[Dnsmasq-discuss] DHCP Issue

Simon Kelley simon at thekelleys.org.uk
Sat Jul 21 15:54:34 BST 2007

Steven wrote:
> Hi,
> I have a problem setting up a dhcp server using dnsmasq.
> Seems that the server is working fine but the client are not able to
> get an ip address.
> The interface on the server is configured to have a static IP address,
> and on the client DHCP is used (obviously).
> The client sends a DHCPDISCOVER packet, dnsmasq receives it and sends
> a DHCPOFFER packet, but the client doesn't recognizes this packet (i
> think).
> With other dhcp server (as dhcp3-server) everything is working fine,
> so i suspect this may be an issue with dnsmasq or my config file.
> I'm using Ubuntu, Feisty Fawn, Linux steven-laptop 2.6.20-16-generic
> #2 SMP Thu Jun 7 20:19:32 UTC 2007 i686 GNU/Linux

The explanation for dhclient ignoring the DHCPOFFERS seems to be that 
the UDP checksum is incorrect. Is the Ubuntu system the client, or the 
server? The checksum problem seems to be  originating on the server, and 
if the server is running Ubuntu (or any Linux distro) then it's a kernel 
problem: on Linux dnsmasq delegates UDP and IP header creation to the 
kernel. If you are running dnsmasq on *BSD, then it could be a dnsmasq 
problem: on those platforms (some of) the UDP and IP headers are created 
by dnsmasq itself.



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