Dynamically assign IP based on MAC

Hi all,

I understand how to assign static IP addresses with DHCP
based on MAC. But here's what I'd like to do: I'd like to dynamically
assign an IP address to a MAC based on some rule (the rule really
is: look up this MAC in a table and give the IP address from that
table,
unless it doesn't exist, in which case assign a new one and enter it
into the table). In other words, you'll get the same IP every time
you
connect with the same MAC, but we don't know all MACs beforehand.

DEBEDb wrote:
> Hi all,
>
> I understand how to assign static IP addresses with DHCP
> based on MAC. But here's what I'd like to do: I'd like to dynamically
> assign an IP address to a MAC based on some rule (the rule really
> is: look up this MAC in a table and give the IP address from that
> table,
> unless it doesn't exist, in which case assign a new one and enter it
> into the table). In other words, you'll get the same IP every time
> you
> connect with the same MAC, but we don't know all MACs beforehand.
>

IME DHCP tends to give the same IP to the same machine unless it
gets starved for addresses and reuses it. Not sure what the underlying
rules are though.

Joe Beanfish wrote:
> DEBEDb wrote:
>
>> Hi all,
>>
>> I understand how to assign static IP addresses with DHCP
>> based on MAC. But here's what I'd like to do: I'd like to dynamically
>> assign an IP address to a MAC based on some rule (the rule really
>> is: look up this MAC in a table and give the IP address from that table,
>> unless it doesn't exist, in which case assign a new one and enter it
>> into the table). In other words, you'll get the same IP every time you
>> connect with the same MAC, but we don't know all MACs beforehand.
>>
>
> IME DHCP tends to give the same IP to the same machine unless it
> gets starved for addresses and reuses it. Not sure what the underlying
> rules are though.

The request for the old address comes from the client. The
DHCP server usually grants it, unless it it already leased
to somewhere else.

--

Tauno Voipio
tauno voipio (at) iki fi

Tauno Voipio wrote:
> Joe Beanfish wrote:
>> DEBEDb wrote:
>>
>>> Hi all,
>>>
>>> I understand how to assign static IP addresses with DHCP
>>> based on MAC. But here's what I'd like to do: I'd like to dynamically
>>> assign an IP address to a MAC based on some rule (the rule really
>>> is: look up this MAC in a table and give the IP address from that table,
>>> unless it doesn't exist, in which case assign a new one and enter it
>>> into the table). In other words, you'll get the same IP every time you
>>> connect with the same MAC, but we don't know all MACs beforehand.
>>>
>>
>> IME DHCP tends to give the same IP to the same machine unless it
>> gets starved for addresses and reuses it. Not sure what the underlying
>> rules are though.
>
> The request for the old address comes from the client. The
> DHCP server usually grants it, unless it it already leased
> to somewhere else.
>

Even after having been turned off?

Hello,

Joe Beanfish a écrit :
>>
>> The request for the old address comes from the client. The
>> DHCP server usually grants it, unless it it already leased
>> to somewhere else.
>
> Even after having been turned off?

That's what lease files are for.

On Jan 31, 2:28 pm, Joe Beanfish <j...@nospam.duh> wrote:
> Even after having been turned off?

A well-behaved server (e.g, ISC) will remember the IP it last gave the
client and offer it again unless its run out of free addresses and
assigned it to another host.

You're describing "automatic allocation" as defined in the RFC:

DHCP supports three mechanisms for IP address allocation. In
"automatic allocation", DHCP assigns a permanent IP address to a
client.

In the server settings, set your leases to something long (or
infinite, if you're *really* sure this is what you want) and make your
pool of addresses big.

David Nicoson wrote:
> On Jan 31, 2:28 pm, Joe Beanfish <j...@nospam.duh> wrote:
>> Even after having been turned off?
>
> A well-behaved server (e.g, ISC) will remember the IP it last gave the
> client and offer it again unless its run out of free addresses and
> assigned it to another host.
>
> You're describing "automatic allocation" as defined in the RFC:
>
> DHCP supports three mechanisms for IP address allocation. In
> "automatic allocation", DHCP assigns a permanent IP address to a
> client.
>
> In the server settings, set your leases to something long (or
> infinite, if you're *really* sure this is what you want) and make your
> pool of addresses big.
>

You can also set IP address allocation based on MAC address. In dhcpd.conf, you include a section like

#
# establish fixed addresses for the computers that previously
# had hard-coded IP addresses. If an ethernet card is changed, the
# machine will receive an address from the dynamic range until the
# new ethernet address has been entered below. The lease time for these
# machines is much longer.
#
group {
option subnet-mask 255.255.255.0;
option broadcast-address 10.10.10.255;
max-lease-time 100000;
#
host printer_1 {
hardware ethernet 00:01:E6:01:528;
fixed-address 10.10.10.3;
}
.......
}

There should be a host block for every address you want to be fixed.

Larry