dnsmasq with gpxe can not get the filename

Recently I used dnsmasq for pxeboot my virtual machine.
The virtual machine was boot with gPXE.
And it can fetch the IP address, but can't fetch the filename.
Then I captured the network package.
The server did send the filename operation to client.
But the client did not use it.
I can't figure out what happened.
After I added the option "dhcp-no-override" and everything just fine.