ndp: fix macOS IPv6 compatibility by using link-local source addresses

macOS ignores NDP packets that don't originate from link-local addresses,
causing IPv6 connectivity issues with odhcpd. This change ensures NDP
packets (Neighbor Advertisements and ICMP Echo Requests) are sent using
link-local source addresses for RFC 4861 compliance.

Changes:
- Add odhcpd_send_with_src() to allow explicit source address control
- Add odhcpd_get_interface_linklocal_addr() to find link-local addresses
- Update send_na() and ping6() to prefer link-local source addresses
- Maintain backward compatibility with fallback behavior

Fixes: openwrt/openwrt#7561

Author: stephengroat

src/ndp.c

@@ -286,12 +286,21 @@ static void ping6(struct in6_addr *addr,
 	struct icmp6_hdr echo = { .icmp6_type = ICMP6_ECHO_REQUEST };
 	struct iovec iov = { .iov_base = &echo, .iov_len = sizeof(echo) };
 	char ipbuf[INET6_ADDRSTRLEN];
+	struct in6_addr src_addr;
 
 	inet_ntop(AF_INET6, addr, ipbuf, sizeof(ipbuf));
 	syslog(LOG_DEBUG, "Pinging for %s on %s", ipbuf, iface->name);
 
 	netlink_setup_route(addr, 128, iface->ifindex, NULL, 128, true);
-	odhcpd_send(iface->ndp_ping_fd, &dest, &iov, 1, iface);
+
+	/* Use link-local address as source for consistency and macOS compatibility */
+	if (odhcpd_get_interface_linklocal_addr(iface, &src_addr) == 0) {
+		odhcpd_send_with_src(iface->ndp_ping_fd, &dest, &iov, 1, iface, &src_addr);
+	} else {
+		/* Fallback to default behavior if no link-local address is available */
+		odhcpd_send(iface->ndp_ping_fd, &dest, &iov, 1, iface);
+	}
+
 	netlink_setup_route(addr, 128, iface->ifindex, NULL, 128, false);
 }
 
@@ -306,6 +315,7 @@ static void send_na(struct in6_addr *to_addr,
 	struct nd_opt_hdr *opt = (struct nd_opt_hdr*) &pbuf[sizeof(struct nd_neighbor_advert)];
 	struct iovec iov = { .iov_base = &pbuf, .iov_len = sizeof(pbuf) };
 	char ipbuf[INET6_ADDRSTRLEN];
+	struct in6_addr src_addr;
 
 	memset(pbuf, 0, sizeof(pbuf));
 	adv->nd_na_hdr = (struct icmp6_hdr) {
@@ -319,7 +329,13 @@ static void send_na(struct in6_addr *to_addr,
 	inet_ntop(AF_INET6, to_addr, ipbuf, sizeof(ipbuf));
 	syslog(LOG_DEBUG, "Answering NS to %s on %s", ipbuf, iface->ifname);
 
-	odhcpd_send(iface->ndp_ping_fd, &dest, &iov, 1, iface);
+	/* Use link-local address as source for macOS compatibility (RFC 4861 compliance) */
+	if (odhcpd_get_interface_linklocal_addr(iface, &src_addr) == 0) {
+		odhcpd_send_with_src(iface->ndp_ping_fd, &dest, &iov, 1, iface, &src_addr);
+	} else {
+		/* Fallback to default behavior if no link-local address is available */
+		odhcpd_send(iface->ndp_ping_fd, &dest, &iov, 1, iface);
+	}
 }
 
 /* Handle solicitations */

src/odhcpd.c

@@ -189,6 +189,14 @@ int odhcpd_get_flags(const struct interface *iface)
 ssize_t odhcpd_send(int socket, struct sockaddr_in6 *dest,
 		struct iovec *iov, size_t iov_len,
 		const struct interface *iface)
+{
+	return odhcpd_send_with_src(socket, dest, iov, iov_len, iface, NULL);
+}
+
+/* Forwards a packet on a specific interface with optional source address */
+ssize_t odhcpd_send_with_src(int socket, struct sockaddr_in6 *dest,
+		struct iovec *iov, size_t iov_len,
+		const struct interface *iface, const struct in6_addr *src_addr)
 {
 	/* Construct headers */
 	uint8_t cmsg_buf[CMSG_SPACE(sizeof(struct in6_pktinfo))] = {0};
@@ -210,6 +218,10 @@ ssize_t odhcpd_send(int socket, struct sockaddr_in6 *dest,
 	struct in6_pktinfo *pktinfo = (struct in6_pktinfo*)CMSG_DATA(chdr);
 	pktinfo->ipi6_ifindex = iface->ifindex;
 
+	/* Set source address if provided */
+	if (src_addr)
+		pktinfo->ipi6_addr = *src_addr;
+
 	/* Also set scope ID if link-local */
 	if (IN6_IS_ADDR_LINKLOCAL(&dest->sin6_addr)
 			|| IN6_IS_ADDR_MC_LINKLOCAL(&dest->sin6_addr))
@@ -254,6 +266,21 @@ static int odhcpd_get_linklocal_interface_address(int ifindex, struct in6_addr *
 	return ret;
 }
 
+/* Get link-local address for NDP operations - prefer this over ULA for macOS compatibility */
+int odhcpd_get_interface_linklocal_addr(const struct interface *iface, struct in6_addr *addr)
+{
+	/* First try to get link-local address from interface addresses */
+	for (size_t i = 0; i < iface->addr6_len; ++i) {
+		if (IN6_IS_ADDR_LINKLOCAL(&iface->addr6[i].addr.in6)) {
+			*addr = iface->addr6[i].addr.in6;
+			return 0;
+		}
+	}
+
+	/* Fallback to the existing method */
+	return odhcpd_get_linklocal_interface_address(iface->ifindex, addr);
+}
+
 /*
  * DNS address selection criteria order :
  * - use IPv6 address with valid lifetime if none is yet selected

src/odhcpd.h

@@ -437,8 +437,13 @@ void odhcpd_process(struct odhcpd_event *event);
 ssize_t odhcpd_send(int socket, struct sockaddr_in6 *dest,
 		struct iovec *iov, size_t iov_len,
 		const struct interface *iface);
+ssize_t odhcpd_send_with_src(int socket, struct sockaddr_in6 *dest,
+		struct iovec *iov, size_t iov_len,
+		const struct interface *iface, const struct in6_addr *src_addr);
 int odhcpd_get_interface_dns_addr(const struct interface *iface,
 		struct in6_addr *addr);
+int odhcpd_get_interface_linklocal_addr(const struct interface *iface,
+		struct in6_addr *addr);
 int odhcpd_get_interface_config(const char *ifname, const char *what);
 int odhcpd_get_mac(const struct interface *iface, uint8_t mac[6]);
 int odhcpd_get_flags(const struct interface *iface);