ESP Raw Socket: Returned IP packet incorrect

[Michael Tüxen]

On Jul 11, 2011, at 5:26 PM, Matthew Cini Sarreo wrote:

> 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)?
I think Linux provides the tot_len field in network byte order whereas
FreeBSD provides it in host byte order. At least they expect it that way
when using a send call.

So you must take care of this in the source code of the application by
using an #ifdef...

Best regards
Michael
> 
> Any help would be much appreciated.
> Matt
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