if necessary
IP: Fragment Offset = 0 (0x0) bytes
IP: Time to Live = 128 (0x80)
IP: Protocol = UDP - User Datagram
IP: Checksum = 0x38A6
IP: Source Address = 0.0.0.0
IP: Destination Address = 255.255.255.255
IP: Data: Number of data bytes remaining = 308 (0x0134)
DHCP: Discover (xid=3ED14752)
DHCP: Op Code (op) = 1 (0x1)
DHCP: Hardware Type (htype) = 1 (0x1) 10Mb Ethernet
DHCP: Hardware Address Length (hlen) = 6 (0x6)
DHCP: Hops (hops) = 0 (0x0)
DHCP: Transaction ID (xid) = 1053902674 (0x3ED14752)
DHCP: Seconds (secs) = 0 (0x0)
DHCP: Flags (flags) = 0 (0x0)
DHCP: 0............... = No Broadcast
DHCP: Client IP Address (ciaddr) = 0.0.0.0
DHCP: Your IP Address (yiaddr) = 0.0.0.0
DHCP: Server IP Address (siaddr) = 0.0.0.0
DHCP: Relay IP Address (giaddr) = 0.0.0.0
DHCP: Client Ethernet Address (chaddr) = 08002B2ED85E
DHCP: Server Host Name (sname) = <Blank>
DHCP: Boot File Name (file) = <Blank>
DHCP: Magic Cookie = [OK]
DHCP: Option Field (options)
DHCP: DHCP Message Type = DHCP Discover
DHCP: Client-identifier = (Type: 1) 08 00 2b 2e d8 5e
DHCP: Requested Address = 157.54.51.5
DHCP: Host Name = JUMBO-WS
DHCP: Parameter Request List = (Length: 7) 01 0f 03 2c 2e 2f 06
DHCP: End of this option field
DHCP information obtained by the client from a DHCP server will have a
lease time associated with it. The lease time defines how long the client
can use the DHCP-assigned information. When the lease reaches certain
milestones, the client will attempt to renew its DHCP information.
To view IP information on a Windows NT or Windows for Workgroups client, use the IPCONFIG utility. If the client is Windows 95, use WINIPCFG.
REFERENCESFor more information about DHCP, see RFC1541 and RFC2131. RFCs may be obtained via the Internet as follows: Paper copies of all RFCs are available from the NIC, either individually or on a subscription basis (for more information, contact NIC@NIC.DDN.MIL). Online copies are available via FTP or Kermit from NIC.DDN.MIL as rfc/rfc####.txt or rfc/rfc####.PS (#### is the RFC number without leading zeros). Additionally, RFCs may be requested through electronic mail from the automated NIC mail server by sending a message to SERVICE@NIC.DDN.MIL with a subject line of "rfc ####" for text versions or a subject line of "rfc ####.PS" for PostScript versions. To obtain the RFC index, the subject line of your message should read "rfc index". Keywords : NTPROTOCOL NTSrvWkst nttcp kbnetwork Version : WinNT:3.5,3.51,4.0;Windows:95,2.2c,3.11 Platform : winnt |
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