[Dnsmasq-discuss] DHCP and DNS regestration

[Geert Stappers]

On Mon, Mar 24, 2025 at 10:18:23PM +0000, Simon Kelley wrote:
> 
> 
> On 3/24/25 09:47, support | IT visibility via Dnsmasq-discuss wrote:
> > Hi *,
> > 
> > I’m using dnsmasq with a few vlan interfaces and each interface is tagged.
> > 
> > And running tests with pihole as well as native dnsmasq.
> > 
> > An example on how this is setup in both cases (.i.e. pihole as well as
> > native dnsmsaq):
> > 
> > =====
> > # Wired - vlan 210
> > interface=eth0.210
> > domain=wired.lan,192.168.210.0/24,local
> > dhcp-range=set:vlan-210,192.168.210.11,192.168.210.99,168h

See below


> > dhcp-option=tag:vlan-210,option:router,192.168.210.241
> > dhcp-option=tag:vlan-210,option:dns-server,192.168.210.235
> > dhcp-option=tag:vlan-210,option:ntp-server,192.168.210.235
> > dhcp-option=tag:vlan-210,option:domain-name,wired.lan
> > =====
> > 
> > The idea behind this is that devices with multiple vlan interfaces have
> > their IP’s registered with dhcp and dns.
> > 
> > The dhcp part seems to work – all interfaces have the correct IP details.
> > 
> > However, the log has numerous wrong-address errors like this (i.e.
> > pihole as well as native dnsmasq):
> > 
> > =====
> > Mar 24 09:02:03 dnsmasq-dhcp[49]: DHCPREQUEST(eth0.210) 192.168.210.201 bc:24:11:87:be:e5
> > Mar 24 09:02:03 dnsmasq-dhcp[49]: DHCPNAK(eth0.210) 192.168.210.201 bc:24:11:87:be:e5 wrong address
> > Mar 24 09:02:03 dnsmasq-dhcp[49]: DHCPDISCOVER(eth0.210) bc:24:11:87:be:e5
> > Mar 24 09:02:03 dnsmasq-dhcp[49]: DHCPOFFER(eth0.210) 192.168.210.201 bc:24:11:87:be:e5
> > Mar 24 09:02:03 dnsmasq-dhcp[49]: DHCPREQUEST(eth0.210) 192.168.210.201 bc:24:11:87:be:e5
> > Mar 24 09:02:03 dnsmasq-dhcp[49]: DHCPACK(eth0.210) 192.168.210.201 bc:24:11:87:be:e5 osiris
> > =====
> > 
> > *Question-1*: where are these wrong-address messages coming from?
> > 
> > If I look at the dhcp.leases (Pihole) or dnsmasq.leases (native dnsmasq)
> > there is always one entry like this:
> > 
> > 1743408124 bc:24:11:87:be:e5 192.168.230.201 osiris *
> > 
> > *Question-2*: why is there only an entry from the last vlan interface of
> > (in this example) osiris? And not one for every vlan interface?
> > 
> > *Question-3*: the same applies for dns regestrations where there is only
> > an entry for the last vlan interface?
> > 
> > I have tried to force this dns regestration for all fqdn’s for each vlan
> > using the following settings (i.e. pihole as well as native dnsmasq):
> > 
> > dhcp-ignore-clid
> > dhcp-fqdn
> > dhcp-client-update
> > expand-hosts
> > domain-needed
> > domain=lan
> > local=/lan/
> > 
> > Does the above ring any bells? Any suggestion?

4: Does the above ring any bells?  5: Any suggestion?


> > With warm regards – Will
> > 
> > 
> 
> Answer 1: Because a lease exists for the relevant MAC address with a
> different IP address to the address it's requesting.
> 
> Answer 2: Because the device is using the same MAC address on all the VLANS
> it's talking through. The MAC address is supposed to be unique: a DHCP
> server won't allow a MAC address to have more than one lease.
> 
> Answer 3: Partly the same answer as 2, but even if you use unique MAC
> addresses (or client-ids) dnsmasq will only associate a DNS name with at
> most one DHCP lease. That's a design decision.
> 

4: The bell of my early days with dnsmasq   :-)
5: Check the manual page,
   it has something like gratuitous tag for interface name

The
> > # Wired - vlan 210
> > interface=eth0.210
> > domain=wired.lan,192.168.210.0/24,local
> > dhcp-range=set:vlan-210,192.168.210.11,192.168.210.99,168h
> > dhcp-option=tag:vlan-210,option:router,192.168.210.241
> > dhcp-option=tag:vlan-210,option:dns-server,192.168.210.235
> > dhcp-option=tag:vlan-210,option:ntp-server,192.168.210.235
> > dhcp-option=tag:vlan-210,option:domain-name,wired.lan

can be
> > # Wired - vlan 210
> > interface=eth0.210
> > domain=wired.lan,192.168.210.0/24,local
> > dhcp-range=tag:eth0.210,192.168.210.11,192.168.210.99,168h
> > dhcp-option=tag:eth0.210,option:router,192.168.210.241
> > dhcp-option=tag:eth0.210,option:dns-server,192.168.210.235
> > dhcp-option=tag:eth0.210,option:ntp-server,192.168.210.235
> > dhcp-option=tag:eth0.210,option:domain-name,wired.lan



Groeten
Geert Stappers
-- 
Silence is hard to parse