listen -a shows on segmented packets ok, but shows also the defragmented,

packet, but garbled:
bpq2: fm DL9SAU to DL9SAU-15 ctl I00+ pid=8(segment) len 4  remain 0
0000  E2 E3 E4                                         | ...

Reason:
the defragmented packet is passed to netif_rx(skb).
It has  skb->protocol = htons(ETH_P_IP), but still belongs to
ifr.ifr_hwaddr.sa_family == AF_AX25.
listen -a looks for every iface with like ifr.ifr_hwaddr.sa_family == AF_AX25
and thus gets the desegmented packet. This desegmented packet has already
lost his ax25 header information (it's the IP datagram that netif_rx sees),
prefixed by AX25_P_IP = 0xcc:

strace:
recvfrom(3, "\0\210\230r\246\202\252\340\210\230r\246\202\252\177\314\10\201\314E\0\1\1\0\32\0\0?\1!\327,\200\200\n,\200\200\1\0\0\347\300\21\6\0\1\26\23\303V..
                                                               recvfrom(3, "\314E\0\1\1\0\32\0\0?\1!\327,\200\200\n,\200\200\1\0\0\347\300\21\6\0\1\26\23\303V..

\314 == 0xcc == AX25_P_IP

We can make safely make the assumption for first byte == 0xcc and length > 2,
that we safeley can detect those IP frames, and then ignore it.

Signed-off-by: Thomas Osterried <ax25@x-berg.in-berlin.de>

1 files changed, 28 insertions, 0 deletions

diff --git a/listen/listen.c b/listen/listen.c
index bf60281..ca933c6 100644
--- a/listen/listen.c
+++ b/listen/listen.c
@@ -188,6 +188,34 @@ int main(int argc, char **argv)
 			if (sock == -1 || sigint)
 				break;
 			if (ifr.ifr_hwaddr.sa_family == AF_AX25) {
+                                if (size > 2 && *buffer == 0xcc) {
+                                        /* IP packets from the ax25 de-segmenter
+                                           are seen on socket "PF_PACKET,
+                                           SOCK_PACKET, ETH_P_ALL" without
+                                           AX.25 header (just the IP-frame),
+                                           prefixed by 0xcc (AX25_P_IP).
+                                           It's unclear why in the kernel code
+                                           this happens (unsegmentet AX25 PID
+                                           AX25_P_IP have not this behavior).
+                                           We have already displayed all the
+                                           segments and like to ignore this
+                                           data.
+                                           AX.25 packets start with a kiss
+                                           byte (buffer[0]); ax25_dump()
+                                           looks for it.
+                                           There's no kiss command 0xcc
+                                           defined; kiss bytes are checked
+                                           against & 0xf (= 0x0c), which is
+                                           also not defined.
+                                           Kiss commands may have one argument.
+                                           => We can make safely make the
+                                           assumption for first byte == 0xcc
+                                           and length > 2, that we safeley can
+                                           detect those IP frames, and then
+                                           ignore it.
+                                         */
+                                        continue;
+                                }
 				display_port(sa.sa_data);
 #ifdef NEW_AX25_STACK
 				ax25_dump(buffer, size, dumpstyle);