pfSense is my prefered router/firewall distro. It’s rock solid and has tons of features.
You can run pfSense on any PC, but if you want to keep the power and heat down to a minimum you can use an Alix board (http://www.pcengines.ch/alix.htm) APU2 board (https://www.pcengines.ch/apu2.htm) or similar. Alix boards are no long supported, but APU2 works fine.

One of the few things you can’t do in pfSense is modify your dhcpd.conf file directly.
For the vast majority of deployments, the built in control you have will be more than enough. It supports multiple vlans, and custom options. But…
If you need something special, like running a DHCP Server for several IP subnets over Layer 3 switches, then you need a different approach. Or maybe you just like to maintain a single file instead of using the GUI interface.

This is not a common requirement for a network, but it does happen.
I had a test-lab with a few Layer 3 switches and several subnets. I wanted to have an easy one-file configuration for the entire network using only the low-power pfSense router. So I had to figure out a way to do that.

There is no dhcpd.conf file you can edit in pfsense because it’s generated on each boot using the config.xml file (managed by all the settings in the GUI). The problem is that if you want any special settings in your dhcpd.conf file, it might not be supported by the config.xml markup and/or the GUI.

The solution is pretty simple. Just stop the DHCP daemon after boot, replace the dhcpd.conf file and restart the daemon.

DHCP daemon starts in a chrooted environment in /var/dhcpd, so all files are located with this folder as a parent.

If you don’t have a monitor on your router, you can enable SSH or use a serial console cable.
It’s also possible to do almost everything using the Diagnostics -> Edit File menu item in the GUI. To create a file, just enter the full path and click Save.

This solution requires two files.

1) 
The first one is a script, like this:

#/bin/bash
killall -3 dhcpd
cp /var/dhcpd/etc/dhcpd.override /var/dhcpd/etc/dhcpd.conf
/usr/local/sbin/dhcpd -user dhcpd -group _dhcp -chroot /var/dhcpd -cf /etc/dhcpd.conf -pf /var/run/dhcpd.pid le1 le0


Save it as: /usr/local/etc/rc.d/dhcpdoverride.sh

pfSense will run any script in this folder (/urs/local/etc/rc.d/) after it has started. The only requirements is the execute flag and that the file ends with .sh
So you’ll need to run this command as well:

chmod +x /usr/local/etc/rc.d/dhcpdoverride.sh


IMPORTANT NOTE!
The last command in this file may be different from system to system. Notice that one or more interfaces are being listed at the end. It’s easy to get the exact command of your system if you have done the initial configuration in GUI (included the interfaces you’ll be using).

Just run this command:

ps axww | grep dhcpd


The output should look something like this:

[2.1.3-RELEASE][root@pfSense.localdomain]/var(35): ps axww | grep dhcpd
38392  ??  Ss     0:00.04 /usr/local/sbin/dhcpd -user dhcpd -group _dhcp -chroot /var/dhcpd -cf /etc/dhcpd.conf -pf /var/run/dhcpd.pid le1 le0


I’m using two interfaces with DCHPD, so two interfaces are listed at the end.

2)
The second file is your own dhcpd.conf file.
If you have a file already then just save it as:
/var/dhcpd/etc/dhcpd.conf.override

If you don’t have a file already made, you can do the initial configuration in the GUI and just copy the file that is created. Then you have a good starting point.

After you have done your initial configuration in the GUI you can copy it like this:

cp /var/dhcpd/etc/dhcpd.conf /var/dhcpd/etc/dhcpd.conf.override


OR, using the GUI, like this

Go to Using Diagnostics -> Edit file:
open /var/dhcpd/etc/dhcpd.conf
then edit the filepath to /var/dhcpd/etc/dhcpd.conf.override and click Save

And that’s all!

Now you can edit dhcpd.conf.override with IP subnets, or anything you like.
An example of dhcpd.conf with multiple subnets:

option domain-name "localdomain";
default-lease-time 7200;
max-lease-time 86400;
log-facility local7;
ddns-update-style none;
authoritative;

#Subnet Alpha
subnet 10.50.150.0 netmask 255.255.255.0 {
  pool {
    range 10.50.150.100 10.50.150.200;
  }
  option routers 10.50.150.3;
  option domain-name-servers 10.50.150.3;
}

#Subnet Delta
subnet 10.18.200.0 netmask 255.255.255.0 {
  pool {
    range 10.18.200.100 10.18.200.200;
  }
  option routers 10.18.200.2;
  option domain-name-servers 10.50.150.3;# ex. a static IP address for a printer

  host HPPrinter { # example of a static IP address for a device
    hardware ethernet xx:xx:xx:xx:xx:xx; # replace the MAC address
    fixed-address 10.18.200.10;
    option host-name "HPLaserJet";
  }
}

NOTE!
You’ll need to replace the xx:xx:xx…  with the actual MAC address of your device.
And also;
If you are doing this to create subnets, you will also need to enable DHCP relay on your switch or routers between these subnets.

 

You can edit the override file from WebGui using the edit file menu.