ctdb-common: Clarify IPv6 multicast addresses

At the moment these are a little too magic, especially the completely
undocumented one.

Ensure there are clear comments with references to save time when
touching this code in future.

Build the link-level multicast address in its own function to reduce
churn later.

Signed-off-by: Martin Schwenke <mschwenke@ddn.com>
Reviewed-by: Vinit Agnihotri <vagnihot@redhat.com>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>

diff --git a/ctdb/common/system_socket.c b/ctdb/common/system_socket.c
index 626787eada4e39e0a436b508014c8451be5025c8..46011a277275917eab3b4a4abc71b10ed1ef4f13 100644 (file)
--- a/ctdb/common/system_socket.c
+++ b/ctdb/common/system_socket.c
@@ -340,6 +340,25 @@ static int arp_build(uint8_t *buffer,
        return 0;
 }
 
+static int ip6_ll_multicast_build(uint8_t *buffer, size_t buflen)
+{
+       /*
+        * Ethernet multicast: 33:33:00:00:00:01 (see RFC2464, section
+        * 7) - 1st 2 octets are 0x3333, final 4 are the last 4 of the
+        * IPv6 address (see all_nodes_ll_multicast in ip6_na_build())
+        */
+       const uint8_t ethernet_multicast[ETH_ALEN] = {
+               0x33, 0x33, 0x00, 0x00, 0x00, 0x01
+       };
+
+       if (buflen < sizeof(ethernet_multicast)) {
+               return EMSGSIZE;
+       }
+
+       memcpy(buffer, ethernet_multicast, sizeof(ethernet_multicast));
+       return 0;
+}
+
 static int ip6_na_build(uint8_t *buffer,
                        size_t buflen,
                        const struct sockaddr_in6 *addr,
@@ -350,6 +369,11 @@ static int ip6_na_build(uint8_t *buffer,
        size_t l = IP6_NA_BUFFER_SIZE;
        struct ether_header *eh;
        struct ip6_hdr *ip6;
+       /*
+        * IPv6 all nodes link-level multicast address: (see RFC2373,
+        * section 2.7.1)
+        */
+       const char *all_nodes_ll_multicast = "ff02::1";
        struct nd_neighbor_advert *nd_na;
        struct nd_opt_hdr *nd_oh;
        struct ether_addr *ea;
@@ -366,13 +390,10 @@ static int ip6_na_build(uint8_t *buffer,
        memset(buffer, 0 , l);
 
        eh = (struct ether_header *)buffer;
-       /*
-        * Ethernet multicast: 33:33:00:00:00:01 (see RFC2464,
-        * section 7) - note memset 0 above!
-        */
-       eh->ether_dhost[0] = 0x33;
-       eh->ether_dhost[1] = 0x33;
-       eh->ether_dhost[5] = 0x01;
+       ret = ip6_ll_multicast_build(eh->ether_dhost, ETH_ALEN);
+       if (ret != 0) {
+               return ret;
+       }
        memcpy(eh->ether_shost, hwaddr, ETH_ALEN);
        eh->ether_type = htons(ETHERTYPE_IP6);
 
@@ -384,10 +405,10 @@ static int ip6_na_build(uint8_t *buffer,
        ip6->ip6_nxt  = IPPROTO_ICMPV6;
        ip6->ip6_hlim = 255;
        ip6->ip6_src  = addr->sin6_addr;
-       /* all-nodes multicast */
 
-       ret = inet_pton(AF_INET6, "ff02::1", &ip6->ip6_dst);
+       ret = inet_pton(AF_INET6, all_nodes_ll_multicast, &ip6->ip6_dst);
        if (ret != 1) {
+               /* Can't happen */
                return EIO;
        }