[Dnsmasq-discuss] Client DHCP Configuration Question

[bad.cow5843 at fastmail.com]

Sorry I forgot to say that my /etc/dnsmasq.conf is entirely comments and this is the only file /etc/dnsmasq.d

# Never forward plain names (without a dot or domain part)
domain-needed
# Never forward addresses in the non-routed address spaces.
bogus-priv
# Add local-only domains here, queries in these domains are answered
# from /etc/hosts or DHCP only.
local=/localnet/
# If you want dnsmasq to listen for DHCP and DNS requests only on
# specified interfaces (and the loopback) give the name of the
# interface (eg eth0) here.
# Repeat the line for more than one interface.
interface=eth0

# Uncomment this to enable the integrated DHCP server, you need
# to supply the range of addresses available for lease and optionally
# a lease time. If you have more than one network, you will need to
# repeat this for each network on which you want to supply DHCP
# service.
dhcp-range=192.168.2.50,192.168.2.150,12h

# Always allocate the host with Ethernet address e8:6a:64:82:47:17
# The IP address 192.168.2.3
dhcp-host=e8:6a:64:82:47:17,192.168.2.3

# Always allocate the host with Ethernet address 00:26:9e:c1:4b:fa
# The IP address 192.168.2.1
dhcp-host=00:26:9e:c1:4b:fa,192.168.2.1

# Always allocate the host with Ethernet address 00:e0:4c:36:07:25
# The IP address 192.168.2.4
dhcp-host=00:e0:4c:36:07:25,192.168.2.4

# Do Router Advertisements and stateless DHCP for this subnet. Clients
# will not get addresses from DHCP, but they will get other
# configuration information. They will use SLAAC for addresses.

dhcp-range=fd15:89c9:e3d8:0000::, ra-stateless


On Thu, Jul 17, 2025, at 7:53 PM, bad.cow5843 at fastmail.com wrote:
> Dear dnsmasq people,
> 
> This is not a problem with dnsmasq itself, but I don't know where else to ask, so I am going ahead and I apologise in advance of it's off-topic.
> 
> I have been using dnsmasq for months with various clients and had no trouble at all, until I tried a Debian client. The problem I have is that the client attaches to two networks automatically: a wired one which is connected to my dnsmasq server, and a wifi connection to a router which is on another private subnet which is NOT routed by my server. But when both interfaces are up the client tries to use the wrong DNS server. It's the wrong one because it's not on the subnet my server is routing. If only one of either of the Debian client's interfaces is up it works fine.
> 
> So, is there anything I could potentially do in the dnsmasq configuration to stop this behaviour? Is it even the right behaviour? Maybe I should try raise it as a bug against Debian, but I am not sure about this.
> 
> Apologies again if this is off-topic, just ignore it and be thankful that I didn't put my questions in a zip file! :-)
> 
> Ian
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