In Windows Server 2003, the DHCP Server service includes support for the Dynamic Host Configuration Protocol (DHCP), the Multicast Address Dynamic Client Allocation Protocol (MADCAP), and the Bootstrap Protocol (BOOTP).
DHCP servers communicate with DHCP clients by using a series of DHCP messages. The format of DHCP messages is based on the message format used with the BOOTP protocol.
RFC 2131 defines the format for each message sent between a DHCP client and a DHCP server. The following table shows the possible fields in the DHCP messages.
DHCP Message Fields
|Field Name||Friendly Name||Field Length (Octets)||Description|
Hardware Address Type
Hardware address type. Defined at http://www.iana.org/assignments/arp-parameters
Hardware Address Length
Hardware address length in octets
Value is set to zero by DHCP clients. Optionally used to count the number of relay agents that forwarded the message.
A random number used to associate messages and responses between a client and a server.
Seconds elapsed since client began address acquisition or renewal process.
Flags set by client. The Broadcast flag is set if the client cannot receive unicast IP datagrams (for example, before it is configured with an IP address).
Client IP Address
This field is only filled in if the client has an IP address and can respond to ARP requests.
Your IP Address
Address given to the DHCP client by the DHCP server
DHCP Server IP Address
IP address of the server that is offering a lease
Gateway IP Address
DHCP relay agent IP address
Client Hardware Address
Client hardware address
Server Host Name
Optional server host name. Not used in Windows Server 2003
Boot File Name
The name of the file containing the boot image for a BOOTP client
Optional parameters field. In the DHCP protocol packet, each option begins with a single octet tag, which holds the option code, and a second octet, which describes the option data length, in bytes. For a complete list of the DHCP options available by default on a DHCP server running on Windows Server 2003, see “DHCP Tools and Settings.”
For a complete view of how these fields are used in each DHCP message, see RFC 2131 or use a network monitoring tool, such as Netmon, to view the DHCP messages.
Windows Server 2003 includes a Multicast Address Dynamic Client Allocation Protocol (MADCAP) Server service to support dynamic assignment and configuration of IP multicast addresses on TCP/IP-based networks.
Whereas DHCP unicast scopes provide client configurations by allocating ranges of IP addresses for point-to-point communication between two networked computers, multicast scopes provide ranges for multicast IP addresses. These addresses are reserved for multicast operation using directed transmission from one point to multiple points.
A multicast address is shared by many computers. A group of TCP/IP computers can use a single multicast IP address to send directed communication to all computers with which they share the use of the group address. An IP datagram that is sent to the multicast address is forwarded to all members of that multicast group.
Multicast addresses support dynamic membership, allowing individual computers to join or leave the multicast group at any time. The size of the group is not limited, and computers can be members of multiple groups. In addition, any computer that uses TCP/IP can send datagrams to any multicast group.
Multicast Address Ranges
You can permanently reserve multicast group addresses or temporarily assign and use them. A permanent group is made by permanently reserving a multicast IP address (220.127.116.11 to 18.104.22.168) with the Internet Assigned Numbers Authority (IANA). The reserved address then becomes a well-known address, indicating a specific multicast group that exists regardless of whether group member computers are present on the network. Any multicast IP address that is not permanently reserved with the IANA can then be used dynamically to assign and form temporary multicast groups. These temporary groups can exist as long as one or more computers on the network are configured with the group’s address and actively share in its use.
Bootstrap Protocol (BOOTP) is a computer configuration protocol developed before DHCP. DHCP improves on BOOTP and resolves specific limitations that BOOTP had as a computer configuration service. RFC 951 defines BOOTP.
Whereas BOOTP configures diskless workstations with limited boot capabilities, DHCP configures networked computers, that have local hard drives and full boot capabilities.
Likewise, although both BOOTP and DHCP allocate IP addresses to clients during startup, they use different methods of allocation. BOOTP typically provides fixed allocation of a single IP address for each client, permanently reserving this address in the BOOTP server database. DHCP typically provides dynamic, leased allocation of available IP addresses, reserving each DHCP client address temporarily in the DHCP database.
Because of the relationship between BOOTP and DHCP, both protocols share some defining characteristics. BOOTP and DHCP use nearly identical request messages and reply messages. Both protocols enclose each protocol message in a single User Datagram Protocol (UDP) datagram of 576 bytes. Message headers are the same for both BOOTP and DHCP, except for the final message header field that carries optional data. For BOOTP, this optional field is called the vendor-specific area and is limited to 64 bytes. For DHCP, this optional field is called the options field and is at least 312 bytes long.
Both BOOTP and DHCP use the same reserved protocol ports for sending and receiving messages between servers and clients. Both BOOTP and DHCP servers use UDP port 67 to listen for and receive client request messages. BOOTP and DHCP clients typically reserve UDP port 68 for accepting message replies from either a BOOTP server or DHCP server.
Because DHCP and BOOTP messages use nearly identical format types and packet structures, and use the same well-known service ports, BOOTP or DHCP relay agent programs usually treat BOOTP and DHCP messages as the same message type and do not differentiate between them.
BOOTP clients do not rebind or renew configuration with the BOOTP server except when the system restarts, whereas DHCP clients do not require a system restart to rebind or renew configuration with the DHCP server. Instead, clients automatically enter the rebinding state at defined intervals to renew their leased address allocation with the DHCP server. This process occurs in the background and is transparent to the user.
BOOTP uses a two-phase bootstrap configuration process in which clients contact BOOTP servers to perform address determination and boot file name selection, and clients also contact Trivial File Transfer Protocol (TFTP) servers to perform file transfer of their boot image. DHCP uses a single-phase boot configuration process whereby a DHCP client negotiates with a DHCP server to determine its IP address and obtain any other initial configuration details it needs for network operation.
Because BOOTP clients contact TFTP servers to perform file transfer of their boot image and Windows Server 2003 does not provide a TFTP file service, you need a third-party TFTP server to support BOOTP clients that must boot from an image file (usually diskless workstations). You also need to configure your DHCP server to provide supported BOOTP/DHCP options.
DHCP Options Supported for BOOTP Clients
To obtain other options, BOOTP clients must specify DHCP option code 55 (the Options Request List parameter) in the BOOTP request. BOOTP clients that do not specify option 55 can still retrieve the options listed in the following table from DHCP servers running Windows NT Server 4.0 or later, if they are configured on the server.
DHCP Options for BOOTP Clients
NetBIOS over TCP/IP Datagram Distribution Server
NetBIOS over TCP/IP Node Type
NetBIOS over TCP/IP Scope
X Window System Font Server
X Window System Display Manager
DHCP servers running Windows Server 2003 return the options in the order listed above and return as many options as can fit in a single datagram response. For more information about individual DHCP options, see “DHCP Tools and Settings.”
- When configuring client reservations for use with BOOTP clients, remember that DHCP options can apply equally to DHCP and BOOTP clients.
Each record in the BOOTP table has three fields of information that is returned to the BOOTP client:
- Boot Image. Identifies the generic file name (such as “unix”) of the requested boot file, based on the BOOTP client’s hardware type.
- File Name. Identifies the full path of the boot file (such as “/etc/vmunix”) that the BOOTP server returns to the client by using TFTP.
- File Server. Identifies the name of the TFTP server used to store the boot file.
To add entries in the BOOTP table, use the DHCP snap-in.
Computer networks can be of any form like a LAN, WAN etc. If you are connected to a local LAN or an internet connection, the IP addresses form the basis of communication over computer networks. An IP address is the identity of a host or a computer device while connected to any network.
In most of the cases when you connect your computer to a LAN or internet, you’ll notice that the IP address and other information like subnet mask etc are assigned to your computer automatically. Have you ever thought about how this happens? Well, in this article we will understand the concept of DHCP that forms the basis of this functionality.
What is DHCP?
DHCP stands for Dynamic Host Configuration Protocol.
As the name suggests, DHCP is used to control the network configuration of a host through a remote server. DHCP functionality comes installed as a default feature in most of the contemporary operating systems. DHCP is an excellent alternative to the time-consuming manual configuration of network settings on a host or a network device.
DHCP works on a client-server model. Being a protocol, it has it’s own set of messages that are exchanged between client and server. Here is the header information of DHCP :
|op||1||Type of message|
|htype||1||type of hardware address|
|hlen||1||length of hardware address|
|hops||1||used in case of relay agents. Clients sets them to 0.|
|xid||4||Transaction ID used by the client and server for a session.|
|secs||2||Time elapsed (in seconds) since the client requested the process|
|ciaddr||4||Client IP address.|
|yiaddr||4||The IP address assigned by server to the client|
|siaddr||4||Server IP address.|
|giaddr||4||IP address of the relay agent.|
|chaddr||16||Hardware address of the client.|
|sname||64||Host name of the server.|
|file||128||Boot file name.|
Understanding DHCP helps in debugging many network related problems. Read our articles on wireshark and Journey of a packet on network to enhance your understanding on network and network debugging tools.
In the next section, we will cover the working of this protocol.
How DHCP Works?
Before learning the process through which DHCP achieves it’s goal, we first have to understand the different messages that are used in the process.
It is a DHCP message that marks the beginning of a DHCP interaction between client and server. This message is sent by a client (host or device connected to a network) that is connected to a local subnet. It’s a broadcast message that uses 255.255.255.255 as destination IP address while the source IP address is 0.0.0.0
It is DHCP message that is sent in response to DHCPDISCOVER by a DHCP server to DHCP client. This message contains the network configuration settings for the client that sent the DHCPDISCOVER message.
This DHCP message is sent in response to DHCPOFFER indicating that the client has accepted the network configuration sent in DHCPOFFER message from the server.
This message is sent by the DHCP server in response to DHCPREQUEST recieved from the client. This message marks the end of the process that started with DHCPDISCOVER. The DHCPACK message is nothing but an acknowledgement by the DHCP server that authorizes the DHCP client to start using the network configuration it received from the DHCP server earlier.
This message is the exact opposite to DHCPACK described above. This message is sent by the DHCP server when it is not able to satisfy the DHCPREQUEST message from the client.
This message is sent from the DHCP client to the server in case the client finds that the IP address assigned by DHCP server is already in use.
This message is sent from the DHCP client in case the IP address is statically configured on the client and only other network settings or configurations are desired to be dynamically acquired from DHCP server.
This message is sent by the DHCP client in case it wants to terminate the lease of network address it has be provided by DHCP server.
Now as we know about the various DHCP messages, it’s time to go through the the complete DHCP process to give a better Idea of how DHCP works. Note that the steps mentioned below assume that DHCP functionality is enabled by default on the client side.
Here are the steps :
- Step 1: When the client computer (or device) boots up or is connected to a network, a DHCPDISCOVER message is sent from the client to the server. As there is no network configuration information on the client so the message is sent with 0.0.0.0 as source address and 255.255.255.255 as destination address. If the DHCP server is on local subnet then it directly receives the message or in case it is on different subnet then a relay agent connected on client’s subnet is used to pass on the request to DHCP server. The transport protocol used for this message is UDP and the port number used is 67. The client enters the initializing stage during this step.
- Step 2: When the DHCP server receives the DHCPDISCOVER request message then it replies with a DHCPOFFER message. As already explained, this message contains all the network configuration settings required by the client. For example, the yaddr field of the message will contain the IP address to be assigned to client. Similarly the the subnet mask and gateway information is filled in the options field. Also, the server fills in the client MAC address in the chaddr field. This message is sent as a broadcast (255.255.255.255) message for the client to receive it directly or if DHCP server is in different subnet then this message is sent to the relay agent that takes care of whether the message is to be passed as unicast or broadcast. In this case also, UDP protocol is used at the transport layer with destination port as 68. The client enters selecting stage during this step
- Step 3: The client forms a DHCPREQUEST message in reply to DHCPOFFER message and sends it to the server indicating it wants to accept the network configuration sent in the DHCPOFFER message. If there were multiple DHCP servers that received DHCPDISCOVER then client could receive multiple DHCPOFFER messages. But, the client replies to only one of the messages by populating the server identification field with the IP address of a particular DHCP server. All the messages from other DHCP servers are implicitly declined. The DHCPREQUEST message will still contain the source address as 0.0.0.0 as the client is still not allowed to use the IP address passed to it through DHCPOFFER message. The client enters requesting stage during this step.
- Step 4: Once the server receives DHCPREQUEST from the client, it sends the DHCPACK message indicating that now the client is allowed to use the IP address assigned to it. The client enters the bound state during this step.
The Concept of Lease
With all the necessary information on how DHCP works, one should also know that the IP address assigned by DHCP server to DHCP client is on a lease. After the lease expires the DHCP server is free to assign the same IP address to any other host or device requesting for the same. For example, keeping lease time 8-10 hours is helpful in case of PC’s that are shut down at the end of the day. So, lease has to be renewed from time to time. The DHCP client tries to renew the lease after half of the lease time has expired. This is done by the exchange of DHCPREQUEST and DHCPACK messages. While doing all this, the client enters the renewing stage.
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