Dynamic Host Configuration Protocol

gives leased IP addresses to endpoint devices (like phones or computers)

used to dynamically (automatic) assign IP addresses to host (Also includes additional network configuration)

DHCP server – run a service to assign and track IP addresses

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DHCP Initialization Operations fall into four phases:

o Discover: Client attempts to discover a DHCP server

o Offer: IP lease offer from server to client

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

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

These stages are often abbreviated as DORA for discovery, offer, request, and acknowledgement

With just DO, an attacker could spoof lot’s of DHCP requests

o Without a confirmation process, the server would assign all the available IP’s

o That means legitimate clients can’t get IP’s – aka Denial-of- Service!

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DHCP Leases

DHCP assignments are only valid for certain period of time – called the “lease time”

Clients must “renew” their IP address with the DHCP server if they want to keep using it

Prevents systems from tying up addresses if they are no longer on the network

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how long should DHCP Leases Last?

• How frequenlty do clients join and leave subnet?

o Wireless in the Dining Hall vs.

o Lab machines in Foster or Skiff

• Highly static subnets could have lease times in hours or days

o Like our lab systems

o Reduces broadcast traffic and potential address changes on clients

• Highly dynamic subnets could have lease times in minutes

o Like Dining Hall Wireless

o Prevents server continuing to hold leases long after client has left the network