This assumes you are using an OPNSense firewall/router. Procedure taken from the OPNSense forum and syslinux Arch wiki page. Use this for smaller ISOs (100 MB or less), if you don’t want to wait for a long time at each boot. I’m sure a better and more efficient way to boot Talos over network works with syslinux, but this is my first try that has worked. I still need to streamline it for booting it automatically without a prompt.

OPNSense setup

1. Install os-tftp package. Start TFTP server and bind it to an IP. In the WebGUI, it’s in Services > TFTP.
2. SSH into OPNSense and create directory /usr/local/tftp (will need sudo privileges).
3. Create directory pxelinux.cfg and a file pxelinux.cfg/default.

   DEFAULT vesamenu.c32
   PROMPT 0
   MENU TITLE PXE Boot Menu (Main)
   
   LABEL linux
      MENU LABEL Linux
      KERNEL vesamenu.c32
      APPEND pxelinux.cfg/linux
   

4. Create file pxelinux.cfg/linux. (NOTE: I wanted to boot Talos Linux, so I put the .iso in the /usr/local/tftp directory.)

   MENU TITLE PXE Boot Menu (Linux)
   
   LABEL main-menu
      MENU LABEL Main Menu
      KERNEL vesamenu.c32
      APPEND pxelinux.cfg/default
   LABEL talos-iso
      MENU LABEL Boot Talos 1.7 ISO (PXE)
      KERNEL memdisk
      INITRD ../talos.iso
      APPEND iso
   

   memdisk is what allows booting ISOs.
5. For OPNSense 24.x (which is based on FreeBSD 13), you’ll need to manually download the syslinux binary.
   1. cd to /tmp
   2. Download the binary:

      wget https://pkg.freebsd.org/FreeBSD:13:amd64/latest/All/syslinux-6.03_1.pkg
      

   3. Extract it:

      sudo tar -C /tmp/syslinux -xvf syslinux-6.03_1.pkg
      

   4. Move the necessary files to /usr/local/tftp directory:

      sudo cp /tmp/syslinux/usr/local/share/syslinux/bios/core/lpxelinux.0 /usr/local/tftp/pxelinux.0
      sudo cp /tmp/syslinux/usr/local/share/syslinux/bios/com32/elflink/ldlinux/ldlinux.c32 /usr/local/tftp/
      sudo cp /tmp/syslinux/usr/local/share/syslinux/bios/com32/menu/vesamenu.c32 /usr/local/tftp/
      sudo cp /tmp/syslinux/usr/local/share/syslinux/bios/com32/lib/libcom32.c32 /usr/local/tftp/
      sudo cp /tmp/syslinux/usr/local/share/syslinux/bios/com32/libutil/libutil.c32 /usr/local/tftp/
      sudo cp /tmp/syslinux/usr/local/share/syslinux/bios/com32/modules/pxechn.c32 /usr/local/tftp/
      sudo cp /tmp/syslinux/usr/local/share/syslinux/bios/memdisk/memdisk /usr/local/tftp/
      

6. In the same subnet as you bound the TFTP to, set up the DHCP settings. You will need the TFTP server address and the bootfile (pxelinux.0 for syslinux).