Week 3: DHCP
Week 3: 9/12/22
PreviousWeek 2: Subnetting, VLANS, & Cisco CommandsNextWeek 4 & 5: Campus and Enterprise Network Design Layers
Last updated
Week 3: 9/12/22
Last updated
The DHCP Server and Routers
Allows hosts to dynamically obtain its IP address
Above shows a DHCP server setup on a network which has 3 subnets
The router must be setup in a way that it knows the IP address of the DHCP server
DHCP must work across routers or through the intervention of BOOTP relay agents
DHCP DORA (Discover, Offer, Request, Accept)
DHCP Port Client 68 and Server 67
DHCP options field:
Subnet mask
Default Gateway
Lease Time
DNS Server
Most DHCP servers are configured to let a client reuse a previously allocated IP
This can reduce the amount of broadcast traffic
A client will broadcast a DHCPREQUEST message on its local subnet.
This message will contain a “Requested IP”
DHCP (Dynamic Host Configuration Protocol)
What is DHCP?
Statically: Manually entered by the administrator (Boo static is for nerds in data centers)
Dynamically: Automatically assigned by the network
Key information that we need:
IP address
Subnet mask
Default gateway
DNS (Domain Name Server)
How does DHCP Work?
Send out Broadcast (Discover)
“I need an IP” - New device
Dest IP: 255.255.255.255:68
Src IP: 0.0.0.0:67
Broadcast will go out any ports on the VLAN
DHCP Server sends out Offer (Offer)
DHCP over different VLANs:
Relay Agent (DHCP Relay, DHCP helper) Cisco calls it the IP helper
Router needs to know the IP address of DHCP Server
This is so it can unicast it and send over the information
DHCP server: 192.168.10.100 | Relay Agent: 192.168.10.100
Router would forwards as unicast to 192.168.10.100
Src of unicast: The Default gateway address for the VLAN
This is how the DHCP Server knows where to send the Offer
DHCP is like a person lost in new york
DHCP uses UDP
Server Port: 67
Client Port: 68
DHCP has two primary Operation Phases:
Initialization: Client request
Renewal: Client asks to renew its lease
Key Fields
Operation Code:
Hardware Type:
Hardware Length:
DORA
Discover: Client attempts to discover a DHCP server
If you spoof Discover you can take all the IP addresses
Offer: IP lease offer from the server to client
Request: Client requests to use the IP lease sent by the server
Acknowledgement: Server sends ack to client that the lease was accepted
Without this process you can have DHCP exhaustion attacks
DHCP snooping
Looks at the access port and if one port is asking for more than 1 IP it will shut it down
BOOTP: RFC 951
Only Discover and Offer
DHCP Renewal
T1 Renewing: Process for client to request continued use of its lease
This is 50% through the lease time.
Just sends to direct IP adress
Using unicast because it knows the IP address of the DHCP server
The client sends DHCP Request packets directly to the server
If the server responds with a DHCP Ack, the IP lease is renewed and its time clock restarts.
If the server doesn’t respond at T1 then it does to T2
T2 Rebinding: If the server doesn’t respond to the clients renewal requests we eventually reach the rebinding phase
This is at 87.5% through the lease time
Gets angry and sends a broadcast to look for another DHCP server
So it’s allowed to continue using the same IP
DHCP Expiration: If nothing responds by the time the lease is over
IP goes to a 169.125.4.0
Self-assign address
DHCP Relay:
Broadcast are Layer 2 only
Need a DHCP relay or Helper
Picks up broadcast and turns it to unicast and gets it to the DHCP Server
CISCO:
CISCO IOS uses the “ip helper-address”
If DHCP server is 10.16.1.50
(Config) interface vlan 100
(Config-IP) ip helper-address 10.16.1.50
