[Dnsmasq-discuss] disable Router does not work

[Simon Kelley]

Jürgen Steinel wrote:
> Hi,
> 
> today I wanted to disable routing for a single host. That machine gets
> its IP from dnsmasq.
> 
> In the dnsmasq.conf I added an entry to assign the host to a
> network-group and set an option for this group that should disable the
> gateway.
> 
> dhcp-host=<host>,<ip>,net:norouting
> dhcp-option=norouting,3,0.0.0.0
> 
> Unfortunately this did not have the expected effect. The host got the IP
> mentioned in dhcp-host but was still told where the (actual) router is.
> 
> I captured the dhcp-request/reply to make sure the Information came from
> dnsmasq (and was not cached by the client). Dnsmasq *did* deliver the
> Information.
> 
> To test if dhcp-option does anything at all I changed the above line to
> this:
> 
> dhcp-option=norouting,3,1.2.3.4
> 
> After requesting a new IP the client got 1.2.3.4 as default gateway.
> 
> 
> I think this behaviour is wrong (=ignore option if given IP is 0) and
> even if it is intented it should be mentioned in the manual.
> 
> Is there a different way to set an option to a zero-value?
> 

Short answer:

dhcp-option=norouting,3

long answer:

Using 0.0.0.0 doesn't work, since that's used as shorthand for "the 
address of the host running dnsmasq" - you saw this in your experiments.

For most DHCP options, doing

dhcp-option=<option number>

will send the option, with no extra data, but for the four special 
options which dnsmasq sends by default (router, DNS server, netmask and 
broadcast address) then  doing

dhcp-option=<option number>

completely suppresses sending the default.

I hope that this is actually more logical than my explanation of it....


Cheers,

Simon.