Re: Proxy ARP

[Ryota Ozaki <ozaki-r%netbsd.org@localhost>]

On Thu, Feb 25, 2016 at 10:25 PM, Greg Troxel <gdt%ir.bbn.com@localhost> wrote:
>
> Ryota Ozaki <ozaki-r%netbsd.org@localhost> writes:
>
>> I have questions about the Proxy ARP feature.
>>
>> arp(8) has two options: "pub" and "pub proxy".
>> What's the different between them and what
>> are expected behaviors of them?
>
> Besides the use case Christos mentioned, the other one is when you have
> an Ethernet with a subnet, and one host you have some link to another
> host that you'd like to also be addressed within the subnet.  Consider
> 10.0.0.0/8, with a host 10.0.0.20, and another computer without an
> ethernet but with a SLIP or PPP link to 10.0.0.20, that you want to be
> 10.0.0.21.
>
> On 10.0.0.20, you would publish a proxy arp entry for 10.0.0.21, using
> 10.0.0.20's ethernet address.  This would be "pub", to cause replies to
> be sent to queries (normally queries are processed without looking at
> the arp cache, but only your addresses, I think).  And "proxy", so that
> 10.0.0.20 would not use it.

I understand the "proxy" case but don't understand well "pub" case well.
In the above case, a route to 10.0.0.21 should direct to the other
computer while the proxy arp entry should be on an interface where
10.0.0.20 is assigned, to reply ARP requests for 10.0.0.21. IIUC, this
is "proxy" case and so I don't know when "pub" case is useful.

Thanks,
  ozaki-r

>
> Sometimes this would be done for dialup hosts, especially homegrown ad
> hoc to one's computer at campus.
>
> This setup does not require a proxy arp daemon, because there are small
> numbers of non-changing entries.
>
> Besides "why don't you plug .21 into the Ethernet", the other question
> is "why don't you allocate  a subnet for the PPP link and run a routing
> protocol", and the answer was usually "I dno't have any subnets to
> allocate" or "it's too hard to make it work".