net: ipv6: Add a sysctl to make optimistic addresses useful candidates

Add a sysctl that causes an interface's optimistic addresses
to be considered equivalent to other non-deprecated addresses
for source address selection purposes.  Preferred addresses
will still take precedence over optimistic addresses, subject
to other ranking in the source address selection algorithm.

This is useful where different interfaces are connected to
different networks from different ISPs (e.g., a cell network
and a home wifi network).

The current behaviour complies with RFC 3484/6724, and it
makes sense if the host has only one interface, or has
multiple interfaces on the same network (same or cooperating
administrative domain(s), but not in the multiple distinct
networks case.

For example, if a mobile device has an IPv6 address on an LTE
network and then connects to IPv6-enabled wifi, while the wifi
IPv6 address is undergoing DAD, IPv6 connections will try use
the wifi default route with the LTE IPv6 address, and will get
stuck until they time out.

Also, because optimistic nodes can receive frames, issue
an RTM_NEWADDR as soon as DAD starts (with the IFA_F_OPTIMSTIC
flag appropriately set).  A second RTM_NEWADDR is sent if DAD
completes (the address flags have changed), otherwise an
RTM_DELADDR is sent.

Also: add an entry in ip-sysctl.txt for optimistic_dad.

Signed-off-by: Erik Kline <ek@google.com>
Acked-by: Lorenzo Colitti <lorenzo@google.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

4 files changed, 59 insertions, 2 deletions

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 9028b879a97b..368e3251c553 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -1460,6 +1460,19 @@ suppress_frag_ndisc - INTEGER
         1 - (default) discard fragmented neighbor discovery packets
         0 - allow fragmented neighbor discovery packets
 
+optimistic_dad - BOOLEAN
+        Whether to perform Optimistic Duplicate Address Detection (RFC 4429).
+                0: disabled (default)
+                1: enabled
+
+use_optimistic - BOOLEAN
+        If enabled, do not classify optimistic addresses as deprecated during
+        source address selection.  Preferred addresses will still be chosen
+        before optimistic addresses, subject to other ranking in the source
+        address selection algorithm.
+                0: disabled (default)
+                1: enabled
+
 icmp/*:
 ratelimit - INTEGER
         Limit the maximal rates for sending ICMPv6 packets.
diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
index ff560537dd61..7121a2e97ce2 100644
--- a/include/linux/ipv6.h
+++ b/include/linux/ipv6.h
@@ -42,6 +42,7 @@ struct ipv6_devconf {
         __s32           accept_ra_from_local;
 #ifdef CONFIG_IPV6_OPTIMISTIC_DAD
         __s32           optimistic_dad;
+        __s32           use_optimistic;
 #endif
 #ifdef CONFIG_IPV6_MROUTE
         __s32           mc_forwarding;
diff --git a/include/uapi/linux/ipv6.h b/include/uapi/linux/ipv6.h
index efa2666f4b8a..e863d088b9a5 100644
--- a/include/uapi/linux/ipv6.h
+++ b/include/uapi/linux/ipv6.h
@@ -164,6 +164,7 @@ enum {
         DEVCONF_MLDV2_UNSOLICITED_REPORT_INTERVAL,
         DEVCONF_SUPPRESS_FRAG_NDISC,
         DEVCONF_ACCEPT_RA_FROM_LOCAL,
+        DEVCONF_USE_OPTIMISTIC,
         DEVCONF_MAX
 };
 
diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 50b95b2db87c..8d12b7c7018f 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -1170,6 +1170,9 @@ enum {
         IPV6_SADDR_RULE_PRIVACY,
         IPV6_SADDR_RULE_ORCHID,
         IPV6_SADDR_RULE_PREFIX,
+#ifdef CONFIG_IPV6_OPTIMISTIC_DAD
+        IPV6_SADDR_RULE_NOT_OPTIMISTIC,
+#endif
         IPV6_SADDR_RULE_MAX
 };
 
@@ -1197,6 +1200,15 @@ static inline int ipv6_saddr_preferred(int type)
         return 0;
 }
 
+static inline bool ipv6_use_optimistic_addr(struct inet6_dev *idev)
+{
+#ifdef CONFIG_IPV6_OPTIMISTIC_DAD
+        return idev && idev->cnf.optimistic_dad && idev->cnf.use_optimistic;
+#else
+        return false;
+#endif
+}
+
 static int ipv6_get_saddr_eval(struct net *net,
                                struct ipv6_saddr_score *score,
                                struct ipv6_saddr_dst *dst,
@@ -1257,10 +1269,16 @@ static int ipv6_get_saddr_eval(struct net *net,
                 score->scopedist = ret;
                 break;
         case IPV6_SADDR_RULE_PREFERRED:
+            {
                 /* Rule 3: Avoid deprecated and optimistic addresses */
+                u8 avoid = IFA_F_DEPRECATED;
+
+                if (!ipv6_use_optimistic_addr(score->ifa->idev))
+                        avoid |= IFA_F_OPTIMISTIC;
                 ret = ipv6_saddr_preferred(score->addr_type) ||
-                      !(score->ifa->flags & (IFA_F_DEPRECATED|IFA_F_OPTIMISTIC));
+                      !(score->ifa->flags & avoid);
                 break;
+            }
 #ifdef CONFIG_IPV6_MIP6
         case IPV6_SADDR_RULE_HOA:
             {
@@ -1306,6 +1324,14 @@ static int ipv6_get_saddr_eval(struct net *net,
                         ret = score->ifa->prefix_len;
                 score->matchlen = ret;
                 break;
+#ifdef CONFIG_IPV6_OPTIMISTIC_DAD
+        case IPV6_SADDR_RULE_NOT_OPTIMISTIC:
+                /* Optimistic addresses still have lower precedence than other
+                 * preferred addresses.
+                 */
+                ret = !(score->ifa->flags & IFA_F_OPTIMISTIC);
+                break;
+#endif
         default:
                 ret = 0;
         }
@@ -3222,8 +3248,15 @@ static void addrconf_dad_begin(struct inet6_ifaddr *ifp)
          * Optimistic nodes can start receiving
          * Frames right away
          */
-        if (ifp->flags & IFA_F_OPTIMISTIC)
+        if (ifp->flags & IFA_F_OPTIMISTIC) {
                 ip6_ins_rt(ifp->rt);
+                if (ipv6_use_optimistic_addr(idev)) {
+                        /* Because optimistic nodes can use this address,
+                         * notify listeners. If DAD fails, RTM_DELADDR is sent.
+                         */
+                        ipv6_ifa_notify(RTM_NEWADDR, ifp);
+                }
+        }
 
         addrconf_dad_kick(ifp);
 out:
@@ -4330,6 +4363,7 @@ static inline void ipv6_store_devconf(struct ipv6_devconf *cnf,
         array[DEVCONF_ACCEPT_SOURCE_ROUTE] = cnf->accept_source_route;
 #ifdef CONFIG_IPV6_OPTIMISTIC_DAD
         array[DEVCONF_OPTIMISTIC_DAD] = cnf->optimistic_dad;
+        array[DEVCONF_USE_OPTIMISTIC] = cnf->use_optimistic;
 #endif
 #ifdef CONFIG_IPV6_MROUTE
         array[DEVCONF_MC_FORWARDING] = cnf->mc_forwarding;
@@ -5155,6 +5189,14 @@ static struct addrconf_sysctl_table
                         .proc_handler   = proc_dointvec,
 
                 },
+                {
+                        .procname       = "use_optimistic",
+                        .data           = &ipv6_devconf.use_optimistic,
+                        .maxlen         = sizeof(int),
+                        .mode           = 0644,
+                        .proc_handler   = proc_dointvec,
+
+                },
 #endif
 #ifdef CONFIG_IPV6_MROUTE
                 {