cnremctrl - remote control server application
cnremctrl [-st] [-l /path/logfile] [--cmd01=/path/cmd] [--cmd02=/path/cmd] ... [--cmd10=/path/cmd]
This application is the receiver or server of a remote control for your linux server. The sender or client is a Microsoft Windows program called cnRemoteControl.exe.
The client has 1 to 10 buttons. If the user pushes e.g. button no. 3, the client sends a string Button03 via TCP/IP on a reserved port (default 26056/tcp) to the linux server.
cnremctrl uses the well known TCP wrapper xinetd(8). xinetd is bind on the cnremctrl port and verifies, if the sender IP address is allowed to communicate to the server. If the result is ok, xinetd pipes the string Button03 to cnremctrl.
cnremctrl looks for an appropriate command in its conf file /etc/xinetd.d/cnremctrl. If program option --cmd03 is defined, it will be executed via a system call. All command output to stdout and the command exitstate is piped back to the client. The client has an log window to display this output and statusinfo.
Configuration
The whole program configuration is under control of /etc/xinetd.d/cnremctrl. If xinetd starts, the daemon scans the directory /etc/xinetd.d and parses each file. The default file for cnremctrl is:
service cnremctrl
{
type = UNLISTED
port = 26056
socket_type = stream
protocol = tcp
user = root
server = /usr/local/bin/cnremctrl
server_args = -s --cmd01=/path/to/command1 --cmd02=/command2%20-para
wait = no
instances = 1
per_source = 1
only_from = 1.2.3.4 1.2.3.5 192.168.0.0/24
log_type = SYSLOG daemon
log_on_success = PID HOST EXIT DURATION
log_on_failure = HOST ATTEMPT
}
Usually you have to change only:
All xinetd options are well documented in the man page xinetd(8).
Note: If you change the conf file, you have to reload the configuration! On Red Hat type /etc/rc.d/init.d/xinted reload, under SuSE type rcxinetd reload. Look for the exit value of xinetd after restarting. If the conf file is buggy, xinetd might not start properly.
Space workaround
If a command contains one or more spaces, e.g. ls -al, you have to replace every space with %20, so the correct command string runs --cmdXY=ls%20-al.
You have to proceed this way because the string has to be interpreted as one program option and xinetd is incapable of handling quotes on server-args.
Security
Be aware which commands should be called by cnremctrl. Be aware which IP addresses should have access to the server (parameter only_from). cnremctrl is not insecure as long as you do not configure it inproper.
The only strings the client sends to the server are ButtonXY. No system calls were send over TCP/IP! If someone wants to hack your system, he cannot use this communication. If you configure commands like --cmd01=/bin/rm%20-rf%20/ and set only_from = 0.0.0.0/32 you might accept to send the server into nirvana ;)
cnremctrl behind a firewall
If your linux server is blocked with iptables(8), you have to install two rules to let pass cnremctrl communication. Example:
iptables -A INPUT -i eth0 -s 192.168.0.0/24 -p tcp --dport 26056 -j ACCEPT iptables -A OUTPUT -o eth0 -d 192.168.0.0/24 -p tcp --sport 26056 -j ACCEPT
(...)
/usr/local/bin/cnremctrl
/etc/xinetd.d/cnremctrl
/usr/share/man/man1/cnremctrl.1.gz
cnremctrl is normally called from xinetd.
For test purposes you can start cnremctrl on commandline. Example:
cnremctrl -t --cmd01=ls%20-al Button01As result you should see:
Testmode: System command "ls -al" not executed
Copyright (c) 2020 Christian Nause-Müller. This software is released for free use under the terms of the GNU General Public License, version 2 or higher.
Version 0.1.1, January 29, 2020
Written by Christian Nause-Müller <cn at cntec dot de>
Newest version, the Windows client cnRemoteControl.exe and more information: https://sourceforge.net/projects/cnremctrl