diff --git a/DDNS.pm b/DDNS.pm
index 448cc4f..d6f8f63 100644
--- a/DDNS.pm
+++ b/DDNS.pm
@@ -4,6 +4,9 @@ use strict;
 use warnings;
 use Net::DNS;
 use File::Temp qw/tempfile/;
+use Memoize;
+
+memoize('_gethosts');
 
 our $VERSION = '0.2';
 
@@ -21,7 +24,7 @@ sub _validateTypeOrDie {
     my ($t) = @_;
 
     die "Invalid type"
-	unless ($t =~ /^_(vhosts|hostedservers|pureslave|custom)$/);
+	unless ($t =~ /^_(vhosts|pureslave|custom)$/);
 }
 
 sub _fqdn {
@@ -121,11 +124,34 @@ sub _gethosts {
     return @vh;
 }
 
+# Find the type of domin that $dom is. If we don't find it, return
+# undef.  (The domain $dom ends in a dot; the DNS info we find won't;
+# hence the concat of the extra "." after the lc.)
+sub type {
+    my ($this, $dom) = @_;
+
+    _validateOrDie($dom);
+
+    my @vh = $this->get();
+    foreach my $i (@vh) {
+	if (lc($i->{zone})."." eq lc($dom)) {
+	    return $i->{type};
+	}
+    }
+
+    # Didn't find it.
+    return undef;
+}
+
 sub add {
     my ($this, $dom, $master, $type) = @_;
     _validateOrDie($dom);
     my $fqdn = _fqdn($dom, $type);
 
+    if (my $type = $this->type($dom)) {
+	die "Domain $dom already exists [of type $type]";
+    }
+    
     $this->__docmd("update add $fqdn 60 TXT $master");
     $this->cleanup();
 }
diff --git a/Makefile.PL b/Makefile.PL
index 9819a95..a5a8a01 100644
--- a/Makefile.PL
+++ b/Makefile.PL
@@ -20,6 +20,7 @@ WriteMakefile(
 			'bin/sync-slave',
 			'bin/list-all',
 			'bin/is-managed',
+			'bin/validate-master',
     ],
     'AUTHOR'       => 'Jorj Bauer <jorj@jorj.org>',
     );
diff --git a/bin/validate-master b/bin/validate-master
new file mode 100755
index 0000000..6a23e7e
--- /dev/null
+++ b/bin/validate-master
@@ -0,0 +1,60 @@
+#!/usr/bin/perl
+
+use strict;
+use warnings;
+use Martnet::DDNS;
+
+my $ddns = Martnet::DDNS->new();
+
+my $path = '/etc/bind';
+
+my %files = ( 'custom.zones.9' => '_custom',
+	      'martnet.zones.9' => '_custom',
+	      'hostedservers.zones.9' => '_custom',
+	      'vhost.zones.9' => '_vhosts',
+	      'martnet.slave.zones.9' => '*' );
+
+our %fixes = ( '_custom' => 'add-custom',
+	       '_vhosts' => 'add-vhost',
+	       '_pureslave' => 'add-slave',
+	       '*' => 'one of the add-* scripts',
+    );
+
+foreach my $i (keys %files) {
+    validate_file($path . '/' . $i, $files{$i});
+}
+
+exit 0;
+
+sub validate_file {
+    my ($path, $expected_type) = @_;
+
+    open(my $fh, $path) || die "Can't open $path: $!";
+
+    while (<$fh>) {
+	my ($zonename) = ($_ =~ /zone \"([^\"]+)\"/i);
+	next unless ($zonename);
+	$zonename = lc($zonename);
+	unless ($zonename =~ /\.$/) {
+	    $zonename .= '.'; # must end with a dot
+	}
+
+	findOrDie($path, $zonename, $expected_type);
+    }
+    close $fh;
+}
+
+sub findOrDie {
+    my ($path, $zonename, $expected_type) = @_;
+    
+    my $type = $ddns->type($zonename);
+    unless ($type) {
+	my $fix = $fixes{$expected_type};
+	die "domain $zonename is not managed, but is in $path [expected $expected_type]; to fix, either remove the domain from the file, or use $fix to fix it";
+    }
+    if ($expected_type ne '*') {
+	unless ($type eq $expected_type) {
+	    die "domain $zonename is managed in $path of type [$type] but should be [$expected_type] if it's in that file";
+	}
+    }
+}