Set up a DHCP Server for your LAN (2022-04-11)¶
Overview¶
In this assignment, we'll be standing up a DHCP service on the Raspberry Pi. As we've discussed in class, DHCP is a function that is sometimes performed by routers and is a necessary service whenever we are creating a new network (of almost any type).
Before you Start¶
Before starting any of the work within this checkpoint, make sure that you have completed all steps from checkpoint one, including the networkd setup.
You must also have completed the Address Planning Exercise correctly. You will use the parameters you selected in that exercise to configure the IP address for the Pi as well as the configuration for isc-dhcp-server.
Configure Static Addresses for your Raspberry Pi¶
In this step, you will add a new configuration file to networkd in order to set up static addresses on eth0. The addresses you use here will be the one you defined for your DHCP Server and Default Gateway in the LAN Planning Exercise.
Please do not try to follow other tutorials for setting up addresses
Every quarter, we have students who choose to follow online tutorials instead of the instructions we've provided. Please be aware that we are not using the default Raspberry Pi networking. These tutorials will often instruct you to configure static addresses in the /etc/dhcpcd.conf file or /etc/network/interfaces. Both of these methods were disabled when we set up networkd.
Create a file named 20-eth0.network in /etc/systemd/network/ (you'll need to use sudo) and define a static configuration that includes your chosen IP address and CIDR range. This path should already contain a default configuration for ethernet interfaces, e.g., 99-eth.network. We are overriding this configuration for eth0 by naming the file in such a way that networkd will load it first and by ensuring that the configuration will match eth0 exactly.
Your file should look something like:
[Match]
# We only want to match the eth0 interface
Name=eth0
# Provide full address config in abbreviated notation
[Network]
# Default Gateway / DHCP Server
Address=192.168.0.1/24
# Enable link local addresses for IPv6 (FE80::)
LinkLocalAddressing=ipv6
Warning
Some online references will have you add a gateway and DNS settings for your new interface. You don't need to (and definitely shouldn't) add either to this interface.
This interface does not provide a path for Internet traffic, but your Pi will try to use it that way if it believes it can be used for a default route.
Likewise, we don't need to give the Pi DNS settings on Ethernet. The Pi will get it’s DNS from the wireless interface, which received it’s DNS settings from DHCP.
In order to test these new settings:
Call sudo systemctl restart systemd-networkd.service or reboot your Pi.
Once you log back in, verify that networkd is running by calling systemctl status systemd-networkd and use the ip addr command to check that eth0 is online with the new address.
Attention
Make note that the Pi is now assigned to our new subnet, but our computer will still be using it's autoconfigured IPv4 address in the 169.254/16 range.
In an IPv4 only world, this would prevent us from talking to the Pi. To fix the problem, we'd have to manually configure a static IP on our laptop in the same range as the Pi's network ID.
Fortunately, both devices have autoconfigured IPv6 addresses that will be discovered through mDNS. By default, this is the address that SSH will use.
Install and Configure DHCP¶
For this part of the exercise, we'll use the ISC DHCP Server. The ISC server is available in apt repositories, so it can be installed using:
Warning
Right after installation, isc-dhcp-server will report that it failed to run, and spit out lots of nasty-looking errors. This is normal, as the DHCP server has not yet been configured.
Edit /etc/default/isc-dhcp-server to specify the interfaces (eth0 only) on which to run the DHCP server. Comment out any options pertaining to DHCP for IPv6.
The stock DHCP configuration is located at /etc/dhcp/dhcpd.conf. This file contains a number of example subnet declarations that you are meant to pick and choose from in order to configure the DHCP server and to distribute addresses within your predefined address range. For this assignment, our configuration is quite minimal:
Specifications
Before you start to configure your subnet, comment out the lines near the top of the file that specify global settings for nameservers and domain (we won’t use them right now).
- Create a new subnet configuration block using the network ID and network mask you established in the LAN Planning Exercise.
- Inside the subnet block, define a contiguous
rangeof IP addresses for the DHCP pool out of the address range selected in the LAN Planning Exercise. - You may browse the othe options available, but at this point you should not specify anything other than the DHCP lease range.
Why don't we configure the default gateway or name servers yet?
Students will frequently jump ahead and fill in additional network parameters. The reason we don't want to include these parameters yet is that our Pi is not yet ready to provide an Internet connection. Sending a default route back to your workstation will create a black hole of sorts for Internet traffic. Your computer will try to route the traffic through the Pi, and the Pi will simply drop everything it receives.
Test DHCP Server¶
Use systemctl to restart the DHCP service as shown here:
sudo systemctl restart isc-dhcp-server.service
If you see errors such as the one shown here, you’ll need to continue troubleshooting the DHCP server (see next section).
pi@titan:~ $ sudo systemctl restart isc-dhcp-server.service
Job for isc-dhcp-server.service failed because the control process exited with error code.
See "systemctl status isc-dhcp-server.service" and "journalctl -xe" for details.
Check your local machine to confirm that an IP address in the specified network range was provided.
Try running ping from both directions (laptop -> pi and pi -> laptop) to confirm that the IP address and related settings are configured correctly.
Warning
If you are on Windows, incoming ping requests (those from pi -> laptop) will not get any response. This is because by default Windows Firewall blocks these requests from being answered. To disable this, run a PowerShell prompt as Administrator and enter these commands:
Troubleshooting¶
Raspberry Pi OS provides us with several commands to troubleshoot and find errors related to services. Check status and recent log output using systemctl status isc-dhcp-server.service or journalctl -xe .
Search the system logs for relevant errors journalctl -u isc-dhcp-server
Here are some additional considerations:
- A misplaced space or bracket may cause DHCP to fail, so pay close attention to syntax.
- Your Pi will keep it's static address, so be sure that you excluded the address from the lease range in
dhcpd.conf. - Double check that you’ve configured the server defaults with the correct interface names and commented out IPv6 related settings.
- Make sure both
dhcpdandsystemdare declaring the same static IP for the Pi. - Inspect DHCP leases for clues. They can be found at
/var/lib/dhcp/dhcpd.leases.