networkd: Add support to configure IPV6 hop limit

This patch adds support to configure IPV6 hop limit.

For example:
/proc/sys/net/ipv6/conf/wlp3s0/hop_limit

src/network/networkd-link.c

@@ -1952,6 +1952,37 @@ static int link_set_ipv6_dad_transmits(Link *link) {
         return 0;
 }
 
+static int link_set_ipv6_hop_limit(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_hop_limit < 0)
+                return 0;
+
+        p = strjoina("/proc/sys/net/ipv6/conf/", link->ifname, "/hop_limit");
+
+        xsprintf(buf, "%u", link->network->ipv6_hop_limit);
+
+        r = write_string_file(p, buf, 0);
+        if (r < 0) {
+                /* If the right value is set anyway, don't complain */
+                if (verify_one_line_file(p, buf) > 0)
+                        return 0;
+
+                log_link_warning_errno(link, r, "Cannot set IPv6 hop limit for interface: %m");
+        }
+
+        return 0;
+}
+
 static int link_configure(Link *link) {
         int r;
 
@@ -1983,6 +2014,10 @@ static int link_configure(Link *link) {
         if (r < 0)
                 return r;
 
+        r = link_set_ipv6_hop_limit(link);
+        if (r < 0)
+                return r;
+
         if (link_ipv4ll_enabled(link)) {
                 r = ipv4ll_configure(link);
                 if (r < 0)

src/network/networkd-network-gperf.gperf

@@ -52,6 +52,7 @@ Network.IPMasquerade,                   config_parse_bool,
 Network.IPv6PrivacyExtensions,          config_parse_ipv6_privacy_extensions,           0,                             offsetof(Network, ipv6_privacy_extensions)
 Network.IPv6AcceptRouterAdvertisements, config_parse_tristate,                          0,                             offsetof(Network, ipv6_accept_ra)
 Network.IPv6DuplicateAddressDetection,  config_parse_int,                               0,                             offsetof(Network, ipv6_dad_transmits)
+Network.IPv6HopLimit,                   config_parse_int,                               0,                             offsetof(Network, ipv6_hop_limit)
 Network.BindCarrier,                    config_parse_strv,                              0,                             offsetof(Network, bind_carrier)
 Address.Address,                        config_parse_address,                           0,                             0
 Address.Peer,                           config_parse_address,                           0,                             0

src/network/networkd-network.c

@@ -127,6 +127,7 @@ static int network_load_one(Manager *manager, const char *filename) {
         network->ipv6_privacy_extensions = IPV6_PRIVACY_EXTENSIONS_NO;
         network->ipv6_accept_ra = -1;
         network->ipv6_dad_transmits = -1;
+        network->ipv6_hop_limit = -1;
 
         r = config_parse(NULL, filename, file,
                          "Match\0"

src/network/networkd-network.h

@@ -122,6 +122,7 @@ struct Network {
 
         int ipv6_accept_ra;
         int ipv6_dad_transmits;
+        int ipv6_hop_limit;
 
         union in_addr_union ipv6_token;
         IPv6PrivacyExtensions ipv6_privacy_extensions;