IPv6: add the option to use anycast addresses as source addresses in echo reply

This change allows to follow a recommandation of RFC4942.

- Add "anycast_src_echo_reply" sysctl to control the use of anycast addresses
  as source addresses for ICMPv6 echo reply. This sysctl is false by default
  to preserve existing behavior.
- Add inline check ipv6_anycast_destination().
- Use them in icmpv6_echo_reply().

Reference:
RFC4942 - IPv6 Transition/Coexistence Security Considerations
   (http://tools.ietf.org/html/rfc4942#section-2.1.6)

2.1.6. Anycast Traffic Identification and Security

   [...]
   To avoid exposing knowledge about the internal structure of the
   network, it is recommended that anycast servers now take advantage of
   the ability to return responses with the anycast address as the
   source address if possible.
Signed-off-by: Francois-Xavier Le Bail <fx.lebail@yahoo.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -1094,6 +1094,13 @@ bindv6only - BOOLEAN
 
 	Default: FALSE (as specified in RFC3493)
 
+anycast_src_echo_reply - BOOLEAN
+	Controls the use of anycast addresses as source addresses for ICMPv6
+	echo reply
+	TRUE:  enabled
+	FALSE: disabled
+	Default: FALSE
+
 IPv6 Fragmentation:
 
 ip6frag_high_thresh - INTEGER

include/net/ip6_route.h

@@ -152,6 +152,13 @@ static inline bool ipv6_unicast_destination(const struct sk_buff *skb)
 	return rt->rt6i_flags & RTF_LOCAL;
 }
 
+static inline bool ipv6_anycast_destination(const struct sk_buff *skb)
+{
+	struct rt6_info *rt = (struct rt6_info *) skb_dst(skb);
+
+	return rt->rt6i_flags & RTF_ANYCAST;
+}
+
 int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *));
 
 static inline int ip6_skb_dst_mtu(struct sk_buff *skb)

include/net/netns/ipv6.h

@@ -73,6 +73,7 @@ struct netns_ipv6 {
 #endif
 	atomic_t		dev_addr_genid;
 	atomic_t		rt_genid;
+	int			anycast_src_echo_reply;
 };
 
 #if IS_ENABLED(CONFIG_NF_DEFRAG_IPV6)

net/ipv6/icmp.c

@@ -556,7 +556,9 @@ static void icmpv6_echo_reply(struct sk_buff *skb)
 
 	saddr = &ipv6_hdr(skb)->daddr;
 
-	if (!ipv6_unicast_destination(skb))
+	if (!ipv6_unicast_destination(skb) &&
+	    !(net->ipv6.anycast_src_echo_reply &&
+	      ipv6_anycast_destination(skb)))
 		saddr = NULL;
 
 	memcpy(&tmp_hdr, icmph, sizeof(tmp_hdr));

net/ipv6/sysctl_net_ipv6.c

@@ -24,6 +24,13 @@ static struct ctl_table ipv6_table_template[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec
 	},
+	{
+		.procname	= "anycast_src_echo_reply",
+		.data		= &init_net.ipv6.anycast_src_echo_reply,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec
+	},
 	{ }
 };
 
@@ -51,6 +58,7 @@ static int __net_init ipv6_sysctl_net_init(struct net *net)
 	if (!ipv6_table)
 		goto out;
 	ipv6_table[0].data = &net->ipv6.sysctl.bindv6only;
+	ipv6_table[1].data = &net->ipv6.anycast_src_echo_reply;
 
 	ipv6_route_table = ipv6_route_sysctl_init(net);
 	if (!ipv6_route_table)