[Simh] VAX XQ and wireless networking

[TJ Merritt]

The TUN interface will get the packets injected into the system, but
will not get them bridged across the wireless network which uses the MAC
address for managing the wireless network.  Since your MAC address for
the virtual interface is not the same as the mac address for the host,
reply packets over wireless will get dropped.  The trick is to get the
packets to go out with the host's MAC address and replies routed back to
the virtual interface.

VirtualBox does this by sending packets out with the original IP address
but the MAC address of the host, they then record the virtual interface
that the IP address came from, when the reply packet comes back, they
use the IP address to look up the virtual interface associated with that
IP address and route the packet to that interface.  There are special
cases of course for things like broadcast, multicast, and IP addresses
that are shared between multiple VM's.  VirtualBox does not currently
support IPv6 addresses though. This would also not help for DECnet.
This is effectively a layer 3 switching capability that works around the
lack of layer 2 switching in the wireless network.

Another approach would be to setup NAT so that the NAT layer can take
care of the MAC address rewriting.  It will also rewrite the IP address
though.  If this is satisfactory, you there should be plenty of NAT
resources available to you on Mint such that no code writing is
required.

Adding virutal networking support for wireless networks would be a major
undertaking for SimH, but if done, could eliminate most of the hiccups
with virtual networking over wireless networks.

Of course, not of these issues occur over wired netowrks.

-- TJ

On Sat, 2012-11-17 at 15:24 -0500, Dan Gahlinger wrote:
> It works...
> use the TUN interface and the bridging, works just fine.
> 
> 
> Dan.
> 
> 
> > Date: Sat, 17 Nov 2012 14:14:24 -0600
> > From: tivo.overo at gmail.com
> > To: simh at trailing-edge.com
> > Subject: [Simh] VAX XQ and wireless networking
> > 
> > Is this even possible? I have SIMH 3.9 on Linux Mint 12, running the
> > VAX with OpenVMS 7.3. It works great, except for networking. The
> > hardware is ASUS EeePC with Atheros AR5001 wireless network adapter
> > and Atheros L2 Fast Ethernet adapter.
> > 
> > I have libpcap version 1.1.1 (which is also callable as version 0.8)
> > installed on Linux. The VAX module was compiled with networking
> > enabled.
> > 
> > The configuration file has an "attach xq0 wlan0" or "attach xq0
> eth0"
> > depending on which interface I'm trying to use. With the wired
> > ethernet (eth0) I am able to use both TCP/IP and DECNET Plus. With
> the
> > wireless (wlan0) neither one works.
> > 
> > I've tried disabling the DECNET IV compatibility mode, which was
> > recommended in one source for wireless compatibility, and also tried
> > adding "set xq xx-yy-zz-aa-bb-cc" (with the actual MAC address of
> the
> > wireless adapter) to the configuration. The first did nothing I
> could
> > discern, and the second generated an error message during startup of
> > the SIMH module, saying that the MAC address was already in use and
> > could not be assigned.
> > 
> > Obviously the workaround is to use a cabled connection when the VAX
> > emulator is needed, but this isn't very practical in most cases. Is
> > there any other possibility I've overlooked?
> > 
> > I notice that this is a frequently found question when using Google
> to
> > look for answers, but I found no responses other than the two
> > solutions mentioned above. I suppose a different network card
> > emulation for VAX could make the wi-fi adapter look like a normal
> > ethernet interface, but apparently that hasn't been done yet and
> it's
> > probably beyond my very rudimentary C coding abilities.
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> 
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