[Dnsmasq-discuss] dhcp-host problem/questrions

[Simon Kelley]

On 19/12/12 11:06, Gene Czarcinski wrote:
> I need dsmasq's dhcp service to provide "nailed" (that is, fixed) IPv4
> and IPv6 addresses to a NIC on a specific system.  My reasons for
> needing this are to have a single point where this is specified amd to
> keep the general general usefullness while having a fixed address that
> can be used fro routing.
> 
> For IPv4, I have been using something like:
> dhcp-host=00:11:22:33:44:55,aloha,192.168.22.2,12h
> 
> and for IPv6, something like:
> dhcp-host=alpha,[fd00:10:11:12::2:2],12h
> 
> THe dnsmasq man page implies that the following is possible but I have
> not tried it:
> dhcp-host=00:11:22:33:44:55,alpha,192.168.22.2,12h,[fd00:10:11:12::2:2],12h
> 
> or maybe it should be specified as (I am not clear about this and have
> not tried it):
> dhcp-host=00:11:22:33:44:55,alpha,192.168.22.2,[fd00:10:11:12::2:2],12h
> 
> OK for IPv4 with the MAC-address specification, this seems to work very
> reliably and I have not had a problem.  However for IPv6 where the only
> thing used is the host's name, I have had a undesirable incident (cause
> unknown) where the IPv6 got a valid IPv6 address assigned by DHCPv6 but
> it wass not the "nailed" address while the IPv4 address continue to have
> effect.
> 
> I believe that once alpha got the wrong IPv6 address, it continued to as
> k for it and dnsmasq continued to provide it rather than the "nailed"
> address.
> 
> Is there a way to for dnsmasq to hand out the "nailed" IPv6 address no
> matter what alpha is asking for?  If not a guarantee  the at least
> improve the chance that the right thing is done.
> 
> That last example above, is that a good way?  Or, do  I need to use the
> dhcpv6.client-id?

You can't use the MAC address with DHCPv6, since the MAC address is not
provided to the DHCP server. (there is some standards activity that
might change this in some circumstances, but not at present)

Using the hostname is just as good, as long as the DHCPv6 client always
send the hostname. If not then the only alternative is the client-id.

There is a big difference between DHCPv4 and DHCPv6 that might affect
this. A DHCPv4 client can only have one DHCP lease and therefore one
DHCP address. If the client has a lease for address other then the
nailed address, dnsmasq, will refuse to renew the lease, forcing the
client to get a new address and move to the nailed address.

DHCPv6 is designed to provide a client with many addresses: just because
a client has a lease on a non-nailed address, doesn't stop if from
requesting the nailed address too, so the "refuse to renew lease"
behaviour detailed above for DHCPv4 is _not_ implemented  for DHCPv6.
This may be relevant in this case.

This is a manifestation of a bigger difference between IPv4 and IPv6.
DHCPv6 addresses are much more ephemeral, and a host will likely have
many of them at any time (from router advertisements, privacy
extensions, DHCPv6, ULA.......) Schemes which relay on fixed mappings
between hosts and IP addresses don't tend to work as well from IPv6.
It's not just IPv4 with bigger addresses. (sadly).


Simon.







> 
> Gene
> 
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