Re: Duplicate IP false alerts from arping

[unni krishnan <unnikrishnan.a@gmail.com>]

Ok, then what is the best method to find the duplicate IP ( same IP
address assigned to different machines ) ?

On Sat, Apr 17, 2010 at 2:28 PM, Pascal Hambourg
<pascal.mail@plouf.fr.eu.org> wrote:
> Hello,
>
> unni krishnan a écrit :
>>
>> I am trying to find a duplicate IP in the network using arping.
>>
>>  -------------------------
>>  [root@vps1 ~]# ping -c 3 192.168.1.212
>>  PING 192.168.1.212 (192.168.1.212) 56(84) bytes of data.
>>  64 bytes from 192.168.1.212: icmp_seq=1 ttl=64 time=1.33 ms
>>  64 bytes from 192.168.1.212: icmp_seq=2 ttl=64 time=0.280 ms
>>  64 bytes from 192.168.1.212: icmp_seq=3 ttl=64 time=0.306 ms
>>
>>  --- 192.168.1.212 ping statistics ---
>>  3 packets transmitted, 3 received, 0% packet loss, time 1999ms
>>  rtt min/avg/max/mdev = 0.280/0.641/1.339/0.494 ms
>>  [root@vps1 ~]# arping -D -I eth0 -c 5 192.168.1.212 ; echo $?
>>  ARPING 192.168.1.212 from 0.0.0.0 eth0
>>  0
>>  -------------------------
>>
>>  As per arping that IP is duplicate.
>
> I disagree. According to man arping :
>
>    -D  Duplicate  address  detection  mode  (DAD).  See RFC2131, 4.4.1.
>        Returns 0, if DAD succeeded i.e. no replies are received
>                                         ^^^^^^^^^^^^^^^^^^^^^^^
> -D (DAD) is meant for DHCP to find out if the proposed IP address is not
> already assigned to another host. Its purpose is not to find out if
> multiple hosts have the same IP address. Besides, a return value of 0
> means that no ARP replies were received (IOW -D inverts the return value
> logic), which is weird since the target IP address replies to ICMP ping
> unless that address is assigned to the local host.
>
> Here :
>
> # arping -DI eth0 -c 1 192.168.0.246 ; echo result=$?
> ARPING 192.168.0.246 from 0.0.0.0 eth0
> Unicast reply from 192.168.0.246 [xx:xx:xx:xx:xx:xx]  0.964ms
> Sent 1 probes (1 broadcast(s))
> Received 1 response(s)
> result=1
>
> # arping -DI eth0 -c 1 192.168.0.24 ; echo result=$?
> ARPING 192.168.0.24 from 0.0.0.0 eth0
> Sent 1 probes (1 broadcast(s))
> Received 0 response(s)
> result=0
>
>> But if I go ahead and ifdown the
>>  IP in the known location I cant ping that IP ( That means that IP is
>>  not duplicated ? ). This is the result after shutting down the IP.
>>
>>  --------------------------
>>  [root@vps1 ~]# ping -c 3 192.168.1.212
>>  PING 192.168.1.212 (192.168.1.212) 56(84) bytes of data.
>>  From 192.168.1.63 icmp_seq=1 Destination Host Unreachable
>>  From 192.168.1.63 icmp_seq=2 Destination Host Unreachable
>>  From 192.168.1.63 icmp_seq=3 Destination Host Unreachable
>
> Ok, that means no ARP reply.
>
>>  [root@vps1 ~]# arping -D -I eth0 -c 5 192.168.1.212 ; echo $?
>>  ARPING 192.168.1.212 from 0.0.0.0 eth0
>>  Sent 5 probes (5 broadcast(s))
>>  Received 0 response(s)
>>  0
>
> Same as above.
>
>>  My question is, in this case IP 192.168.1.212 is not duplicated. But
>>  still arping gives duplicate status. Why it is like that ?
>
> A situation of real duplicate ARP replies may occur when the address is
> assigned to a host which has multiple interfaces connected to the same
> network, so it receives and replies to ARP queries on each interface.
>



-- 
Regards,
Unni
http://mutexes.org/
http://twitter.com/webofunni