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Re: strange DHCP behaviour [SOLVED]




On 24/10/2018 13:34, john doe wrote:
> On 10/24/2018 1:28 PM, tony wrote:
>> On 24/10/2018 08:19, Reco wrote:
>>> 	Hi.
>>>
>>> On Tue, Oct 23, 2018 at 03:56:34PM +0200, Tony van der Hoff wrote:
>>>> The DHCP server is another Stretch system, with the stanzas
>>>> host tony-lt {
>>>> 	hardware ethernet 0c:60:76:6c:e6:6f;
>>>> 	fixed-address 192.168.1.199;
>>>> 	}
>>>> subnet 192.168.1.0 netmask 255.255.255.0
>>>> 	{
>>>>         range 192.168.1.200 192.168.1.254;
>>>>         option routers 192.168.1.10;
>>>> 	}
>>>>
>>>> That MAC is correct for the laptop.
>>>>
>>>
>>>> Finally, the server shows:
>>>> Oct 23 14:23:38 routerpi dhcpd[1068]: DHCPREQUEST for 192.168.1.253 from
>>>> 0c:60:76:6c:e6:6f (tony-lt) via eth0
>>>> Oct 23 14:23:38 routerpi dhcpd[1068]: DHCPACK on 192.168.1.253 to
>>>> 0c:60:76:6c:e6:6f (tony-lt) via eth0
>>>> Oct 23 14:23:53 routerpi dhcpd[1068]: reuse_lease: lease age 15 (secs)
>>>> under 25% threshold, reply with unaltered, existing lease for 192.168.1.253
>>>> Oct 23 14:23:53 routerpi dhcpd[1068]: DHCPREQUEST for 192.168.1.253 from
>>>> 0c:60:76:6c:e6:6f (tony-lt) via eth0
>>>> Oct 23 14:23:53 routerpi dhcpd[1068]: DHCPACK on 192.168.1.253 to
>>>> 0c:60:76:6c:e6:6f (tony-lt) via eth0
>>>>
>>>> So, my question: why is the server handing out .253, when it is
>>>> configured to provide .199?
>>>
>>> Because client specifically asks for .253 address:
>>>
>>>> Oct 23 14:23:38 routerpi dhcpd[1068]: DHCPREQUEST for 192.168.1.253 from
>>>> 0c:60:76:6c:e6:6f (tony-lt) via eth0
>>>
>>
>> Agree fully.
>>
>>> And, to quote dhcpd.conf:
>>>
>>> There may be a host declaration matching the client’s identification. If
>>> that host declaration contains a fixed-address declaration that lists an
>>> IP address that is valid for the network segment to which the client is
>>> connected. In this case, the DHCP server will never do dynamic address
>>> allocation. In this case, the client is required to take the address
>>> specified in the host declaration. If the client sends a DHCPREQUEST for
>>> some other address, the server will respond with a DHCPNAK.
>>>
>>
>> Yes, agree again.
>>
>>> When the DHCP server allocates a new address for a client (remember,
>>> this only happens if the client has sent a DHCPDISCOVER), it first looks
>>> to see if the client already has a valid lease on an IP address, or if
>>> there is an old IP address the client had before that hasn’t yet been
>>> reassigned. In that case, the server will take that address and check it
>>> to see if the client is still permitted to use it. If the client is no
>>> longer permitted to use it, the lease is freed if the server thought it
>>> was still in use - the fact that the client has sent a DHCPDISCOVER
>>> proves to the server that the client is no longer using the lease.
>>>>
>>> You assume that a 'host' entry overrides any previous IP assignment.
>>> It's not.
>>>
>>
>> No, When a client issues a DHCPREQUEST which the server doesn't like,
>> the server should issue a DHCPNAK, to which the client will respond with
>> a DHCPDISCOVER. The server is not sending the NAK. This, I believe to be
>> the crux of the matter. The 'host' assignment should cause the server to
>> send a DHCPNAK.
>>
>>>
>>>> What is this 'reuse-lease' all about?
>>>
>>> Your DHCP client renews the leased IP although the lease time is not
>>> expired. Not relevant to this problem.
>>>>
>>>> I've tried 'dhclient -r wlan 0; dhclient -v wlan0' on the laptop, to no
>>>> avail.
>>>
>>> I'd be really surprised if it did change something. Your DHCP server has
>>> a problem, client does not.
>>>
>>
>> I agree again, but when one gets desperate, one starts to grasp at straws.
>>
>>>
>>>> Any suggestions, please?
>>>
>>> Clear your DHCP lease file on DHCP server. Bounce the thing. Check
>>> again.
>>>
>>
>> Yes, I've done that. No change I'm afraid.
>>
> 
> By doing:
> 
> $ systemctl stop isc-dhcp-server
> rm /var/lib/dhcpd/dhcpd.leases
> $ systemctl start isc-dhcp-server
> 
> Make sure that the client is disconnected before doing the above.
> 

Ew, this time it worked. I wonder why deleting the leases file was
better than clearing it out?

Ah, well; thank you all for your help.

Cheers, Tony