Contents

1 Funci’on

• DHCP makes the process of assigning new IP addresses almost transparent. DHCP assigns IP addresses and other important network configuration information dynamically. Because desktop clients typically make up the bulk of network nodes, DHCP is an extremely useful and timesaving tool for network administrators. RFC 2131 describes DHCP.
• RFC 2131.
• Protocolo cliente-servidor, UDP, puerto 67. DHCP uses User Datagram Protocol (UDP) as its transport protocol. The client sends messages to the server on port 67. The server sends messages to the client on port 68.
• Los clientes lo utilizan para obtener de forma autom’atica los par’ametros de conexi’on (dir IP, m’ascara, gateway y DNS) de la red.
• Especialmente utilizado en el caso de hosts m’oviles.
• T’ipicamente se utiliza para asignar un pool de $X$ dirs IP a $Y$ hosts de forma din’amica, donde generalmente $X\le Y$.
• Cuando un host arranca se pone en contacto con un servidor DHCP preguntando a la dir de broadcast¹ (255.255.255.255) y usando la dir IP origen 0.0.0.0.
• Las configuraciones pueden asignarse de forma temporal o de forma indefinida.

2 Operation

DHCP includes three different address allocation mechanisms to provide flexibility when assigning IP addresses:

1. Manual Allocation: The administrator assigns a pre-allocated IP address to the client and DHCP only communicates the IP address to the device.
2. Automatic Allocation: DHCP automatically assigns a static IP address permanently to a device, selecting it from a pool of available addresses. There is no lease and the address is permanently assigned to a device.
3. Dynamic Allocation: DHCP automatically dynamically assigns, or leases, an IP address from a pool of addresses for a limited period of time chosen by the server, or until the client tells the DHCP server that it no longer needs the address.

DHCP works in a client/server mode and operates like any other client/server relationship. When a PC connects to a DHCP server, the server assigns or leases an IP address to that PC. The PC connects to the network with that leased IP address until the lease expires. The host must contact the DHCP server periodically to extend the lease. This lease mechanism ensures that hosts that move or power off do not hold onto addresses that they do not need. The DHCP server returns these addresses to the address pool and reallocates them as necessary.

3 Clientes, servidores y agentes de retransmisi’on

• El DHCP es un protocolo cliente-servidor. Un cliente es una computadora que desea configurar su IP y un servidor es otra que sabe con qu’e par’ametros hacerlo.
• Aunque es raro, en una misma red pueden existir varios servidores DHCP. En principio todos contestar’ian y es el cliente quien elige.
• Aunque es raro, puede que en una red no exista un servidor DHCP y sin embargo, el servicio se est’e prestando. En este caso, un router que funciona como relay agent conoce la dir IP de un servidor externo y permite que los clientes accedan a ’el. In a complex hierarchical network, enterprise servers are usually contained in a server farm. These servers may provide DHCP, DNS, TFTP, and FTP services for the clients. The problem is that the network clients typically are not on the same subnet as those servers. Therefore, the clients must locate the servers to receive services and often these services are located using broadcast messages.
• El proceso de consesi’on se lleva a cabo, siempre, bajo demanda del cliente.

4 Configuraciones est’aticas y din’amicas

• Las configuraciones del IP son est’aticas cuando los mismos hosts (en funci’on de su direcci’on f’isica, por ejemplo) reciben siempre las mismas direcciones IP. En caso contrario, son di’amicas.
• Al rango de direcciones IP est’aticas se le suele llamar intervalo de exclusi’on y al conjunto de direcciones IP din’amicas, ’ambito. La uni’on de ambos conjuntos forman el conjunto de direcciones IP disponibles.
• Clients lease the information from the server for an administratively defined period. Administrators configure DHCP servers to set the leases to time out at different intervals. Most ISPs and large networks use default lease durations of up to three days. When the lease expires, the client must ask for another address, although the client is typically reassigned the same address.
• The DHCPREQUEST message also addresses the dynamic DHCP process. The IP information sent in the DHCPOFFER might have been offered to another client during the dynamic allocation. Each DHCP server creates pools of IP addresses and associated parameters. Pools are dedicated to individual, logical IP subnets. The pools allow multiple DHCP servers to respond and IP clients to be mobile. If multiple servers respond, a client can choose only one of the offers.

5 El proceso de concesi’on

• Se ejecuta en 4 pasos (v’ease la Figura

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  Figure 1: Proceso de concesi’on utilizando el DHCP [1]. La notaci’on es la siguiente: “src”: dirección IP y puerto origen del paquete, “dest”: dirección IP y puerto destino el paquete, “yiaddr”: Your Internet ADDRess y “ID”: IDentification.

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  1):

  1. Solicitud (DHCP DISCOVER packet).
  2. Oferta (DHCP OFFER packet).
  3. Petici’on (DHCP REQUEST packet).
  4. Reconocimiento (DHCP ACK) / No reconocimiento (DHCP NAK packet).

6 La solucitud (DHCP Discover)

• Broadcast message.
• In the first step, the client broadcasts a DHCPDISCOVER message. The DHCPDISCOVER message finds DHCP servers on the network. Because the host has no valid IP information at bootup, it uses L2 and L3 broadcast addresses to communicate with the server. If a client is configured to receive its IP settings dynamically, it transmits a DHCPDISCOVER message on its local physical subnet when it boots or senses an active network connection. Because the client has no way of knowing the subnet to which it belongs, the DHCPDISCOVER is an IP broadcast (destination IP address of 255.255.255.255). The client does not have a configured IP address, so the source IP address of 0.0.0.0 is used. The client IP address (CIADDR), default gateway address (GIADDR), and subnetwork mask are all marked with question marks.
• Cuando un host es encendido y no tiene configurado el IP a mano, o desea renovar su configuraci’on, emite un paquete UDP dirigido a la direcci’on de broadcast de su red, puerto 67 (servicio bootps² ). Dicho paquete contiene un mensaje DHCP Discover. Este mensaje llegar’a a todos los adaptadores de red de la subred, y entre ellos al del servidor DHCP o del relay agent.
• El cliente esperar’a un tiempo (determinado por ’el) a la contestaci’on del servidor.

7 La oferta (DHCP Offer)

• Unicast message.
• When the DHCP server receives a DHCDISCOVER message, it finds an available IP address to lease, creates an ARP entry consisting of the MAC address of the requesting host and the leased IP address, and transmits a binding offer with a DHCPOFFER message. The DHCPOFFER message is sent as a unicast, using the L2 MAC address of the server as the source address and the L2 address of the client as the destination.³
• This message contains initial configuration information for the client, including the MAC address of the client, followed by the IP address that the server is offering, the subnet mask, the lease duration, and the IP address of the DHCP server making the offer. The subnet mask and default gateway are specified in the options field, subnet mask, and router options, respectively. The DHCPOFFER message can be configured to include other information, such as the lease renewal time, domain name server, and NetBIOS Name Service (Microsoft Windows Internet Name Service [Microsoft WINS]).
• The server determines the configuration, based on the hardware address of the client as specified in the CHADDR field.
• Administrators set up DHCP servers to assign addresses from predefined pools. Most DHCP servers also allow the administrator to define specifically which client MAC addresses can be serviced and automatically assign them the same IP address each time.
• Uno o m’as servidores DHCP contestan (en un paquete UDP) al cliente con un mensaje del tipo DHCP Offer. Este contiene una direcci’on IP, una m’ascara de red, la direcci’on del gateway, la/s direcci’on/es IP con el/los servidores DNS y el tiempo de pr’estamo (lease time).
• Como el cliente no tiene todav’ia una direcci’on IP v’alida, el servidor env’ia la oferta de nuevo a la direcci’on de broadcast.

8 La petici’on (DHCP Request)

• Broadcast message.
• When the client receives the DHCPOFFER from the server, it sends back a DHCPREQUEST message. This message has two purposes: lease origination and lease renewal and verification. When used for lease origination, the DHCPREQUEST of the client is requesting that the IP information be verified just after it has been assigned. The message provides error checking to ensure that the assignment is still valid. The DHCPREQUEST also serves as a binding acceptance notice to the selected server and an implicit decline to any other servers that may have provided the host a binding offer. ⁴
• El cliente seleciona una de las ofertas y env’ia un paquete UDP a la direcci’on de broadcast de la subred con el mensaje DHCP Request. Dicho mensaje incluye los datos de la oferta seleccionada.
• Los servidores DHCP van a recibir este mensaje y el que realiz’o la oferta va a emitir, de nuevo a la direcci’on de broadcast un paquete UDP con el mensaje DHCP Ack (Acknowledgment).

9 El reconocimiento (DHCP Ack)

• Unicast message.
• On receiving the DHCPREQUEST message, the server verifies the lease information, creates a new ARP entry for the client lease, and replies with a unicast DHCPACK message. The DHCPACK message is a duplicate of the DHCPOFFER, except for a change in the message type field. When the client receives the DHCPACK message, it logs the configuration information and performs an ARP lookup for the assigned address. If it does not receive a reply, it knows that the IP address is valid and starts using it as its own.
• Cuando el cliente recibe el DHCP Ack, aplica la configuraci’on recibida.
• Si el servidor finalmente negara al cliente el uso de la configuraci’on ofertada⁵ , en lugar de transmitirse un DHCP Ack se transmitir’ia un DHCP NAck (Negative Ack) y todo el proceso comienza de nuevo.

10 Atacks

• Man-in-the-middle: Because DHCP Discover messages are broadcasted, a attacker can gain access to network traffic spoofing responses that would be sent by a valid DHCP server. If the spoofing device replies faster than the legimate server, the attaker can configure the IP of the DPCH client sending its IP address as the Gateway in the DHCP Offer. After that, all the trafic that the new host sent to the Internet can be seen by the attacker.

  To fight againts this attack, some switches can be configured to receibe DHCP Offers throught a pre-defined port (in which the real DHCP server is attached).

11 BOOTP (Bootstrap Protocol)

The Bootstrap Protocol (BOOTP), defined in RFC 951, is the predecessor of DHCP and shares some operational characteristics. BOOTP is a way to download address and boot configurations for diskless workstations. A diskless workstation does not have a hard drive or an operating system. For example, many automated cash register systems at your local super market are examples of diskless workstations. Both DHCP and BOOTP are client/server based and use UDP ports 67 and 68. Those ports are still known as BOOTP ports.

DHCP and BOOTP have two components. The server is a host with a static IP address that allocates, distributes, and manages IP and configuration data assignments. Each allocation (IP and configuration data) is stored on the server in a data set called a binding. The client is any device using DHCP as a method for obtaining IP addressing or supporting configuration information.

There are three primary differences between DHCP and BOOTP:

1. The main difference is that BOOTP was designed for manual pre-configuration of the host information in a server database, while DHCP allows for dynamic allocation of network addresses and configurations to newly attached hosts. When a BOOTP client requests an IP address, the BOOTP server searches a predefined table for an entry that matches the MAC address for the client. If an entry exists, the corresponding IP address for that entry is returned to the client. This means that the binding between the MAC address and the IP address must have already been configured in the BOOTP server.
2. DHCP allows for recovery and reallocation of network addresses through a leasing mechanism. Specifically, DHCP defines mechanisms through which clients can be assigned an IP address for a finite lease period. This lease period allows for reassignment of the IP address to another client later, or for the client to get another assignment if the client moves to another subnet. Clients may also renew leases and keep the same IP address. BOOTP does not use leases. Its clients have reserved IP address which cannot be assigned to any other host.
3. BOOTP provides a limited amount of information to a host. DHCP provides additional IP configuration parameters, such as WINS and domain name.

12 DHCP message format

• Operation Code (OP) - Specifies the general type of message. A value of 1 indicates a request message; a value of 2 is a reply message.
• Hardware Type - Identifies the type of hardware used in the network. For example, 1 is Ethernet, 15 is Frame Relay, and 20 is a serial line. These are the same codes used in ARP messages.
• Hardware Address length - 8 bits to specify the length of the address. Hops - Set to 0 by a client before transmitting a request and used by relay agents to control the forwarding of DHCP messages.
• Transaction Identifier - 32-bit identification generated by the client to allow it to match up the request with replies received from DHCP servers. Seconds - Number of seconds elapsed since a client began attempting to acquire or renew a lease. Busy DHCP servers use this number to prioritize replies when multiple client requests are outstanding.
• Flags - Only one of the 16 bits is used, which is the broadcast flag. A client that does not know its IP address when it sends a request, sets the flag to 1. This value tells the DHCP server or relay agent receiving the request that it should send the reply back as a broadcast.
• Client IP Address - The client puts its own IP address in this field if and only if it has a valid IP address while in the bound state; otherwise, it sets the field to 0. The client can only use this field when its address is actually valid and usable, not during the process of acquiring an address.
• Your IP Address - IP address that the server assigns to the client. Server IP Address - Address of the server that the client should use for the next step in the bootstrap process, which may or may not be the server sending this reply. The sending server always includes its own IP address in a special field called the Server Identifier DHCP option.
• Gateway IP Address - Routes DHCP messages when DHCP relay agents are involved. The gateway address facilitates communications of DHCP requests and replies between the client and a server that are on different subnets or networks. Client Hardware Address - Specifies the Physical layer of the client.
• Server Name - The server sending a DHCPOFFER or DHCPACK message may optionally put its name in this field. This can be a simple text nickname or a DNS domain name, such as dhcpserver.netacad.net.
• Boot Filename - Optionally used by a client to request a particular type of boot file in a DHCPDISCOVER message. Used by a server in a DHCPOFFER to fully specify a boot file directory and filename.
• Options - Holds DHCP options, including several parameters required for basic DHCP operation. This field is variable in length. Both client and server may use this field.

References