[ECOS] Re: Redboot responds to ICMP echo when it shouldn't.

[Gary Thomas]

On 03/21/2011 02:36 PM, Grant Edwards wrote:
> On 2011-03-21, Gary Thomas<gary@mlbassoc.com>  wrote:
>> On 03/21/2011 02:21 PM, Grant Edwards wrote:
>>> While testing my rewrite of Redboot DHCP support, I've noticed that
>>> Redboots "ping" support is also broken.  Redboot responds to ICMP echo
>>> requests that it shouldn't.
>>>
>>> Redboot will respond to ICMP echo requests _before_ it has received an
>>> IP address from the BOOTP/DHCP server.  The destination IP address in
>>> the ICMP echo packet _does_not_match_ Redboot's IP address, but it
>>> responds anyway with a source IP address of 0.0.0.0.
>>>
>>> Is this behavior intentional?
>>
>> Not as far as I know.
>>
>> How are those packets even being received?
>
> The "pinging" host's ARP cache still has RedBoot's MAC address
> associated with the IP address bing pinged.
>
>> Are they going to a broadcast address (IP or ESA)?
>
> Unicast Ethernet MAC, unicast IP.

This is happening because of a stale ARP cache on your host.

>
>> Normally incoming packets are filtered by ESA by the network driver
>> before they get pushed up the stack and processed.
>
> Right.  And then the IP layer should filter them by IP address, right?
>
>> You might find that RedBoot replies to [some, maybe all] packets
>> which somehow match it's ESA, but not IP address as well.
>
> That appears to be the case.
>
> I assumed that devices shouldn't respond to ICMP ping requests that
> don't match their IP address.  Am I wrong?

No, I think you are correct.  The IP code may match if the local IP address
is set to IPANY (i.e. 0.0.0.0).

>
> The DHCP server (which is sending the ICMP echo request before
> offering an IP address) appears to be ignoring the reply from IP
> address 0.0.0.0, but I can imagine somewhat contrived situations where
> it might cause problems.

This seems to be a pretty rare condition, caused by a stale ARP cache.

-- 
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Gary Thomas                 |  Consulting for the
MLB Associates              |    Embedded world
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