Setting up DHCPD on MacOS

One of the frustrations with my current network setup is that the router is very inflexible. For example, it would not allow me to set the Domain Name servers that I wanted to use for my internal devices. It would only deliver the DNS servers that my internet provider delivered to it. And who wants that?

Well, since I have set up many DHCP servers in the past, I did not think this would be a problem. I have an older Mac that I could use, as it stays plugged into the network and is already operating as a Apple iCloud Cache server.

I also wanted to be able to deliver IPv6 DNS servers to my machines as well.

Well, it was not quite as easy as I thought it was going to be. I had to learn how to use launchctl and launchd. I come from a background of using init and systemd so I assumed this would be fairly easy.

Here are the steps that I found to work on a machine running 10.15. I set up IPv4 first.

Step 1

Install the DHCP server via brew.

~ ☯ brew install isc-dhcp

Step 2

Set up the IPv4 plist and config files.

The config file was pretty simple:

/usr/local/etc ☯ cat dhcpd.conf 
# dhcpd.conf
#

default-lease-time 600;
max-lease-time 7200;

# Use this to enble / disable dynamic dns updates globally.
#ddns-update-style none;

# If this DHCP server is the official DHCP server for the local
# network, the authoritative directive should be uncommented.
authoritative;

# Use this to send dhcp log messages to a different log file (you also
# have to hack syslog.conf to complete the redirection).
#log-facility local7;

# My First Subnet
subnet 192.168.1.0 netmask 255.255.255.0 {
  range 192.168.1.10 192.168.1.200;
  option domain-name-servers 1.1.1.1, 1.0.0.1;
  option routers 192.168.1.1;
  option broadcast-address 192.168.1.255;

  # Let's let folks keep their IP's for a while
  default-lease-time 6000;
  max-lease-time 72000;
}

Next up was getting it installed as a service. The brew stuff was unhelpful, and, honestly I wanted to play with the underlying plist files anyways.

After some gnashing of teeth and frantic googling, I came across a helpful article by Steven Diver that outlined the syntax of the org-isc-dhcpd.plist file.

Anyway, this is what I came up with:

/Library/LaunchDaemons ☯ cat org.isc.dhcpd-6.plist 
<?xml version=\"1.0\" encoding=\"UTF-8\"?>
<!DOCTYPE plist PUBLIC \"-//Apple Computer//DTD PLIST 1.0//EN\" \"http://www.apple.com/DTDs/PropertyList-1.0.dtd\">
<plist version=\"1.0\">
<dict>
     <key>Label</key>
          <string>org.isc.dhcpd</string>
     <key>OnDemand</key>
          <false/>
     <key>ProgramArguments</key>
          <array>
          <string>/usr/local/sbin/dhcpd</string>
          <string>-q</string>
          <string>-f</string>
          </array>
     <key>RunAtLoad</key>
          <true/>
     <key>ServiceDescription</key>
          <string>ISC DHCP Server</string>
</dict>
</plist>

Next I figured out that I had to load the plist, and then it would hopefully start up.

~ ☯ sudo launchctl load /Library/LaunchDaemons/org.isc.dhcpd.plist

I could then use sudo launchctl list |grep dhcp to see that the service was in fact started.

~ ☯ sudo launchctl list |grep dhcp
47891   -15 org.isc.dhcpd

I also rebooted the machine to insure that it would survive a reboot - no one wants their DHCP server vanishing, right?

Step 3

Next up was setting up IPv6.

I did a bit of reading, and figured out that you would have to have 2 DHCPD services running, one for IPv4 and one for IPv6. I also learned a bit about IPv6 and SLAAC. It seemed that I could keep using my ISP’s SLAAC to hand out proper addresses, and use DHCP to deliver additional information to the hosts, like a proper DNS server’s address.

This is the config file that I came up with:

/usr/local/etc ☯ cat dhcpd6.conf 
# dhcpd6.conf

default-lease-time 600;
max-lease-time 7200;

# Use this to send dhcp log messages to a different log file (you also
# have to hack syslog.conf to complete the redirection).
# log-facility local7;

# Subnet declaration

subnet6 2601:601:cc00:370::/64 {
        # keep it simple, just hand out DNS
        option dhcp6.name-servers 2606:4700:4700::1111, 2606:4700:4700::1001;
}

And the plist:

/Library/LaunchDaemons ☯ cat org.isc.dhcpd-6.plist 
<?xml version=\"1.0\" encoding=\"UTF-8\"?>
<!DOCTYPE plist PUBLIC \"-//Apple Computer//DTD PLIST 1.0//EN\" \"http://www.apple.com/DTDs/PropertyList-1.0.dtd\">
<plist version=\"1.0\">
<dict>
     <key>Label</key>
          <string>org.isc.dhcpd-6</string>
     <key>OnDemand</key>
          <false/>
     <key>ProgramArguments</key>
          <array>
          <string>/usr/local/sbin/dhcpd</string>
          <string>-f</string>
      <string>-6</string>
          <string>-f</string>
      <string>-cf</string><string>/usr/local/etc/dhcpd6.conf</string>
      <string>-lf</string><string>/usr/local/var/dhcpd/dhcpd6.leases</string>
          </array>
     <key>RunAtLoad</key>
          <true/>
     <key>ServiceDescription</key>
          <string>ISC DHCP-6 Server</string>
</dict>
</plist>

The only thing left was to load the plist and see if this all worked. Again, with the reboot test.

~ ☯ sudo launchctl list |grep dhcp
47891   -15 org.isc.dhcpd
48425   0   org.isc.dhcpd-6

I also restarted the network interface on another device, and watched the logs on the server to see if this all worked together:

dhcpd: Information-request message from a0a0::a0a0:a0a0:a0a0:a0a0 port 546, transaction ID 0x158E3D00
dhcpd: Sending Reply to a0a0::a0a0:a0a0:a0a0:a0a0 port 546
dhcpd: DHCPREQUEST for 192.168.1.49 from a0:a0:a0:a0:a0:a0 via en6
dhcpd: DHCPACK on 192.168.1.49 to a0:a0:a0:a0:a0:a0 via en6

Presto, devices started to use the correct DNS servers.