dnsresponse_: enable verifying responses

add option verify to enable graphing the number of successful responses.
if verify is enabled (default it is not) then a warning is emitted when
we do not get a response for all queries and critical if no query
succeeded.

add option timeout to be able to limit the time the plugin takes for
unresponsive servers. old max time was 75 seconds times 20 (25minutes).
new default max is 30 seconds timeout times 20 (10 minutes). with
setting a timeout of a few seconds this can now be reduced even more.

added warnings if average and or mean get close to the timeout.

plugins/network/dns/dnsresponse_

@@ -19,10 +19,14 @@ The following shows the default configuration.
   [dnsresponse_*]
     env.site www.google.com
     env.times 20
+    env.verify no
+    env.timeout 30
 
 Where the plugin suffix represents the DNS-Server which should be queried.
-The site which domain-name should be queried.
-Times represents how often a value should be queried.
+env.site: which domain-name should be queried.
+env.times: how often a value should be queried.
+env.verify: yes: check that we get a response back from the server and show the number of verified responses
+env.timeout: timeout in seconds to use when querying servers
 
 =head1 INTERPRETATION
 
@@ -44,6 +48,7 @@ $Id: dnsresponse_ 61 2009-04-14 09:11:00Z stsimb $
 =head1 AUTHOR
 
 Copyright (c) 2009 by Sotiris Tsimbonis.
+Copyright (c) 2023 by Andreas Perhab, WT-IO-IT GmbH.
 
 =head1 LICENSE
 
@@ -57,9 +62,11 @@ GPLv2
 use strict;
 use warnings;
 
-my $DEBUG=0;
+my $DEBUG = exists $ENV{'MUNIN_DEBUG'} ? $ENV{'MUNIN_DEBUG'} : 0;
 my $site = exists $ENV{'site'} ? $ENV{'site'} : "www.google.com";
 my $times = exists $ENV{'times'} ? $ENV{'times'} : "20";
+my $verify = exists $ENV{'verify'} ? lc($ENV{'verify'}) eq "yes" : 0;
+my $timeout = exists $ENV{'timeout'} ? $ENV{'timeout'} : 30;
 my $resconf="/etc/resolv.conf";
 
 use File::Basename;
@@ -75,6 +82,7 @@ if ( defined $ARGV[0] and $ARGV[0] eq "config" ) {
 	print "graph_scale no\n";
 	print "graph_category dns\n";
 	print "graph_info Time taken by $dnsip to resolve $site $times times.\n";
+	print "graph_info Time taken by $dnsip to resolve $site.\n";
 	#my @val = ("min", "avg", "median", "max");
 	my @val = ("avg", "median", "stddev");
 	my $value;
@@ -88,20 +96,17 @@ if ( defined $ARGV[0] and $ARGV[0] eq "config" ) {
 #		print "$value.type DERIVE\n";
 #		print "$value.min 0\n";
 #		print "$value.warning 100\n";
+		if ($value ne "stddev") {
+			print "$value.warning ".($timeout*1000)."\n";
+		}
 #		print "$value.critical 600\n";
 	}
-	exit 0;
-}
-
-if ( defined $ARGV[0] and $ARGV[0] eq "autoconf" ) {
-	my $ret;
-	if (! eval "require Net::DNS;") { $ret .= "Net::DNS not found. "; }
-	if (! eval "require Time::HiRes;") { $ret .= "Time::HiRes not found. "; }
-	if (! -s $resconf) { $ret .= "$resconf not found. "; }
-	if ($ret) {
-		print "no ($ret)\n";
-	} else {
-		print "yes\n";
+	if ( $verify ) {
+		print "verified.label Responses\n";
+		print "verified.info Number of responses received (sent $times queries)\n";
+		print "verified.min 0\n";
+		print "verified.warning $times:\n";
+		print "verified.critical 1:\n";
 	}
 	exit 0;
 }
@@ -132,15 +137,25 @@ my $res = Net::DNS::Resolver->new(
 	recurse     => 1,
 	debug       => $DEBUG,
 );
+# as we send multiple queries a retry in the DNS resolver would multiply the timeout value
+$res->retry(0);
+# set retrans time and udp timeout to the same time to effectively act as a timeout value
+$res->retrans($timeout);
+$res->udp_timeout($timeout);
 
 my $i;
 my @restimes;
+my @verified_answers;
 for ($i=1; $i<=$times; $i++) {
 	my $t0 = [gettimeofday];
 	my $answer = $res->send($site);
 	my $elapsed = tv_interval ($t0);
 	push(@restimes, $elapsed);
 	print "DBG: count $i elapsed $elapsed\n" if ($DEBUG>0);
+	if ($verify && $answer && $answer->string =~ /^;; (Answer|Response) received.*/) {
+		print "DBG: answer verified\n" if ($DEBUG>0);
+		push(@verified_answers, $answer)
+	}
 }
 
 
@@ -157,6 +172,9 @@ print "avg.value $average\n";
 print "median.value $median\n";
 print "stddev.value $stddev\n";
 #print "max.value $max\n";
+if ($verify) {
+	print "verified.value " . scalar(@verified_answers) . "\n";
+}
 
 sub mean {
 	my $result;