ITEM: CF5079L

SLIP routing problems on local subnets - how to do proxy arp ?

ENV:
AIX 3.2.5
7013-570
SLIP

DESC:
Our network looks something like this:

All subnet masks for IP addresses shown are 255.255.255.192

The PC can get to the Internet, no problem... The router has a route
that forwards packets destined for .80 to .66 so that works fine. But
The PC (.80) can't ping the mainframe (.68)

ACT:
Explained this is because the two addresses are on the same subnet.
The mainframe is most likely getting the packet but when sending a
reply back, based on the destination IP (.80) and the subnet mask,
the two IP's are on the same subnet. The packet is being put out on
the ethernet, but no .80 is responding.

It may be possible to add a route to the mainframe using .66 as a
gateway, but it's not likely the mainframe would consult its routing
table, after all - the two hosts are on the same subnet, why route?

I suggest changing the SLIP links to another subnet (something not
between 64-127) This was not a possibility because of the sheer
number of PCs configured to dial in this way.

An alternative is to use a pseudo-proxy-arp - letting the RS/6000's
ethernet adapter respond to packets destined for the PC's IP address.
To accomplish this:

1) Get the RS/6000's ethernet network card hardware address:
netstat -v

2) the RS/6000 must publish an arp address for the PC. This is done by
piggybacking the PC on the Risc's network card's hardware address via
the arp command:

arp -s ether \ \ pub

Be sure to include the colons (:) in the network card's hardware
address.

The pub makes it a permanent entry. This line can be added to the end
of /etc/rc.net so as to invoke this command on each system boot.

These steps have resolved your problem.

Support Line: SLIP routing problems on local subnets - how to do proxy arp ? ITEM: CF5079L

Dated: October 1996 Category: N/A

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