4 files changed, 52 insertions, 6 deletions

diff --git a/src/dhcpv6.c b/src/dhcpv6.c
index 4cd9e02..ebb0c80 100644
--- a/src/dhcpv6.c
+++ b/src/dhcpv6.c
@@ -232,7 +232,7 @@ static void handle_client_request(void *addr, void *data, size_t len,
 	size_t dns_cnt = iface->dns_cnt;
 
 	if ((dns_cnt == 0) &&
-		!odhcpd_get_linklocal_interface_address(iface->ifindex, &dns_addr)) {
+		!odhcpd_get_interface_dns_addr(iface, &dns_addr)) {
 		dns_addr_ptr = &dns_addr;
 		dns_cnt = 1;
 	}
@@ -464,7 +464,7 @@ static void relay_server_response(uint8_t *data, size_t len)
 		size_t rewrite_cnt = iface->dns_cnt;
 
 		if (rewrite_cnt == 0) {
-			if (odhcpd_get_linklocal_interface_address(iface->ifindex, &addr))
+			if (odhcpd_get_interface_dns_addr(iface, &addr))
 				return; // Unable to get interface address
 
 			rewrite = &addr;
diff --git a/src/odhcpd.c b/src/odhcpd.c
index 8a18fbf..8a1c66e 100644
--- a/src/odhcpd.c
+++ b/src/odhcpd.c
@@ -336,7 +336,7 @@ out:
 	return ctxt.ret;
 }
 
-int odhcpd_get_linklocal_interface_address(int ifindex, struct in6_addr *lladdr)
+static int odhcpd_get_linklocal_interface_address(int ifindex, struct in6_addr *lladdr)
 {
 	int status = -1;
 	struct sockaddr_in6 addr = {AF_INET6, 0, 0, ALL_IPV6_ROUTERS, ifindex};
@@ -354,6 +354,51 @@ int odhcpd_get_linklocal_interface_address(int ifindex, struct in6_addr *lladdr)
 	return status;
 }
 
+/*
+ * DNS address selection criteria order :
+ * - use IPv6 address with valid lifetime if none is yet selected
+ * - use IPv6 address with a preferred lifetime if the already selected IPv6 address is deprecated
+ * - use an IPv6 ULA address if the already selected IPv6 address is not an ULA address
+ * - use the IPv6 address with the longest preferred lifetime
+ */
+int odhcpd_get_interface_dns_addr(const struct interface *iface, struct in6_addr *addr)
+{
+	time_t now = odhcpd_time();
+	ssize_t m = -1;
+
+	for (size_t i = 0; i < iface->ia_addr_len; ++i) {
+		if (iface->ia_addr[i].valid <= (uint32_t)now)
+			continue;
+
+		if (m < 0) {
+			m = i;
+			continue;
+		}
+
+		if (iface->ia_addr[m].preferred >= (uint32_t)now &&
+				iface->ia_addr[i].preferred < (uint32_t)now)
+			continue;
+
+		if (IN6_IS_ADDR_ULA(&iface->ia_addr[i].addr)) {
+			if (!IN6_IS_ADDR_ULA(&iface->ia_addr[m].addr)) {
+				m = i;
+				continue;
+			}
+		} else if (IN6_IS_ADDR_ULA(&iface->ia_addr[m].addr))
+			continue;
+
+		if (iface->ia_addr[i].preferred > iface->ia_addr[m].preferred)
+			m = i;
+	}
+
+	if (m >= 0) {
+		*addr = iface->ia_addr[m].addr;
+		return 0;
+	}
+
+	return odhcpd_get_linklocal_interface_address(iface->ifindex, addr);
+}
+
 int odhcpd_setup_route(const struct in6_addr *addr, const int prefixlen,
 		const struct interface *iface, const struct in6_addr *gw,
 		const uint32_t metric, const bool add)
diff --git a/src/odhcpd.h b/src/odhcpd.h
index 9733090..4ddadbe 100644
--- a/src/odhcpd.h
+++ b/src/odhcpd.h
@@ -203,7 +203,8 @@ ssize_t odhcpd_send(int socket, struct sockaddr_in6 *dest,
 		const struct interface *iface);
 ssize_t odhcpd_get_interface_addresses(int ifindex,
 		struct odhcpd_ipaddr *addrs, size_t cnt);
-int odhcpd_get_linklocal_interface_address(int ifindex, struct in6_addr *lladdr);
+int odhcpd_get_interface_dns_addr(const struct interface *iface,
+		struct in6_addr *addr);
 struct interface* odhcpd_get_interface_by_name(const char *name);
 int odhcpd_get_interface_config(const char *ifname, const char *what);
 int odhcpd_get_mac(const struct interface *iface, uint8_t mac[6]);
diff --git a/src/router.c b/src/router.c
index 6e5111a..feae5c7 100644
--- a/src/router.c
+++ b/src/router.c
@@ -319,7 +319,7 @@ static uint64_t send_router_advert(struct interface *iface, const struct in6_add
 	struct in6_addr dns_pref, *dns_addr = &dns_pref;
 	size_t dns_cnt = 1;
 
-	odhcpd_get_linklocal_interface_address(iface->ifindex, &dns_pref);
+	odhcpd_get_interface_dns_addr(iface, &dns_pref);
 
 	for (ssize_t i = 0; i < ipcnt; ++i) {
 		struct odhcpd_ipaddr *addr = &addrs[i];
@@ -617,7 +617,7 @@ static void forward_router_advertisement(uint8_t *data, size_t len)
 			size_t rewrite_cnt = iface->dns_cnt;
 
 			if (rewrite_cnt == 0) {
-				if (odhcpd_get_linklocal_interface_address(iface->ifindex, &addr))
+				if (odhcpd_get_interface_dns_addr(iface, &addr))
 					continue; // Unable to comply
 
 				rewrite = &addr;