[Simh] FW: FTP client connecting to SIMH/OpenVMS?

[Brad Parker]

Jeremy Begg wrote:
>Peter,
>
>>I just found that I *can* FTP and Telnet into the VMS machine from
>>another Physical host on the LAN - so it seems as though TCP/IP traffic
>>between VMS and the PC running SIMH isn't routing, but so long as I
>>connect to the VMS machine from another PC on the LAN, then the VAX
>>responds as it should.
>
>I believe this is a known problem with SIMH (actually I believe it's a
>problem in the underlying networking library used by SIMH).  To
>communicate between the host PC and the emulated VAX, the host PC
>needs to have an ethernet NIC which is dedicated to the VAX.

if simh is running on linux, it's a long standing issue with linux (and
possibly every other unix)

Depending on what sort of network "tap" you use to simulate the ethernet
you may not be able to access the simulated host from the host machine.
You can, however, access it from any other host on the network.

I figured out exactly what the issue was years ago and asked the linux
networking community about it - the response was that most people didn't
notice or care and fixing it was hard.

It happens more and more now that people are doing emulators as machines
get faster.  I've run into before.  I think I got around it last by
using an IP level tap internet (i.e. creating a new interface) but that
has it's own set of issues.

I as I recall (dimly), if you use an ethernet level tap, the ethernet
driver has to duplicate each packet sent and pass them back up to the
simulator socket.  Likewise, all packets received have to be duplicated
and sent to the simulator socket.  I think right now the receive side
happens but not the transmit side.

Typing it here is seems simple to fix but it didn't look easy at the
time.  Ah yes.  the problem is you also want to send packets back to the
host if they originated in the sim.  but you don't know who sent them in
the ethernet driver (very low level).  so you risk sending packets back
to the host which originated in the host and packets back to the sim
which originated in the sim.

basically, you want to simulate a mini ethernet wire.  I guess you
could do some sort of simple learning bridge.

Hopefully this illustrates the problem and some of the complexity.

anyway, simh suffers this problem as do many emulators.

-brad