RESOLVED: dhcpd sporadically ignoring one of two "copies" of a DHCP request

Phil Mayers p.mayers at
Wed Mar 12 11:33:42 UTC 2014

On 12/03/14 11:21, Simon Hobson wrote:
> Phil Mayers <p.mayers at> wrote:
>> This would solve a number of problems (e.g. giaddr is not routeable because it's a local-scoped RFC1918 block)
> You mean, it would workaround (not fix) a broken IP network ;-)

No, I don't mean that.

> By definition, if you have an address in use which is not routable, then your IP networking is broken - don't even get me started on NAT !

No. It is not routeable *from the DHCP server*. That doesn't mean it's 
not routeable anywhere. It might be an RFC1918 range policy-routed into 
a TTW registration server inside your POP, but not inside the backbone.

Or it might be routed down a GRE tunnel and you can't send the DHCP 
packets that way because the (ahem) "interesting" networking code in ISC 
dhcpd doesn't use a UDP socket like normal people, and can thus only 
receive packets on interface types someone has bothered to code for.

(and the last time I tried USE_SOCKETS it was a mess of compile errors 
on Linux)

Or you might be using an MPLS L3VPN, but punting DHCP packets into a 
separate "services" VRF, and want the replies to go back to the PE 
router IP inside the services VRF, not the giaddr in the customer VRF.

There are a bunch of reasons to want to send DHCP replies to addresses 
which aren't giaddr, and none of them constitute "broken" IP network 


More information about the dhcp-users mailing list