22 Dynamic Host Configuration Protocol – MikroTik DHCP Relay

Typically, larger networks are segmented into smaller LANs. However, one concern that can arise from this is the issue of distributing IP addresses across individual subnets. This is because DHCP discover packets are designed to be broadcasted within local networks. While a network administrator could configure a dedicated DHCP server for each LAN, we demonstrated in Static Networking Part 1 that this can quickly become tedious to configure and maintain. Luckily, DHCP relays can be configured to re-transmit IP requests to remote servers. This way, one server can lease addresses to multiple networks at once. In this chapter, we will configure a DHCP relay with a MikroTik router to service two networks.

Estimated time for completion: 60 minutes

Learning Objectives

• Create three functional LANs
  • RED – PCs
  • BLUE – PCs
  • GRAY – DHCP Server
• Configure a router to serve as a functional DHCP relay
• Configure a DHCP server successfully

Prerequisites

• Chapter 16 – Introduction to Routers
• Chapter 17 – IPv4 Addressing
• Chapter 18 – DHCP using Linux

Deliverables

• Screenshot of GNS Workspace with labels
• Screenshot of Wireshark DHCP
  • LAN1 end devices requesting and receiving IP addresses
  • LAN2 end devices requesting and receiving IP addresses

Resources

• MikroTik RouterOS Documentation – “DHCP Relay” – https://wiki.mikrotik.com/wiki/Manual:IP/DHCP_Relay

Contributors and Testers

• Dante A. Rocca, Cybersecurity Student, ERAU-Prescott

Phase I -Build the Network Topology

The following steps are to create a baseline environment for completing the lab.  It makes assumptions about learner knowledge from completing previous labs.

By the end of this lab your network should look like the following:

1. Start GNS3
   1. Create a new project: LAB_08
2. Build a small network will the following specifications:
   1. Subnet – Red
      1. One switch – Ethernet switch
      2. Two client machines – VPCS
      3. Minimize wasted address space for 250 hosts
         

         Network Information
         Network	100.10.10.0
         Netmask	255.255.255.0 (/24)
         Broadcast	100.10.10.255
         Gateway	100.10.10.1
         DHCP Lower Bound	100.10.10.2
         DHCP Upper Bound	100.10.10.254

   2. Subnet – Blue
      1. One switch – Ethernet switch
      2. Two client machines – VPCS
      3. Minimize wasted address space for 100 hosts
         

         Network Information
         Network	200.20.20.0
         Netmask	255.255.255.128 (/25)
         Broadcast	200.20.20.127
         Gateway	200.20.20.1
         DHCP Lower Bound	200.20.20.2
         DHCP Upper Bound	200.20.20.126

   3. Subnet – Gray
      1. One DHCP server – Ubuntu Server / Windows Server / Tiny Core / MikroTik CHR

         NOTE: This example will use 150.30.30.5 as the server’s static IP address.

      2. Minimize wasted address space for 10 hosts
         

         Network Information
         Network	150.30.30.0
         Netmask	255.255.255.240 (/28)
         Broadcast	150.30.30.15
         Gateway	150.30.30.1

3. Add a MikroTik router to the workspace
   1. Connect all three networks to the router
   2. Configure the router with static IP addresses on all active interfaces (Chapter 16, Phase II, Step 4)

      NOTE: This example uses the following router ports:
      – ether1 -> Red LAN
      – ether2 -> Blue LAN
      – ether3 -> Gray LAN
      

4. Label and organize your network as necessary
   

   

1. Setup the relay
   1. Open the MikroTik console
   2. Configure the Red subnet’s relay path

      > ip dhcp-relay add name=Red-Relay interface=ether1 dhcp-server=150.30.30.5 local-address=100.10.10.1 disabled=no

      Command	Description
      ip dhcp-relay	Access the DHCP relay menu.
      add name=Red-Relay	Name this group of configuration settings “Red-Relay”.
      interface=ether1	Assign the ether1 interface to listen for DHCP requests.
      dhcp-server=150.30.30.5	DHCP request packets will forwarded to 150.30.30.5.
      local-address=100.10.10.1	DHCP response packets will arrive with the address 100.10.10.1.
      disabled=no	The default setting is to disable configurations unless otherwise specified.

   3. Configure the Blue subnet’s relay path

      > ip dhcp-relay add name=Blue-Relay interface=ether2 dhcp-server=150.30.30.5 local-address=200.20.20.1 disabled=no

      NOTE: Don’t forget to disable the DHCP client that is listening by default on ether1!

      > ip dhcp-client remove 0

2. Verify that the router’s settings are configured properly (Figure 3)

   > ip address print

   > ip dhcp-relay print

1. Start the Ubuntu Server VM
2. Configure the server to have a static host address (Figure 4)

   > sudo vi /etc/netplan/00-installer-config.yaml

   > sudo netplan apply

   NOTE: Ensure that the server’s IP is NOT the same address as its gateway and is WITHIN the range of available hosts for your subnet! Also, notice that the configuration provided includes the addition of a default gateway. This is important for the server to know how to respond to DHCP request packets. Check if you have a default route set in your server:

   > ip route

   

   The above image illustrates that the enp0s3 interface is assigned the address 150.30.30.5 on the 150.30.30.0/24 network with a gateway address pointing towards 150.30.30.1.

3. Modify the DHCP daemon configuration file to support all three subnets (Figure 6)

   > sudo vi /etc/dhcp/dhcpd.conf

   NOTE: Declare the Gray subnet. Although there are no clients to service here, it is good practice to help the server understand the layout of our network.

4. Ensure that the daemon is enabled and active (Figure 7)

1. Start capturing packets on the PC1-Relay connection
   1. In PC1’s terminal, request a new IP address

      NOTE: If using a Tiny Core VM, it should automatically request an IP address at startup.

   2. Look for the DHCP broadcast packets on Wireshark (you should see the same packets from when you completed the DHCP labs)
   3. You can check the VPCS’s assigned IP address in its console

      > show ip

   4. If you want to see more traffic, just add more end devices or renew current leases
2. Repeat for the Blue clients
3. Ensure that all three subnets can ping each other and have full connectivity

End of Lab

List of Figures for Print Copy