networkd: Option to use LinkLocalAddressing only when DHCP fails

When LinkLocalAddressing=fallback or LinkLocalAddressing=ipv4-fallback
then IPv4LL will be started only when DHCP fails.

Closes #9648.

man/systemd.network.xml

@@ -326,9 +326,16 @@
           <term><varname>LinkLocalAddressing=</varname></term>
           <listitem>
             <para>Enables link-local address autoconfiguration. Accepts <literal>yes</literal>,
-            <literal>no</literal>, <literal>ipv4</literal>, or <literal>ipv6</literal>. If
+            <literal>no</literal>, <literal>ipv4</literal>, <literal>ipv6</literal>,
-            <varname>Bridge=</varname> is set, defaults to <literal>no</literal>, and if not,
+            <literal>fallback</literal>, or <literal>ipv4-fallback</literal>. If
-            defaults to <literal>ipv6</literal>.</para>
+            <literal>fallback</literal> or <literal>ipv4-fallback</literal> is specified, then an IPv4
+            link-local address is configured only when DHCPv4 fails. If <literal>fallback</literal>,
+            an IPv6 link-local address is always configured, and if <literal>ipv4-fallback</literal>,
+            the address is not configured. Note that, the fallback mechanism works only when DHCPv4
+            client is enabled, that is, it requires <literal>DHCP=yes</literal> or
+            <literal>DHCP=ipv4</literal>. If <varname>Bridge=</varname> is set, defaults to
+            <literal>no</literal>, and if not, defaults to <literal>ipv6</literal>.
+            </para>
           </listitem>
         </varlistentry>
         <varlistentry>

src/libsystemd-network/sd-dhcp-client.c

@@ -32,6 +32,8 @@
 #define MAX_CLIENT_ID_LEN (sizeof(uint32_t) + MAX_DUID_LEN)  /* Arbitrary limit */
 #define MAX_MAC_ADDR_LEN CONST_MAX(INFINIBAND_ALEN, ETH_ALEN)
 
+#define MAX_CLIENT_ATTEMPT 64
+
 #define RESTART_AFTER_NAK_MIN_USEC (1 * USEC_PER_SEC)
 #define RESTART_AFTER_NAK_MAX_USEC (30 * USEC_PER_MINUTE)
 
@@ -1050,8 +1052,10 @@ static int client_timeout_resend(
         case DHCP_STATE_REQUESTING:
         case DHCP_STATE_BOUND:
 
-                if (client->attempt < 64)
+                if (client->attempt < MAX_CLIENT_ATTEMPT)
                         client->attempt *= 2;
+                else
+                        goto error;
 
                 next_timeout = time_now + (client->attempt - 1) * USEC_PER_SEC;
 
@@ -1079,7 +1083,7 @@ static int client_timeout_resend(
                         client->state = DHCP_STATE_SELECTING;
                         client->attempt = 1;
                 } else {
-                        if (client->attempt >= 64)
+                        if (client->attempt >= MAX_CLIENT_ATTEMPT)
                                 goto error;
                 }
 
@@ -1087,7 +1091,7 @@ static int client_timeout_resend(
 
         case DHCP_STATE_SELECTING:
                 r = client_send_discover(client);
-                if (r < 0 && client->attempt >= 64)
+                if (r < 0 && client->attempt >= MAX_CLIENT_ATTEMPT)
                         goto error;
 
                 break;
@@ -1097,7 +1101,7 @@ static int client_timeout_resend(
         case DHCP_STATE_RENEWING:
         case DHCP_STATE_REBINDING:
                 r = client_send_request(client);
-                if (r < 0 && client->attempt >= 64)
+                if (r < 0 && client->attempt >= MAX_CLIENT_ATTEMPT)
                          goto error;
 
                 if (client->state == DHCP_STATE_INIT_REBOOT)

src/network/networkd-dhcp4.c

@@ -511,6 +511,7 @@ static int dhcp_lease_acquired(sd_dhcp_client *client, Link *link) {
 
         return 0;
 }
+
 static void dhcp4_handler(sd_dhcp_client *client, int event, void *userdata) {
         Link *link = userdata;
         int r = 0;
@@ -523,9 +524,24 @@ static void dhcp4_handler(sd_dhcp_client *client, int event, void *userdata) {
                 return;
 
         switch (event) {
-                case SD_DHCP_CLIENT_EVENT_EXPIRED:
                 case SD_DHCP_CLIENT_EVENT_STOP:
+
+                        if (link_ipv4ll_fallback_enabled(link)) {
+                                assert(link->ipv4ll);
+
+                                log_link_debug(link, "DHCP client is stopped. Acquiring IPv4 link-local address");
+
+                                r = sd_ipv4ll_start(link->ipv4ll);
+                                if (r < 0) {
+                                        log_link_warning(link, "Could not acquire IPv4 link-local address: %m");
+                                        return;
+                                }
+                        }
+
+                        _fallthrough_;
+                case SD_DHCP_CLIENT_EVENT_EXPIRED:
                 case SD_DHCP_CLIENT_EVENT_IP_CHANGE:
+
                         if (link->network->dhcp_critical) {
                                 log_link_error(link, "DHCPv4 connection considered system critical, ignoring request to reconfigure it.");
                                 return;

src/network/networkd-ipv4ll.c

@@ -197,7 +197,7 @@ int ipv4ll_configure(Link *link) {
 
         assert(link);
         assert(link->network);
-        assert(link->network->link_local & ADDRESS_FAMILY_IPV4);
+        assert(link->network->link_local & (ADDRESS_FAMILY_IPV4 | ADDRESS_FAMILY_FALLBACK_IPV4));
 
         if (!link->ipv4ll) {
                 r = sd_ipv4ll_new(&link->ipv4ll);

src/network/networkd-link.c

@@ -110,7 +110,7 @@ static bool link_dhcp4_server_enabled(Link *link) {
         return link->network->dhcp_server;
 }
 
-static bool link_ipv4ll_enabled(Link *link) {
+bool link_ipv4ll_enabled(Link *link) {
         assert(link);
 
         if (link->flags & IFF_LOOPBACK)
@@ -128,6 +128,24 @@ static bool link_ipv4ll_enabled(Link *link) {
         return link->network->link_local & ADDRESS_FAMILY_IPV4;
 }
 
+bool link_ipv4ll_fallback_enabled(Link *link) {
+        assert(link);
+
+        if (link->flags & IFF_LOOPBACK)
+                return false;
+
+        if (!link->network)
+                return false;
+
+        if (STRPTR_IN_SET(link->kind, "vrf", "wireguard"))
+                return false;
+
+        if (link->network->bond)
+                return false;
+
+        return link->network->link_local & ADDRESS_FAMILY_FALLBACK_IPV4;
+}
+
 static bool link_ipv6ll_enabled(Link *link) {
         assert(link);
 
@@ -3114,7 +3132,7 @@ static int link_configure(Link *link) {
         if (r < 0)
                 return r;
 
-        if (link_ipv4ll_enabled(link)) {
+        if (link_ipv4ll_enabled(link) || link_ipv4ll_fallback_enabled(link)) {
                 r = ipv4ll_configure(link);
                 if (r < 0)
                         return r;

src/network/networkd-link.h

@@ -160,6 +160,9 @@ int link_ipv6ll_gained(Link *link, const struct in6_addr *address);
 int link_set_mtu(Link *link, uint32_t mtu, bool force);
 
 int ipv4ll_configure(Link *link);
+bool link_ipv4ll_enabled(Link *link);
+bool link_ipv4ll_fallback_enabled(Link *link);
+
 int dhcp4_configure(Link *link);
 int dhcp4_set_client_identifier(Link *link);
 int dhcp4_set_promote_secondaries(Link *link);

src/network/networkd-util.c

@@ -16,6 +16,10 @@ const char *address_family_boolean_to_string(AddressFamilyBoolean b) {
                 return "ipv4";
         if (b == ADDRESS_FAMILY_IPV6)
                 return "ipv6";
+        if (b == ADDRESS_FAMILY_FALLBACK)
+                return "fallback";
+        if (b == ADDRESS_FAMILY_FALLBACK)
+                return "ipv4-fallback";
 
         return NULL;
 }
@@ -35,6 +39,10 @@ AddressFamilyBoolean address_family_boolean_from_string(const char *s) {
                 return ADDRESS_FAMILY_IPV4;
         if (streq(s, "ipv6"))
                 return ADDRESS_FAMILY_IPV6;
+        if (streq(s, "fallback"))
+                return ADDRESS_FAMILY_FALLBACK;
+        if (streq(s, "ipv4-fallback"))
+                return ADDRESS_FAMILY_FALLBACK_IPV4;
 
         return _ADDRESS_FAMILY_BOOLEAN_INVALID;
 }

src/network/networkd-util.h

@@ -11,6 +11,8 @@ typedef enum AddressFamilyBoolean {
         ADDRESS_FAMILY_IPV4           = 1 << 0,
         ADDRESS_FAMILY_IPV6           = 1 << 1,
         ADDRESS_FAMILY_YES            = ADDRESS_FAMILY_IPV4 | ADDRESS_FAMILY_IPV6,
+        ADDRESS_FAMILY_FALLBACK_IPV4  = 1 << 2,
+        ADDRESS_FAMILY_FALLBACK       = ADDRESS_FAMILY_FALLBACK_IPV4 | ADDRESS_FAMILY_IPV6,
         _ADDRESS_FAMILY_BOOLEAN_MAX,
         _ADDRESS_FAMILY_BOOLEAN_INVALID = -1,
 } AddressFamilyBoolean;

src/network/test-network-tables.c

@@ -14,7 +14,6 @@
 #include "test-tables.h"
 
 int main(int argc, char **argv) {
-        test_table(address_family_boolean, ADDRESS_FAMILY_BOOLEAN);
         test_table(bond_ad_select, NETDEV_BOND_AD_SELECT);
         test_table(bond_arp_all_targets, NETDEV_BOND_ARP_ALL_TARGETS);
         test_table(bond_arp_validate, NETDEV_BOND_ARP_VALIDATE);
@@ -42,6 +41,7 @@ int main(int argc, char **argv) {
 
         test_table_sparse(ipvlan_mode, NETDEV_IPVLAN_MODE);
         test_table_sparse(macvlan_mode, NETDEV_MACVLAN_MODE);
+        test_table_sparse(address_family_boolean, ADDRESS_FAMILY_BOOLEAN);
 
         return EXIT_SUCCESS;
 }