Week 3: DHCP

Week 3: 9/12/22

Homework - Reading:

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

DORA

DORA

• 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”

Class:

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?

1. 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

2. 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