diff --git a/include/ipv4_from_ipv6.h b/include/ipv4_from_ipv6.h
new file mode 100644
index 00000000..f6a881f9
--- /dev/null
+++ b/include/ipv4_from_ipv6.h
@@ -0,0 +1,44 @@
+/*
+ * charybdis: An advanced ircd.
+ * ipv4_from_ipv6.h: Finds IPv4 addresses behind IPv6 transition technologies
+ *
+ * Copyright (C) 2012 Jilles Tjoelker
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * 3. The name of the author may not be used to endorse or promote products
+ *    derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT,
+ * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
+ * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ * $Id$
+ */
+
+#ifndef _IPV4_FROM_IPV6_H_
+#define _IPV4_FROM_IPV6_H_
+
+#ifdef RB_IPV6
+int ipv4_from_ipv6(const struct sockaddr_in6 *restrict ip6,
+		struct sockaddr_in *restrict ip4);
+#endif
+
+#endif
diff --git a/src/Makefile.in b/src/Makefile.in
index ab69afbb..f435475b 100644
--- a/src/Makefile.in
+++ b/src/Makefile.in
@@ -66,6 +66,7 @@ SRCS =                          \
   hash.c                        \
   hook.c                        \
   hostmask.c			\
+  ipv4_from_ipv6.c              \
   irc_dictionary.c		\
   ircd.c                        \
   ircd_signal.c                 \
diff --git a/src/ipv4_from_ipv6.c b/src/ipv4_from_ipv6.c
new file mode 100644
index 00000000..421977f1
--- /dev/null
+++ b/src/ipv4_from_ipv6.c
@@ -0,0 +1,64 @@
+/*
+ * charybdis: A slightly useful ircd.
+ * ipv4_from_ipv6.c: Finds IPv4 addresses behind IPv6 transition technologies
+ *
+ * Copyright (C) 2012 Jilles Tjoelker
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * 3. The name of the author may not be used to endorse or promote products
+ *    derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT,
+ * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
+ * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#include "stdinc.h"
+#include "ipv4_from_ipv6.h"
+
+#ifdef RB_IPV6
+int
+ipv4_from_ipv6(const struct sockaddr_in6 *restrict ip6,
+		struct sockaddr_in *restrict ip4)
+{
+	int i;
+
+	if (!memcmp(ip6->sin6_addr.s6_addr, "\x20\x02", 2))
+	{
+		/* 6to4 and similar */
+		memcpy(&ip4->sin_addr, ip6->sin6_addr.s6_addr + 2, 4);
+	}
+	else if (!memcmp(ip6->sin6_addr.s6_addr, "\x20\x01\x00\x00", 4))
+	{
+		/* Teredo */
+		for (i = 0; i < 4; i++)
+			((uint8_t *)&ip4->sin_addr)[i] = 0xFF ^
+				ip6->sin6_addr.s6_addr[12 + i];
+	}
+	else
+		return 0;
+	SET_SS_LEN(ip4, sizeof(struct sockaddr_in));
+	ip4->sin_family = AF_INET;
+	ip4->sin_port = 0;
+	return 1;
+}
+#endif /* RB_IPV6 */