arp who-has? tell?

[John Summerfield]

John Cornelius wrote:
> 
> 
> John Summerfield wrote:
> 
>> John Cornelius wrote:
>>
>>> Actually, these messages are coming from the DHCP server for the 
>>
>>
>> Not so. DCHP uses UDP ports 67 & 68. That's not what I saw.
> 
> Tis too! When the server chooses an address to issue to someone it has 
> to verify that the address isn't already in use. To do that, it asks the 
> network if anyone has the address (ARP) and if no one say's yes the 
> address is issued.

man dhcpd.conf
topic  IP ADDRESS CONFLICT PREVENTION
   DHCP server checks IP addresses to see if they are in use before 
allocating them to clients.   It does this by sending an ICMP Echo 
request message to the IP address being allocated. If no ICMP Echo reply 
is received  within a second, the address is assumed to be free.  This 
is only done for leases that have been specified in range statements, 
and only when the lease is thought by the DHCP server to be free - i.e., 
the DHCP  server  or  its  failover peer has not listed the lease as in use.

ICMP doesn't depend on knowing how IP traffic is transported.


> 
> If (as may be happening in this case) the DHCP server is also the 
> segment's router then any time the router needs to send a packet to the 
> segment it checks its ARP cache to see if it has a physical address for 
> this IP address and if the address has been flushed from the cache it 
> ARPs the network again to get an entry.
> 
> Since there wasn't any subsequent traffic between the router/server and 
> the addresses that it was arping it's likely that it was just 
> housekeeping by the DHCP server.

On a switching hub (as opposed to an ordinary cheap hub), traffic other 
than what I described earlier is not send to just any NIC. The switch 
knows which nic is on which port.




-- 

Cheers
John

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