May Contain Blueberries

the sometimes journal of Jeremy Beker

Just a quick entry that will hopefully help someone else with a similar situation. (Pardon the ton of links and buzzwords, I want to make sure this is easily searchable.) In my network, I use the ISC DHCP server via OPNsense. Most of my hosts get fully dynamic addresses and I have the DHCP server register their names in DNS using RFC 2136. This works very well in that I don’t have to worry about manual IP allocation yet I can still use friendly hostnames to access systems. For the few systems that need staticly defined IP addresses, I have set up static leases via system MAC addresses. This has worked very well so far.

But I recently came across a new situation that hit a snag. As I will now be having 2 laptops that I use regularly, I wanted to be able to attach them to my desktop monitors via my CalDigit TS3+ which has a wired ethernet connection. This means that I will have two computers that will at times be requesting an IP address from the same MAC address. This isn’t an issue particularly except that macOS will set the hostname of the system based on a reverse DNS lookup of the IP address it receives. Given caching and the timing of the RFC 2136 updates, I would open a terminal on laptop minas-tirith and see my prompt saying my computer was hobbiton. This bothered me for aesthetic and possible confusion reasons.

DHCP has the ability to send a “client ID” when requesting an address and the ISC DHCPd server can do a static IP assignment based on that client ID. This seemed the perfect solution. I could set a client ID on each of the laptops for that network interface and each would get the proper IP, register a DNS name for that IP and all would be happy. My first attempt almost worked. I set the client IDs on both the laptops and the DHCP server did give them different IP addresses so I was confident that the client ID was being used, however, they were not the static assignments that I had set in my configuration.

Doing some searching I found very little about this problem but I saw a few mentions that indicated that DHCP clients often prepended some bytes to what they send as the client ID. I dug a bit into what was being sent by looking at the dhcpd.leases file on my server and lo and behold, that was what was happening:

lease {
  starts 4 2023/03/02 12:08:46;
  ends 4 2023/03/02 12:10:05;
  tstp 4 2023/03/02 12:10:05;
  cltt 4 2023/03/02 12:08:46;
  binding state free;
  hardware ethernet 64:4b:f0:13:22:f6;
  uid "\000hobbiton";

The uid line is the client ID. The client was prepending a null byte at the beginning. So, I went back to my DHCP server and set the matching client ID name to be \000hobbiton. I renewed the lease and VOILA! I was now getting the IP address I assigned.

Another step in living the dual laptop life. Now if I can just find a good solution to using Apple bluetooth keyboards with TouchID…