detachtty

-- Text -- is good.

		    detachtty

detachtty lets you run interactive programs non-interactively, and connect to them over the network when you do need to interact with them.

If you've used screen, it's a similar concept. Compared to screen, detachtty has

• no capability to swap between different screens (it doesn't co-opt C-a)

• no tty emulation (you can sanely use it in emacs comint modes)

• no features (less to go wrong)

• no configuration file

• ability to spawn ssh to securely connect across the network

• What it's for - a likely usage scenario

I have a persistent server implemented in Lisp. Usually it sits there serving things, but sometimes (for example when a new version is available and it needs to be upgraded) I need to connect to it and type Lisp at it.

Why don't I use screen? I could - I used to do exactly that - but then I have to talk directly to the CMUCL repl, which means no command line editing. I want to use ILISP, but (1) screen's terminal handling is likely to confuse Emacs, (2) screen isn't network-enabled and the server machine doesn't have Emacs installed anyway.

• How it works:

detachtty has two parts:

** detachtty - the server

opens a pseudoterminal and starts your program on the slave end. It then copies between that tty and a unix-domain socket whose location you specify when you start it

detachtty has options to fork into the background, to specify the socket location, and to govern the placement of its log file and of the dribble file from the application underneath it

** attachtty - the client

When you want to connect to the process, attachtty attaches to the unix-domain socket and copies input/output to and from you.

If you have ssh installed and set up, you can also use attachtty across the network. This requires

• that you also have attachtty installed on the machine that you are running detachtty on, and that it's somewhere on the path

• that you have ssh set up not to need to ask for a password (e.g using ssh-agent)

It's often nice to be able to interrupt a process, especially when it's stuck in some kind of loop. attachtty catches SIGINT and sends the character \003 into the pty master, which causes the controlled process to get a SIGINT of its own. This works at least for using with ILISP and CMUCL.

• Using it - an example

server$ detachtty --help detachtty: unrecognized option `--help' usage: detachtty [--no-detach] [--dribble-file name] [--log-file name] [--pid-file name] socket-path command [arg] [arg] [arg] ...

server$ detachtty --dribble-file /var/log/cliki-dribble --log-file /var/log/detachtty.log --pid-file /var/run/cliki.pid /var/run/cliki-socket /usr/bin/lisp -eval '(load-system :telentweb)' -eval '(progn (telentweb::init) (telentweb::start))' \

client$ attachtty dan@server:/var/run/cliki-socket

** Signals and log files

When detachtty receives a SIGHUP, it closes and reopens its log and dribble files, so it should play nice with whatever log rotation tools you already use. Note that the dribble file especially can grow large quite quickly

The SIGHUP should be sent to detachtty, not to the child. What the child does with signals depends entirely on what the child is ...

• Security considerations

• detachtty performs no authentication of who's attached. You choose
  where the socket is created in the filesystem, so choose somewhere
  that other people can't get to.

(detachtty creates its socket with mode 0700. Some Unix systems - Linux among them - honour file permissions on sockets, so only the original user or root will be able to connect to it. However, "many BSD derived systems [...] ignore permissions for Unix sockets" so you need to check if this will help you or not.)

• communication across the net is done using ssh. If there is a program on your computer called "ssh", then whatever level of security it provides is available here

• no serious effort has been made to examine the attachtty and detachtty code for buffer overruns etc. If you are planning on using these setuid or in some other untrusted-user situation, now is the time to think of an alternative plan.

• Where to get it

detachtty is part of the Debian GNU/Linux system. Debian "woody" and "sid" users should find that "apt-get install detachtty" will download and install it for them.

Users of other platforms are invited to check the CLiki page at URL:http://ww.telent.net/cliki/detachtty which should always contain a link to a download location.

• Feedback, feature requests, patches

to [email protected], or using the Debian bug tracking system (bug reports can be sent to the BTS even if you're not using Debian; it's exactly the same package either way), or leave a note on the CLiki page