Re: tcpdump confused with NAT-T+IPSec Packets

Gopalakrishnan Raman wrote:

Hi
I'm using 2.6.11.7 and debugging why my ESP tunnel mode does
not work between two 2.6 machines one of which is behind a NAT.
I'm using tcpdump to capture NAT-T packets on one of the hosts
and using espdecrypt (http://www.cs.rpi.edu/~flemej/freebsd/espdecrypt/)
to see it in the clear.

Turns out, tcpdump will display an incoming NAT-T packet after it
has been mangled by udp_encap_rcv(). udp_encap_rcv() changes the
protocol field in the IP hdr to ESP from UDP and also moves other
bytes in the sk_buff data area.

The problem is that packet_rcv() calls skb_clone() which is the
right thing to do in all cases except when the data portion of the
incoming skb is being modified in place. I replaced it with a pskb_copy()
in the case when the packet is likely to be NAT-T or ESP. The patch
for this follows the end of this mail and seems to work quite well.

Note that af_packet.c is the right place for the ESP/NAT-T check.
Can't do it in ESP or UDP code because we can't tell if these packets
are also being captured by tcpdump/ethereal.

Herbert already fixed it with this patch.