diff --git a/authd/res.c b/authd/res.c
index f9db7afd..d68e402a 100644
--- a/authd/res.c
+++ b/authd/res.c
@@ -465,6 +465,52 @@ static void do_query_name(struct DNSQuery *query, const char *name, struct resli
 	query_name(request);
 }
 
+/* Build an rDNS style query - if suffix is NULL, use the appropriate .arpa zone */
+void build_rdns(char *buf, size_t size, const struct rb_sockaddr_storage *addr, const char *suffix)
+{
+	const unsigned char *cp;
+
+	if (GET_SS_FAMILY(addr) == AF_INET)
+	{
+		const struct sockaddr_in *v4 = (const struct sockaddr_in *)addr;
+		cp = (const unsigned char *)&v4->sin_addr.s_addr;
+
+		(void) snprintf(buf, size, "%u.%u.%u.%u.%s",
+			(unsigned int)(cp[3]),
+			(unsigned int)(cp[2]),
+			(unsigned int)(cp[1]),
+			(unsigned int)(cp[0]),
+			suffix == NULL ? "in-addr.arpa" : suffix);
+	}
+#ifdef RB_IPV6
+	else if (GET_SS_FAMILY(addr) == AF_INET6)
+	{
+		const struct sockaddr_in6 *v6 = (const struct sockaddr_in6 *)addr;
+		cp = (const unsigned char *)&v6->sin6_addr.s6_addr;
+
+		(void) snprintf(buf, size,
+			"%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%s",
+			(unsigned int)(cp[15] & 0xf), (unsigned int)(cp[15] >> 4),
+			(unsigned int)(cp[14] & 0xf), (unsigned int)(cp[14] >> 4),
+			(unsigned int)(cp[13] & 0xf), (unsigned int)(cp[13] >> 4),
+			(unsigned int)(cp[12] & 0xf), (unsigned int)(cp[12] >> 4),
+			(unsigned int)(cp[11] & 0xf), (unsigned int)(cp[11] >> 4),
+			(unsigned int)(cp[10] & 0xf), (unsigned int)(cp[10] >> 4),
+			(unsigned int)(cp[9] & 0xf), (unsigned int)(cp[9] >> 4),
+			(unsigned int)(cp[8] & 0xf), (unsigned int)(cp[8] >> 4),
+			(unsigned int)(cp[7] & 0xf), (unsigned int)(cp[7] >> 4),
+			(unsigned int)(cp[6] & 0xf), (unsigned int)(cp[6] >> 4),
+			(unsigned int)(cp[5] & 0xf), (unsigned int)(cp[5] >> 4),
+			(unsigned int)(cp[4] & 0xf), (unsigned int)(cp[4] >> 4),
+			(unsigned int)(cp[3] & 0xf), (unsigned int)(cp[3] >> 4),
+			(unsigned int)(cp[2] & 0xf), (unsigned int)(cp[2] >> 4),
+			(unsigned int)(cp[1] & 0xf), (unsigned int)(cp[1] >> 4),
+			(unsigned int)(cp[0] & 0xf), (unsigned int)(cp[0] >> 4),
+			suffix == NULL ? "ip6.arpa" : suffix);
+	}
+#endif
+}
+
 /*
  * do_query_number - Use this to do reverse IP# lookups.
  */
@@ -480,40 +526,7 @@ static void do_query_number(struct DNSQuery *query, const struct rb_sockaddr_sto
 		request->name = (char *)rb_malloc(IRCD_RES_HOSTLEN + 1);
 	}
 
-	if (GET_SS_FAMILY(addr) == AF_INET)
-	{
-		const struct sockaddr_in *v4 = (const struct sockaddr_in *)addr;
-		cp = (const unsigned char *)&v4->sin_addr.s_addr;
-
-		sprintf(request->queryname, "%u.%u.%u.%u.in-addr.arpa", (unsigned int)(cp[3]),
-			   (unsigned int)(cp[2]), (unsigned int)(cp[1]), (unsigned int)(cp[0]));
-	}
-#ifdef RB_IPV6
-	else if (GET_SS_FAMILY(addr) == AF_INET6)
-	{
-		const struct sockaddr_in6 *v6 = (const struct sockaddr_in6 *)addr;
-		cp = (const unsigned char *)&v6->sin6_addr.s6_addr;
-
-		(void)sprintf(request->queryname, "%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x."
-			      "%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.%x.ip6.arpa",
-			      (unsigned int)(cp[15] & 0xf), (unsigned int)(cp[15] >> 4),
-			      (unsigned int)(cp[14] & 0xf), (unsigned int)(cp[14] >> 4),
-			      (unsigned int)(cp[13] & 0xf), (unsigned int)(cp[13] >> 4),
-			      (unsigned int)(cp[12] & 0xf), (unsigned int)(cp[12] >> 4),
-			      (unsigned int)(cp[11] & 0xf), (unsigned int)(cp[11] >> 4),
-			      (unsigned int)(cp[10] & 0xf), (unsigned int)(cp[10] >> 4),
-			      (unsigned int)(cp[9] & 0xf), (unsigned int)(cp[9] >> 4),
-			      (unsigned int)(cp[8] & 0xf), (unsigned int)(cp[8] >> 4),
-			      (unsigned int)(cp[7] & 0xf), (unsigned int)(cp[7] >> 4),
-			      (unsigned int)(cp[6] & 0xf), (unsigned int)(cp[6] >> 4),
-			      (unsigned int)(cp[5] & 0xf), (unsigned int)(cp[5] >> 4),
-			      (unsigned int)(cp[4] & 0xf), (unsigned int)(cp[4] >> 4),
-			      (unsigned int)(cp[3] & 0xf), (unsigned int)(cp[3] >> 4),
-			      (unsigned int)(cp[2] & 0xf), (unsigned int)(cp[2] >> 4),
-			      (unsigned int)(cp[1] & 0xf), (unsigned int)(cp[1] >> 4),
-			      (unsigned int)(cp[0] & 0xf), (unsigned int)(cp[0] >> 4));
-	}
-#endif
+	build_rdns(request->queryname, IRCD_RES_HOSTLEN + 1, addr, NULL);
 
 	request->type = T_PTR;
 	query_name(request);
diff --git a/authd/res.h b/authd/res.h
index 5a177257..1fabe0b2 100644
--- a/authd/res.h
+++ b/authd/res.h
@@ -32,5 +32,6 @@ extern void init_resolver(void);
 extern void restart_resolver(void);
 extern void gethost_byname_type(const char *, struct DNSQuery *, int);
 extern void gethost_byaddr(const struct rb_sockaddr_storage *, struct DNSQuery *);
+extern void build_rdns(char *, size_t, const struct rb_sockaddr_storage *, const char *);
 
 #endif