network: wireguard: allow to run NDisc and RADV when IPv6LL address is manually configured

Fixes #17380.

src/network/networkd-address.c

@@ -752,7 +752,7 @@ int link_drop_ipv6ll_addresses(Link *link) {
         /* IPv6LL address may be in the tentative state, and in that case networkd has not received it.
          * So, we need to dump all IPv6 addresses. */
 
-        if (link_ipv6ll_enabled(link))
+        if (link_may_have_ipv6ll(link))
                 return 0;
 
         r = sd_rtnl_message_new_addr(link->manager->rtnl, &req, RTM_GETADDR, link->ifindex, AF_INET6);

src/network/networkd-link.c

@@ -126,6 +126,43 @@ bool link_ipv6ll_enabled(Link *link) {
         return link->network->link_local & ADDRESS_FAMILY_IPV6;
 }
 
+bool link_may_have_ipv6ll(Link *link) {
+        assert(link);
+
+        /*
+         * This is equivalent to link_ipv6ll_enabled() for non-WireGuard interfaces.
+         *
+         * For WireGuard interface, the kernel does not assign any IPv6LL addresses, but we can assign
+         * it manually. It is necessary to set an IPv6LL address manually to run NDisc or RADV on
+         * WireGuard interface. Note, also Multicast=yes must be set. See #17380.
+         *
+         * TODO: May be better to introduce GenerateIPv6LinkLocalAddress= setting, and use algorithms
+         *       used in networkd-address-generation.c
+         */
+
+        if (link_ipv6ll_enabled(link))
+                return true;
+
+        /* IPv6LL address can be manually assigned on WireGuard interface. */
+        if (streq_ptr(link->kind, "wireguard")) {
+                Address *a;
+
+                if (!link->network)
+                        return false;
+
+                ORDERED_HASHMAP_FOREACH(a, link->network->addresses_by_section) {
+                        if (a->family != AF_INET6)
+                                continue;
+                        if (in6_addr_is_set(&a->in_addr_peer.in6))
+                                continue;
+                        if (in6_addr_is_link_local(&a->in_addr.in6))
+                                return true;
+                }
+        }
+
+        return false;
+}
+
 bool link_ipv6_enabled(Link *link) {
         assert(link);
 

src/network/networkd-link.h

@@ -215,6 +215,7 @@ bool link_has_carrier(Link *link);
 
 bool link_ipv6_enabled(Link *link);
 bool link_ipv6ll_enabled(Link *link);
+bool link_may_have_ipv6ll(Link *link);
 int link_ipv6ll_gained(Link *link);
 
 bool link_ipv4ll_enabled(Link *link);

src/network/networkd-ndisc.c

@@ -37,7 +37,7 @@ bool link_ipv6_accept_ra_enabled(Link *link) {
         if (!link->network)
                 return false;
 
-        if (!link_ipv6ll_enabled(link))
+        if (!link_may_have_ipv6ll(link))
                 return false;
 
         assert(link->network->ipv6_accept_ra >= 0);

src/network/networkd-radv.c

@@ -54,7 +54,7 @@ void network_adjust_radv(Network *network) {
 bool link_radv_enabled(Link *link) {
         assert(link);
 
-        if (!link_ipv6ll_enabled(link))
+        if (!link_may_have_ipv6ll(link))
                 return false;
 
         return link->network->router_prefix_delegation;