Permission denied

Posted on 2000-03-28
Medium Priority
Last Modified: 2013-12-27
I'm using a SUN server and while using

echo "Yoh" > /dev/pts/x (x is a number)

I can send the message to myself, or when I'm root, I can send mesg to user but user cannot send message to each other, error message is "permission denied", what can I do with this ?
Question by:ahTri
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Expert Comment

ID: 2666008
Well you're not supposed to be able to do that as an ordinary user. If you could write to just any user's terminal a lot of malicious things would be possible and there wouldn't be any way for the user to control it. Don't do it that way.

There are facilities within Unix specifically for this purpose. Look at the man pages for wall(1M), write(1), mesg(1), and talk(1). Probably the one you want is write(1).

Author Comment

ID: 2666098
it gotta be something relate to tty because currently every user have mesg on, can talk, can write ...

Expert Comment

ID: 2667138
Have you checked the permissions on the /dev/pts/#?  I just checked my nonsecure development machine (IBM RS/6000) and the permissions are set to rw--w--w- so everyone has write access to my tty so a "echo message > /dev/pts/#" would work.
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Author Comment

ID: 2667491
I did try but onn SUN /dev/pts/# is just a link to ../devices/pseudo/..... so cannot change access right

Accepted Solution

festive earned 150 total points
ID: 2667657
As stated already by jlevie this is not something that you would normally do,
however if you must do it you need to setup a shell script or program as "setuid" root.
you could then redirect the output to the target pty, ie something like the following:

#!/bin/sh -p
ARG1="$1" # pty number
ARG2="$2" message in quotes
cat $ARG2 > $ARG1

This is not recommended, and perhaps a compromise may be to put setuid on one of the mentioned utilities.

LVL 40

Expert Comment

ID: 2667728
I don't think you need suid on write(1) and I know that you don't on talk(1).

Author Comment

ID: 2668051
festive, is that a script to run on root ?

#!/bin/sh -p

this line doesn't run, maybe I don't have sh

Expert Comment

ID: 2668440
this script to run needs to be setuid root, and the top line means that it is a bourne shell script.

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