Duplicate pings

I have a WLAN with two wireless stations, the first one with ip
192.168.0.193 and the other 192.168.0.194, both obtained via dhcp.

I do a ping from the .194 to the .193 and I obtain duplicate packets,
the output from the ping is:

# ping 192.168.0.193
PING 192.168.0.193 (192.168.0.193) 56(84) bytes of data.
64 bytes from 192.168.0.193: icmp_seq=0 ttl=128 time=7.38 ms
64 bytes from 192.168.0.193: icmp_seq=0 ttl=128 time=7.98 ms (DUP!)
64 bytes from 192.168.0.193: icmp_seq=1 ttl=128 time=1.85 ms
64 bytes from 192.168.0.193: icmp_seq=1 ttl=128 time=2.00 ms (DUP!)
64 bytes from 192.168.0.193: icmp_seq=2 ttl=128 time=2.60 ms
64 bytes from 192.168.0.193: icmp_seq=2 ttl=128 time=3.16 ms (DUP!)
64 bytes from 192.168.0.193: icmp_seq=3 ttl=128 time=2.77 ms
64 bytes from 192.168.0.193: icmp_seq=3 ttl=128 time=3.00 ms (DUP!)
64 bytes from 192.168.0.193: icmp_seq=4 ttl=128 time=2.66 ms
64 bytes from 192.168.0.193: icmp_seq=4 ttl=128 time=3.43 ms (DUP!)

I captured that output with Ethereal, and what is actually happening
is that the .194 machine is generating each time two ping requests
with the same sequence code 0, 1 and so on, and this is why the .193
machine responses two requests with the same sequence code, and .194
thinks that the response is duplicated.

Why is this happening ? Why the .194 machine is generating two
requests each time? btw the two requests with the same sequence code
are always generated in a row, before the first reply to that sequence
code arrives.

Does icmp have any kind of retransmission procedure ?


Thanks


Dani Camps

Dani Camps wrote:
> I have a WLAN with two wireless stations, the first one with ip
> 192.168.0.193 and the other 192.168.0.194, both obtained via dhcp.
>
> I do a ping from the .194 to the .193 and I obtain duplicate packets,
> the output from the ping is:
>
> # ping 192.168.0.193
> PING 192.168.0.193 (192.168.0.193) 56(84) bytes of data.
> 64 bytes from 192.168.0.193: icmp_seq=0 ttl=128 time=7.38 ms
> 64 bytes from 192.168.0.193: icmp_seq=0 ttl=128 time=7.98 ms (DUP!)
> 64 bytes from 192.168.0.193: icmp_seq=1 ttl=128 time=1.85 ms
> 64 bytes from 192.168.0.193: icmp_seq=1 ttl=128 time=2.00 ms (DUP!)
[snip]
>
> I captured that output with Ethereal, and what is actually happening
> is that the .194 machine is generating each time two ping requests

..193 is _receiving_ two ping _replies_, but can you be _sure_ they are
from .194? See below.

> with the same sequence code 0, 1 and so on, and this is why the .193
> machine responses two requests with the same sequence code, and .194
> thinks that the response is duplicated.
>
> Why is this happening ? Why the .194 machine is generating two
> requests each time? btw the two requests with the same sequence code
> are always generated in a row, before the first reply to that
sequence
> code arrives.
>
> Does icmp have any kind of retransmission procedure ?

There could be several reasons, but the couple of times I've run into
this it was _very_ similar to this (or just a duplicate IP on the
network):

https://www.redhat.com/archives/redh.../msg00058.html

Can be tricky to spot and figure out ;-)

Hope that's the only problem you are seeing,
hth,
prg
email above disabled