DHCP Relay

Tim Peiffer peiffer at umn.edu
Tue Apr 28 12:13:56 UTC 2009


Glenn Satchell wrote:
>> Date: Tue, 28 Apr 2009 13:17:36 +0300
>> From: William Jobs <wllmjbs at gmail.com>
>>
>> I'd also appreciate it if someone could let me know, if I'm attempting
>> something that is just not possible. That way I could stop banging my head
>> against the wall :)
>>
>>
>> On Tue, Apr 28, 2009 at 12:49 PM, William Jobs <wllmjbs at gmail.com> wrote:
>>
>>     
>>> Hi all,
>>>
>>> I've been trying out ISC's dhcp software as a relay server and everything
>>> is working fine except for the remote-id sub-option of Option 82. For some
>>> reason dhcrelay will not include this sub-option in the packets it forwards
>>> to the DHCP server. I get the circuit-id sub-option, just fine.
>>>
>>> Is there a specific way to enable this feature? I'm currently running
>>> dhcrelay as follows for debugging purposes:
>>>
>>>         dhcrelay -d -a 10.0.0.1
>>>
>>> Any help would be appreciated.
>>>
>>> Thanks.
>>>       
>
>
> Which version of dhcrelay and dhcpd are you using? What platform? (linux, *bsd, 
> solaris, hp and so on...)
>
> This is from the 4.1.1b1 dhcrelay man page, note the last sentence.
>
>      -a   Append an agent option field  to  each  request  before
>           forwarding  it  to the server.   Agent option fields in
>           responses sent from servers to clients will be stripped
>           before  forwarding  such  responses back to the client.
>           The agent option field will contain two agent  options:
>           the  Circuit  ID suboption and the Remote ID suboption.
>           Currently, the Circuit ID will be the printable name of
>           the interface on which the client request was received.
>           The client supports inclusion of a Remote ID  suboption
>           as well, but this is not used by default.
>
> This is the equivalent section from the 3.1.1 dhcrelay man page.
>
>      If the -a flag is set the relay agent will append  an  agent
>      option  field  to  each  request before forwarding it to the
>      server.   Agent option fields in responses sent from servers
>      to clients will be stripped before forwarding such responses
>      back to the client.
>
>      The agent option field will contain two agent  options:  the
>      Circuit   ID   suboption   and   the  Remote  ID  suboption.
>      Currently, the Circuit ID will be the printable name of  the
>      interface  on  which  the  client request was received.  The
>      client supports inclusion of a Remote ID suboption as  well,
>      but this is not used by default.
>
> So it seems the remote-id is not added by default. The remote-id is
> mentioned in the source, but there is no way to set it. Looks like if
> the packet arrives with a remote-id then the remote-id will not be
> removed.
>
>   
I don't run the dhc relay code, so I am not an authority there, but 
there is also the '-A <size>' option to consider:

       It  is  recommended  that  with  the  Internet Systems Consortium 
DHCP
       server, the maximum packet size be set to about 1400, allowing  
plenty
       of  extra  space  in  which  the  relay agent can put the agent 
option
       field, while still fitting into the Ethernet MTU size.   This  
can  be
       done by specifying the -A flag, followed by the desired maximum 
packet
       size (e.g., 1400).

So dhcrelay -a -A 1400 -d 10.0.0.1 ??

Regards,
Tim

> Sorry it's not much use to you...
>
> regards,
> -glenn
>
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> dhcp-users at lists.isc.org
> https://lists.isc.org/mailman/listinfo/dhcp-users
>   


-- 
Tim Peiffer
Network Support Engineer
Office of Information Technology
University of Minnesota/NorthernLights GigaPOP




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