IP address classes vs CIDR (was Re: Reviving ARPAnet)

Grant Taylor cctalk at gtaylor.tnetconsulting.net
Thu Jan 18 12:35:57 CST 2018

On 01/18/2018 11:00 AM, Eric Smith wrote:
> Years ago I added a configurable "bozo-arp" feature to the Telebit 
> NetBlazer router, which would respond to ARP requests for non-local 
> addresses and reply with the router's MAC address (on that interface), 
> specifically in order to make classful-only hosts work on a CIDR 
> network.

That functionality sounds exactly like my understanding of what Proxy 
ARP is supposed to do.

> Later someone paid me to write a NetBSD daemon ("anyipd") to do the same 
> thing, though for an entirely different reason.


Since you stated that anyipd "…would respond to ARP requests for 
non-local addresses…" I"m assuming that you are talking IP and not 
another protocol.  Please correct me if I'm assuming incorrectly.

> Recently I've needed that functionality on Linux, as I have multiple 
> old systems that only understand classful, including the AT&T UnixPC 
> (7300 or 3B1). I suppose I should rewrite and open-source it.

I'm trying to make sure that I understand what you're wanting / needing 
to do and evaluate if Proxy ARP can do it or not.

I'm guessing that you have a host, AT&T Unix PC, that's at (for the sake 
of discussion) and you'd like to communicate with another 
machine that's at  Obviously is a subset 
of, so the AT&T Unix PC thinks that is local.  - 
Does this accurately represent your use case?

Unless you correct me, I'm going to assume that this is accurate enough 
for the sake of discussion.

I /think/ (it's been too long since I've done this) that you would 
configure one classless interface with and another 
classless interface with -and- enable Proxy ARP on both 
(?) interfaces.  You will likely need to enter the target machine's IP 
addresses in a file that the Proxy ARP sub-system references to learn 
what target IPs that it needs to Proxy ARP for.

I might not have the nuances exactly correct because I've not done this 
in a long time.  But I have made this scenario work with the Proxy ARP 
support that currently exists in the Linux kernel.

So … I wonder what additional functionality your anyipd would provide. 
-  I'm actually quite curious to learn.

Grant. . . .
unix || die

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