Interference between multiple DHCP instances on the same server

Glenn Satchell glenn.satchell at
Mon May 3 14:26:00 UTC 2010

Which interface is used for the outgoing packets is kind of arbitrary, 
since they are both on the same subnet. The kernel decides how to route 
the outgoing packets through whatever means it uses.

So in this case you have two possible source IPs for the oputgoing 
packet, depending on which interface it goes out on. In this situation 
using server-identifier lets the dhcp client know which IP to send the 
answer back to. Your case here is exactly how we used to do multiple IPs 
before IP aliasing was available, so it really is exactly the same case: 
two physical interfaces, versus one physical and one virtual.


On 05/03/10 17:24, Abu Abdulla alhanbali wrote:
> what i understand from the "server-identifier" description is that it
> is useful when you have virtual ips on the same physical interface:
> " The usual case where the server-identifier statement needs to be
> sent is when a physical interface has more than one IP address, and
> the one being sent by default isn't appropriate for some or all
> clients served by that interface. Another common case is when an alias
> is defined for the purpose of having a consistent IP address for the
> DHCP server, and it is desired that the clients use this IP address
> when contacting the server. "
> but in my case i have two physical interfaces so i don't know why it
> is sending the response through another physical interface (even
> though they are in the same subnet).
> On Thu, Apr 29, 2010 at 8:47 AM, Glenn Satchell
> <glenn.satchell at>  wrote:
>> If the two interfaces are on the same subnet it shouldn't matter which
>> interface they send their responses out. This is a function of the kernel's
>> network stack and is outside the control of dhcpd.
>> Use of server-identifier should tell the clients to send their response back
>> to that particular server ip, rather than the source IP of the packet. I
>> would assume that you have different IPs on the two interfaces.
>> Apart from observing the traffic, are there any errors reported, or dhcpd
>> not functioning correctly?
>> regards,
>> -glenn
>> On 04/28/10 20:55, Abu Abdulla alhanbali wrote:
>>> Hi,
>>> I have installed additional DHCP instance on the same physical machine
>>> (different interface) that has another DHCP.
>>> I’m binding each server with the interface at the startup: dhcpd
>>> <options>    <interface>
>>> On the other hand, I’m noticing strange behavior:
>>> The second server is receiving the request from the correct interface
>>> but the response is sent from the other interface. I checked this by
>>> ‘snoop’ the two interfaces. I tried also compiling the server
>>> (solaris) with USE_SOCKETS support (even though it is designed for
>>> another purpose) and define local-address / server-identifier without
>>> success.
>>> Any hint where could be the problem.

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