Persisted SSH Remote Control (PSRC)

Run (bash) scripts across a set of hosts over persisted ssh control sessions.

Installing

Just download psrc.sh.

psrc.sh

Configuring

Create a subdir called commands.d under the directory containing psrc.sh. And add whatever set of remote scripts you would like to use. For each remote script, defined a shell function with the same name as the script. For example:

#!/bin/bash

# File: commands.d/foo.sh

foo() {
  local arg="$1"
  shift
  case "$arg" in
    xyz)
      foo_xyz "$@"
      ;;
    *)
      return 1
      ;;
  esac
}

foo_xyz() {
  blah blah
}

All commands should try to be "nice" regarding global function and global variable scope, as all commands are run in the same shell context. This allows sharing of data between functions, but also can lead to unintentional conflicts. Use the bash local keyword for most variables.

Running

Once some commands have been defined, start a remote control session from one host. For the following examples, the remote hosts to be controlled are called slave1 .. slave3. If the location where psrc.sh and the commands directory is not shared among hosts, you can first bootstrap the hosts to be controlled. Bootstrapping pushes the control script and command files to the remote hosts (to a path under $TMPDIR or /tmp, make sure $TMPDIR is set to somewhere that allows execute):

./psrc.sh bootstrap slave1 slave2 slave3

Next, start a session for a set of hosts (more hosts can be added at any time by repeating this command):

./psrc.sh connect slave1 slave2 slave3

Now run as many remote commands as desired over any / all of the remote hosts.

./psrc.sh run slave1 foo xyz
./psrc.sh run ALL bar abc
./psrc.sh run slave3 baz 1 2 3

Note that the exit status from the remote script is returned and can be tested for locally. If the command was run on ALL hosts, then the exit status will be 1 (fail) if any of the hosts returned non-zero (failed).

When done with the session, close all the connections and cleanup:

./psrc.sh stop

Individual hosts can be added or removed at any time with:

./psrc.sh connect slaveX
./psrc.sh disconnect slaveX

The ALL operations always act on all currently connected hosts.

Also run psrc.sh --help for other usage.

Security

THIS TOOL ALLOWS ARBITRARY REMOTE CODE EXECUTION TO ANYONE WITH WRITE PERMISSION ON THE LOCAL (CONTROLLING HOST) CONTROL FIFO.

The FIFO is made with mkfifo -m 0600 so this exploit should be limited to the user who ran connect and anyone with root on the controlling host. But hey, this tool already assumes the running user can start a full remote shell on the remote hosts, so who cares.

If all remote commands are issued via the psrc.sh script itself, propper quoting / escaping is used to "do the right thing". For instance, running a remote command such as:

./psrc.sh run ALL foo '`date`'

will actually pass the literal string `date` to the foo command, it will not be evaluated.

However, writing directly to the control socket, passing `date` unescaped to the remote can cause the date command to actually be evaluated on the remote. Likewise passing other unescaped metacharacters like |&;()<> can cause arbitrary remote code execution.

Customizing

The logic for psrc itself is contained entirely in the one script, so just tweak away. And feel free to rename the script to any name you like, it should not care what it is named.

Dependencies

On the local (controlling) host:

• bash
• find (only for bootstrap)
• pkill (part of procps)
• sed
• shar (only for bootstrap)
• ssh
• GNU coreutils
  • basename
  • cat
  • cut
  • dirname
  • env
  • ls
  • mkdir
  • mkfifo
  • rm
  • rmdir
  • sort
  • test
  • tail

On the remote (controlled) hosts:

• bash
• sed
• ssh
• GNU coreutils
  • dirname
  • env
  • mkdir
  • sort
  • stdbuf
  • test