Openssh + Screen + Bash = Remote assistance, question about local shell switch

[Rexilion]

Hello everyone, this is my first post in this section. I hope this question is appropiate.

What I'm doing: Using OpenSSH's remote port forwarding to provide remote assistance to Windows boxes from people I help (you can call them customers if you want). Every now and then, <snip> decides to roll an update that breaks ... random stuff (I unintentionally earn money on this insanity, really).

My question is regarding the server side OpenSSH setup. I have rules like this in my authorized_keys file (I stripped the public key for your viewing convenience):

no-user-rc,no-agent-forwarding,no-pty,
no-X11-forwarding,permitopen="127.255.255.255:65535",
environment="SSH_SIG=customer_name",
command="env PS1=\"$SSH_SIG\"' \t  ' screen -OqxRRS \"$SSH_SIG\" bash -n"

My question is specifically about the bash command:

bash -n

This allows me to abuse bash and use it for chatting. The '-n' switch (or noexec option) in bash makes bash to not execute anything. Is this safe for using over OpenSSH as a chat mechanism? The weird thing is, this bash command is an interactive session, and the manual of bash states:

    -n      Read commands but do not execute them.  This may be used to check a
    shell  script  for  syntax  errors.  This is ignored by interactive
    shells.

Thanks in advance for any tips or remarks.

P.S. As you might have already figured, looking for google for bash + noexec mostly yields results about the noexec mount flag (ugh)...

[sudodus]

I didn't know about

bash -n

but I see that it works even though I can't find it in man bash.

I don't know enough about internet security to tell how safe it is.

However, I sometimes use the old write command. See

man write

This is an old chat tool, used way back in Unix, but I played with it to teach my daughter some basic command line tools, and it is still there in Ubuntu linux.

You might find this tool convenient

[Rexilion]

I didn't know about

bash -n

but I see that it works even though I can't find it in man bash.

Yes, it's kind of hidden between all the other '-n' 'test' switches. If you search for 'noexec' first and then scroll up to the first instance of '-n' you will find it.

I don't know enough about internet security to tell how safe it is.

Yes, this is always hard to asses. I agree.

However, I sometimes use the old write command. See

man write

This is an old chat tool, used way back in Unix, but I played with it to teach my daughter some basic command line tools, and it is still there in Ubuntu linux.

You might find this tool convenient

A tool that fits my requirements is better than using an entire shell =) . Thanks I'll try and experiment with this!

[Rexilion]

I gave it a good try (also found the 'who' command), and it has some drawbacks over the old 'bash -n' method:

- It talks to itself if I'm not logged in (the remote user logs in under the same name as the local user)
- Chat history disappears on the other side if the connection breaks

I'm appealed by it's simplicity though. Maybe I have to change some stuff about my setup...

[sudodus]

I gave it a good try (also found the 'who' command), and it has some drawbacks over the old 'bash -n' method:

- It talks to itself if I'm not logged in (the remote user logs in under the same name as the local user)
- Chat history disappears on the other side if the connection breaks

I'm appealed by it's simplicity though. Maybe I have to change some stuff about my setup...

I hope there will also be an answer about the security issue

Good luck

[Rexilion]

I hope there will also be an answer about the security issue

Good luck

Again, thank you for your help. I have figured out a secure 'middleground'.

I'm using a seperate user now that starts screen sessions. This user does not have a home directory, but I can specify an alternative screenrc file. This way I can make this screen session 'multiuser' allowing my own user to connect to this.

The reason I did not do this before, is that I assumed that since I blocked a certain range of ports in order to inhibit other users to access someone else his/her remote machine. These ports are only accessible by me.

However, I assumed that *binding* to these ports is also inhibited by other users. This was not the case.

So, I can keep my firewall simple (and this safe), it doesn't matter now if 'bash -n' is 'safe' since it is ran as an user comparable to 'nobody' and I get to have dedicated screenrc file as a bonus!

However, the 'write' you mentioned might come in handy someday =) Thanks!

[Ms. Daisy]

Do you know if there's a command that will turn off the -n switch? I have no idea but it seems like a crucial thing to know.

You could just use http://webchat.freenode.net/ and create your own channel, then give your clients a link to your channel using the "add webchat to your site" feature.
irc.png

[Rexilion]

Do you know if there's a command that will turn off the -n switch? I have no idea but it seems like a crucial thing to know.

Yes, that is kind of what I'm asking in this thread. However, common sense (to me) says that should not be possible. Because if a command can change this parameter, then it's not suitable for use in scripts.

Furthermore, I looked at other ways of communicating with bash and it seems that signals cannot interfere either...

You could just use http://webchat.freenode.net/ and create your own channel, then give your clients a link to your channel using the "add webchat to your site" feature.
irc.png

I have considered setting up my own IRC chat server for this. But I don't like to involve third parties. This setup (right now) is just me and the other side.

[Rexilion]

I'm not posting what I have and what I use right now. Just to let you know and in case someone else is curious.

- user remote: unix user dedicated for remote control sessions
- user admin: unix user corresponding to an actual uses who manages remote assistance
- env SSH_SIG: environment variable to differentiate between customers, usefull since they all login as user 'remote' so differentiating becames kind of tedious.

There is one caveat to all of this: Remote users can remote forward to any port on my host! I have seen users on OpenSSH mailing lists requesting config settings to limit this, but this has yet to be implemented.

What could happen? Example: You always use 127.0.0.1:80 for a local filtering proxy. Your proxy crashes. A malicious user could attach @ 127.0.0.1:80 and view all your browsing habbits. For me, this is not the case, but a customer that forwards port 5000 for remote assistance could also forward port 5500 and could pretend to be a different person!

However, I can live with this.

This is in chronological order, containing only relevant snippets:

#Example customer; rport 57000; pass vnc
no-user-rc,environment="SSH_SIG=Customer Name" ssh-rsa <KEY>

AllowUsers remote
Protocol 2
ChallengeResponseAuthentication no
PasswordAuthentication no
RSAAuthentication no

AuthorizedKeysFile /etc/ssh/authorized_keys/%u # restricted authorized keys management
PermitUserEnvironment yes # allow environment variable passing
UsePrivilegeSeparation sandbox # not working yet, need seccomp for this

Match user remote # unix user for remote assistance
AllowAgentForwarding no # disabling makes since there is no shell access
PermitOpen none # disable local and dynamic forwarding
ForceCommand screen -c /etc/custom_scripts/remote_screenrc -qxRS "$SSH_SIG" # start screen session

multiuser on
acladd admin
bindkey "^D" exec # prevent remote user from shutting down session
screen bash /etc/custom_scripts/remote_bashrc # start new screen with custom start parameters
chacl remote -x ? # disable remote user from doing *anything* (e.g. screen exec can execute commands!)

echo Dear "$SSH_SIG"
echo Welcome to Remote Assistance
echo Please use this screen to communicate
echo Type short textnotes, end them by pressing [ENTER]
echo
echo
export HISTFILE=~/log_"$SSH_SIG" # set up dedicated logfile
export HISTTIMEFORMAT='%F %T ' # enable history (man bash)
touch "$HISTFILE"
chmod 606 "$HISTFILE" # allow admin to view and modify these files
echo Session started on: "$(date)" >> "$HISTFILE"
export PS1="${SSH_SIG} \t : " # Make PS1 echo customers name and date
exec bash -n # execute dumb and restricted shell for chat only functionality

Initial setup:

# user: remote
groupadd -g 501 remote
useradd -p "" -M -u 501 -g 501 -d /var/log/remote remote
mkdir -p /var/log/remote
chown remote:admin /var/log/remote
chmod 370 /var/log/remote

Cliënt side:

- VNC server attaching to looback only (security).
- PuTTY + private key remote port forwarding VNC to my PC.

One caveat, PuTTY exhibits 'non default window behaviour': Minimizing the PuTTY window, by clicking on the _ symbol of it's window, from within the VNC connection makes the tunnels go down (and thereby hanging your VNC connection indefinitly). I have not found a good way around this (yet). Minimizing from the taskbar works, though...

I think I have it all pretty much under control right now. If anyone has questions/suggestions/critique please do not hesistate to post.

[CharlesA]

TBH, it would be way easier to just use something like gotomeeting or TeamViewer for what you want to do.