DHCPINFORM via a relay agent

[Kan Sasaki]

From: "David W. Hankins" <David_Hankins at isc.org>
Subject: Re: DHCPINFORM via a relay agent
Date: Sun, 20 Aug 2006 15:47:13 +0000

> This is a compounded bug.  Existing code that attempted to work with
> clients that specify a ciaddr of 0.0.0.0 was 'enhanced' to deal even
> better with them.  In the meantime, we appear to have brkoen clients
> that specify non-zero ciaddr via relays.
> 
> You will have noted from your careful reading of rfc already that
> such zero-ciaddr-DHCPINFORM clients are already not implementing
> rfc2131.

Do you mean that this is zero-ciaddr-client-handling issue?

The following scenario describes my question.

0. basis

   Client has an IP Address CCC.CCC.CCC.CCC which has been assigned by
   server. Relay agent has two IP Addresses, GGG.GGG.GGG.GGG (client
   side) and XXX.XXX.XXX.XXX (server side). Server has an IP Addresses
   SSS.SSS.SSS.SSS. Client is MS windows XP. Server and relay agent
   are ISC's.

1. Client broadcasts DHCPINFORM.

   ip src  CCC.CCC.CCC.CCC
   ip dst  255.255.255.255
   ciaddr  CCC.CCC.CCC.CCC
   giaddr  0.0.0.0
   yiaddr  0.0.0.0

2. Relay agent receives the message and relays the following to the
   server.

   ip src  XXX.XXX.XXX.XXX
   ip dst  SSS.SSS.SSS.SSS
   ciaddr  CCC.CCC.CCC.CCC
   giaddr  GGG.GGG.GGG.GGG
   yiaddr  0.0.0.0

3. Server receives the message and replies the following to the relay
   agent.

   ip src  SSS.SSS.SSS.SSS
   ip dst  GGG.GGG.GGG.GGG
   ciaddr  CCC.CCC.CCC.CCC
   giaddr  GGG.GGG.GGG.GGG
   yiaddr  0.0.0.0

   The ISC dhcpd always replies to giaddr if it has giaddr. I guess
   that the server should send the DHCPACK reply to the client
   directly.

4. Relay agent receives the reply message and relays to the 0.0.0.0.

   ip src  GGG.GGG.GGG.GGG
   ip dst  0.0.0.0
   ciaddr  CCC.CCC.CCC.CCC
   giaddr  GGG.GGG.GGG.GGG
   yiaddr  0.0.0.0

   The ISC dhcrelay always forwards the reply message to yiaddr if it
   has no broadcast flag.

Is it wrong to send the DHCPACK reply to ciaddr instead of giaddr
whether the DHCPINFORM has giaddr?

-- 
Fusion Communications Corp.
Kan Sasaki  sasaki at fcc.ad.jp