ESP Raw Socket: Returned IP packet incorrect

[Matthew Cini Sarreo]

Hello all;

I have recently encountered a problem when using raw sockets on FreeBSD 8
(8.0-RELEASE) when using ESP raw sockets.

I have created a raw esp socket using:
socket(AF_INET, SOCK_RAW, 50);
which works fine. However, when there is a packet on the socket, recvfrom()
returns a packet where the length bytes in the IP header are incorrect; they
are swapped (MSB is placed in the LSB and vice-versa)

tcpdump shows the following:

tcpdump: listening on le0, link-type EN10MB (Ethernet), capture size 96
bytes
15:00:53.993810 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto ESP
(50), length 120)
    10.0.251.228 > 10.0.252.231: ESP(spi=0xa0534f17,seq=0x3), length 100
        0x0000:  4500 0078 0000 4000 4032 2d88 0a00 fbe4
        0x0010:  0a00 fce7 a053 4f17 0000 0003 6885 8abd
        0x0020:  2222 5ded 44dc 842f 3081 8fa3 bde4 2265
        0x0030:  7438 2bf4 049c 664b 7dc4 44ef 1f6f 5e7d
        0x0040:  b8c1 482f 8c3b f488 a19a 3d9a d5fe ed9d
        0x0050:  b1c2


However, recvfrom() returns the following buffer:
4500 6400 0000 0040 4032 2D88 0A00 FBE4
0A00 FCE7 A053 4F17 0000 0003 6885 8ABD
2222 5DED 44DC 842F 3081 8FA3 BDE4 2265
7438 2BF4 049C 664B 7DC4 44EF 1F6F 5E7D
B8C1 482F 8C3B F488 A19A 3D9A D5FE ED9D
B1C2

As it is easy to see, the length is not correct (bytes 2 and 3 are 0x6400
instead of 0x0064) and it does not correspond to the value returned by
recvfrom().

Is this a known issue? Am I missing some options for raw sockets that are
required for FreeBSD? I have attempted this on a socket to a TUN interface
(not with an ESP socket) and the buffer had the proper length; it seems to
only happen with ESP. This code runs fine on multiple Linux distributions
and on Windows; it was only noticed with FreeBSD. Could it be that there is
some other ESP application running and interfering (I have not installed
any; don't know if there are by default and I'm quite new to any of the
BSDs)?

Any help would be much appreciated.
Matt