Dynamic Host Configuration Protocol (DHCP)

What is DHCP?

How does a computer receive an IP address?

Statically:

• Manually entered by the administrator

Dynamically:

• Automatically assigned by the network

DHCP:

• is used to automatically assign an IP address to a host
• May include a lot more network information too

Key information that we need:

• IP Address
• Subnet Mask
• Default Gateway/Router
• DNS Server Address

How does DHCP Work

The client (computer) has no idea what network it's even on

So it sends out a broadcast asking for help

If a DHCP server hears this broadcast, it will respond with an IP address that the client can use.

The DCHP employs a connectionless service model of UDP

It is implemented with two known UDP port numbers:

• Server port: 67
• Client port: 68

DHCP has two primary Operation Phases:

• Initialization: Client requests, reviews, receives, and confirms an IP address
• Renewal: Client asks to renew its "lease" of the IP address

DHCP Header

Key Fields:

• Operation Code: Indicates if this is a request or a reply

• Hardware Type: Type of HW address

• Hardware Length: Length of HW address

• Transaction ID: Random number used to pair requests and replies (since UDP is connectionless)

• Client IP Address: Eventual client IP

• Your IP Address: Offered client IP

• Server IP Address: IP of DHCP server

• Gateway IP Address: Default gateway IP

• Client Hardware Address: Client's MAC

DHCP Initialization

DORA: Four-step process

1. Discover: Client attempts to discover a DHCP server

2. Offer: IP lease offer from server to client

3. Request: Client requests to use the IP lease sent by the server

4. Acknowledgement: Server sends acknowledgement to client that the lease was accepted.

DHCP Renewal

DHCP Renewal: Process for client to request continued use(renewal) of its lease.

• By default, this begins 50% of the way through the current IP lease.
• The client sends DHCP Request packets directly to the server.
• If the server respond with a DHCP Acknowledgement, the IP lease is renewed and its time clock restarts.
• The amount of time is up to you

DHCP Rebinding: If the server does not respond to the client's renewal requests, we eventually reach the rebinding phase.

• By default, this begins 87.5% of the way through the current IP lease.
• The client begins sending its DHCP Request packets as broadcasts to see if any DHCP server will allow them to continue using their IP.
• If a server responds, the lease is renewed and the timer restarts.

DHCP Expiration: If no server responds before the lease ends, the lease expires and the IP is released.

• All TCP/IP communication stops.
• The client must go through the DORA initialization process once again to get a new IP address.

DHCP Relay

Remember, unconfigured clients have no IP configuration.

• They know nothing about the subnet, gateway, etc.
• All they can do is a broadcast into the wild

But broadcasts are Layer 2 only…what happens if a network doesn't have a local DHCP server?

• Layer 3 devices (routers, servers) can be configured as DHCP relays (aka helpers)
• Pick up broadcasts and forward to the DHCP server

DHCP

DHCP Relay/Helper on Cisco

Cisco IOS uses the “ip helper-address”

Can be assigned to a physical or VLAN interface

If configured, grabs DHCP broadcasts seen on that interface and sends to DHCP server

E.g. If DHCP server is 10.16.1.50

(config) interface vlan 100

(config-if) ip helper-address 10.16.1.50

Sends any DHCP broadcasts on VLAN 100 to the server

VLAN Interfaces

Routers (or Multi-Layer Switches acting as routers) act as the Default Gateway for VLANs

Physical Ports on Router assigned to VLAN (or trunked)

• E.g GigEthernet 0/1 may be trunked for VLAN 2,3, and 10

”Virtual” Interface is configured to assign IP address

• e.g. interface vlan2 ip address 10.1.2.1 /24

The Virtual Interface can act as the Default Gateway for that subnet/vlan