invalid checksum with vtnet and in_kernel BOOTP

I am trying to run a diskless bhyve client diskless using in-kernel
bootp support and vtnet connected to tap and bridge on the host side
and I am having the problem in the subject which may be vtnet-related.

Packets generated by the in-kernel DHCP arrive to the host tap
interface with a corrupt checksum, which is consistent with vtnet0
having the TXCSUM capability enabled.

Unfortunately, when the packet reaches the bridge0 on the host:
> uname -a
FreeBSD bsd9.casa 10.0-STABLE FreeBSD 10.0-STABLE #0 r269180:
>ifconfig bridge0

    bridge0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
    ether 02:01:85:b1:55:00
    id 00:00:00:00:00:00 priority 32768 hellotime 2 fwddelay 15
    maxage 20 holdcnt 6 proto rstp maxaddr 2000 timeout 1200
    root id 00:00:00:00:00:00 priority 32768 ifcost 0 port 0
    member: re0 flags=143<LEARNING,DISCOVER,AUTOEDGE,AUTOPTP>
	    ifmaxaddr 0 port 1 priority 128 path cost 200000
    member: tap0 flags=143<LEARNING,DISCOVER,AUTOEDGE,AUTOPTP>
	    ifmaxaddr 0 port 4 priority 128 path cost 55

The packet goes out with a broken checksum and the (external)
dhcp server does not respond.

When the guest is up, i can run dhclient on the same guest interface
and packets exit with a correct checksum.
Maybe dhclient generates correct packets ignoring the TXCSUM setting.

In any case i applied a quick fix (disable TXCSUM in if_vtnet.c)
and at least can get the dhcp request reach the router and get
a response back.

Also note that subsequent (tcp) traffic through the interface
is reported on the host with a correct checksum, so i suspect
that either the kernel-bootp code does not pass the correct
checksum flags, or somehow it is sent to the device too early
in the initialization process.

any idea ?

cheers
luigi