Index: INSTALL
===================================================================
--- INSTALL	(revision 233103)
+++ INSTALL	(working copy)
@@ -278,6 +278,13 @@
     to install this module.
 
 
+  - IO::Socket::INET6 (from CPAN)
+
+    This is required if the first nameserver listed in your IP
+    configuration or /etc/resolv.conf file is available only via an IPv6
+    address.
+
+
   - IO::Socket::SSL (from CPAN)
 
     If you wish to use SSL encryption to communicate between spamc and
Index: lib/Mail/SpamAssassin/Util/DependencyInfo.pm
===================================================================
--- lib/Mail/SpamAssassin/Util/DependencyInfo.pm	(revision 233103)
+++ lib/Mail/SpamAssassin/Util/DependencyInfo.pm	(working copy)
@@ -109,6 +109,13 @@
   to install this module.',
 },
 {
+  module => 'IO::Socket::INET6',
+  version => '0.00',
+  desc => 'This is required if the first nameserver listed in your IP
+  configuration or /etc/resolv.conf file is available only via
+  an IPv6 address.',
+},
+{
   module => 'IO::Socket::SSL',
   version => '0.00',
   desc => 'If you wish to use SSL encryption to communicate between spamc and
Index: lib/Mail/SpamAssassin/DnsResolver.pm
===================================================================
--- lib/Mail/SpamAssassin/DnsResolver.pm	(revision 233103)
+++ lib/Mail/SpamAssassin/DnsResolver.pm	(working copy)
@@ -43,6 +43,8 @@
 
 use IO::Socket::INET;
 
+use constant HAS_SOCKET_INET6 => eval { require IO::Socket::INET6 };
+
 our @ISA = qw();
 
 ###########################################################################
@@ -149,16 +151,28 @@
   my $port_offset = int(rand(64511));  # 65535 - 1024
   for (my $i = 0; (!$sock && ($i<64511)); $i++) {
     my $lport = 1024 + (($port_offset + $i) % 64511);
-    $sock = IO::Socket::INET->new (
-                                   Proto => 'udp',
-                                   LocalPort => $lport,
-                                   Type => SOCK_DGRAM,
-                                   );
+
+    my %args = (
+        PeerAddr => $self->{res}->{nameservers}[0],
+        PeerPort => $self->{res}->{port},
+        Proto => 'udp',
+        LocalPort => $lport,
+        Type => SOCK_DGRAM
+    );
+
+    if (HAS_SOCKET_INET6) {
+      $sock = IO::Socket::INET6->new(%args);
+    } else {
+      $sock = IO::Socket::INET->new(%args);
+
+      # did we fail due to the attempted use of an IPv6 nameserver?
+      if (!$sock) {
+        $self->_ipv6_ns_warning();
+      }
+    }
   }
 
   $self->{sock} = $sock;
-  $self->{dest} = sockaddr_in($self->{res}->{port},
-            inet_aton($self->{res}->{nameservers}[0]));
 
   $self->{sock_as_vec} = $self->fhs_to_vec($self->{sock});
 }
@@ -268,9 +282,8 @@
   my $pkt = $self->new_dns_packet($host, $type, $class);
 
   my $data = $pkt->data;
-  my $dest = $self->{dest};
   $self->connect_sock() if !$self->{sock};
-  if (!$self->{sock}->send ($pkt->data, 0, $self->{dest})) {
+  if (!$self->{sock}->send ($pkt->data, 0)) {
     warn "dns: sendto() failed: $@";
     return;
   }
@@ -426,6 +439,26 @@
   $self->connect_sock();
 }
 
+sub _ipv6_ns_warning {
+  my ($self) = @_;
+
+  # warn about the attempted use of an IPv6 nameserver without
+  # IO::Socket::INET6 installed (bug 4412)
+  my $firstns = $self->{res}->{nameservers}[0];
+
+  use Mail::SpamAssassin::Constants qw(:ip);
+  my $ip64 = IP_ADDRESS;
+  my $ip4 = IPV4_ADDRESS;
+
+  # was the nameserver in IPv6 format?
+  if ($firstns =~ /^${ip64}$/o && $firstns !~ /^${ip4}$/o) {
+    my $addr = inet_aton($firstns);
+    if (!defined $addr) {
+      die "IO::Socket::INET6 module is required to use IPv6 nameservers such as '$firstns': $@\n";
+    }
+  }
+}
+
 1;
 
 =back