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Tag Archives: ssh

Sharing a screen session with another administrator on a Linux system

Background

If you aren’t familiar with GNU screen, you really should stop right now and familiarize yourself with it. This is a very powerful utility which allows you to run terminal based programs on a system, disconnect from that session and re-connect later from the same or a different location. You can also start multiple terminals within a given screen session. Whenever I ssh into a system, I almost always launch screen first. If my ssh session gets disconnected unexpectedly, I can simply re-connect and pick up where I left off by re-attaching to the screen session.

The Problem

I was recently working with a client on a process that was going to take quite some time to complete. The command we were running would give a progress indicator, so we could monitor the progress off and on over time. I assumed, since we both had the ability to utilize sudo to change user privileges that he would be able to sudo su – myusername followed by screen -r to take over the screen session I had started which contained this command. When he tried this, however, he was greeted with the following error:

Cannot open your terminal '/dev/pts/1' - please check.

The Solution

Searching around on Google comes up with a couple of different solutions. One of these solutions suggests that the second user should change the permissions on his tty to allow everyone access to it. While this works, it is definitely a Bad Idea for security as any user on the system could then snoop that tty.

The other solution suggests that the second user should issue script /dev/null after escalating themselves to the first user’s account. This works and does not appear to have the same security implications as the method above because everyone retains access to their own tty’s.

But Why Does This Work?

What I found was that none of the posts I ran across which presented the second, preferred solution explained why this works. They merely said, “use this method,” and left it at that. Being naturally curious, and harboring a concern as to whether this also opened up a tty to others’ snooping, I had to investigate.

Prerequisites

Of course, this all assumes that at least the second user has the ability to sudo su – and escalate their privileges. That is all. Let’s move on.

Stepping Through The Process

Here’s how I went about discovering what exactly script /dev/null does and why it allows the second user to access what appeared to be an inaccessible tty.

First, usera logs in via ssh, checks which tty was assigned, checks the permissions on that tty and launches screen:

usera@localhost ~ $ ssh usera@remotehost
usera@remotehost ~ $ tty
/dev/pts/1
usera@remotehost ~ $ ls -l /dev/pts/1
crw--w---- 1 usera tty 136, 1 2011-01-09 20:14 /dev/pts/1
usera@remotehost ~ $ screen

As you can see, usera has RW permissions, group members have W permissions and others have no access at all to this tty. Next, userb logs in to the same system via ssh, checks which tty was assigned and checks the permissions on that tty:

userb@localhost ~ $ ssh userb@remotehost
userb@remotehost ~ $ tty
/dev/pts/2
userb@remotehost ~ $ ls -l /dev/pts/2
crw--w---- 1 userb tty 136, 2 2011-01-09 20:20 /dev/pts/2

Again, the same permissions are present on the tty assigned to userb. So neither user can snoop on the other’s tty at this point. Here’s where it gets interesting, though. Let’s have userb escalate to usera and check the tty assignment and permissions again:

userb@remotehost ~ $ sudo su - usera
[sudo] password for userb:
usera@remotehost ~ $ tty
/dev/pts/2
usera@remotehost ~ $ ls -l /dev/pts/2
crw--w---- 1 userb tty 136, 2 2011-01-09 20:20 /dev/pts/2

This is where I had my “aha moment.” Although userb has changed to usera, the same tty (with the same permissions) is in use. Therefore, all commands issued are now under usera but any command which tries to manipulate the tty (like screen does) will fail because the tty remains under control of userb.

So now let’s take a look at what script /dev/null does to the tty:

usera@remotehost ~ $ script /dev/null
Script started, file is /dev/null
usera@remotehost ~ $ tty
/dev/pts/3
usera@remotehost ~ $ ls -l /dev/pts/3
crw--w---- 1 usera tty 136, 3 2011-01-09 20:36 /dev/pts/3

Ahh, we now have a new tty assigned to this user. Therefore, when screen -r is issued, the currently assigned tty, /dev/pts/3 is accessible to usera and the command succeeds! Also note that this new tty has the same permissions as the original usera tty, so it should be just as secure from snooping.

Conclusion

If you need to share a screen session with another (admin-rights holding) user, then the script /dev/null method is much preferred over mucking around with tty permissions. It appears that the script /dev/null method is just as secure as the original user’s tty because the permissions on the new tty are exactly the same.

On a more general note, be aware that solutions you find on the Internet might work, but they may not always be the best solution for the task at hand. Be sure you understand the implications of what you are doing instead of blindly copying and pasting commands you found on someone’s blog. If you are not sure what a particular solution does, I encourage you to test as I did (on a non-production system, of course) to make sure you understand it before you put it to use.

Using OpenSSH for Linux administration

Here are some hints and tips for those who are new to using ssh/OpenSSH for Linux system administration. Most of these tips have come from my recent work with a large number of Linux servers hosted on a VMware ESXi 4.x server farm.

Password authentication VS ssh key authentication

  • If you are administering only a few systems on a closed network (i.e. accessible only locally or by a secure VPN connection), then password authentication is probably OK, but you should consider using ssh keys anyway.
  • If your network needs to allow ssh access directly from the Internet or you are administering a large number of systems, then you should definitely use ssh keys.

Ssh-agent, scripting and cron

  • ssh-agent can save you typing in the password to your ssh key every time you need it.
  • This site gives a good overview of ssh-agent and includes some code you can add to your .bash_profile script to ensure your keys get added upon login.
  • Although there are hack-ish ways to get ssh-agent and cron to work together, you are probably better off setting up special keys to use with scripts that must be called via cron. Just keep in mind that keys without passwords are a security risk.
  • If you cannot risk using keys without passwords, consider running those cron scripts locally on each system. Utilize shared file space or e-mail to collect the results.

Bash one-liners and ssh with ssh keys

  • I’ve become a fan of using bash “one-liner” scripts to keep abreast of server stats such as load averages, available patches and disk usage.
  • Keep an up-to-date list of hosts in a file called hostlist.
  • Run your one-liners while ssh-agent has your ssh keys cached.
  • Here’s a template one-liner which checks uptime on each host listed in the file hostlist:

for e in `cat hostlist`; do echo $e; ssh $e "uptime"; done

  • In the above example, you can replace uptime with just about any command which exists on the remote host.
  • You can also synchronize some of the configurations under /etc with the above by utilizing either scp or rsync instead of ssh in that one-liner.

Turn your one-liners into scripts

  • If you find yourself using the same one-liner over and over, it is time to save yourself some typing and turn it into a script.
  • I like to keep these sorts of scripts under ~/bin. I also like to add that to my $PATH and create a simlink ~/scripts.
  • Some one-liners are good candidates to be turned in to cron scripts. Just keep in mind the risks of using ssh keys without passwords, and include logic to detect conditions you want to monitor. For example, you can run /proc/loadavg through awk to isolate one of the three figures and send yourself an e-mail if that average is too high.