You're supposed to be running LTSP5 with dnsmasq as DHCP and TFTP server. This will switch you from PXELinux to iPXE and GRUB, in order to support UEFI clients even with 32 bit LTSP images.
To download and run the installer:
wget https://raw.githubusercontent.com/alkisg/ltsp5-uefi/master/ltsp5-uefi -O /tmp/ltsp5-uefi
sudo sh /tmp/ltsp5-uefi
Afterwards, you might want to customize the following files:
- /etc/dnsmasq.d/ltsp-server-dnsmasq.conf
- /var/lib/tftpboot/ltsp/ltsp.ipxe
Do not run ltsp-config dnsmasq
after installing ltsp5-uefi,
as it will overwrite the UEFI-capable /etc/dnsmasq.d/ltsp-server-dnsmasq.conf file.