Support router solicitation without SLLA option

Many clients don't include SLLA option in router solicitations:
https://github.com/radvd-project/radvd/issues/63#issuecomment-287172252
Answer these router solicitations by triggering a neighbour
solicitation when SLLA is not included, and then waiting for the
neighbour advertisment.

netlink.c

@@ -9,6 +9,7 @@
 #include <netlink/msg.h>
 #include <netlink/attr.h>
 #include <netlink/route/addr.h>
+#include <netlink/route/neighbour.h>
 #include <netlink/route/route.h>
 #include <netlink/errno.h>
 
@@ -71,6 +72,51 @@ static int family_handler(struct nl_msg *msg, void *arg)
     return NL_SKIP;
 }
 
+struct nl_sock *rtnl_sock = NULL;
+
+int nl_get_mac(int ifindex, unsigned char * dst, unsigned char * mac)
+{
+    struct nl_cache * cache = NULL;
+    struct rtnl_neigh * neigh = NULL;
+    struct nl_addr *dst_addr = NULL;
+    struct nl_addr *mac_addr = NULL;
+    int rc;
+
+    if(rtnl_sock == NULL) {
+        rtnl_sock = nl_socket_alloc();
+        if(rtnl_sock == NULL)
+            return -NLE_NOMEM;
+
+        rc = nl_connect(rtnl_sock, NETLINK_ROUTE);
+        if(rc < 0) {
+            nl_socket_free(rtnl_sock);
+            rtnl_sock = NULL;
+            return rc;
+        }
+    }
+
+    rc = rtnl_neigh_alloc_cache(rtnl_sock, &cache);
+    if(rc)
+        return rc;
+
+    dst_addr = nl_addr_build(AF_INET6, dst, 16);
+
+    neigh = rtnl_neigh_get(cache, ifindex, dst_addr);
+    if(neigh == NULL) {
+        nl_cache_free(cache);
+        return 1;
+    }
+    mac_addr = rtnl_neigh_get_lladdr(neigh);
+    if(mac_addr == NULL) {
+        nl_cache_free(cache);
+        return 1;
+    }
+    memcpy(mac, nl_addr_get_binary_addr(mac_addr), 6);
+    rtnl_neigh_put(neigh);
+    nl_cache_free(cache);
+    return 0;
+}
+
 int nl_get_multicast_id(struct nl_sock *sock,
                         const char *family, const char *group)
 {
@@ -281,7 +327,6 @@ netlink_disassociate(int ifindex, const unsigned char *mac,
     return -1;
 }
 
-struct nl_sock *rtnl_sock = NULL;
 
 int
 netlink_route(int ifindex, int add, int ipv6, const unsigned char *dst, int dlen)

netlink.h

@@ -8,3 +8,4 @@ int netlink_disassociate(int ifindex, const unsigned char *mac,
                          const unsigned char *mymac);
 int netlink_route(int ifindex, int add, int ipv6,
                   const unsigned char *dst, int dlen);
+int nl_get_mac(int ifindex, unsigned char * dst, unsigned char * mac);

ra.c

@@ -13,6 +13,7 @@
 #include "flood.h"
 #include "util.h"
 #include "ra.h"
+#include "netlink.h"
 
 unsigned char dnsv6[16][16];
 int numdnsv6 = 0;
@@ -163,6 +164,7 @@ receive_rs()
     int buflen = 1500, rc;
     unsigned char buf[buflen];
     unsigned char *mac;
+    unsigned char mac_cache[6];
     struct sockaddr_in6 from;
     struct interface *interface;
     struct iovec iov[1];
@@ -231,7 +233,14 @@ receive_rs()
     }
     if(mac == NULL) {
         debugf("No source address option in router solicitation.\n");
-        return -1;
+        rc = nl_get_mac(interface->ifindex, from.sin6_addr.s6_addr, mac_cache);
+        if(rc < 0)
+            return rc;
+        if (rc) {
+            send_ns(interface, from.sin6_addr.s6_addr);
+            return -1;
+        }
+        mac = mac_cache;
     }
 
     debugf("<- RS %s\n", interface->ifname);
@@ -300,6 +309,37 @@ send_gratuitous_na(struct interface *interface)
     return sendto(ra_socket, buf, i, 0, (struct sockaddr*)&to, sizeof(to));
 }
 
+int
+send_ns(struct interface *interface, unsigned char * ipv6)
+{
+    int buflen = 1024;
+    unsigned char buf[buflen];
+    struct sockaddr_in6 to;
+    int i = 0;
+
+    memset(&to, 0, sizeof(to));
+    to.sin6_family = AF_INET6;
+    memcpy(&to.sin6_addr, all_nodes, 16);
+    to.sin6_scope_id = interface->ifindex;
+
+    CHECK(24);
+    BYTE(135);
+    BYTE(0);
+    SHORT(0);
+    LONG(0);
+    BYTES(ipv6, 16);
+
+    if(memcmp(interface->mac, zeroes, 6) != 0) {
+        CHECK(8);
+        BYTE(1);
+        BYTE(1);
+        BYTES(interface->mac, 6);
+    }
+
+ sendit:
+    debugf("-> Neighbour Solicitation\n");
+    return sendto(ra_socket, buf, i, 0, (struct sockaddr*)&to, sizeof(to));
+}
 
 int
 ra_setup()

ra.h

@@ -35,5 +35,6 @@ int send_ra(const unsigned char *prefix, int tm,
             const struct sockaddr_in6 *to, struct interface *interface);
 int receive_rs(void);
 int send_gratuitous_na(struct interface *interface);
+int send_ns(struct interface *interface, unsigned char * ipv6);