Relay agent with dhcp server on same subnet

Autuori Gianluigi Autuori.Gianluigi.Wintime at ansaldobreda.it
Wed Nov 18 12:53:09 UTC 2009


Hi,
I have a simple network with one dhcp server (isc dhcp), one managged switch and two clients.

                      ___________
                     |          4|------- Client 1 to sw port 4
DHCPD  Server--------| Switch   6|------- Client 2 to sw port 6
(192.168.1.30)       |___________|
                    (192.168.1.101)

I want to assign ip address according to switch ports whitch clients are connected.
So I set Relay Agent on switch to relaying to DHCPD  Server (192.168.1.30) with Option-82.
Then I configure DHCPD  Server like follow:

class "port4" {
    match (binary-to-ascii(16, 8, ":",option agent.circuit-id)); }

subclass "port4" "b8:6:0:0:1:1:1:4";

class "port6" {
    match (binary-to-ascii(16, 8, ":",option agent.circuit-id)); }

subclass "port6" "b8:6:0:0:1:1:1:6";

subnet 192.168.1.0 netmask 255.255.255.0 {    
    pool {
	allow members of "port4";
	range 192.168.1.4 192.168.1.4;
    }
    pool {
	allow members of "port6";
	range 192.168.1.6 192.168.1.6;
    }
}

Now I test this with only one client connected to the right port and I obtain this traffic:

No.     Time        Source                Destination           Protocol Info
      1 0.000000    0.0.0.0               255.255.255.255       DHCP     DHCP Discover - Transaction ID 0xeaa79f3f
      2 0.005068    192.168.1.101         192.168.1.30          DHCP     DHCP Discover - Transaction ID 0xeaa79f3f
      3 0.005072    192.168.1.30          192.168.1.101         DHCP     DHCP Offer    - Transaction ID 0xeaa79f3f
      4 0.010307    192.168.1.101         192.168.1.4           DHCP     DHCP Offer    - Transaction ID 0xeaa79f3f
      5 0.010796    0.0.0.0               255.255.255.255       DHCP     DHCP Request  - Transaction ID 0xeaa79f3f
      6 0.010819    192.168.1.30          255.255.255.255       DHCP     DHCP NAK      - Transaction ID 0xeaa79f3f
      7 0.015958    192.168.1.101         192.168.1.30          DHCP     DHCP Request  - Transaction ID 0xeaa79f3f
      8 0.031054    192.168.1.30          192.168.1.101         DHCP     DHCP ACK      - Transaction ID 0xeaa79f3f
      9 0.035778    192.168.1.101         192.168.1.4           DHCP     DHCP ACK      - Transaction ID 0xeaa79f3f

as can you see there is a NAK (packet n°6) so the transaction fails.
Can I configure DHCP Server to not send this NAK?

Thanks
Gianluigi Autuori

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