Two IPs on one NIC

Simon Hobson dhcp1 at
Wed Sep 16 15:30:58 UTC 2009

Alan Buxey wrote:

>does any DHCP client handle being given 2 addresses by DHCP?
>I thought the original request asked for DHCP from 2 ranges to be given..
>2 addresses can be handed out (and more!) from DHCPD but as far as I'm aware
>the client will check which address is valid for the link its on and
>then use it.... so unless its a case of 1 DHCP address and one static, then
>cant see this working

Wrong. Until the device is configured, it has no way of knowing what 
address/subnet might be valid - that's what DHCP is for. It is the 
server that will check which of several possible addresses are valid 
for the clients network and offer something appropriate.

The problem is that the server could offer two different leases (but 
not in one offer message) - the client would simply pick one and 
ignore the other. There are two problems to be addressed - both need 
doing at the client and that means you might as well just manually 
configure the client !

1) The client need to make two requests, using different <something> 
so the server will treat them as <different> and offer two different 
leases. With the ISC server, the <something> would be client-id - 
either two different values, or one value and not-present.

2) having got two leases, the client would need to configure both 
addresses - that's going to require some significant hacking of the 
code to cope with all the permutations
Simon Hobson

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