Fun with Pseudo-Terminals (PTY) & TTY ("Teletypewriter") over `socat` & `websocat`
Fun with Pseudo-Terminals (PTY) & TTY (“Teletypewriter”) over socat & websocat
In the context of Enola.dev #1368,
I recently learned a few things related to openpty and
how to execute a program under a “pseudo (virtual) terminal” so that it e.g. prints ANSI color codes,
and allows interactive control, like a Shell or Editor requires.
socat
socat, the “better netcat++” is one way do this. In one Terminal, run:
socat file:/dev/tty,raw,echo=0 tcp-listen:12345
In another Terminal, run:
socat tcp-connect:localhost:12345 exec:'/bin/bash -li',pty,stderr,setsid,sigint,sane
The former can now use the latter - with working ANSI escape sequences and keystrokes.
(You might need to export TERM=xterm-256color, if that’s not already set.)
Just the terminal size won’t be correct: echo $LINES $COLUMNS (likely) will show the default 24 80.
Running stty over socat will also show that default. But running it “locally” and then “fixing” it with stty rows 24 cols 182 can do the trick.
Unfortunately with socat it’s difficult to do this automagically on connect, or even continuously on resize.
PS: file:/dev/tty is easier than $(tty)
or `file:``tty``` (which is Bash specific and does not work in Fish, and is also difficult to render correctly in Markdown).
websocat
https://github.com/vi/websocat is a cool socat-like tool which would allow to do something similar to above but over ws:// WebSocketss.
Alternatives
SSH automatically handles all aspects of TTY allocation, environment propagation ($TERM), and
window resizing (SIGWINCH to TIOCGWINSZ, TIOCSWINSZ, TIOCSCTTY).
(For Enola.dev’s planned “AI Shell”, I might persue this avenue further.)
Another option would be a custom TTY binary, written e.g. with Python’s pty and termios (minimally even just python -c 'import pty; pty.spawn("/bin/bash")'),
or in Go (e.g. with https://github.com/creack/pty; or https://github.com/mattn/go-tty),
or in Rust (with nix and tokio crates),
or even in Node.js (with node-pty),
or even Java with Pty4J,
which catches SIGWINCH and somehow sends the terminal dimensions over to the remote process,
which then performs the TIOCSWINSZ ioctl.
Such tools obviously already exist, and are sometimes referred to as “remote shells” (or “web shells”) or “reverse shell” or “bind shell” when they are associated with “penetration testing” (AKA PenTest) [or worse] related projects: