prevent ip conflict in dhcp client

Frank Leonhardt freebsd-doc at
Thu Jul 11 12:47:23 UTC 2013

This all sounds like a very strange thing to be doing! But I hate it 
when people answer my questions with "Why would you want to do that", so 
I won't.

Binding an IPv4 address using a MAC address, which is the answer to a 
lot of DHCP problems. But your explanation "my client acts like a 
router" set alarm bells ringing. What exactly are you trying to do, and 
are you aware that routers aren't (normally) configured using DHCP? If 
you've got any kind of normal Internet line it will receive it's IP 
address using LCP (the NCP part, and the IPCP to be precise). Or at 
least, that's how I think it normally works.

Regards, Frank.

On 11/07/2013 12:43, krad wrote:
> ops %s/rand/range/
> On 11 July 2013 12:42, krad<kraduk at>  wrote:
>> alter the pool rand on the network to use say, x.x.x.1-199 on a /24, and
>> then allocate your statics >200 but <= 254 or add something similar to your
>> isc-dhcp config
>> host host.intranet {
>>    hardware ethernet c8:60:33:1d:f3:57;
>>    fixed-address;
>>    option host-name "host.intranet";
>>   }
>> Alternatively use ipv6 as the automatic ip address configuration tests
>> exactly like you commented on
>> On 11 July 2013 12:18, s m<sam.gh1986 at>  wrote:
>>> thanks Eugene,
>>> you're right but i forgot to say that my client acts like a router. i mean
>>> none of interfaces should have ip address in same range (this is conflict
>>> for me). i can manage each interface to get ip address from DHCP or
>>> manually. so one interface may get ip address from dhcp server whereas all
>>> others have ip addresses which are set manually.
>>> for this situation, do you have any ideas to avoid ip conflict?
>>> thanks again for your attention
>>> SAM
>>> On Thu, Jul 11, 2013 at 3:06 PM, Eugene<genie at>  wrote:
>>>> Hi Sam,
>>>> Actually I think this is wrong approach. Correctly configured networks
>>>> should be consistent and should not need such 'fixes'. Also you should
>>>> observe the IP provided by upstream DHCP server otherwise it is an
>>>> invitation for trouble (both technical and possibly legal).
>>>> Are the 'other' interfaces in your internal networks? Then you should
>>>> change them to use different address block from that used in your
>>>> provider's network (there are many address blocks for private networks).
>>>> And/or you should talk to your admin and discuss the address policy,
>>> maybe
>>>> they can give you a fixed address.
>>>> Best wishes
>>>> Eugene
>>>> -----Original Message----- From: s m
>>>> Sent: Thursday, July 11, 2013 2:19 PM
>>>> To: freebsd-questions
>>>> Subject: prevent ip conflict in dhcp client
>>>> hello all
>>>> i have a question about dhcp client. i want to know if there is any way
>>> to
>>>> understand the ip address which is offered by server before it assigned
>>> to
>>>> the interface.
>>>> i have a freebsd system which one of its interfaces should get ip
>>> address
>>>> from dhcp server whereas other interfaces have ip addresses and their ip
>>>> address change many times. so i want to prevent ip conflict.  is there
>>> any
>>>> way to prevent ip conflict in this situation?
>>>> i think the best way is to know the ip address which is offered by dhcp
>>>> server before assigning it to interface  and check if it has conflict
>>> with
>>>> others or not. is it possible? if yes, how i can do this?
>>>> any comments or hints are appreciated.
>>>> thanks in advance
>>>> SAM
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