pxe + nfs + microsoft dhcp

[pacija]

Dear list readers,

I am having a problem with pxe loader on FreeBSD 9.0 i386 release. No
matter what value I put for DHCP option 017 (Root Path) in Microsoft
DHCP server, pxe always sets root path:
pxe_open: server path: /

I've read src/sys/boot/i386/libi386/pxe.c as instructed in handbook, and
i learned there that root path is a failover value which gets set if no
valid value is supplied by DHCP server. At first i thought that
Microsoft DHCP does not send it but i confirmed with windump it does:

--
15:46:49.505748 IP (tos 0x0, ttl 128, id 6066, offset 0, flags [none],
proto: UDP
(17), length: 392) dhcp.domain.tld.67 > 255.255.255.255.68: [bad udp
cksum 4537!]
BOOTP/DHCP, Reply, length 364, xid 0xdcdb5309, Flags [ none ] (0x0000)
          Your-IP 192.168.218.32
          Server-IP dhcp.domain.tld
          Client-Ethernet-Address 00:19:db:db:53:09 (oui Unknown)
          file "FreeBSD/install/boot/pxeboot"
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Offer
            Subnet-Mask Option 1, length 4: 255.255.255.0
            RN Option 58, length 4: 345600
            RB Option 59, length 4: 604800
            Lease-Time Option 51, length 4: 691200
            Server-ID Option 54, length 4: dhcp.domain.tld
            Default-Gateway Option 3, length 4: gate.domain.tld
            Domain-Name-Server Option 6, length 4: dhcp.domain.tld
            Domain-Name Option 15, length 1: "^@"
            RP Option 17, length 42:
"192.168.218.32:/b/tftpboot/FreeBSD/install/^@"
            BF Option 67, length 29: "FreeBSD/install/boot/pxeboot^@"
--

I do not understand code well enough to fix it, or at least send
pxeloader static value of /b/tftpboot/FreeBSD/install/, so if someone
would instruct me how to do it i would be very grateful.

Thank you in advance for your help.