I see. I've been trying to figure out everything that's going on with your udplisten.pl. I've made some changes primarily to introduce time measurement in a way that I could use for the higher-level abstractions of DateTime. Also, I couldn't understand where the newline after mypi was coming in, but I think that's what Data::Dumper gives the string in dumpstr. Regarding this syntax:

I searched for an operator !! in perlop, and not finding it have to believe it's a double application of !, essentially "Not-not". It seems to have the effect of making LHS the same as RHS, whether RHS is defined or not. That was torturous to say; do I have it right?

The revision in Markdown did help me make an effective crontab command with this as the result:

$ crontab -l
# Edit this file to introduce tasks to be run by cron.
...
# m h  dom mon dow   command
*/17 * * * *  hostname | socat -s - UDP-DATAGRAM:255.255.255.255:12340
+,broadcast 2>/dev/null
$
[download]

Source:

#!/usr/bin/env perl
use warnings;
use strict;
use 5.010;
no feature 'switch';

=head1 SYNOPSIS

Waits for message(s) on a UDP port and when a message is received,
prints the IP address from which each message was received.

 udplisten.pl [-m] [-c COUNT] [-a] [-p PORT] [-e EXPR] [-b RXSZ]
 OPTIONS:
   -m       - Output the entire message, not just the IP address
   -c COUNT - Exit after receiving this many messages (default=1)
   -a       - Continuously output all messages (overrides -c)
   -p PORT  - UDP port number (default=12340)
   -e EXPR  - Output only messages which match this Perl expression
   -b RXSZ  - Receive length (default=1024)

=head1 DETAILS

The message can be transmitted, for example, via the following
L<crontab(5)> entry (note: C<sudo apt-get install socat>):

 * * * * *  echo "HELLO xyZ129" | socat - UDP-DATAGRAM:255.255.255.255
+:12340,broadcast

The string used can be completely random; the idea is for it to be uni
+que
to your device so you can identify it, for example:

 udplisten.pl -e '/HELLO xyZ129/'

Two alternate ways to listen are via L<netcat(1)> or L<socat(1)>
(note these will receive and print I<any> messages on that port):

 netcat -ul 12340           # may need to use -p12340 instead
 socat -u udp-recv:12340 -

Don't forget to open the port for incoming UDP traffic on your local f
+irewall,
for example for L<UFW|https://wiki.ubuntu.com/UncomplicatedFirewall>:

 ufw allow in 12340/udp

B<Note> that the choice of port number above is completely random.
At the time of writing, this port appears to be unused
(L<http://www.iana.org/assignments/port-numbers>),
but if you've got other things on your network that use this port, cho
+ose a different one.
You're also free to, for example, use different ports for different de
+vices.

=head1 AUTHOR, COPYRIGHT, AND LICENSE

Copyright (c) 2016 Hauke Daempfling (haukex@zero-g.net)
at the Leibniz Institute of Freshwater Ecology and Inland Fisheries (I
+GB),
Berlin, Germany, L<http://www.igb-berlin.de/>

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program. If not, see L<http://www.gnu.org/licenses/>.

=cut

use Getopt::Std 'getopts';
use Pod::Usage 'pod2usage';
use IO::Socket::INET ();
use Data::Dumper;
use DateTime;
use DateTime::Format::Duration;

sub HELP_MESSAGE { pod2usage( -output => shift ); return }
sub VERSION_MESSAGE { say {shift} q$udplisten.pl v2.00$; return }
$Getopt::Std::STANDARD_HELP_VERSION = 1;
getopts( 'mc:ap:e:b:', \my %opts ) or pod2usage;
my $FULLMSG = !!$opts{m};
my $COUNT   = $opts{c} // 1;
pod2usage("Bad count") unless $COUNT && $COUNT =~ /^\d+$/ && $COUNT > 
+0;
my $ALLMSGS = !!$opts{a};
my $PORT    = $opts{p} // 12340;
pod2usage("Bad port") unless $PORT && $PORT =~ /^\d+$/ && $PORT > 0;
my $EXPR = $opts{e};
my $RXSZ = $opts{b} // 1024;
pod2usage("Bad receive length") unless $RXSZ && $RXSZ =~ /^\d+$/ && $R
+XSZ > 0;
pod2usage("Extra arguments") if @ARGV;

my $sock = IO::Socket::INET->new(
  LocalPort => $PORT,
  Proto     => 'udp',
) or die "error in socket creation: $!";

my $count = 0;
my $dt;
my $start = DateTime->now;
say "Start time is: ", $start->hms;
my $later;
RXLOOP: while (1) {
  defined $sock->recv( my $rx, $RXSZ )
    or die "error during recv: $!";
  my $peeraddr = $sock->peerhost;
  my $match    = 1;
  if ( defined $EXPR ) {
    local $_ = $rx;
    eval "\$match = do { package CodeEval; $EXPR }; 1"
      or die "Perl expression failed: " . ( $@ || "Unknown error" );
  }
  next RXLOOP unless $match;
  say "rx is $rx";
  say $FULLMSG ? "From $peeraddr: " . dumpstr($rx) :  "$peeraddr";
  $dt = DateTime->now;
  say "Time is: ", $dt->hms;
  $later = $dt;
  last RXLOOP if !$ALLMSGS && ++$count >= $COUNT;
}

$sock->close;

my $d =
  DateTime::Format::Duration->new(
  pattern => '%Y years, %m months, %e days, '
    . '%H hours, %M minutes, %S seconds' );

my $duration_object = $later - $start;
say $d->format_duration($duration_object);

sub dumpstr {
  chomp( my $s =
      Data::Dumper->new( [ '' . shift ] )->Terse(1)->Useqq(1)->Dump );
  return $s;
}
[download]

Slightly-redacted output:

$ ./2.udplisten.pl -m -c 2
Start time is: 23:30:43
rx is mypi

From 192.168.red.acted: "mypi\n"
Time is: 23:34:01
rx is mypi

From 192.168.red.acted: "mypi\n"
Time is: 23:51:01
0 years, 00 months, 0 days, 00 hours, 20 minutes, 18 seconds
$
[download]

so, everything's looking pretty good, but I wanted to complete step 4 in your rpi setup, with some attempt to understand it along the way:

On my first attempt, I just tacked it on the end:

# patch 4/20/2022
-*.*;auth,authpriv.none          -/var/log/syslog
+*.*;cron,auth,authpriv.none     -/var/log/syslog
[download]

, and restarted:

, but rsyslog did not want to parse these lines:

Apr 20 16:41:59 mypi systemd[1]: Stopped System Logging Service.
Apr 20 16:41:59 mypi systemd[1]: Starting System Logging Service...
Apr 20 16:41:59 mypi rsyslogd: error during parsing file /etc/rsyslog.
+conf, on or before line 94: errors occured in file '/etc/rsyslog.conf
+' around line 94 [v8.1901.0 try https://www.rsyslog.com/e/2207 ]
Apr 20 16:41:59 mypi rsyslogd: error during parsing file /etc/rsyslog.
+conf, on or before line 95: invalid character '+' - is there an inval
+id escape sequence somewhere? [v8.1901.0 try https://www.rsyslog.com/
+e/2207 ]
Apr 20 16:41:59 mypi systemd[1]: Started System Logging Service.
[download]

, so I recalled that I didn't see any notation for a leading + or -, and went with this instead in /etc/rsyslog.conf:

###############
#### RULES ####
###############

#
# First some standard log files.  Log by facility.
#
auth,authpriv.*            /var/log/auth.log
#*.*;auth,authpriv.none        -/var/log/syslog
*.*;cron,auth,authpriv.none     -/var/log/syslog
#cron.*                /var/log/cron.log
daemon.*            -/var/log/daemon.log
kern.*                -/var/log/kern.log
lpr.*                -/var/log/lpr.log
mail.*                -/var/log/mail.log
user.*                -/var/log/user.log
[download]

, and that looks more like it, the parser having no objections. The man page for rsyslog had promised an html page, which I was enjoined to find. When running

after a successful start, this url was posted as output: www.rsyslog.com, where I finally learned what the r is for in rsyslog. (rocket-fast)

So, yay, at the pace of a tortoise I think I've got task 4 covered. Thank you for your comments.

Gruss aus Amiland,