subnet seems selected on source address not gateway address

Chuck Anderson cra at WPI.EDU
Wed Jan 7 14:19:42 UTC 2009

On Wed, Jan 07, 2009 at 05:18:30AM +0000, Peter Grandi wrote:
> > Are you using shared-network? If you are, then that is a
> > misconfiguration.
> Of course, that's why I mentioned it. I wondered if there are
> other hints and known mishaps to look for. But I don't know yet
> if GIADDR is used in the configuration or now, see below.

GIADDR is used automatically.  It is a basic function built-in to all 
DHCP servers without requiring any special configuration beyond 
properly specifying the subnets and their relationships to each other. 
E.g. if there is more than one subnet connected to the same wire/same 
broadcast domain, that's when you use shared-network to enclose those 
specific subnets that are shared.  You must not use shared-network 
unless that is true.  Also, several 802.1Q-tagged VLANs on a single 
Ethernet interface doesn't count as "same wire" for those VLANs--they 
should be considered completely separate subnets, not shared.

Shared-network tells the server "these subnets are all equivalent, so 
feel free to assign an address from any of the subnets within the 
shared-network when a request comes in with a GIADDR (or local DHCP 
server interface, unrelayed, with a GIADDR of in any of those 
subnets".  There is no automatic way to distinguish clients as being 
on any particular subnet of multiple subnets shared on the same 
wire/same broadcast domain.  You can apply manual matching in this 
case with conditional statements and classes to force certain clients 
to be assigned to certain subnets of the shared-network, but you would 
have to match on client-specific data other than GIADDR, such as 
listing every MAC address that belongs in one subnet vs. the other(s), 
or matching on specially configured hostname patterns, etc.

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