Setting up DHCPD on MacOS

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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.