Comment 4 for bug 1410280

The most obvious fix seems to be to replace pxe-service with the following two lines:
pxe-service=net:!gpxe,x86PC,"Install",pxelinux,10.20.0.3 <- replace # to !
pxe-service=net:gpxe,x86PC,"Install",gpxelinux.0,10.20.0.3 <- add gpxelinux.0 image

And it would be necessary to add gpxelinux.0 to tftproot.

But we could short circuit this possibly on the dhcrelay side by rewriting option 54 (Server IP field).

I should point out that in Fuel 6.1 we will move to host networking and remove all the Docker NAT issues and this issue should correct itself. For 6.0.x and 5.1.x we will probably need a solution like above.