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{ config, lib, pkgs, ... }:

with lib;

let
  targetOpts = { name, config, ... }: {
    options = {
      name = mkOption {
        type = types.str;
      };
      backingStore = mkOption {
        type = types.str;
      };
      index = mkOption {
        type = types.int;
        description = "the index of the target, must be unique within the server";
      };
    };
    config = {
      name = mkDefault name;
    };
  };
      makeService = target: {
        name = target.name;
        value = {
          description = target.name+" auto-starter";
          wantedBy = [ "basic.target" ];
          partOf = [ "tgtd.service" ];
          script = ''
            ${pkgs.tgt}/bin/tgtadm --lld iscsi --op new --mode target --tid ${builtins.toString target.index} -T ${target.name}
            ${pkgs.tgt}/bin/tgtadm --lld iscsi --op new --mode logicalunit --tid ${builtins.toString target.index} --lun 1 -b ${target.backingStore}
            ${pkgs.tgt}/bin/tgtadm --lld iscsi --op bind --mode target --tid ${builtins.toString target.index} -I ALL # gives everybody access
          '';
          serviceConfig = {
            Type = "oneshot";
            RemainAfterExit = true;
            ExecStop = "${pkgs.tgt}/bin/tgtadm --lld iscsi --op delete --mode target --tid ${builtins.toString target.index}";
          };
        };
      };
in
{
  options = {
    services.tgtd = {
      enable = mkOption {
        type = types.bool;
        default = false;
        description = "enable tgtd running on startup";
      };
      targets = mkOption {
        default = [];
        type = types.loaOf types.optionSet;
        options = targetOpts;
      };
    };
  };
  config = let
      LUNs = builtins.listToAttrs (map makeService (attrValues config.services.tgtd.targets));
      tgtd = {
        description = "tgtd daemon";
        wantedBy = [ "basic.target" ];
        serviceConfig = {
          ExecStart = "${pkgs.tgt}/bin/tgtd -f --iscsi nop_interval=30 --iscsi nop_count=10";
          ExecStop = "${pkgs.coreutils}/bin/sleep 30 ; ${pkgs.tgt}/bin/tgtadm --op delete --mode system";
          KillMode = "process";
          Restart = "on-success";
        };
      };
    in
     mkIf config.services.tgtd.enable {
      systemd.services = LUNs // { tgtd = tgtd; };
    };
}

        subnet 192.168.2.0 netmask 255.255.255.0 {
          option subnet-mask 255.255.255.0;
          option broadcast-address 192.168.2.255;
          option routers 192.168.2.1;
          range 192.168.2.100 192.168.2.200;
          next-server 192.168.2.61;
          if exists user-class and option user-class = "iPXE" {
            filename "http://192.168.2.61/boot.php?mac=${net0/mac}&asset=${asset:uristring}";
          } else {
            filename = "undionly.kpxe";
          }
        }

setup dhcp to direct users to the tftp server with ipxe using the dhcp fragment

192.168.2.61 is the IP of the tftp and http server

setup a tgt daemon using the tgt_module and tftp server
configure and rebuild-switch the server with server-configuration.nix

[root@amd-nixos:~]# nix-build '<nixpkgs>' -A ipxe
these paths will be fetched (0.60 MiB download, 1.20 MiB unpacked):
  /nix/store/1xw30y28dfc1wz62wgcjifsp58c8wdkk-ipxe-20141124-5cbdc41
fetching path ‘/nix/store/1xw30y28dfc1wz62wgcjifsp58c8wdkk-ipxe-20141124-5cbdc41’...

*** Downloading ‘https://cache.nixos.org/nar/1sksixb1cc57ypjmm99k274wfmc3mqbrvnbvyilz0sd8h8bnyn0l.nar.xz’ (signed by ‘cache.nixos.org-1’) to ‘/nix/store/1xw30y28dfc1wz62wgcjifsp58c8wdkk-ipxe-20141124-5cbdc41’...
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  614k  100  614k    0     0  1170k      0 --:--:-- --:--:-- --:--:-- 1172k

/nix/store/1xw30y28dfc1wz62wgcjifsp58c8wdkk-ipxe-20141124-5cbdc41

[root@amd-nixos:~]# mkdir /tftproot/

[root@amd-nixos:~]# cp -vi result/undionly.kpxe /tftproot/

{ ... }:

{
  imports = [ ./tgt_service.nix ];
  services = {
    atftpd.enable = true;
    atftpd.root = "/tftproot";
    tgtd = {
      enable = true;
      targets = {
        # backing store can be a file made with truncate(1)
        "iqn.2016-01.laptop-root" = { backingStore = "/dev/naspool/laptop-root"; index = 1; };
      };
    };
  };
}