[LARTC] problem with the "arp" command (using the pub flag)

[LARTC] problem with the "arp" command (using the pub flag)

[Antonio Pinizzotto]

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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Re: [LARTC] problem with the "arp" command (using the pub flag)

[gypsy]

Antonio Pinizzotto wrote:
>    ___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

You got Trouble, Antonio!

I've been running for 2+ months with no "disappearing" PUB arps.  arp -n
works flawlessly.

Wondering:  Could it be a bad NIC or cable?
--
gypsy
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Re: [LARTC] problem with the "arp" command (using the pub flag)

[Arkadiusz Binder]

and what shows 
#ip ro sh cache
?
It might be that you have not arp of some host who respond for icmp and arp's in case of some recieved icmp redirect packet.



-----Original Message-----
    From: "gypsy"<gypsy@iswest.com>
    Sent: 05-05-18 05:27:02
    To: "Antonio Pinizzotto"<antonio.pinizzotto@iit.cnr.it>
    Cc: "lartc@mailman.ds9a.nl"<lartc@mailman.ds9a.nl>
    Subject: Re: [LARTC] problem with the "arp" command (using the pub flag)
      Antonio Pinizzotto wrote:
    >    ___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
    
    You got Trouble, Antonio!
    
    I've been running for 2+ months with no "disappearing" PUB arps.  arp -n
    works flawlessly.
    
    Wondering:  Could it be a bad NIC or cable?
    --
    gypsy
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    LARTC mailing list
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Re: [LARTC] problem with the "arp" command (using the pub flag)

[Antonio Pinizzotto]

I've monitored the status of the arp table using
"arp -n"
and
"cat /proc/net/arp"
at the same time (for a long time, every 1 second).

The strange thing that I have found is that sometimes, in the "arp -n", 
the publishing entry (P flag) is missing, while in the "cat 
/proc/net/arp"  it is always present.
I have no idea why it happens, but I suppose (and hope :)) that if the 
entry is  present in the "cat /proc/net/arp" output then the arp proxy 
behavious is ok.

Is it right?

Antonio



Arkadiusz Binder wrote:
> and what shows 
> #ip ro sh cache
> ?
> It might be that you have not arp of some host who respond for icmp and arp's in case of some recieved icmp redirect packet.
> 
> 
> 
> -----Original Message-----
>     From: "gypsy"<gypsy@iswest.com>
>     Sent: 05-05-18 05:27:02
>     To: "Antonio Pinizzotto"<antonio.pinizzotto@iit.cnr.it>
>     Cc: "lartc@mailman.ds9a.nl"<lartc@mailman.ds9a.nl>
>     Subject: Re: [LARTC] problem with the "arp" command (using the pub flag)
>       Antonio Pinizzotto wrote:
>     >    ___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
>     
>     You got Trouble, Antonio!
>     
>     I've been running for 2+ months with no "disappearing" PUB arps.  arp -n
>     works flawlessly.
>     
>     Wondering:  Could it be a bad NIC or cable?
>     --
>     gypsy

> 


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