[Dnsmasq-discuss] Same IP for a range of MAC addresses?
tmetro+dnsmasq at gmail.com
Sat Apr 4 00:29:46 BST 2009
Stephen Lee wrote:
> I have a linux box running dnsmasq that needs to assign the
> same IP to whichever dhcp client it happens to be connected to.
Can you elaborate on your setup? It sounds rather unusual.
> Is it possible to for dnsmasq to assign the same IP to a range of MAC
I think I ran across the answer to that in the FAQ recently (see below),
but it sounds like you have to list each MAC address and I wonder if
there is a better solution for your situation. Can you get the clients
to issue a consistent ID or host name?
I thought maybe you could use dhcp-mac to map from MAC addresses to a
network ID tag, but dhcp-host doesn't use network ID tag as a selector
(instead it does the equivalent of a dhcp-mac to apply the tag for the
benefit of other options). Which is logical, given that network ID tags
are meant to apply to groups of machines, and dhcp-host is normally
meant to apply to a single machine.
Q: My laptop has two network interfaces, a wired one and a wireless
one. I never use both interfaces at the same time, and I'd like the
same IP and configuration to be used irrespective of which
interface is in use. How can I do that?
A: By default, the identity of a machine is determined by using the
MAC address, which is associated with interface hardware. Once an
IP is bound to the MAC address of one interface, it cannot be
associated with another MAC address until after the DHCP lease
expires. The solution to this is to use a client-id as the machine
identity rather than the MAC address. If you arrange for the same
client-id to sent when either interface is in use, the DHCP server
will recognise the same machine, and use the same address. The
method for setting the client-id varies with DHCP client software,
dhcpcd uses the "-I" flag. Windows uses a registry setting,
From version 2.46, dnsmasq has a solution to this which doesn't
involve setting client-IDs. It's possible to put more than one MAC
address in a --dhcp-host configuration. This tells dnsmasq that it
should use the specified IP for any of the specified MAC addresses,
and furthermore it gives dnsmasq permission to sumarily abandon a
lease to one of the MAC addresses if another one comes along. Note
that this will work fine only as longer as only one interface is
up at any time. There is no way for dnsmasq to enforce this
constraint: if you configure multiple MAC addresses and violate
this rule, bad things will happen.
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