[Dnsmasq-discuss] dnsmasq not responding to DHCPv6 DHCPCONFIRM messages attempting to confirm an unknown lease

Michael Marley michael at michaelmarley.com
Fri Jul 20 15:43:32 BST 2018


On 2018-07-20 09:01, Michael Marley wrote:

> Hi,
> 
> I have dnsmasq set to be a DHCPv6 and DHCPv6 server for my local network.  Here is the relevant part of the configuration:
> 
> interface=vlan1
> interface=vlan2
> interface=vlan3
> interface=vlan4
> interface=vlan5
> dhcp-authoritative
> dhcp-range=interface:vlan1,10.1.0.2,10.1.255.253,1h
> dhcp-range=interface:vlan2,10.2.0.2,10.2.255.253,1h
> dhcp-range=interface:vlan3,10.3.0.2,10.3.255.253,1h
> dhcp-range=interface:vlan4,10.4.0.2,10.4.255.253,1h
> dhcp-range=interface:vlan5,10.5.0.2,10.5.255.253,1h
> dhcp-range=interface:vlan1,fdda:5f29:421b:1::2,fdda:5f29:421b:1::ffff,64,1h
> dhcp-range=interface:vlan2,fdda:5f29:421b:2::2,fdda:5f29:421b:2::ffff,64,1h
> dhcp-range=interface:vlan3,fdda:5f29:421b:3::2,fdda:5f29:421b:3::ffff,64,1h
> dhcp-range=interface:vlan4,fdda:5f29:421b:4::2,fdda:5f29:421b:4::ffff,64,1h
> dhcp-range=interface:vlan5,fdda:5f29:421b:5::2,fdda:5f29:421b:5::ffff,64,1h
> 
> My problem is that if a DHCPv6 client attempts to confirm an unknown lease (from another network, for example if I unplug a computer from vlan4 and plug it into vlan3), dnsmasq doesn't respond to the DHCPCONFIRM messages that the client sends.  I just get a long string of
> 
> DHCPCONFIRM(vlan4) 00:01:00:01:20:65:91:d7:3c:97:0e:7f:f5:ba
> 
> until the client finally gives up and acquires a new lease from scratch.  This also happens if I connect a client to my network that was previously connected to another network with a DHCPv6 lease.  For DHCPv4 in the same situation, it works correctly and sends a DHCPNAK, causing the client to retry from scratch immediately and get a new lease quickly as described in the documentation for the "dhcp-authoritative" option.  It seems to me this ought to take place for DHCPv6 as well, preventing the client from having to time out before obtaining a lease.  Have I configured something wrong or is there a bug?
> 
> Thanks,
> 
> Michael Marley
> 
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I apologize for my ineptness, but it seems dnsmasq does actually send a
NOTONLINK reply in this case (rfc3315.c:1105).  It just doesn't log that
it is doing so, which is what led me to believe that it wasn't.  My
actual problem appears to be caused by defective/bugged DHCPv6 snooping
on a switch, which I will report to the manufacturer.  I do think that
logging the NOTONLINK reply would probably be a good idea though, to
avoid confusion. 

Michael
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