DHCP Relay agent - Duplicating messages

Gero Palacio gero.palacio at gmail.com
Mon Jun 29 22:12:56 UTC 2015


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.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.isc.org/pipermail/dhcp-users/attachments/20150629/9365fe05/attachment.html>


More information about the dhcp-users mailing list