DHCP Relay agent - Duplicating messages

Gero Palacio gero.palacio at gmail.com
Tue Jun 30 16:45:27 UTC 2015


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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