about concept "group", "shared-network", and "subnet", thanks.
Simon Hobson
dhcp1 at thehobsons.co.uk
Tue Apr 19 08:42:54 UTC 2011
Guo, Fei wrote:
> what is the relationship between "group", "shared-network", and
>"subnet" in ISC configuration?? can someone give me some clue??
>thank you very much in advance.
Have you read "man dhcpd-conf" yet ?
Group simply allows you to apply options collectively to a number of
host statements like this :
group {
option ...
option ...
host ...
host ...
}
The option will apply to each host in the group without having to
repeat them inside each host statement.
Groups are unconnected with subnet and shared-network. Note that you
should always declare hosts at the global level (ie not inside any
subnet declaration) as they are always global in scope but do inherit
options from an enclosing subnet - this leads to very strange, and
almost always incorrect, option inheritance.
"subnet" simply defines a subnet.
"shared-network" is used for the situation where you run multiple IP
subnets on the same wire. Note - this is **NOT** the case for most
users, and is also completely different to using VLANs to trunk
several networks over one circuit.
Eg :
shared-network demo {
subnet 192.168.1.0 ...
subnet 10.0.0.0 ...
}
defines a case where computers on that physical network may use
192.168.1.x or 10.0.0.x addresses interchangeably. The DHCP server
will allocate addresses from either subnet (assuming you have ranges
defined). In the case of a remote network, the GI Addr field filled
in by the relay agent on the remote network may be in either subnet
and the DHCP server can work out from the shared-network statement
that both sets of IP addresses are valid.
DO NOT use shared-network where (for example) there are two remote
networks. While the relayed request packets arrive at the server on
the same interface, the subnets are still separate.
Similarly, do not use it where you have a VLAN capable switch and
trunk multiple VLANs to your server. Each VLAN on the server acts as
a separate ethernet interface as far as software is concerned.
So, inheritance rules go like this, going more specific from left to
right, and with more specific option overriding the more general ones
:
global - [shared-network] - subnet - pool
And for hosts with a host declaration (I think)
global - [group] - host - [shared-network] - subnet - pool
--
Simon Hobson
Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
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