Strange behaviour from dhcp server

jeffrey j donovan donovan at beth.k12.pa.us
Fri Sep 7 15:23:17 UTC 2012


On Sep 7, 2012, at 10:27 AM, Tobias Szyndler wrote:

> Hello list
> 
> my first post, so hopefully I do not ask something obvious :-)
> We have a strange problem with one of our dhcp servers. The problem is that the server offers an IP to the client via DHCPOFFER but in the following DHCPREQUEST from the client the server returns 'unknown lease'.
> After some of these 'unknown lease' the client comes again with DHCPDISCOVER and this time the server offers another IP which seems to work for the server and after the DHCPREQUEST from the client for that IP the server sends the expected DHCPACK

hello,
I think what you are seeing are clients requesting pervious leases. lets say Mac1 was on Net1 and had a lease of 1.2.3.4, then when mac1 moves to net2 it trys to renew the previous lease , gives up and try's for new.
-j

> I put some of the loglines here (ip addresses anonymized):
> <<
> Sep  7 09:12:14 dhcp1 dhcpd: DHCPDISCOVER from 00:0f:93:00:10:ba via x.y.z.1
> Sep  7 09:12:23 dhcp1 dhcpd: DHCPDISCOVER from 00:0f:93:00:10:ba via x.y.z.1
> Sep  7 09:12:24 dhcp1 dhcpd: DHCPOFFER on x.y.z.112 to 00:0f:93:00:10:ba via x.y.z.1
> Sep  7 09:12:24 dhcp1 dhcpd: DHCPREQUEST for x.y.z.112 (x.y.z.1) from 00:0f:93:00:10:ba via x.y.z.1: unknown lease x.y.z.112.
> Sep  7 09:12:25 dhcp1 dhcpd: DHCPREQUEST for x.y.z.112 (x.y.z.1) from 00:0f:93:00:10:ba via x.y.z.1: unknown lease x.y.z.112.
> Sep  7 09:12:27 dhcp1 dhcpd: DHCPREQUEST for x.y.z.112 (x.y.z.1) from 00:0f:93:00:10:ba via x.y.z.1: unknown lease x.y.z.112.
> Sep  7 09:12:27 dhcp1 dhcpd: DHCPDISCOVER from 00:0f:93:00:10:ba via x.y.z.1
> Sep  7 09:12:27 dhcp1 dhcpd: DHCPOFFER on x.y.z.112 to 00:0f:93:00:10:ba via x.y.z.1
> Sep  7 09:12:27 dhcp1 dhcpd: DHCPREQUEST for x.y.z.112 (x.y.z.1) from 00:0f:93:00:10:ba via x.y.z.1: unknown lease x.y.z.112.
> Sep  7 09:12:28 dhcp1 dhcpd: DHCPREQUEST for x.y.z.112 (x.y.z.1) from 00:0f:93:00:10:ba via x.y.z.1: unknown lease x.y.z.112.
> Sep  7 09:12:30 dhcp1 dhcpd: DHCPREQUEST for x.y.z.112 (x.y.z.1) from 00:0f:93:00:10:ba via x.y.z.1: unknown lease x.y.z.112.
> Sep  7 09:12:33 dhcp1 dhcpd: DHCPREQUEST for x.y.z.112 (x.y.z.1) from 00:0f:93:00:10:ba via x.y.z.1: unknown lease x.y.z.112.
> Sep  7 09:12:37 dhcp1 dhcpd: DHCPREQUEST for x.y.z.112 (x.y.z.1) from 00:0f:93:00:10:ba via x.y.z.1: unknown lease x.y.z.112.
> Sep  7 09:12:41 dhcp1 dhcpd: DHCPREQUEST for x.y.z.112 (x.y.z.1) from 00:0f:93:00:10:ba via x.y.z.1: unknown lease x.y.z.112.
> <<here the dhcp server restarts>>
> Sep  7 09:14:10 dhcp1 dhcpd: DHCPDISCOVER from 00:0f:93:00:10:ba via x.y.z.1
> Sep  7 09:14:11 dhcp1 dhcpd: DHCPOFFER on x.y.z.221 to 00:0f:93:00:10:ba via x.y.z.1
> Sep  7 09:14:11 dhcp1 dhcpd: DHCPREQUEST for x.y.z.221 (x.y.z.1) from 00:0f:93:00:10:ba via x.y.z.1
> Sep  7 09:14:11 dhcp1 dhcpd: DHCPACK on x.y.z.221 to 00:0f:93:00:10:ba via x.y.z.1
> >>
> I really do not understand why the server first offers an IP which it does not allow in the following DHCPREQUEST by the client. For me it sounds odd that the server tries to offer the 112 IP several times and then suddenly (after dhcp restart) offers 221 and everything works
> 
> Does anyone have an idea where we could check more to narrow down this problem?
> 
> Furthermore I wonder if there is a config setting in dhcpd.conf to force the dhcp to only give one IP per mac address. Sometimes we have the case that the same MAC seems to receive more than one IP.
> 
> Thanks a lot for any hint and have a nice weekend
> 
> tobi
> 
> 
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