UDP listener

[eulaersi]

I would like to integrate musicpd into my home automation. My home automation system can send UDP telegrams.

Therefor, I was wondering if it would be possible to set up a udp listener or daemon that listens to a specific port. Whenever there is a telegram on that port, it compares it with a predefined list of telegrams. Each predefined telegram should be linked to a command shell program.

By doing so, I would be able to start, stop, pauze, ... Music by using the musicpd command shell app (mpc).

Can you help me (I must admit that I have no experience with udp telegrams).

Ivan

[Irithori]

Well, listening to a udp port is trivial.
For first tests, netcat or socat can be used.

netcat -ul 2000

This will listen to udp port 2000 and send every incoming udp datagram to standardout.
An actual implementation may use xinetd or a dedicated server.

The problem is to understand, what your home automation sends, and what it expects in return.
This may or may not be documented.
If it is documented, some coding may be needed, as you have already indicated in your post.

[eulaersi]

Tx for your reply. My home automation can send whatever string by an UDP connection. E.g. I can send a UDP telegram by specifying "sendudp(3807u,192.168.1.53,$Start$)". This will send the string "start" to port 3807 on the specified IP address.

When listening to this port (thanks to your post), I can see that "Start" is coming over ("netstat -ulp 3807").

Can you point me in the right direction so I can write a shell script that runs as a daemon. The script should react as follows:
- when receiving "Start" -> run "/usr/bin/mpc -h 192.168.1.53 play"
- when receiving "Stop" -> run "/usr/bin/mpc -h 192.168.1.53 stop"
- when receiving "Next" -> run "/usr/bin/mpc -h 192.168.1.53 next"
...

Tx for your help

[Irithori]

Ok, I got a skeleton implementation.
There is surely a lot room for improvements.

It works with tcp only for now.
For the udp version, there is a problem with (presumably) the socket_type.

Need to investigate some more here, but maybe s/o else has a hint?

What I know: /usr/local/bin/mpdproxy.sh gets called,
but it stalls on the read command, unless a timeout is used.

It probably doesnt receive anything via stdin,
and instead the udp socket opened by xinetd needs to be handed over to the script.


Anyway, you will need an installed xinetd daemon.

Disclaimer:
Make sure you have a backup of this file:
/etc/services

cat >> /etc/services <<EOF
mpdproxy         3807/tcp                       # receive commands and use them with mpd
mpdproxy         3807/udp                       # receive commands and use them with mpd
EOF

cat >> /etc/xinetd.d/mpdproxy <<EOF
# default: off
# description: An xinetd service which triggers mpd actions 
# based on commands received via udp/tcp

# This is the udp version.
service mpdproxy
{
        disable         = no
        socket_type     = dgram
        protocol        = udp
        port            = 3807
        server          = /usr/local/bin/mpdproxy.sh
        user            = nobody
        wait            = yes
}

# This is the tcp version.
service mpdproxy
{
        disable         = no
        socket_type     = stream
        port            = 3807
        server          = /usr/local/bin/mpdproxy.sh
        user            = nobody
        wait            = no
}
EOF

mkdir -p /usr/local/bin

cat >> /usr/local/bin/mpdproxy.sh <<EOF
#!/usr/bin/env bash

## get command,
## strip windows style line feed and perform basic input validation
read -t 3 STDIN
INCOMING_COMMAND=$(echo ${STDIN}|sed 's/\x0D$//'|egrep '^(Start|Stop|Next)$)

## tools
MPC_BIN="/usr/bin/mpc"
MPC_OPT="-h 192.168.1.53"

LOGGER_BIN="/usr/bin/logger"
LOGGER_OPT="-t mpdproxy -p local0.err"

## depending on the command, call the mpd. 
## Log failed calls to syslog 
case "${INCOMING_COMMAND}" in
    Start)
        if ! $MPC_BIN $MPC_OPT play; then $LOGGER_BIN $LOGGER_OPT ERROR while executing mpd play; fi
            ;;
    Stop)
        if ! $MPC_BIN $MPC_OPT stop; then $LOGGER_BIN $LOGGER_OPT ERROR while executing mpd stop; fi
            ;;
    Next)
        if ! $MPC_BIN $MPC_OPT stop; then $LOGGER_BIN $LOGGER_OPT ERROR while executing mpd next; fi
            ;;
    *)
        $LOGGER_BIN $LOGGER_OPT ERROR unknown command received: $STDIN
            ;;
esac

exit 0

EOF

chown nobody /usr/local/bin/mpdproxy.sh
chmod 700 /usr/local/bin/mpdproxy.sh

Restart the xinetd daemon

[Irithori]

Ok.
It seems, in case of udp the server itself needs to read and make sense of the datagram.
comp.unix.programmer: Re: Writting a UDP server program under inetd/xinetd
Which might be messy in bash.

I thought, that xinetd would do some of the magic and just provide the actual data.
But meh.

Any chance, that you can implement your server in python/perl/ruby/etc?
http://www.java2s.com/Code/Python/Ne...verforWait.htm
Or alternatively: Can that home automation system send tcp as well?

[eulaersi]

Ok.
Or alternatively: Can that home automation system send tcp as well?

TCP is also possible, but I have to check how this should be done.

Nevertheless thanks for thinking with me. I tought it should be easy, but I was mistaken.

Ivan

[Irithori]

Well, for tcp it is fairly simple, even in bash. The above scripts should work.
udp needs some lines more and something else than bash

P.S.: Forgot a '

INCOMING_COMMAND=$(echo ${STDIN}|sed 's/\x0D$//'|egrep '^(Start|Stop|Next)$')