Re: Does the debian kernel sends the gratuitous arp ?

[debian-user@howorth.org.uk]

Geert Stappers <stappers@debian.org> wrote:
> On Fri, Oct 06, 2023 at 09:19:32PM +0200, Geert Stappers wrote:
> > On Fri, Oct 06, 2023 at 05:52:16PM +0530, Balaji G wrote:  
> > > Hi,
> > >     I am using "Debian GNU/Linux 11 (bullseye)" with kernel
> > > version 5.16.12. When i do a link up/down i don't see any
> > > Gratuitous ARP being sent.  
> > 
> > ARP  
> 
> Address Resolution Protocol
> 
>   
> > > # echo 1 > /proc/sys/net/ipv4/conf/eno5np0/arp_notify  
> > 
> > Probably transmitted "I tried to enforce ARP notification"
> > 
> >   
> > > # ip link set down dev eno5np0
> > > # ip link set up dev eno5np0  
> > 
> > link  
> 
> As in "the link layer"
>   
>  
> > > Captured all the packets via tcpdump & the tcpdump is not showing
> > > any Gratuitous ARP packets.  
> > 
> > Why should it?     (Yes, that is a serious question.)  
>  
> As opening question for further discussion.
> 
> Because I like the question
>   Does the Debian kernel sends the gratuitous ARP ?
> 
> I think the answer is   YES
> and I very curious when gratuitous ARP is ommitted.
> 
>   
> > > But, with the same commands i could see the Gratuitous ARP being
> > > sent in Red hat.9.0 (Plow).  
> > 
> > I think more factors have been change as kernel.  
>  
> As invitation for telling how the comparsion was done.
> 
> 
> > > So, please let me know if this is a specific scenario in Debian
> > > 11 ??  
> > 
> > I state that the OP, Original Poster, is the specific scenario.
> > Yeah, that is a blunt statement.  Now hear me out.
> > 
> > Gratuitous ARP is "linking" MAC-address and  IP-address.
> > 
> > During `ip link set down dev eno5np0` and `ip link set up dev
> > eno5np0` are NO IP-addresses involved.
> > 
> > So there is no need for ARP.
> > 
> > 
> > I do hope to see a follow-up message like:
> > 
> >    Re-did the test with a static IP-address on the interface
> >    and indeed see a gratuitous ARP
> > 
> > or some thing like
> > 
> >    To reproduce the missing gratuitous ARP do ...
> > 
> > Yeah, that might reveal more information about dev eno5np0.
> > 
> >    
> > > Thanks,
> > > Balaji  
> > 
> > Groeten
> > Geert Stappers  
> 
> Because in https://lists.debian.org/debian-user/2023/10/msg00212.html
> I wrote "Lets follow-up
> https://lists.debian.org/debian-user/2023/10/msg00181.html "

If you'll forgive me stepping in ...

I suspect neither Geert's nor Balaji's first language is English, and
independently notice that Geert writes in a somewhat terse style whilst
Balaji omits potentially useful information (such as the actual tcpdump
output). So I'll add my summary of what I understand:

Gert is pointing out that ARP packets are sent in connection with
resolving the IP address associated with a MAC address, and that
nothing that Balaji has posted suggests that an IP address has been
allocated yet, so there would be no reason for a Gratuitous ARP packet
to be sent.

So he is interested to learn how the IP address is set up on both the
Debian system and the Red Hat system (static or DHCP or something
else?) and I suspect would ideally like to see sufficient message logs
to demonstrate the truth of the answers.

Specifically, on the Debian system is the IP address set statically,
and are there any packets containing the IP address in the logs? Either
transmitted or received. Similarly for the Red Hat system but perhaps
less important.

> Groeten
> Geert Stappers