Configuration question..
Glenn Satchell
Glenn.Satchell at uniq.com.au
Wed Sep 13 13:10:06 UTC 2006
>Date: Wed, 13 Sep 2006 08:50:33 -0400
>From: "B. Cook" <bcook at poklib.org>
>To: dhcp-users at isc.org
>Subject: Re: Configuration question..
>
>Glenn Satchell wrote:
>>> To: dhcp-users at isc.org
>>> Subject: Re: Configuration question..
>>> From: Keith.Neufeld at wichita.edu
>>> Date: Tue, 12 Sep 2006 09:20:54 -0500
>>>
>>>> . . .
>>>>
>>>> The webmin gui doesn't show us the defined hosts we have, and we can't
>>>> figure out where/how to add them.. so I started to think that somehow
>>>> our config is incorrect..
>>>>
>>>> . . .
>>>>
>>>> We use hosts in pools, b/c I have one setup for unknown clients and one
>>>> for known clients.. and well, in the pools I can't see the defined hosts
>>>> that we have in the webmin gui.
>>> The fact that the host declarations are inside the pool declaration is
>>> actually irrelevant to whether they can get a lease from that pool or
>>> not--the determination is made (in your configuration) by whether they're
>>> known (listed anywhere in the configuration) or unknown.
>>>
>>> To demonstrate that, take the host declaration for ronlapbop [sic] and
>>> move it to the 10.0.0.0 "Public Network" section of your configuration,
>>> then try to boot it on your 192.168.1.0 subnet. It'll still get an
>>> address from one of your known-host ranges, _not_ from the 192.168.1.20-25
>>> range for unknown clients.
>>>
>>> There's been a thread on the mailing list recently about proper placement
>>> of host declarations; but the common wisdom is to put them _outside_ of
>>> all blocks, in the global scope, UNLESS you specifically want them to use
>>> some of the options from their "home" subnet even when they're roaming to
>>> other subnets. [Did I say that about right?] Putting host declarations
>>> inside subnet or pool declarations can lead people to think the hosts are
>>> tied to getting addresses only from the subnet or pool (which isn't
>>> actually the case), so it's nicer to put them globally and avoid the
>>> confusion.
>>>
>>>
>>> It may be just the way my browser is displaying the file, but I prefer
>>> indenting blocks visually to match their syntactic nesting--so it'd be
>>> easier for me to read quickly if e.g. the stuff inside the 10.0.0.65-94
>>> pool were indented by another tabstop, and the hosts inside it (if they
>>> stay there) indented two tabstops from where they appear now.
>>>
>>> My $.02.
>>
>> One other idea is to use the group { } function when you have some
>> common options for different groups of hosts. See below.
>>
>> I also agree with the wisdom of moving the host declarations to the
>> global scope. They will still pick up appropriate subnet and pool
>> settings based on whichever subnet or pool they get assigned an address
>> from. I would venture to say this seems to be the collective "best
>> practice" of the mailing list.
>>
>> I would then move the option settings from the pool to the subnet scope.
>>
>> The file would start to look a bit like this:
>>
>> #global options
>>
>> group {
>> option routers 0.0.0.0;
>> host ... { }
>> host ... { }
>> }
>> host ... { }
>> host ... { }
>>
>> subnet 192.168.1.0 netmask 255.255.255.0 {
>> option domain-name "adriance.poklib.org";
>> option domain-name-servers 192.168.1.2;
>> option netbios-name-servers 192.168.1.248;
>> option netbios-node-type 8;
>> #option netbios-node-type 4;
>> option routers 192.168.1.3;
>> #option routers 192.168.1.2;
>>
>> pool {
>> deny unknown clients;
>> range ...
>> ...
>> } # close pool
>> } # close subnet
>>
>> subnet 10.0.0.0 netmask 255.255.255.128 {
>> option ...
>> range ...
>> allow unknown clients;
>> }
>>
>> You have authoritative in the global scope, so no need to repeat it in
>> subnet or pool scopes.
>>
>> My two cents worth, hope it helps.
>>
>> regards,
>> -glenn
>> --
>> Glenn Satchell mailto:glenn.satchell at uniq.com.au | Heard about
>> Uniq Advances Pty Ltd http://www.uniq.com.au | the new Unix
>> PO Box 70 Paddington NSW Australia 2021 | sports car?
>> tel:0409-458-580 tel:02-9380-6360 fax:02-9380-6416 | The rwx
>>
>>
>Ok,
>
>I'm trying to understand this.. and thinking I'm still missing
>something (as I'm still getting the warning..)
>
>So here is the rough skeleton of what I'm doing..
>
>
># dhcpd.conf
>
>authoritative;
>other_global_options;
>
>subnet 192.168.1.0 netmask 255.255.255.0 {
>global_settings_for_this_subnet;
>
>pool {
> deny unknown clients;
> range 192.168.1.100 192.168.1.105;
> }
>
>group {
> option routers 0.0.0.0;
> host one { hardware ethernet aa:bb:cc:dd:ee:f1; fixed-address
>192.168.1.99; }
> }
>
>host two { hardware ethernet aa:bb:cc:dd:ee:f2; fixed-address
>192.168.1.98; }
>host three { hardware ethernet aa:bb:cc:dd:ee:f3; }
>
>pool {
> allow unknown clients;
> range 192.168.1.20 192.168.1.25;
> option domain-name-servers 0.0.0.0;
> }
>
>} # close subnet 192.168.1.0/24
>
>
># public network
>subnet 10.0.0.0 netmask 255.255.255.128 {
>other_options_here;
>option router 10.0.0.87;
>
>pool {
> allow unknown clients;
> range 10.0.0.95 10.0.0.125;
> }
>
>group {
> option routers 10.0.0.62;
> host something { hardware ethernet 11:22:33:44:55:61; fixed-address
>10.0.0.5; }
> }
>
>host another { hardware 11:22:33:44:55:62; }
>
>} # close subnet 10.0.0.0/25
>
>
>This still gives me warnings:
>WARNING: Host declarations are global. They are not limited to the
>scope you declared them in
>
>do I need to define the hosts outside the subnets even?
Yes. Have a look at the section EXAMPLES in the dhcpd.conf man page,
especially Figure 1. Note that the group { host { ... } ... } comes
after the subnet definition is closed.
You want something like this:
global options
subnet {
subnet options
pool {
pool options
}
}
group {
host { ... }
host { ... }
...
}
host { ... }
host { ... }
> but if I do that
>how will it know what hosts to apply what to?
Ok, from memory the 192.168.1.0 subnet was local and the 10.0.0.0 came
via a relay.
The dhcp server sees the dhcp broadcast from the hosts on 192.168.1.0
network. Also the giaddr (gateway interface address) will not be set.
Thus the dhcp server knows they are in the local subnet and selects
from the 192.168.1.0 definitions.
A host on the 10.0.0.0 subnet broadcasts, the dhcp relay picks up the
broadcast and forwards it to the dhcp server as a unicastpacket. the
dhcp relay also fills in a field in the dhcp options, the giaddr, with
the address of the interface that the request came in on. Using the
giaddr the dhcp server now knows which subnet the request originally
came from. It selects an appropriate address from the 10.0.0.0 subnet
and replies to the dhcp relay which thenm broadcasts it back onthye
subnet for the client ot pickup.
> how would it tell the
>different between hosts one, two and another? Or am I just missing
>something that should be glaringly obvious?
It works this way because consider if you had, say, a laptop that could
be connected to either subnet and you wanted it to have a fixed address
in each subnet. This would be a perfectly valid host declaration:
host bcook {
hardware ethernet aa:bb:cc:dd:ee:ff;
fixed address 192.168.1.5, 10.0.0.5;
}
The dhcp server selects the appropriate address based on the subnet
where the request came from. Then it applies all the options defined in
global scope, then for that subnet and then for the pool.
You only need to use the group { ... } around the host options if you
want to supply an option that is different to what it would otherwise
get in the subnet.
HTH.
regards,
-glenn
--
Glenn Satchell mailto:glenn.satchell at uniq.com.au | Heard about
Uniq Advances Pty Ltd http://www.uniq.com.au | the new Unix
PO Box 70 Paddington NSW Australia 2021 | sports car?
tel:0409-458-580 tel:02-9380-6360 fax:02-9380-6416 | The rwx
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