DHCP Relay agent - Duplicating messages

[Gero Palacio]

Correction: ISC DHCP v *4.2.8*

On Tue, Jun 30, 2015 at 1:45 PM, Gero Palacio <gero.palacio at gmail.com>
wrote:

> Hi,
>
> I've done further testing and it's seems that the problem is that the
> DHCPREQUEST unicast messages are being duplicated by the relay agent which
> is running on the router that connects the client subnet to the server
> subnet.
>
> I also found and old thread (link here
> <https://lists.isc.org/pipermail/dhcp-users/2012-April/015310.html>)
> which reports there very same problem. The person who started that thread
> already filled a bug report (ISC-Bugs #29053) but I was not able to find
> in the release notes if this problem was fixed.
> I'm running DHCP v 2.4.8. Any help would be appreciated. Thanks. Gero.
>
> On Mon, Jun 29, 2015 at 7:12 PM, Gero Palacio <gero.palacio at gmail.com>
> wrote:
>
>> Hi All,
>>
>> I found a weird behavior on the DHCP relay agent which is duplicating the
>> outgoing messages from the client when it's not supposed to.
>>
>> This is the set up:
>>
>> [client] <-> (192.168.1.1) [relay_agent] (10.0.0.1) <-> [pair of failover
>> servers](10.0.1.10 & 10.0.1.11)
>>
>>
>>
>> *What I found is this:*
>>
>> *No.  Source        Destination    Info*
>>
>> -- We're good here. Relay agent duplicate the discovery messages
>> -- to send to both pair of servers
>> *44*  10.0.0.1       10.0.1.11      DHCP Discover - Transaction ID
>> 0xd2728f7c
>> *45*  10.0.0.1       10.0.1.10      DHCP Discover - Transaction ID
>> 0xd2728f7c
>>
>> -- Receives only 1 response from 1 of the servers. So far, so good.
>> *47*  10.0.1.10      192.168.1.1    DHCP Offer    - Transaction ID
>> 0xd2728f7c
>>
>> -- This bit is also okay. Client broadcasts the request message
>> -- relay agent sends 2 messages (one to each server)
>> -- receives only 1 response (good).
>> *48*  10.0.0.1       10.0.1.11      DHCP Request  - Transaction ID
>> 0xd2728f7c
>> *49*  10.0.0.1       10.0.1.10      DHCP Request  - Transaction ID
>> 0xd2728f7c
>> *50*  10.0.1.10      192.168.1.1    DHCP ACK      - Transaction ID
>> 0xd2728f7c
>>
>> -- This is the part where it gets weird.
>> -- Client sends 1 requst (WireShark on the client confirms this, not
>> shown)
>> -- This message contains:
>> -- Relay agent IP address: 0.0.0.0 (0.0.0.0)
>> *52*  192.168.1.60   10.0.1.10      DHCP Request  - Transaction ID
>> 0xd2728f7c
>>
>> -- Relay agent (source 10.0.0.1) makes 4 messages!! I dont get this part
>> -- These 2 messages contains:
>> -- Relay agent IP address: 192.168.1.1 (192.168.1.1) ???
>> *53*  10.0.0.1       10.0.1.11      DHCP Request  - Transaction ID
>> 0xd2728f7c
>> *54*  10.0.0.1       10.0.1.10      DHCP Request  - Transaction ID
>> 0xd2728f7c
>> -- These 2:
>> -- Relay agent IP address: 10.0.0.1 (10.0.0.1) ???
>> *55*  10.0.0.1       10.0.1.11      DHCP Request  - Transaction ID
>> 0xd2728f7c
>> *56*  10.0.0.1       10.0.1.10      DHCP Request  - Transaction ID
>> 0xd2728f7c
>>
>> -- The response are received with the client ip and with
>> -- the relay agent ip that's facing the client
>>
>> *57*  10.0.1.11      192.168.1.1    DHCP ACK      - Transaction ID
>> 0xd2728f7c
>> *58*  10.0.1.10      192.168.1.60   DHCP ACK      - Transaction ID
>> 0xd2728f7c
>> *59*  10.0.1.10      192.168.1.1    DHCP ACK      - Transaction ID
>> 0xd2728f7c
>>
>>
>> Why is this happening? Why is the DHCP Relay Agent IP is changing in the
>> DHCP message?
>> I've read the DHCP Handbook chapters discussing relay agents (Ch 7 and 9)
>> and also the chapter that describes the ISC relay agent but it doesn't seem
>> to behave in that way.
>>
>> I'm using version 4.2.8
>>
>> Thanks for your help.
>> Gero.
>>
>>
>
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