networkd: add support to configure IPv6 MTU (#8664)

This patch supports to configure IPv6 MTU.

Closes #8632

man/systemd.network.xml

@@ -666,6 +666,12 @@
           sections for more configuration options.
           </para></listitem>
         </varlistentry>
+        <varlistentry>
+          <term><varname>IPv6MTUBytes=</varname></term>
+          <listitem><para>Configures IPv6 maximum transmission unit (MTU).
+          An integer greater than or equal to 1280 bytes. Defaults to unset.
+          </para></listitem>
+        </varlistentry>
         <varlistentry>
           <term><varname>Bridge=</varname></term>
           <listitem>

src/network/networkd-link.c

@@ -2469,6 +2469,32 @@ static int link_set_ipv6_hop_limit(Link *link) {
         return 0;
 }
 
+static int link_set_ipv6_mtu(Link *link) {
+        char buf[DECIMAL_STR_MAX(unsigned) + 1];
+        const char *p = NULL;
+        int r;
+
+        /* Make this a NOP if IPv6 is not available */
+        if (!socket_ipv6_is_supported())
+                return 0;
+
+        if (link->flags & IFF_LOOPBACK)
+                return 0;
+
+        if (link->network->ipv6_mtu == 0)
+                return 0;
+
+        p = strjoina("/proc/sys/net/ipv6/conf/", link->ifname, "/mtu");
+
+        xsprintf(buf, "%u", link->network->ipv6_mtu);
+
+        r = write_string_file(p, buf, 0);
+        if (r < 0)
+                log_link_warning_errno(link, r, "Cannot set IPv6 MTU for interface: %m");
+
+        return 0;
+}
+
 static int link_drop_foreign_config(Link *link) {
         Address *address;
         Route *route;
@@ -2623,6 +2649,10 @@ static int link_configure(Link *link) {
         if (r < 0)
                 return r;
 
+        r = link_set_ipv6_mtu(link);
+        if (r < 0)
+                return r;
+
         if (link_ipv4ll_enabled(link)) {
                 r = ipv4ll_configure(link);
                 if (r < 0)

src/network/networkd-network-gperf.gperf

@@ -69,6 +69,7 @@ Network.IPv6AcceptRouterAdvertisements, config_parse_tristate,
 Network.IPv6DuplicateAddressDetection,  config_parse_int,                               0,                             offsetof(Network, ipv6_dad_transmits)
 Network.IPv6HopLimit,                   config_parse_int,                               0,                             offsetof(Network, ipv6_hop_limit)
 Network.IPv6ProxyNDP,                   config_parse_tristate,                          0,                             offsetof(Network, ipv6_proxy_ndp)
+Network.IPv6MTUBytes,                   config_parse_ipv6_mtu,                          0,                             0
 Network.ActiveSlave,                    config_parse_bool,                              0,                             offsetof(Network, active_slave)
 Network.PrimarySlave,                   config_parse_bool,                              0,                             offsetof(Network, primary_slave)
 Network.IPv4ProxyARP,                   config_parse_tristate,                          0,                             offsetof(Network, proxy_arp)

src/network/networkd-network.c

@@ -247,6 +247,7 @@ static int network_load_one(Manager *manager, const char *filename) {
         network->arp = -1;
         network->ipv6_accept_ra_use_dns = true;
         network->ipv6_accept_ra_route_table = RT_TABLE_MAIN;
+        network->ipv6_mtu = 0;
 
         dropin_dirname = strjoina(network->name, ".network.d");
 
@@ -1416,6 +1417,42 @@ int config_parse_dhcp_route_table(const char *unit,
         return 0;
 }
 
+int config_parse_ipv6_mtu(
+                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) {
+
+        Network *network = data;
+        uint32_t mtu;
+        int r;
+
+        assert(filename);
+        assert(lvalue);
+        assert(rvalue);
+
+        r = safe_atou32(rvalue, &mtu);
+        if (r < 0) {
+                log_syntax(unit, LOG_ERR, filename, line, r, "Failed to parse IPv6 MTU, ignoring assignment: %s", rvalue);
+                return 0;
+        }
+
+        if (mtu < IPV6_MIN_MTU) {
+                log_syntax(unit, LOG_ERR, filename, line, r, "Failed to parse IPv6 MTU for interface. Allowed minimum MTU is 1280 bytes, ignoring: %s", rvalue);
+                return 0;
+        }
+
+        network->ipv6_mtu = mtu;
+
+        return 0;
+}
+
 DEFINE_CONFIG_PARSE_ENUM(config_parse_dhcp_use_domains, dhcp_use_domains, DHCPUseDomains, "Failed to parse DHCP use domains setting");
 
 static const char* const dhcp_use_domains_table[_DHCP_USE_DOMAINS_MAX] = {

src/network/networkd-network.h

@@ -33,6 +33,8 @@
 #define BRIDGE_VLAN_BITMAP_MAX 4096
 #define BRIDGE_VLAN_BITMAP_LEN (BRIDGE_VLAN_BITMAP_MAX / 32)
 
+#define IPV6_MIN_MTU 1280
+
 typedef enum DHCPClientIdentifier {
         DHCP_CLIENT_ID_MAC,
         DHCP_CLIENT_ID_DUID,
@@ -199,6 +201,7 @@ struct Network {
         int ipv6_hop_limit;
         int ipv6_proxy_ndp;
         int proxy_arp;
+        unsigned ipv6_mtu;
 
         bool ipv6_accept_ra_use_dns;
         bool active_slave;
@@ -291,7 +294,7 @@ int config_parse_dhcp_use_domains(const char *unit, const char *filename, unsign
 int config_parse_lldp_mode(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_dhcp_route_table(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_ntp(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_ipv6_mtu(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);
 /* Legacy IPv4LL support */
 int config_parse_ipv4ll(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);