Re: [PLUG] Slow Down an ssh Session

["N. Albert via plug" <plug@lists.phillylinux.org>]

On 9/27/2024 6:03 AM, Casey Bralla via plug wrote:
> In the "good old days", I used a DEC PDP-8 with a hulking noisy 
> teletype.  To relive my youth, I bought a PiDP-8, which is just the 
> front panel of the PDP-8, run by a Raspberry Pi.
>
> The Pi, of course, is waaay faster than the PDP-8, so I set some 
> parameters in the simulator software to slow it down to the speed of 
> an actual PDP-8 from the early 70's.
>
> Since I don't have a real teletype (I actually had one and gave it 
> away years ago.  Ugh!  I wish I kept it!) I'm using an ssh session on 
> an old laptop as a terminal.
>
> But this means the effective terminal baud rate is like warp speed 
> compared to the teletype.
>
> I'd like to mimic the original 110 baud teletype.  Other than actually 
> connecting via 110 baud serial connection, is there a way to throttle 
> the effective baud rate of ssh? 

If you're using a serial connection, it's relatively straightforward, 
something like:
struct termios slow;
tcgetattr(STDOUT_FILENO, &slow);
cfsetispeed(&slow, B300);
cfsetospeed(&slow, B1200);
tcsetattr(STDOUT_FILENO, TCSANOW, &slow);

For that, you'd need a serial monitor, as you said.

To do this for an IP connection like SSH, it's a lot more complicated.

I wrote a "feature" that does in my BBS software, that throttles the 
output speed for the entire session, regardless of the actual speed of 
the connection, primarily for effect for BBS users connecting via IP.

The way I do this is simply by adding the appropriate delay in the 
pseudoterminal layer, i.e. between reading data from the slave and 
writing it to the master. I wouldn't say it's the most *efficient* way 
to do this, from a CPU perspective, since the number of system calls is 
linear in the number of characters, but for very slow speeds, like 
110/300/1200/2400, etc. it works very well, and the CPU overhead isn't 
enormous per se, either, and any program that gets launched, like a 
shell, is blissfully unaware that anything funky is going on.

I remember looking for a way to do this in the kernel, just using a 
constant number of system calls, but I don't think I ever found anything 
that would help me. Maybe it would make a good project for a kernel mod.

The one thing you can NOT do is the same thing you would with a serial 
connection, using cfsetispeed/cfsetospeed. Those only work for serial 
connections. Otherwise, they are basically no-ops.

Another way you could do this without modifying the system is just dial 
into it using a modem. Using a modem on your PC + softmodem on the 
server + telnet to proxy from the modem to IP, it's pretty easy to just 
dial in at 300 using your computer's terminal software. HyperTerminal 
even supports 110 but I've never actually used that speed, your modem 
might have to support it. My USR modem seems a little cranky when I try 
using that.
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