Set host name via DHCP when PXE booting

1

Goal: PXE booting several back-end boxes behind a front-end box to create an “appliance”.

Problem: back-end boxes are not setting their host names from the information being supplied via DHCP. (They all end up with “nixos” as their host name.)

Details:

  • NixOS 24.11
  • Prototyping with VMs via Incus before setting up physical machines
  • pixiecore to supply the kernel image, initrd, and cmdline for PXE booting
  • back-end box and pixiecore configuration:
{inputs, lib, ...}:
let
  system = "x86_64-linux";
  config = { config, pkgs, lib, modulesPath, ... }: {
    imports = [
      "${modulesPath}/installer/netboot/netboot-minimal.nix"
    ];
    config = {
      virtualisation.incus.agent.enable = true;
      system.nixos.tags = [ "netboot" ];
      system.stateVersion = config.system.nixos.release;
      networking.hostName = "";  # get from DHCP rather than set it here
    };
  };
  mkSys = system: config: inputs.nixpkgs.lib.nixosSystem {
    inherit system;
    modules = [ config ];
  };
  sys = mkSys system config;
  build = sys.config.system.build;
in {
  services.pixiecore = {
    enable = true;
    openFirewall = true;
    dhcpNoBind = true;  # use existing DHCP server
    port = 64172;
    statusPort = 64172;

    mode = "boot";
    kernel = "${build.kernel}/bzImage";
    initrd = "${build.netbootRamdisk}/initrd";
    cmdLine = "init=${build.toplevel}/init";
    debug = true;
  };
}
  • MAC addresses set in Incus when VMs are created
  • dnsmasq configuration to assign IP addresses and host names based on MAC addresses
  services.dnsmasq.settings = {
    ...
    host-record = [
      "frontbox,10.0.8.2"
      "backbox1,10.0.8.11"
      ...
    ];
    dhcp-host   = [
      "00:16:3e:00:00:01,backbox1,10.0.8.11,infinite"
      ...
    ];
    ...
  };

According to the hostname man page the order followed in setting the host name is:

  1. cmdline=system.hostname=<name>
  2. /etc/hostname
  3. systemd-hostnamed.service (/etc/machine-info)
  4. localhost

There is no system.hostname on the boot cmdline. The files /etc/nostname and /etc/machine-info do not exist. Somehow the transient host name built into the netboot image is being used.

$ incus exec backbox1 -- bash

# cat /proc/cmdline
kernel initrd=initrd0 init=/nix/store/iaxdv94fwsvd6m29nv611rm0jwnksbxy-nixos-system-unnamed-netboot-24.11.20250412.26d499f/init

# cat /etc/hostname /etc/machine-info
cat: /etc/hostname: No such file or directory
cat: /etc/machine-info: No such file or directory

# hostnamectl | grep hostname
     Static hostname: (unset)
  Transient hostname: nixos

The journal on the front-end box shows that it is correctly sending DHCP option 12 “hostname” as “backbox1”

May 08 15:14:43 frontbox dnsmasq-dhcp[54250]: 402364354 DHCPACK(eth1) 10.0.8.11 00:16:3e:00:00:01 backbox1
May 08 15:14:43 frontbox dnsmasq-dhcp[54250]: 402364354 requested options: 1:netmask, 121:classless-static-route, 3:router,
May 08 15:14:43 frontbox dnsmasq-dhcp[54250]: 402364354 requested options: 6:dns-server, 12:hostname, 15:domain-name,
May 08 15:14:43 frontbox dnsmasq-dhcp[54250]: 402364354 requested options: 26:mtu, 28:broadcast, 33:static-route, 42:ntp-server,
May 08 15:14:43 frontbox dnsmasq-dhcp[54250]: 402364354 requested options: 51:lease-time, 58:T1, 59:T2, 119:domain-search
May 08 15:14:43 frontbox dnsmasq-dhcp[54250]: 402364354 next server: 10.0.8.2
May 08 15:14:43 frontbox dnsmasq-dhcp[54250]: 402364354 sent size:  1 option: 53 message-type  5
May 08 15:14:43 frontbox dnsmasq-dhcp[54250]: 402364354 sent size:  4 option: 54 server-identifier  10.0.8.2
May 08 15:14:43 frontbox dnsmasq-dhcp[54250]: 402364354 sent size:  4 option: 51 lease-time  infinite
May 08 15:14:43 frontbox dnsmasq-dhcp[54250]: 402364354 sent size:  4 option:  1 netmask  255.255.255.0
May 08 15:14:43 frontbox dnsmasq-dhcp[54250]: 402364354 sent size:  4 option: 28 broadcast  10.0.8.255
May 08 15:14:43 frontbox dnsmasq-dhcp[54250]: 402364354 sent size:  4 option: 15 domain-name  internal
May 08 15:14:43 frontbox dnsmasq-dhcp[54250]: 402364354 sent size:  7 option: 12 hostname  backbox1
May 08 15:14:43 frontbox dnsmasq-dhcp[54250]: 402364354 sent size: 16 option: 42 ntp-server  1.1.1.1
May 08 15:14:43 frontbox dnsmasq-dhcp[54250]: 402364354 sent size:  6 option:119 domain-search  08:69:6e:74:65:72:6e:61:6c:00
May 08 15:14:43 frontbox dnsmasq-dhcp[54250]: 402364354 sent size:  4 option:  6 dns-server  10.0.8.2
May 08 15:14:43 frontbox dnsmasq-dhcp[54250]: 402364354 sent size:  4 option:  3 router  10.0.8.1

but still the host name is “nixos”.

Even if a system.hostname option is added to the cmdLine in the pixiecore configuration, the host name of the back-end box is still “nixos” even though the command line says it should be backbox1.

# hostname
nixos

# cat /proc/cmdline 
kernel initrd=initrd0 init=/nix/store/iaxdv94fwsvd6m29nv611rm0jwnksbxy-nixos-system-unnamed-netboot-24.11.20250412.26d499f/init system.hostname=backbox1

What am I missing?

2 Likes
2

Support for setting the hostname via dhcpcd was very recently added to NixOS (nixos/dhcpcd: fix hostname via DHCP by rnhmjoj · Pull Request #385348 · NixOS/nixpkgs · GitHub). Try using nixos-unstable.

2 Likes
3

Thanks for the pointer. I got it working after updating to unstable by adding the following to the configuration above:

+      networking.hostName = "";  # get from DHCP rather than set it here
+      networking.dhcpcd.setHostname = true;  # set DHCP hostname
+      security.polkit.enable = true;  # required to set hostname
+      networking.dhcpcd.allowSetuid = true;  # weaken sandbox to allow hostname to be set

Note: it takes about 5s after the VM allows login before the host name is changed.

4

What is the failure mode if one or two of the last lines are missing? Why don’t we have networking.dhcpd.setHostname = true also enable polkit and setuid?

5

Good catch. It looks like networking.dhcpcd.allowSetuid = true is being done for me and isn’t needed. It works with commented out. Likewise for security.polkit.enable = true. Neither need to be enabled explicitly.