problem with the "arp" command (using the pub flag)

[Antonio Pinizzotto <antonio.pinizzotto@xxxxxxxxxx>]

Hi all.

I experienced a strange behaviour in the "arp" command when I use the 
"pub" (publishing) flag.

This is the scenario (I know it's an unusual one, but it's just for a test):

        __________
       |          |
       |  host C  |
       |          |
       |__________|
            |
  __________|________________________________ 192.168.30.0/24
       |                                      (SUBNET  S1)
       | eth0  192.168.30.5/24
   ____|_____
  |          |
  |  host A  |
  |          |
  |__________|
       | eth1
       | 192.168.30.129/30
   ____|_____________________________________ 192.168.30.128/30
                                  |           (SUBNET  S2)
             192.168.30.130/30____|_____
                   IP_B      |          |
                             |  host B  |
                             |          |
                             |__________|

Note: "S2" is a subnet of "S1".

I want to allow the host C to communicate with the host B through the 
host A using a proxy arp function on host A. I don't want to enable the 
proxy arp on host A for every known destination of host A, but I want to 
decide, on host A, for which IP address to answer to ARP requests.
In other words, I want to use the "host A" for a "selective" proxy arp 
on "SUBNET_S1". I want "host A" to reply to ARP Request for IP_B 
(192.168.30.130) with its ("host A") MAC address.

So I performed, on host A, the following command:

arp -s 192.168.30 -i eth0 -D eth0 pub

If I perform the command "arp -n" I get:


debiantest:~# arp -n
Address           HWtype  HWaddress           Flags Mask     Iface
192.168.30.130    ether   00:D0:59:C4:F3:A1   C              eth1
192.168.30.130    *       *                   MP             eth0


In this situation it works fine. Host A replies to the ARP Request from 
host C for host IP_B, allowing host C and B to exchange IP packets.


  ___PROBLEM___

The problems is that, sometimes, the second line (the one with the P 
Flag in the "arp -n" output) desappears for some time and then appears 
again... !!?

Note.
Do you know if it is normal or if it could be a bug in "arp"?
I didn't verify if, when the second line is not present, the host A 
continues to answer to ARP Request for 192.168.30.130 on eth0.
In other words I didn't verify if it's just a problem in the output (of 
"arp -n" command) and not in the behaviour.


Antonio





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