check_dns: use system default DNS server if none was provided

check_dns.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 
 # Default values
-dns_server="1.1.1.1"
+dns_server=""
 query_domain=""
 warning_time=200
 critical_time=250
@@ -20,16 +20,23 @@ while getopts "s:d:w:c:" flag; do
   esac
 done
 
-# Check if DNS server is provided
-if [ -z "$dns_server" ]; then
-  echo "Error: DNS server not provided"
+if [[ -z $query_domain && ${query_domain+x} ]]; then
+  echo "Error: query domain not provided."
   echo "Usage: $0 -s <dns_server> -d <query_domain> -w <warning_time> -c <critical_time>"
   exit 1
 fi
 
 # Perform DNS resolution check and measure the time
 start_time=$(date +%s%N)
-result=$(dig @$dns_server $query_domain +short)
+
+if [[ -z "$dns_server" ]]; then
+  # The DNS server was not provided.
+  result=$(dig "$query_domain" +short)
+  dns_server="the default server"
+else
+  result=$(dig @$dns_server "$query_domain" +short)
+fi
+
 end_time=$(date +%s%N)
 response_time=$(((end_time - start_time) / 1000000))
 perfdata="response_time=${response_time}ms;${warning_time};${critical_time};0;"