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This question is mostly about "socat", but here's some background to go with it:

I am trying -- for reasons having mostly to do with nostalgia -- to write a virtual modem driver for use with VirtualBox. It should listen to the socket that VirtualBox connects to the virtual guest, and emulate (a) a standard Hayes command set and (b) let one connect to remote systems using "atd some.host.name". Mostly it works, but I've run into problems with data transfers. I assume the problem is mine, because I seldom have the chance to muck about with mult-channel communication, select loops, and the like...

...so I though I would prototype my solution using the "socat" command, like this:

socat UNIX-CONNECT:/tmp/vbox.port EXEC:"telnet -E8 some.remote.host"

This works, sort of, just like my solution -- basic interactive typing seems fine, but try a file transfer and it just falls over. I've also tried this, just in case there was some sort of tty line discipline in the way:

socat UNIX-CONNECT:/tmp/vbox.port EXEC:"telnet -E8 some.remote.host",pty,raw,cs8

That didn't work any better.

I'm wondering if anyone here has thoughts on solving this. The problem is not with telnet; using -E8 provides an 8-bit clean path that works fine by itself (e.g., when not involved in this sort of pty-proxying).

This is obviously not a critical problem, but I'm hoping that someone else out there finds it at least mildly interesting.

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I don't think socat has any problem with arbitrary byte streams, so I suspect the fault is elsewhere. Can you post a small reproducible example? – Gilles Mar 11 '11 at 20:59
@Gilles: (1) Install DOS under virtualbox and assign com1 to a socket on the remote host. (2) Install a terminal program; txzm seems like a nice one. (3) Connect the com1 socket to a remote host using socat. (4) Try to transfer a file with zmodem (e.g., using sz from the lrzsz package). – larsks Mar 17 '11 at 1:03
...noting, again, that I'm not suggesting you actually go through any of this unless you find this problem interesting. – larsks Mar 17 '11 at 1:04
I might try it if I could do Linux to Linux, but DOS? No way. Though what's DOS for anyway? I do have a Linux VM on Linux. – Gilles Mar 17 '11 at 1:09
You could probably demonstrate the problem using minicom on Linux, too. – larsks Mar 17 '11 at 1:18

1 Answer

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Although you say telnet is not the culprit, I would test taking it away.

Have you tried the following?

socat UNIX-CONNECT:/tmp/vbox.port tcp:some.remote.host:23

This should be 8-bit clean by itself.

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Unfortunately, telnet is required, and demonstrably is not the problem -- for example, one can use qodem over a telnet connection without a hitch. I'm beginning to suspect the VirtualBox socket itself, but I'm not sure exactly how to verify that. A simple "send characters 0x00 through 0xFF" seems to work as expected. – larsks Mar 17 '11 at 1:00

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