ISC-dhcp-server cannot detect ip address conflict

Question

I had a hub connected with a dhcp server(debian 8) and two hosts A and B, as B's interface was shut down at the beginning. Then I had dhcp-server give host A 192.168.2.170 automatically, at that time everything worked well. Next I set the interface of B with the same IP 192.168.2.170 manually, and enabled that interface, host B can ping the server (with maybe 40% loss), but host A would still be able to renew that 192.168.2.170 from server successfully roughly every 10 mins each time (I tried 5+ times). Eventually, server would abandon IP 192.168.2.170 and allocate another IP to host A in each try, but still after an unpredictable time period, from 2 mins to 15 mins.

In my opinion, as the 2131 says, server will ping that IP before send it to client, and once that ICMP is replied, server will send another IP. So in my case I think server should be able to detect that conflict as soon as it first time received the request from client, and send a new IP. But server still ACK that IP for many times before it does ICMP check and allocate new one.

Answer 1

To avoid IP address conflicts, you configure "static" and "dynamic" ranges on your DHCP server. For example, 192.168.2.1 to 192.168.2.100 is your dynamic range, the DHCP server is allowed to distribute those addresses freely, 192.168.2.101 to 192.168.2.150 is you static range, DHCP server is NOT allowed to distribute these IP addresses freely. You can map IP addresses to MAC addresses to create "static leases", for these, you use the "static" range. You have a third range, 192.168.2.151 to 192.168.2.254, that is for "playing around". Note that the ranges here are for home use, right, if you were in charge of a corporate network, it would look very different.

You claim the DHCP server detected the IP address conflict and gave host A another IP ? I think that is a bug in the DHCP server!

The DHCP server should only do so when host A requests a new lease. The DHCP server will check if the address to give is already in use before assigning it, if it is in use, it is marked as such and the address will only ever be considered again when the DHCP server runs out of addresses.