[Dnsmasq-discuss] Client DHCP Configuration Question

[B F]

On Fri, 18 Jul 2025, at 04:53, 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.

Assuming your Debian is using Network Manager I was under the impression that you could modify the network configuration of your client so that the DNS settings received via DHCP on the particular SSID is ignored. 

There's a few ways of modifying things in Network Manager, but assuming your SSID is "MyWifi" and you wanted to modify things via the cli you could use something like

# View the current settings...scroll down to the "ipv4" section in the output
sudo nmcli connection show MyWifi 

# Turn off auto dns
sudo nmcli connection modify MyWifi ipv4.ignore-auto-dns on

I prefer to use "sudo nmtui" for Network Manager configuration. "nmtui" is a text based gui and it a bit easier to navigate than "nmcli".
 
Let us know how you go!

> 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

Berto.

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