networkd: IPv6 router discovery - follow IPv6AcceptRouterAdvertisemnt=

The previous behavior:
When DHCPv6 was enabled, router discover was performed first, and then DHCPv6 was
enabled only if the relevant flags were passed in the Router Advertisement message.
Moreover, router discovery was performed even if AcceptRouterAdvertisements=false,
moreover, even if router advertisements were accepted (by the kernel) the flags
indicating that DHCPv6 should be performed were ignored.

New behavior:
If RouterAdvertisements are accepted, and either no routers are found, or an
advertisement is received indicating DHCPv6 should be performed, the DHCPv6
client is started. Moreover, the DHCP option now truly enables the DHCPv6
client regardless of router discovery (though it will probably not be
very useful to get a lease withotu any routes, this seems the more consistent
approach).

The recommended default setting should be to set DHCP=ipv4 and to leave
IPv6AcceptRouterAdvertisements unset.

man/systemd.network.xml

@@ -227,7 +227,14 @@
             <literal>yes</literal>, <literal>no</literal>,
             <literal>ipv4</literal>, or <literal>ipv6</literal>.</para>
 
-            <para>Please note that, by default, the domain name
+            <para>Note that DHCPv6 will by default be triggered by Router
+            Advertisment, if that is enabled, regardless of this parameter.
+            By enabling DHCPv6 support explicitly, the DHCPv6 client will
+            be started regardless of the presence of routers on the link,
+            or what flags the routers pass. See
+            <literal>IPv6AcceptRouterAdvertisements=</literal>.</para>
+
+            <para>Furthermore, note that by default the domain name
             specified through DHCP is not used for name resolution.
             See option <option>UseDomains=</option> below.</para>
           </listitem>
@@ -414,6 +421,9 @@
           When unset, the kernel default is used, and router
           advertisements are accepted only when local forwarding
           is disabled for that interface.
+          When router advertisements are accepted, they will
+          trigger the start of the DHCPv6 client if the relevant
+          flags are passed, or if no routers are found on the link.
           Takes a boolean. If true, router advertisements are
           accepted, when false, router advertisements are ignored,
           independently of the local forwarding state.</para>

src/network/networkd-link.c

@@ -124,6 +124,28 @@ static bool link_ipv6_forward_enabled(Link *link) {
         return link->network->ip_forward & ADDRESS_FAMILY_IPV6;
 }
 
+bool link_ipv6_accept_ra_enabled(Link *link) {
+        if (link->flags & IFF_LOOPBACK)
+                return false;
+
+        if (!link->network)
+                return false;
+
+        /* If unset use system default (enabled if local forwarding is disabled.
+         * disabled if local forwarding is enabled).
+         * If set, ignore or enforce RA independent of local forwarding state.
+         */
+        if (link->network->ipv6_accept_ra < 0)
+                /* default to accept RA if ip_forward is disabled and ignore RA if ip_forward is enabled */
+                return !link_ipv6_forward_enabled(link);
+        else if (link->network->ipv6_accept_ra > 0)
+                /* accept RA even if ip_forward is enabled */
+                return true;
+        else
+                /* ignore RA */
+                return false;
+}
+
 static IPv6PrivacyExtensions link_ipv6_privacy_extensions(Link *link) {
         if (link->flags & IFF_LOOPBACK)
                 return _IPV6_PRIVACY_EXTENSIONS_INVALID;
@@ -485,13 +507,13 @@ static int link_stop_clients(Link *link) {
                         r = log_link_warning_errno(link, r, "Could not stop IPv4 link-local: %m");
         }
 
-        if(link->ndisc_router_discovery) {
-                if (link->dhcp6_client) {
-                        k = sd_dhcp6_client_stop(link->dhcp6_client);
-                        if (k < 0)
-                                r = log_link_warning_errno(link, r, "Could not stop DHCPv6 client: %m");
-                }
+        if (link->dhcp6_client) {
+                k = sd_dhcp6_client_stop(link->dhcp6_client);
+                if (k < 0)
+                        r = log_link_warning_errno(link, r, "Could not stop DHCPv6 client: %m");
+        }
 
+        if (link->ndisc_router_discovery) {
                 k = sd_ndisc_stop(link->ndisc_router_discovery);
                 if (k < 0)
                         r = log_link_warning_errno(link, r, "Could not stop IPv6 Router Discovery: %m");
@@ -1242,6 +1264,16 @@ static int link_acquire_conf(Link *link) {
         }
 
         if (link_dhcp6_enabled(link)) {
+                assert(link->dhcp6_client);
+
+                log_link_debug(link, "Acquiring DHCPv6 lease");
+
+                r = sd_dhcp6_client_start(link->dhcp6_client);
+                if (r < 0)
+                        return log_link_warning_errno(link, r,  "Could not acquire DHCPv6 lease: %m");
+        }
+
+        if (link_ipv6_accept_ra_enabled(link)) {
                 assert(link->ndisc_router_discovery);
 
                 log_link_debug(link, "Discovering IPv6 routers");
@@ -2041,6 +2073,12 @@ static int link_configure(Link *link) {
         }
 
         if (link_dhcp6_enabled(link)) {
+                r = dhcp6_configure(link);
+                if (r < 0)
+                        return r;
+        }
+
+        if (link_ipv6_accept_ra_enabled(link)) {
                 r = ndisc_configure(link);
                 if (r < 0)
                         return r;

src/network/networkd-link.h

@@ -157,6 +157,7 @@ bool link_ipv6ll_enabled(Link *link);
 bool link_dhcp4_server_enabled(Link *link);
 bool link_dhcp4_enabled(Link *link);
 bool link_dhcp6_enabled(Link *link);
+bool link_ipv6_accept_ra_enabled(Link *link);
 
 const char* link_state_to_string(LinkState s) _const_;
 LinkState link_state_from_string(const char *s) _pure_;

src/network/networkd-manager.c

@@ -1091,7 +1091,8 @@ static bool manager_check_idle(void *userdata) {
                     link_ipv4ll_enabled(link) ||
                     link_dhcp4_server_enabled(link) ||
                     link_dhcp4_enabled(link) ||
-                    link_dhcp6_enabled(link))
+                    link_dhcp6_enabled(link) ||
+                    link_ipv6_accept_ra_enabled(link))
                         return false;
         }
 

src/network/networkd-ndisc.c

@@ -42,7 +42,7 @@ static void ndisc_router_handler(sd_ndisc *nd, uint8_t flags, const struct in6_a
                         dhcp6_request_address(link);
 
                 r = sd_dhcp6_client_start(link->dhcp6_client);
-                if (r < 0 && r != -EBUSY)
+                if (r < 0 && r != -EALREADY)
                         log_link_warning_errno(link, r, "Starting DHCPv6 client on NDisc request failed: %m");
         }
 }
@@ -61,7 +61,7 @@ static void ndisc_handler(sd_ndisc *nd, int event, void *userdata) {
                 dhcp6_request_address(link);
 
                 r = sd_dhcp6_client_start(link->dhcp6_client);
-                if (r < 0 && r != -EBUSY)
+                if (r < 0 && r != -EALREADY)
                         log_link_warning_errno(link, r, "Starting DHCPv6 client after NDisc timeout failed: %m");
                 break;
         case SD_NDISC_EVENT_STOP: