about concept "group", "shared-network", and "subnet", thanks.

Guo, Fei Fei.Guo at windriver.com
Tue Apr 19 09:35:29 UTC 2011

Cool, thank you very much. It is very helpful. 

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-----Original Message-----
From: dhcp-users-bounces+fei.guo=windriver.com at lists.isc.org [mailto:dhcp-users-bounces+fei.guo=windriver.com at lists.isc.org] On Behalf Of Simon Hobson
Sent: 2011年4月19日 16:43
To: Users of ISC DHCP
Subject: Re: about concept "group", "shared-network", and "subnet", thanks.

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 ...
   subnet ...
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 Christmas stocking fillers. Some available as e-books.
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