Strange DHCP behaviour

[Bob Harold]

On Wed, Oct 24, 2018 at 7:15 AM tony <lists at vanderhoff.org> wrote:

> Hi,
> I'm running a DHCP server (4.3) under Debian Stretch to serve a small
> local domain. It behaves well for most clients, but I have a laptop
> (also Debian Stretch) for which I wish to assign a fixed address. My
> server configuration is this (tony-lt being the laptop):
> ------------------------------------------------------------------
> # The ddns-updates-style parameter controls whether or not the server will
> # attempt to do a DNS update when a lease is confirmed. We default to the
> # behavior of the version 2 packages ('none', since DHCP v2 didn't
> # have support for DDNS.)
> ddns-update-style none;
>
> # option definitions common to all supported networks...
> #option domain-name "example.org";
> option domain-name "chalet.vanderhoff.org";
> #option domain-name-servers ns1.example.org, ns2.example.org;
> option domain-name-servers 192.168.1.1;
>
> default-lease-time 600;
> max-lease-time 7200;
>
> # If this DHCP server is the official DHCP server for the local
> # network, the authoritative directive should be uncommented.
> authoritative;
>
> # Use this to send dhcp log messages to a different log file (you also
> # have to hack syslog.conf to complete the redirection).
> log-facility local7;
>
> # No service will be given on this subnet, but declaring it helps the
> # DHCP server to understand the network topology.
>
> #subnet 10.152.187.0 netmask 255.255.255.0 {
> #}
>
> # This is a very basic subnet declaration.
>
> 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;
> }
>
> host tony-lt
> {
>         hardware ethernet 0c:60:76:6c:e6:6f;
>         fixed-address 192.168.1.199;
> }
> ------------------------------------------------------------------
>
> Despite this configuration I have failed to get that address for the
> laptop; it seems to be stuck on 192.168.1.254, currently. It may have
> legitimately had that address in the past.
>
> I have stopped and cleared the client via 'dhclient -r', deleted the
> files in /var/lib/dhclient, before restarting it with 'dhclient -v, It
> immediately requests ..254
>
> I'm seeing this in the server log:
> ------------------------------------------------------------------
> Oct 24 12:52:04 router-pi dhcpd[1068]: DHCPREQUEST for 192.168.1.254
> from 0c:60:76:6c:e6:6f (tony-lt) via eth0
> Oct 24 12:52:04 router-pi dhcpd[1068]: DHCPACK on 192.168.1.254 to
> 0c:60:76:6c:e6:6f via eth0
> Oct 24 12:52:24 router-pi dhcpd[1068]: DHCPREQUEST for 192.168.1.244
> from f4:81:39:82:53:c4 via eth0
> Oct 24 12:52:24 router-pi dhcpd[1068]: DHCPACK on 192.168.1.244 to
> f4:81:39:82:53:c4 via eth0
> Oct 24 12:52:32 router-pi dhcpd[1068]: DHCPREQUEST for 192.168.1.254
> from 0c:60:76:6c:e6:6f via eth0
> Oct 24 12:52:32 router-pi dhcpd[1068]: DHCPACK on 192.168.1.254 to
> 0c:60:76:6c:e6:6f (tony-lt) via eth0
> Oct 24 12:52:41 router-pi dhcpd[1068]: reuse_lease: lease age 9 (secs)
> under 25% threshold, reply with unaltered, existing lease for 192.168.1.254
> Oct 24 12:52:41 router-pi dhcpd[1068]: DHCPREQUEST for 192.168.1.254
> from 0c:60:76:6c:e6:6f (tony-lt) via eth0
> Oct 24 12:52:41 router-pi dhcpd[1068]: DHCPACK on 192.168.1.254 to
> 0c:60:76:6c:e6:6f (tony-lt) via eth0
> ------------------------------------------------------------------
>
> Apart from the rather frequent lease request, which I suspect is due to
> NetworkManager on this laptop, what is the 'reuse-lease' comment?
>
> Has anyone any suggestions on how to fix this, please?
>
> Cheers, Tony
>

In this situation I typically create a 'fake' host reservation for the old
IP with a dummy MAC address, which forces the DHCP server to refuse to give
out that address.  Once the laptop gets the right IP, then the fake entry
can be removed.

-- 
Bob Harold
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