DHCP (dynamic host configuration protocol)

DHCP (Dynamic Host Configuration Protocol)
The Dynamic Host Configuration Protocol (DHCP) is a standardized network protocol used on Internet
Protocol (IP) networks for dynamically distributing network configuration parameters, such as IP
addresses for interfaces and services. With DHCP, computers request IP addresses and networking
parameters automatically from a DHCP server, reducing the need for a network administrator or a user
to configure these settings manually.
Depending on implementation, the DHCP server may have three methods of allocating
IP-addresses:
1. Dynamic Allocation: A network administrator reserves a range of IP addresses for DHCP, and
each client computer on the LAN is configured to request an IP address from the DHCP server
during network initialization. The request-and-grant process uses a lease concept with a
controllable time period, allowing the DHCP server to reclaim (and then reallocate) IP addresses
that are not renewed.
2. Automatic Allocation: The DHCP server permanently assigns an IP address to a requesting client
from the range defined by the administrator. This is like dynamic allocation, but the DHCP server
keeps a table of past IP address assignments, so that it can preferentially assign to a client the
same IP address that the client previously had.
3. Static Allocation: The DHCP server allocates an IP address based on a preconfigured mapping to
each client's MAC address. This feature is variously called static DHCP assignment by DD-WRT,
fixed-address by the dhcpd documentation, address reservation by Netgear, DHCP reservation or
static DHCP by Cisco and Linksys, and IP address reservation or MAC/IP address binding by
various other router manufacturers.
Figure 1 DHCP (Dynamic Host Configuration Protocol)

DHCP is used for Internet Protocol version 4 (IPv4), as well as IPv6. While both versions serve the same
purpose, the details of the protocol for IPv4 and IPv6 are sufficiently different that they may be
considered separate protocols. For IPv6 operation, devices may alternatively use stateless address auto
configuration. IPv4 hosts may also use link-local addressing to achieve operation restricted to the local
network link.
DHCP is a collection of software that implements all aspects of the DHCP (Dynamic Host
Configuration Protocol) suite. It includes:
1. DHCP Server, which receives clients’ requests and replies to them.
2. DHCP Client, which can be bundled with the operating system of a client computer or other IP
capable device and which sends configuration requests to the server. Most devices and
operating systems already have DHCP clients included.
3. DHCP Relay Agent, which passes DHCP requests from one LAN to another so that there need not
be a DHCP server on every LAN.
The DHCP server, client and relay agent are provided both as reference implementations of the protocol
and as working, fully-featured sample implementations.
History?
In 1984, the Reverse Address Resolution Protocol (RARP), defined in RFC 903, was introduced to allow
simple devices such as diskless workstations to dynamically obtain a suitable IP address. However,
because it acted at the data link layer it made implementation difficult on many server platforms, and
also required that a server be present on each individual network link. Soon afterwards it was
superseded by the "Bootstrap Protocol" (BOOTP) defined in RFC 951. This introduced the concept of a
relay agent, which allowed the forwarding of BOOTP packets across networks, allowing one central
BOOTP server to serve hosts on many IP subnets.
DHCP is based on BOOTP but can dynamically allocate IP addresses from a pool and reclaim them when
they are no longer in use. It can also be used to deliver a wide range of extra configuration parameters
to IP clients, including platform-specific parameters. It was first defined in RFC 1531 in October 1993;
but due to errors in the editorial process was almost immediately reissued as RFC 1541.
Four years later the DHCPINFORM message type and other small changes were added by RFC 2131;
which as of 2014 remains the standard for IPv4 networks.
DHCPv6 was initially described by RFC 3315 in 2003, but this has been updated by many subsequent
RFCs. RFC 3633 added a DHCPv6 mechanism for prefix delegation, and stateless address auto
configuration was added by RFC 3736.

Why use DHCP?
Every device on a TCP/IP-based network must have a unique unicast IP address to access the network
and its resources. Without DHCP, IP addresses for new computers or computers that are moved from
one subnet to another must be configured manually; IP addresses for computers that are removed from
the network must be manually reclaimed.
With DHCP, this entire process is automated and managed centrally. The DHCP server maintains a pool
of IP addresses and leases an address to any DHCP-enabled client when it starts up on the network.
Because the IP addresses are dynamic (leased) rather than static (permanently assigned), addresses no
longer in use are automatically returned to the pool for reallocation.
How DHCP works?
Before understanding the process of IP address assignment, it is important to know some key technical
terms that are used in context of the DHCP server.
 DHCP Address Pool
DHCP address pool is a virtual container that contains all the IP addresses that have been configured in
the DHCP range to make available to the client computers. As soon as any IP address from the address
pool is assigned to a client computer, the address is temporarily removed from the pool.
 DHCP Lease
When the DHCP server assigns an IP address to a DHCP client computer, the address is assigned for
specific time duration. The time duration for which an IP address is assigned to a DHCP client computer
by the DHCP server is technically called the DHCP lease. When the DHCP lease expires, the IP address is
revoked from the DHCP client computer and is sent back to the DHCP address pool.
 IP Assign Operation (DORA)
The DHCP protocol employs a connectionless service model, using the User Datagram Protocol (UDP). It
is implemented with two UDP port numbers for its operations which are the same as for the BOOTP
protocol. UDP port number 67 is the destination port of a server, and UDP port number 68 is used by
the client.
DHCP operations fall into four phases: server discovery, IP lease offer, IP request, and IP lease
acknowledgment. These stages are often abbreviated as DORA for discovery, offer, request, and
acknowledgment.

Figure 2 DHCP DORA Process
Figure 3 DHCP DORA Packets Interchange

1. DHCP Discovery
D in the term DORA stands for the DHCP Discover packet. The DHCP Discover packet is broadcasted by
the DHCP client computer in order to find the available DHCP server(s) in the network. Since the DHCP
client computer sends the DHCP Discover packet as a broadcast, all the DHCP servers that are present in
the network receive the packet and respond accordingly.
Figure 4 DHCPDISCOVER Message

2. DHCP Offer
O in the term DORA stands for the DHCP Offer packet. The DHCP Offer is a unicast packet that is sent by
the DHCP server who receives the DHCP Discover packet from the DHCP client computer. The DHCP
Offer packet contains the available IP address that the DHCP server offers to the client computer.
The server determines the configuration based on the client's hardware address as specified in the
CHADDR (client hardware address) field. Here the server, 192.168.1.1, specifies the client's IP address in
the YIADDR (your IP address) field.
Figure 5 DHCPOFFER Message

3. DHCP Request
R in the term DORA stands for the DHCP Request packet. In response to the DHCP offer, the client
replies with a DHCP request, broadcast to the server, requesting the offered address. A client can
receive DHCP offers from multiple servers, but it will accept only one DHCP offer. Based on required
server identification option in the request and broadcast messaging, servers are informed whose offer
the client has accepted. When other DHCP servers receive this message, they withdraw any offers that
they might have made to the client and return the offered address to the pool of available addresses.
Figure 6 DHCPREQUEST Message

4. DHCP Acknowledgement
A in the term DORA stands for the DHCP Acknowledge packet. When the DHCP server receives the
DHCPREQUEST message from the client, the configuration process enters its final phase. The
acknowledgement phase involves sending a DHCPACK packet to the client. This packet includes the
lease duration and any other configuration information that the client might have requested. At this
point, the IP configuration process is completed.
Figure 7 DHCPACK Message

The protocol expects the DHCP client to configure its network interface with the negotiated parameters.
After the client obtains an IP address, it should probe the newly received address (e.g. with ARP Address
Resolution Protocol) to prevent address conflicts caused by overlapping address pools of DHCP servers.
 DHCP Relaying
In small networks, where only one IP subnet is being managed, DHCP clients communicate directly with
DHCP servers. However, DHCP servers can also provide IP addresses for multiple subnets. In this case, a
DHCP client that has not yet acquired an IP address cannot communicate directly with the DHCP server
using IP routing, because it does not have a routable IP address, nor does it know the IP address of a
router.
In order to allow DHCP clients on subnets not directly served by DHCP servers to communicate with
DHCP servers, DHCP relay agents can be installed on these subnets. The DHCP client broadcasts on the
local link; the relay agent receives the broadcast and transmits it to one or more DHCP servers using
unicast. The relay agent stores its own IP address in the GIADDR field of the DHCP packet. The DHCP
server uses the GIADDR to determine the subnet on which the relay agent received the broadcast, and
allocates an IP address on that subnet. When the DHCP server replies to the client, it sends the reply to
the GIADDR address, again using unicast. The relay agent then retransmits the response on the local
network.
Advantages of DHCP
Its capability to automatically allocate IP addresses to clients booting on the TCP/IP network for the first
time.
1. Using DHCP reduces the labour involved in managing the network.
2. Because the DHCP server automatically dispenses IP addresses and other configuration
information, the process of connecting a new computer to the network is much simpler.
3. DHCP is very flexible and allows the network administrator to set up the server one time to serve
many thousands of clients.
Disadvantages of DHCP
1. When client make query to DHCP server (DHCP Discover) it is UDP query it consume more
bandwidth. When DHCP server is unavailable client unable to access enterprises network.
2. Your machine name does not change when you get a new IP address.

3. Unauthorized DHCP servers providing false information to clients.
4. Unauthorized clients gaining access to resources.
5. Resource exhaustion attacks from malicious DHCP clients.
DHCP Security Issues?
Not only does DHCP run over IP and UDP, which are inherently insecure, the DHCP protocol itself have in
fact no security provisions whatsoever. This is a fairly serious issue in modern networks, because of the
sheer power of DHCP: the protocol deals with critical configuration information.
There are two different classes of potential security problems related to DHCP:
1. Unauthorized DHCP Servers: If a malicious person plants a “rogue” DHCP server, it is possible
that this device could respond to client requests and supply them with spurious configuration
information. This could be used to make clients unusable on the network, or worse, set them up
for further abuse later on. For example, a hacker could exploit a bogus DHCP server to direct a
DHCP client to use a router under the hacker's control, rather than the one the client is supposed
to use.
2. Unauthorized DHCP Clients: A client could be set up that masquerades as a legitimate DHCP
client and thereby obtain configuration information intended for that client; this could then be
used to compromise the network later on. Alternately, a “bad guy” could use software to
generate lots of bogus DHCP client requests to use up all the IP addresses in a DHCP server's
pool. More simply, this could be used by a thief to steal an IP address from an organization for
his own use.
Adding Security to DHCP
These are obviously serious concerns. The normal recommended solutions to these risks generally
involve providing security at lower layers. For example, one of the most important techniques for
preventing unauthorized servers and clients is careful control over physical access to the network: layer
one security. Security techniques implemented at layer two may also be of use, for example, in the case
of wireless LANs. Since DHCP runs over UDP and IP, one could use IPSec at layer three to provide
authentication.
DHCP Authentication
To try to address some of the more specific security concerns within DHCP itself, in June 2001 the IETF
published RFC 3118, Authentication for DHCP Messages. This standard describes an enhancement that
replaces the normal DHCP messages with authenticated ones. Clients and servers check the
authentication information and reject messages that come from invalid sources. The technology

involves the use of a new DHCP option type, the Authentication option, and operating changes to
several of the leasing processes to use this option.
Unfortunately, 2001 was pretty late in the DHCP game, and there are millions of DHCP clients and
servers around that don't support this new standard. Both client and server must be programmed to use
authentication for this method to have value. A DHCP server that supports authentication could use it
for clients that support the feature and skip it for those that do not. However, the fact that this option is
not universal means that it is not widely deployed, and most networks must rely on more conventional
security measures.

DHCP (dynamic host configuration protocol)

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