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>

5 files changed, 26 insertions, 1 deletions

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index d71afa8bd828..7373115407e4 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/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
diff --git a/include/net/ip6_route.h b/include/net/ip6_route.h
index 1fb6cddbd448..017badb1aec7 100644
--- a/include/net/ip6_route.h
+++ b/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)
diff --git a/include/net/netns/ipv6.h b/include/net/netns/ipv6.h
index 0fb2401197c5..76fc7d1dbfd3 100644
--- a/include/net/netns/ipv6.h
+++ b/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)
diff --git a/net/ipv6/icmp.c b/net/ipv6/icmp.c
index 5d420095190f..9a809a4b3d86 100644
--- a/net/ipv6/icmp.c
+++ b/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));
diff --git a/net/ipv6/sysctl_net_ipv6.c b/net/ipv6/sysctl_net_ipv6.c
index 107b2f1d90ae..6b6a2c83027e 100644
--- a/net/ipv6/sysctl_net_ipv6.c
+++ b/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)