[Dnsmasq-discuss] using dchp-option 121 breaks default route used with public address range

Simon Kelley simon at thekelleys.org.uk
Tue Aug 18 14:15:57 BST 2009

Tim Allingham wrote:
> I'm not sure if this issue is being caused by my ignorance, or if its 
> actually a bug/issue with dnsmasq
> what I'm trying to achieve is routing particular public addresses via a 
> certain router - essentially need to push out a static route to a 
> machine connected to 2 networks,  both with internet access, and 
> enforcing a connection to the mail server of one ISP to be routed via 
> the correct connection.
> when I add the following line to dnsmasq
> dhcp-option=121,,
> The default route is lost on the client, leaving me with no routing 
> rules for public addresses outside of the address range.
> adding the line
> dhcp-option=3,
> has no effect on the routing either.  is there any reason that I can't 
> force routing in this manner, or is there something else I'm missing?

The only role dnsmasq has in this is to include the information you 
supply into the DHCP packets its sends. The interesting and variable bit 
is what the client on the other end does with the information.

What OS is the client running, and what DHCP client?

Note that sending option 121 will not stop dnsmasq sending the normal 
default route as option 3, but it may well cause the client to ignore 
that. Have you tried setting a general default route in the option 121 
with something like

dhcp-option=121,,,,<normal default 
router IP>



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