[Simh] SimH networking on Ubuntu (dapper)

[Mark Pizzolato - Info Comm]

Edward Berry wrote:
> Mark Pizzolato - Info Comm wrote:
> > Let's go back to the simplest case which is with  2 NIC cards.
> >
> > What you need to do in this case is:
> > 1) The primary NIC (eth0) will be setup normally and you should be
> > able to ping and otherwise access nodes on the LAN.
> > 2) The second NIC should be plugged into the SAME network as the
> > primary NIC.
> > 3) The second NIC SHOULD NOT be used by the networking on the Ubuntu
> > system (i.e. NO IP Address explicitly assigned and none set by
DHCP).
> > You should have to explicitly turn on that NIC with something like
> > "/sbin/ifconfig eth2 up"
> 
> I'm also interested in these instructions. On the ubuntu host, is
there a file
> /etc/sysconfig/network-scripts/ifcfg-eth2 ?
> (I think there must be or ifup will fail (configuration for eth2 not
found).
> Does it have "PROTOCOL STATIC"? and no IP address or netmask?
> 
> I'm using a (now very old) simh vax, sharing the NIC with the linux
host,
> When the vax starts it puts the NIC in promiscuous mode so so it can
receive
> packets at the vax's IP, but of course in linux the NIC is configured
with the
> linux box's IP. 

The age of your SIMH vax doesn't matter as long as it is somewhat
younger than about 7 years.  The basics of the Networking capabilities
haven't changed.

It is simple for the SIMH vax and the Linux host to share the same NIC.
However, without some other bridging monkey business or the use of a
second NIC, the SIMH vax and the Linux host can't actually talk to each
other.  Operating in that mode may be useful for some folks, but many
others will want to be able to move files and/or other data between the
host and the SIMH vax.  You certainly can get into the Console of the
SIMH vax on the local system without any Network capabilities, but more
natural networking is probably useful.

> So it may not matter if a static IP is assigned to the vax-dedicated
nic in linux (it just won't be used).

Well, you really want the Linux box to have a single IP which is
unambiguous.  If there are more than one IP's on the same LAN segment,
then the Linux box will probably have no problem talking to other hosts
on the LAN, BUT there definitely will be a potential ambiguity talking
to the SIMH vax.  If the Linux box has an IP address assigned on the
vax-dedicated NIC on the same LAN as the SIMH vax's IP, the kernel
(randomly) chooses the IP address when sending packets to VAX.  This
could probably work if the Linux side's IP Address were statically set
to be in some other random network which you never wanted to talk to,
but from my point of view working like that is more of an accident
rather than what you're really trying to do by dedicating the NIC to the
SIMH vax.  Hence the recommendation that the NIC not have any IP
assigned on the Linux, but it merely be enabled (/sbin/ifconfig eth2 up)
so that packets can move.

> 
> >   You can avoid the need to run the SIMH vax as root by leveraging
> > Linux "Filesystem Capabilities" to give the SIMH vax executable the
> > needed capabilities.
> 
> (maybe a dumb question from security point of view):
> Why not run the vax console as root, then telnet in as a regular user
to do
> work. When you need to do SYSOPER stuff like mounting tapes or disks,
> switch back to the console to do it.
> Starting the vax from inside the linux "screen" command lets you put
away
> the console and resume it later, and only users logged in as root will
be able
> to resume the root's screen console.

Feel free to do that.    However, over the years, many folks have asked
for ways to be able to run SIMH vax a a standard user.  The only
functionality that the SIMH vax needs root access for is to provide
networking support.  The rest of the SIMH vax functionality runs just
fine as a standard user.

- Mark Pizzolato