Archive for the ‘rbash’ Tag

A very restricted shell

I was recently asked to set up access for someone so that he could connect from the internet to a machine running on our company network. Securing the machine on the company network was easy enough but I needed to create a route through to it.
The approach I took was to build a CentOS 6 machine running sshd which was accessible via a NAT rule on our outside firewall to act as a gateway to our network. This meant that the computer to which the user wanted to connect wasn’t directly accessible via the internet which made me happier.

The user had to forward a local port to the required port on the computer he wanted to access on our network. In other words, conventional ssh tunnelling and port forwarding. As a further security restriction, on the gateway computer I used the local iptables firewall to restrict not just the inbound traffic, but the outbound as well, only opening the ports that were needed.

I also decided I wanted to restrict the shell that the user was given on the computer he connected to via ssh. The restricted bash option seemed reasonable (i.e. bash -r option) but even that allowed more than was needed. My solution was to write a very simple c program that acted as the most restricted shell I could think of, i.e. it understands one command only – “exit”.

File: smallsh.c

#include <stdio.h>
#include <string.h>

/* Very simple command shell. Only supports one command "exit". */
/* Output is inelegant if the user types more than 60 chars as command */

/* Version 1.0 14/10/2014 */

main()
{
  const char cmdEXIT[5] = "exit\n";
  char userCmd[60];
  printf ("Type exit when done\n$ ");
  while (fgets(userCmd, sizeof userCmd, stdin)) {
    if ( ! strncmp( cmdEXIT, userCmd, 5 ) ) {
      return;
    } else {
      printf ("I only understand 'exit'\n$ ");
    }
  }
}

When this was compiled, I moved the result (smallsh) to /usr/local/bin. The program is simple, and if the user types more than 60 characters in a single command at the input prompt the output doesn’t look elegant, but I, and he, can live with that!

The suitable user account was created on the gateway machine with:

useradd -m -d /home/<username> -s /usr/local/bin/smallsh <username>

The sshd_config file then needed the following settings changed from the default and the sshd service needed restarting.

PermitTunnel yes
AllowUsers <usernname>
PermitRootLogin no

(OK, I’ll accept that “PermitRootLogin no” is not needed, but as far as I’m concerned it is).

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