Today is one of those days where you want to get something "simple" done (like testing some new pptp conntrack helper code), and where everything goes wrong.
My test boxes are small embedded network booting devices. For some strange reason, they failed to obtain DHCP leases from the DHCP server.
Since I couldn't spot anything wrong while looking at the packets in ethereal, I added lots and lots of debug statements to the etherboot DHCP client code.
And there it was: etherboot refuses to accept a DHCPOFFER that doesn't have the "siaddr" field set in the DHCP/BOOTP header. According to the DHCP specifications (rfc1335, rfc2131), this indicates the address for the "next server in bootup process", i.e. tftp and alike.
A browse through the isc DHCP changelog indicated that version starting from 3.0.2 default this field to "0.0.0.0" unless "next-server" is explicitly set in dhcpd.conf.
Unfortunately the man-page states the exact opposite: That it defaults to the DHCPD's IP address.
After some more issues with some strange interaction between my USB2.0 hub, the ehci-hcd host and two different smartcard readers, I can probably finally start to do some real work..