[Dnsmasq-discuss] [PATCH] DHCPv6: Add support for more than one hardware address per IPv6 address

[Pali Rohár]

On Saturday 02 June 2018 15:48:58 Pali Rohár wrote:
> On Tuesday 23 May 2017 09:39:11 Pali Rohár wrote:
> > On Monday 22 May 2017 23:11:02 Simon Kelley wrote:
> > > On 12/05/17 16:32, Pali Rohár wrote:
> > > > On Friday 12 May 2017 17:15:20 Simon Kelley wrote:
> > > >> There are so many layers of quotes here that I've completely lost
> > > >> track of what we were trying to achieve, and how to achieve it. My
> > > >> memory is that we'd failed to come up with any consensus on either
> > > >> of those.
> > > > 
> > > > Goal 1:
> > > > 
> > > > Ability to assign one IPv4 address to two different MAC addresses. 
> > > > Currently it is possible by misusing concept of "more mac addresses" 
> > > > (where IPv4 address can be "steal" by later DHCP client).
> > > > 
> > > > Goal 2:
> > > > 
> > > > Achieve Goal 1 also for DHCPv6.
> > > > 
> > > >> Using MAC addresses with DHCPv6 AT ALL is quite difficult - it's not
> > > >> a concept that the RFCs deal with.
> > > > 
> > > > I read DHCPv6 RFC and it does not refuse assigning IPv6 address based on 
> > > > link layer MAC address. Anyway, this is already supported by dnsmasq.
> > > > 
> > > > But what I want to achieve has ability to assign one IPv6 address to 
> > > > more MAC addresses at same time. This DHCPv6 RFC does not allow, but in 
> > > > some situations it is useful and I think such options could be provided 
> > > > by DHCPv6 server with explicit warning in documentation.
> > > > 
> > > >> Doing the sleight-of-hand trick
> > > >> that works with DHCPv4 doesn't seem feasible to me for DHCPv6.
> > > > 
> > > > Do you completely disagree with fact that dnsmasq could support this 
> > > > scenario for assigning one IP address to more network cards (identified 
> > > > by MAC address)? Or you just do not like my current implementation?
> > > 
> > > The whole point of DHCP is to avoid an IP address being used by more
> > > than one network card. The current two-MAC addresses for one IP facility
> > > in DHCPv4 doesn't contradict this. It's specified to be used only when
> > > there's a guarantee that both MAC address are never simultaneously in use.
> > 
> > I know. But as I wrote, lot of people misuse this feature to assign one
> > IPv4 address to more network cards. As there is use case for such state
> > and dnsmasq can do it.
> > 
> > So instead of misusing that feature I'm asking how to implement it
> > properly.
> 
> Hi Simon!
> 
> Do you have any opinion about this? Or do you fully disagree and such
> feature should not be in dnsmasq?
> 
> > > Cheers,
> > > 
> > > Simon.
> > > 
> > > 
> > > > 
> > > > In previous email I wrote that Goal 2 can be achieved by storing tuple 
> > > > DUID, IAID, MAC address and IPv6 address into DHCPv6 leases file.
> > > > 
> > 
> 
> In IPv6 it is a more complicated, e.g. when network administrator wants
> to assign one IPv6 address for specific computer.
> 
> Imagine that you have one computer with more OS (dua-boot) and each OS
> has its own DUID and IAID (MAC address is stable).
> 
> Problem: dnsmasq assign IPv6 address to that computer when OS1 is
> running. Computer is then rebooted to OS2 which has different DUID and
> IAID. Therefore dnsmasq assigns a new (different) IPv6, because old one
> is still "used" in server lease file.
> 
> To "solve" this problem it is either needed to extend dnsmasq to allow
> assigning one IPv6 address to more DUIDs/IAIDs.
> 
> Or to assign IPv6 addresses based on MAC address and then dnsmasq leases
> file needs to be extended to included also MAC address for IPv6
> addresses.

Currently in lease file for DHCPv6 records there is line:

  expire_time iaid ipv6_addr hostname duid

and for DHCPv4 is:

  expire_time mac ipv4_addr hostname clid

To have similar format DHCPv6 records as DHCPv4 could be changed and
extended for mac address to:

  expire_time mac ipv6_addr hostname duid iaid

Or to have iaid on same position, to:

  expire_time iaid ipv6_addr hostname duid mac

And then allow assigning IPv6 address for IAID and correctly from lease
file for IPv6 address takes value relevant for configuration. E.g. when
IPv6 address is assigned based on MAC address, took mac. When is
assigned for DUID, then duid. And when iaid, then IAID.

So when configured this would allow "stealing" IPv6 address when there
is one computer which uses two different DHPv6 clients with different
DUIDs or IAIDs. (E.g. dual-boot Linux-Windows setup).


Also this extended information in lease file could allow to implement
that assigning one IPv6 address to more MAC addresses properly as in
lease file would be all relevant information about dhcp client.

-- 
Pali Rohár
pali.rohar at gmail.com
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