From 3a0ee630ed9232063375e74ec791a51304a3a506 Mon Sep 17 00:00:00 2001
From: "Peter J. Holzer" <hjp@wsr.ac.at>
Date: Thu, 11 Aug 2022 15:46:31 +0200
Subject: [PATCH] Make fqdn more resilient and more polite

---
 dns/GNUmakefile |  2 +-
 dns/fqdn.pl     | 27 +++++++++++++++++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)
 create mode 100755 dns/fqdn.pl

diff --git a/dns/GNUmakefile b/dns/GNUmakefile
index 1ad5eb3..83650a7 100644
--- a/dns/GNUmakefile
+++ b/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
 
diff --git a/dns/fqdn.pl b/dns/fqdn.pl
new file mode 100755
index 0000000..c4e4235
--- /dev/null
+++ b/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);
+}