Make fqdn more resilient and more polite

dns/GNUmakefile

@@ -22,7 +22,7 @@ cfg/%:
 
 gethostbyname: gethostbyname.o hstrerror.o
 
-fqdn: fqdn.o hstrerror.o
+fqdn: fqdn.pl
 
 hstrerror.o: cfg/have_hstrerror.h
 

dns/fqdn.pl

@@ -0,0 +1,27 @@
+#!/usr/bin/perl
+use v5.26;
+
+sub fqdn {
+    my ($h) = @_;
+
+    if ($h =~ /\./) {
+        return $h;
+    }
+    open(my $fh, '-|', '/usr/bin/host', $h);
+    while (<$fh>) {
+        if (/^(\S+\.\S+) has address/) {
+            return $1;
+        }
+    }
+    say STDERR "$0: cannot determine FQDN of $h";
+    return "$h.invalid";
+}
+
+if (!@ARGV) {
+    $ARGV[0] = `/bin/hostname -f`;
+    chomp($ARGV[0]);
+}
+
+for my $h (@ARGV) {
+    say fqdn($h);
+}