networkd: bump MTU to 1280 for interfaces which have IPv6 enabled (#3077)

IPv6 protocol requires a minimum MTU of 1280 bytes on the interface.
This fixes #3046.

Introduce helper link_ipv6_enabled() to figure out whether IPV6 is enabled.
Introduce network_has_static_ipv6_addresses() to find out if any static
ipv6 address configured.
If IPv6 is not configured on any interface that is SLAAC, DHCPv6 and static
IPv6 addresses not configured, then IPv6 will be automatically disabled for that
interface, that is we write "1" to /proc/sys/net/ipv6/conf//disable_ipv6.

man/systemd.network.xml

@@ -202,6 +202,8 @@
           <para>The maximum transmission unit in bytes to set for the
           device. The usual suffixes K, M, G, are supported and are
           understood to the base of 1024.</para>
+          <para>Note that if IPv6 is enabled on the interface, and the MTU is chosen
+          below 1280 (the minimum MTU for IPv6) it will automatically be increased to this value.</para>
         </listitem>
       </varlistentry>
       <varlistentry>
@@ -210,6 +212,15 @@
           <para>Identity Association Identifier for the interface, a 32-bit unsigned integer.</para>
         </listitem>
       </varlistentry>
+      <varlistentry>
+      <listitem>
+        <para>Note that an interface without any static IPv6 addresses configured, and neither
+        DHCPv6 nor IPv6LL enabled, shall be considered to have no IPv6 support. IPv6 will be
+        automatically disabled for that interface by writing "1" to
+        <filename>/proc/sys/net/ipv6/conf/<replaceable>ifname</replaceable>/disable_ipv6.</filename>
+        </para>
+      </listitem>
+      </varlistentry>
     </variablelist>
   </refsect1>
 

src/basic/missing.h

@@ -828,6 +828,10 @@ struct btrfs_ioctl_quota_ctl_args {
 #define IPV6_UNICAST_IF 76
 #endif
 
+#ifndef IPV6_MIN_MTU
+#define IPV6_MIN_MTU 1280
+#endif
+
 #ifndef IFF_MULTI_QUEUE
 #define IFF_MULTI_QUEUE 0x100
 #endif

src/network/networkd-link.c

@@ -99,6 +99,15 @@ static bool link_ipv6ll_enabled(Link *link) {
         return link->network->link_local & ADDRESS_FAMILY_IPV6;
 }
 
+static bool link_ipv6_enabled(Link *link) {
+        assert(link);
+
+        if (!socket_ipv6_is_supported())
+                return false;
+
+        return link_dhcp6_enabled(link) || link_ipv6ll_enabled(link) || network_has_static_ipv6_addresses(link->network);
+}
+
 static bool link_lldp_rx_enabled(Link *link) {
         assert(link);
 
@@ -218,6 +227,31 @@ static IPv6PrivacyExtensions link_ipv6_privacy_extensions(Link *link) {
         return link->network->ipv6_privacy_extensions;
 }
 
+static int link_enable_ipv6(Link *link) {
+        const char *p = NULL;
+        bool disabled;
+        int r;
+
+        if (link->flags & IFF_LOOPBACK)
+                return 0;
+
+        disabled = !link_ipv6_enabled(link);
+
+        p = strjoina("/proc/sys/net/ipv6/conf/", link->ifname, "/disable_ipv6");
+
+        r = write_string_file(p, one_zero(disabled), WRITE_STRING_FILE_VERIFY_ON_FAILURE);
+        if (r < 0)
+                log_link_warning_errno(link, r, "Cannot %s IPv6 for interface %s: %m", disabled ? "disable" : "enable", link->ifname);
+        else {
+                if (disabled)
+                        log_link_info(link, "IPv6 disabled for interface: %m");
+                else
+                        log_link_info(link, "IPv6 enabled for interface: %m");
+        }
+
+        return 0;
+}
+
 void link_update_operstate(Link *link) {
         LinkOperationalState operstate;
         assert(link);
@@ -1510,7 +1544,21 @@ static int link_up(Link *link) {
                         return log_link_error_errno(link, r, "Could not set MAC address: %m");
         }
 
+        /* If IPv6 not configured (no static IPv6 address and neither DHCPv6 nor IPv6LL is enabled)
+           for this interface then disable IPv6 else enable it. */
+        (void) link_enable_ipv6(link);
+
         if (link->network->mtu) {
+                /* IPv6 protocol requires a minimum MTU of IPV6_MTU_MIN(1280) bytes
+                   on the interface. Bump up MTU bytes to IPV6_MTU_MIN. */
+                if (link_ipv6_enabled(link) && link->network->mtu < IPV6_MIN_MTU) {
+
+                        log_link_warning(link, "Bumping MTU to " STRINGIFY(IPV6_MIN_MTU) ", as "
+                                         "IPv6 is requested and requires a minimum MTU of " STRINGIFY(IPV6_MIN_MTU) " bytes: %m");
+
+                        link->network->mtu = IPV6_MIN_MTU;
+                }
+
                 r = sd_netlink_message_append_u32(req, IFLA_MTU, link->network->mtu);
                 if (r < 0)
                         return log_link_error_errno(link, r, "Could not set MTU: %m");
@@ -1520,7 +1568,7 @@ static int link_up(Link *link) {
         if (r < 0)
                 return log_link_error_errno(link, r, "Could not open IFLA_AF_SPEC container: %m");
 
-        if (socket_ipv6_is_supported()) {
+        if (link_ipv6_enabled(link)) {
                 /* if the kernel lacks ipv6 support setting IFF_UP fails if any ipv6 options are passed */
                 r = sd_netlink_message_open_container(req, AF_INET6);
                 if (r < 0)

src/network/networkd-network.c

@@ -399,6 +399,19 @@ int network_apply(Manager *manager, Network *network, Link *link) {
         return 0;
 }
 
+bool network_has_static_ipv6_addresses(Network *network) {
+        Address *address;
+
+        assert(network);
+
+        LIST_FOREACH(addresses, address, network->static_addresses) {
+                if (address->family == AF_INET6)
+                        return true;
+        }
+
+        return false;
+}
+
 int config_parse_netdev(const char *unit,
                 const char *filename,
                 unsigned line,

src/network/networkd-network.h

@@ -186,6 +186,8 @@ int network_get_by_name(Manager *manager, const char *name, Network **ret);
 int network_get(Manager *manager, struct udev_device *device, const char *ifname, const struct ether_addr *mac, Network **ret);
 int network_apply(Manager *manager, Network *network, Link *link);
 
+bool network_has_static_ipv6_addresses(Network *network);
+
 int config_parse_netdev(const char *unit, const char *filename, unsigned line, const char *section, unsigned section_line, const char *lvalue, int ltype, const char *rvalue, void *data, void *userdata);
 int config_parse_domains(const char *unit, const char *filename, unsigned line, const char *section, unsigned section_line, const char *lvalue, int ltype, const char *rvalue, void *data, void *userdata);
 int config_parse_tunnel(const char *unit, const char *filename, unsigned line, const char *section, unsigned section_line, const char *lvalue, int ltype, const char *rvalue, void *data, void *userdata);