[Dnsmasq-discuss] Query about solving a DHCPNAK issue

Mon Jun 1 13:22:49 BST 2015

>That's my opinion too (although I can see a flip side to the coin: if
>clients honor NAKs from any sever, then a rogue machine could break a
>local noetwork by NAKing any and all DHCPREQUESTs, effectively DoSing
>the whole segment at least).

Oh definitely. DHCPNAKs are definitely not a good way to protect against
rogue DHCP servers. Packet filtering (e.g. DHCP snooping or UDP drop rules)
are much better at that. I was just pointing out that if we are teaching
clients to ignore NAKs from other servers, we might as well have the
servers stop sending NAKs in response to traffic to other servers. "dhcp
authoritative" in the dnsmasq case would then just mean it accepts any
renewal.

>RFC 2131 says a server SHOULD send a DHCPNAK if it detects a DHCP
>request for e.g. the wrong network. It also says "If the client
>receives a DHCPNAK message, the client restarts the configuration
>process", which, while there is no "MUST" in it, seems to me like a
>non-optional requirement to honor DHCPNAKs.

Right, so then wouldn't the referenced busy-box patch be making the client
non-compliant?
http://git.busybox.net/busybox/commit/?id=e2318bbad786d6f9ebff704490246bfe52e588c0

On Mon, Jun 1, 2015 at 2:51 AM, Albert ARIBAUD <albert.aribaud at free.fr>
wrote:

> Hi Kevin,
>
> Le Mon, 1 Jun 2015 02:02:27 -0700, Kevin Benton <blak111 at gmail.com> a
> écrit :
>
> > I understand, but that eliminates the whole 'correcting rouge dhcp
> offers'
> > part of the authoritative mode.
> >
> > If we are teaching clients to ignore NAKs from other DHCP servers, why do
> > DHCP servers like dnsmasq generate them in the first place? Wouldn't it
> be
> > logically consistent to make a change to dnsmasq to prevent it from
> > generating a NAK if the client is communicating with a different server?
>
> That's my opinion too (although I can see a flip side to the coin: if
> clients honor NAKs from any sever, then a rogue machine could break a
> local noetwork by NAKing any and all DHCPREQUESTs, effectively DoSing
> the whole segment at least).
>
> > Is the client behavior regarding NAKs outlined in one of the DHCP RFCs?
> It
> > does seem like there is a lot of confusion around what authoritative
> > servers should be doing.
>
> RFC 2131 says a server SHOULD send a DHCPNAK if it detects a DHCP
> request for e.g. the wrong network. It also says "If the client
> receives a DHCPNAK message, the client restarts the configuration
> process", which, while there is no "MUST" in it, seems to me like a
> non-optional requirement to honor DHCPNAKs.
>
> > On Tue, May 26, 2015 at 10:40 PM, Vladislav Grishenko <themiron at mail.ru>
> > wrote:
> >
> > > Hi Kevin,
> > >
> > > Ignoring all naks – would be, but the fix is different.
> > >
> > > That fix ignores all naks except from the selected/requested server
> only,
> > > it’s ok.
> > >
> > >
> > >
> > > Best Regards, Vladislav Grishenko
> > >
> > >
> > >
> > > *From:* Kevin Benton [mailto:blak111 at gmail.com]
> > > *Sent:* Wednesday, May 27, 2015 9:32 AM
> > > *To:* Vladislav Grishenko
> > > *Cc:* Brian Haley; Simon Kelley;
> dnsmasq-discuss at lists.thekelleys.org.uk
> > > *Subject:* Re: [Dnsmasq-discuss] Query about solving a DHCPNAK issue
> > >
> > >
> > >
> > > That fix is interesting. Doesn't ignoring a NAK sort of defeat the
> point
> > > of the 'authoritative' NAKing in the first place?
> > >
> > >
> > >
> > > On Tue, May 26, 2015 at 2:26 PM, Vladislav Grishenko <themiron at mail.ru
> >
> > > wrote:
> > >
> > > > On 02/02/2015 05:47 PM, Brian Haley wrote:
> > > > >>
> > > > >>> The one thing I'm curious about is if dnsmasq is restarted while
> a
> > > > >>> VM holds a lease, how will it respond?  As someone else has
> > > > >>> pointed-out to me - isc-dhcp will respond with a DHCPNAK in that
> > > > >>> case, and wondered why there would be a difference with dnsmasq.
> > > > >>> Different interpretation of an RFC?
> > > > >>
> > > > >>
> > > > >> If by "dnsmasq is restarted" you mean "dnsmasq is restarted and
> > > > >> therefore has its lease database deleted", then the RFC says that
> if
> > > > >> a server gets a renewal for an unknown lease, it should return
> > > > >> DHCPNAK. That's what dnsmasq does _unless_ --dhcp-authoritative is
> > > > >> set, when instead it quietly re-creates the lease.
> > > > >
> > > > > Yes, your assumption is correct, as --leasefile-ro is used it
> knows of
> > > > > no current leases, and by default get a DHCPNAK.
> > > > >
> > > > >> dhcp-authoritative gives permission to dnsmasq to violate the RFC
> in
> > > > >> a way which is useful in certain circumstances.
> > > > >
> > > > > Thanks, it does seem to do what I want with my initial testing.
> > > >
> > > > Hi Simon,
> > > >
> > > > Replying to my old thread since using --dhcp-authoritative seems to
> have
> > > > introduced an issue where a DHCP client can get a NAK when using
> multiple
> > > > dnsmasq servers on the same subnet (they both have the same host
> > > > information, >1 running just to get HA).
> > > >
> > > > Short story is that both dnsmasq's return the same lease info, but
> when
> > > the
> > > > client ACKs (sending to broadcast), one agent ACKs and the other
> agent
> > > > NAKs.
> > > > The tcpdump shows this better than I'm describing:
> > > >
> > > > https://launchpadlibrarian.net/207180476/dhcp_neutron_bug.html
> > > >
> > > > Does that seem like normal operation to you?  Does this second
> dnsmasq
> > > > assume this response is from a rogue server and NAKs since it didn't
> send
> > > out
> > > > the offer?
> > > >
> > >
> > > Hi Brian,
> > >
> > > Second dnsmasq assume the client request is to another server and
> responds
> > > with NAK in authoritative mode.
> > > The root of loop issue is in that busybox 1.20.x udhcpc client, it
> doesn't
> > > check server id for anything but offer packet.
> > > Bug is already fixed in bb 1.23.x, see commit
> > >
> > >
> http://git.busybox.net/busybox/commit/?id=e2318bbad786d6f9ebff704490246bfe52
> > > e588c0
> > >
> > > Best Regards, Vladislav Grishenko
> > >
> > >
> > >
> > >
> > > _______________________________________________
> > > Dnsmasq-discuss mailing list
> > > Dnsmasq-discuss at lists.thekelleys.org.uk
> > > http://lists.thekelleys.org.uk/mailman/listinfo/dnsmasq-discuss
> > >
> > >
> > >
> > >
> > >
> > > --
> > >
> > > Kevin Benton
> > >
> >
> >
> >
>
>
> Amicalement,
> --
> Albert.
>

-- 
Kevin Benton
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