[Dnsmasq-discuss] dnsmasq always answer dhcp NAK

Albert ARIBAUD albert.aribaud at free.fr
Fri Jan 20 22:55:13 GMT 2017

Hi again Nikita,

Le Fri, 20 Jan 2017 13:24:10 -0800
"Nikita N." <nikitan at operamail.com> a écrit:

> Hi Albert,
> thank you for your answer, but my config already has
> --dhcp-authoritative.

OK, then. Have you tested that it does indeed work? (and you have also
tested that the normal/correct DHCP leasing scenario indeed works?

> I will try to explain the problem in more details, showing the
> Wireshark-style "bugged" frame, popping up on the wire:
> -Ethernet II, Src: correct_mac_aa:bb:cc (mac_client), Dst:
> correct_gateway_dd:ee:ff (mac_gateway)
> -Internet Protocol Version 4, Src: (, Dst:
> (correct_gateway_ip)
> -User Datagram Protocol, Src Port: 68 (68), Dst Port: 67 (67)
> -Bootstrap Protocol (Request)
> --Client IP address: (
> --Your (client) IP address: (
> --Client MAC address: correct_mac_aa:bb:cc (mac_client)
> --Option: (53) DHCP Message Type (Request)
> --Option: (61) Client identifier
> --Option: (60) Vendor class identifier
> --Option: (55) Parameter Request List
> ---Parameter Request List Item: (1) Subnet Mask
> ---Parameter Request List Item: (121) Classless Static Route
> ---Parameter Request List Item: (33) Static Route
> ---Parameter Request List Item: (3) Router
> ---Parameter Request List Item: (6) Domain Name Server
> ---Parameter Request List Item: (15) Domain Name
> ---Parameter Request List Item: (28) Broadcast Address
> ---Parameter Request List Item: (51) IP Address Lease Time
> ---Parameter Request List Item: (58) Renewal Time Value
> ---Parameter Request List Item: (59) Rebinding Time Value
> ---Parameter Request List Item: (119) Domain Search
> --Option: (255) End
> The mac correct_mac is the correct mac of the bugged client, that is
> always correct.
> The ip is the bug, this value changes randomly time by time
> (no workaround), it can be anything: but luckily is coherent (same)
> in the relevant positions of the single DHCP frame.

And it always matches the IP layer address? Because if it does, then
the frame is valid; that is what a machine moved from one subnet to
another might do, and a DHCP server (dnsmasq or other) is designed to
handle this.

> Finally, as you notice, the relevant "Option: (50) Requested IP
> Address" is always missing.

IIUC option 50 is not required, so I don't think its absence causes
dnsmasq to skip answering this.

> What I need is: dnsmasq sends a DHCP Answer NAK with
> Dst:correct_mac_aa:bb:cc (and possibly also ip Dst: whatever)
> How can I set this?

That's what --dhcp-authoritative is about. Hence my suggestion to
test with a working client that the server is indeed running in
authoritative mode.

> Thanks


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