#+TITLE: Setting up a DHCP server on OpenBSD
#+DATE: 2020-07-10T16:10:09-04:00
#+DRAFT: false
#+DESCRIPTION: Cofiguring dhcpd for a small home network on OpenBSD
#+TAGS[]: openbsd dhcp
#+KEYWORDS[]: openbsd dhcp
#+SLUG:
#+SUMMARY:

#+ATTR_HTML: :alt OpenBSD DHCP Server
#+ATTR_HTML: :title OpenBSD DHCP Server
[[file:cover.png]]

[[https://www.openbsd.org/faq/faq4.html#Download][OpenBSD]] makes a great router. It's simplicity and ease of
configuration makes it perfect for network infrastructure
applications. Everything you need to build a network
of any size is built into the base system, plus its man pages and
examples cover everything you'd need to know.

While I've been an OpenBSD user for years, I'm finally in the process
of replacing the router provided by my ISP with a [[{{< ref "pcengines-comparison" >}}][PC Engines]] APU2E4
[[{{< ref "installing-openbsd-on-pcengines" >}}][running OpenBSD]].

One of the key services that I have to setup for this is the [[https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol][DHCP]]
server, which is responsible for giving out IP addresses to machines
when they join the network.

This task is incredibly easy on OpenBSD, as it comes with =dhcpd= on
the base system. The OpenBSD project provides excellent documentation
for this system under [[http://man.openbsd.org/dhcpd][=dhcpd(8)=]] for the DHCP server itself,
[[http://man.openbsd.org/dhcpd.conf.5][=dhcpd.conf(5)=]] for the configuration file, and [[http://man.openbsd.org/dhcpd.leases.5][=dhcpd.leases(5)=]] for
the lease database format.

The example we'll be going over in this post will be extremely simple,
as it's only for a small home network without any advanced options. I
encourage you to read the man pages if you're ever setting up your
own, as I might not cover what you need to know here.

For my case, I'll be handing out IP addresses in the =192.168.0.0/24=
subnet, with addresses =192.168.0.30= through =192.168.0.200= assigned
with DHCP. The gateway router will be at the address =192.168.0.1=.

I will assign the devices on the network the =home.local= domain. I'll
use =192.168.0.23= as the DNS server, as I have a caching DNS server
which filters out advertisers setup at that address, with =8.8.8.8= as
the secondary DNS server in case my DNS server ever goes down.

I'll also set the default lease time to one week, and the maximum
lease time to one month.

To jumpstart the configuration, I'll first copy the file from
=/etc/examples/dhcpd.conf= to =/etc/dhcpd.conf= so I don't have to
start from scratch.

#+BEGIN_SRC
doas cp /etc/examples/dhcpd.conf /etc/
#+END_SRC

Then I'll edit the file with =mg=, an emacs-like editor that comes
with OpenBSD.

#+BEGIN_SRC
doas mg /etc/dhcpd.conf
#+END_SRC

Here's the finished configuration file.

#+BEGIN_SRC
# Network:          192.168.0.0/255.255.255.0
# Domain name:      home.local
# Name servers:     192.168.0.23 and 8.8.8.8
# Default router:   192.168.0.1
# Addresses:        192.168.0.30 - 192.168.0.200
#
option  domain-name "home.local";
option  domain-name-servers 192.168.0.23, 8.8.8.8;

subnet 192.168.0.0 netmask 255.255.255.0 {
    option routers 192.168.0.1;

    range 192.168.0.30 192.168.0.200;
    default-lease-time 604800;
    max-lease-time 2592000;

    host example-static-client {
            hardware ethernet 22:33:44:55:66:77;
            fixed-address 192.168.0.201;
    }
}
#+END_SRC

Let's go through this line-by-line to get a better understanding of
what's going on here.

The comments at the top are just to help understand the file at a
glance, and don't serve any practical function.

An =option= can be specified globally or per-subnet, and can also be
specified per-client for [[https://en.wikipedia.org/wiki/Bootstrap_Protocol][BOOTP]] clients. Here I set the domain name
and DNS servers globally.

#+BEGIN_SRC
option domain-name "home.local"
option domain-name-servers 192.168.0.23, 8.8.8.8;
#+END_SRC

Then I declare the configuration for my subnet.

#+BEGIN_SRC
subnet 192.168.0.0 netmask 255.255.255.0 {

}
#+END_SRC

The subnet-specific configuration goes inside the braces. You may
specify as many subnets as you need in the configuration file,
although for a home network like this I only need one.

Inside of the subnet configuration block, I set the gateway router and
the range of IPs I'll be handing out with DHCP.

#+BEGIN_SRC
option routers 192.168.0.1;

range 192.168.0.30 192.168.0.200;
#+END_SRC

I also specify the default lease time and max lease time. The default
lease time is the number of seconds the DHCP server will hold a lease
for a client if the client doesn't ask for a specific lease length,
and the max lease length is the maximum allowed lease length. Here I
set the default to 604800 (one week), and the max to 2592000 (one
month).

#+BEGIN_SRC
default-lease-time 604800;
max-lease-time 2592000;
#+END_SRC

Finally I have an example static client configuration. Here the client
with the MAC address =22:33:44:55:66:77= will all ways be given the IP
=192.168.0.201=. This is useful when you want to assign a specific IP
to a certain client without having to manually configure the IP.

#+BEGIN_SRC
host example-static-client {
        hardware ethernet 22:33:44:55:66:77;
        fixed-address 192.168.0.201;
}
#+END_SRC

To run the DHCP server, I first enable, then start it using =rcctl=.

#+BEGIN_SRC
doas rcctl enable dhcpd
doas rcctl start dhcpd
#+END_SRC

To see the leases the =dhcpd= is currently holding, you can check
=/var/db/dhcpd.leases=.

If you followed along these steps, you not have a running DHCP sever!