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from this article: http://backreference.org/2010/03/26/tuntap-interface-tutorial/ the author created a tun interface

# openvpn --mktun --dev tun2
# ip link set tun2 up
# ip addr add 10.0.0.1/24 dev tun2

then he did a ICMP ping to 10.0.0.2

# ping 10.0.0.2
PING 10.0.0.2 (10.0.0.2) 56(84) bytes of data.
From 10.0.0.1 icmp_seq=2 Destination Host Unreachable
From 10.0.0.1 icmp_seq=3 Destination Host Unreachable

...

and he used tshark or tcpdump to capture packets on interface tun2, and he can capture packets as below:

 0.000000     10.0.0.1 -> 10.0.0.2     ICMP Echo (ping) request
 0.999374     10.0.0.1 -> 10.0.0.2     ICMP Echo (ping) request
 1.999055     10.0.0.1 -> 10.0.0.2     ICMP Echo (ping) request

I followed the same procedures, but I can't capture packets like he did, and I don't get Destination Host Unreachable. I think the problem is due to local routing, in his test, it seems that the system regard 10.0.0.1 as the default gateway for subnet 10.0.0.0/24, but in my test, it is not the case.

so how can I get the same result as the author's?

BTW, on my machine:

    tun2      Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
              inet addr:10.0.0.1  P-t-P:10.0.0.1  Mask:255.255.255.0
              UP POINTOPOINT NOARP MULTICAST  MTU:1500  Metric:1
              RX packets:7 errors:0 dropped:0 overruns:0 frame:0
              TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
              collisions:0 txqueuelen:100
              RX bytes:420 (420.0 b)  TX bytes:0 (0.0 b)


route -n

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG    0      0        0 wlan0
10.0.0.0        0.0.0.0         255.255.255.0   U     0      0        0 tun2
192.168.1.0     0.0.0.0         255.255.255.0   U     2      0        0 wlan0

I don't understand what is the meaning of 0.0.0.0 for gateway, so I did add a route:

route add -net 10.0.0.0 netmask 255.255.255.0 gw 10.0.0.1

and get

    Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
    0.0.0.0         192.168.1.1     0.0.0.0         UG    0      0        0 wlan0
    10.0.0.0        10.0.0.1        255.255.255.0   UG    0      0        0 tun2
    10.0.0.0        0.0.0.0         255.255.255.0   U     0      0        0 tun2
    192.168.1.0     0.0.0.0         255.255.255.0   U     2      0        0 wlan0

still it doesn't work. I really don't understand why.

share|improve this question
    
Your route table is showing 10.0.0.0 on tun3 not tun2. If you are capturing on the wrong interface, then you aren't going to see anything. –  Zoredache Jun 3 '13 at 22:39
    
this is not the cause. I captured on tun3. Anyway, I edited the question to clear this confusion. –  misteryes Jun 3 '13 at 22:44

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