networkd: Add support to configure proxy arp support to interfaces (#3020)

Fixes: #2889

man/systemd.network.xml

@@ -548,6 +548,15 @@
           Defaults to unset.
         </para></listitem>
         </varlistentry>
+        <varlistentry>
+          <term><varname>ProxyARP=</varname></term>
+          <listitem><para>A boolean. Configures proxy ARP. Proxy ARP is the technique in which one host,
+          usually a router, answers ARP requests intended for another machine. By "faking" its identity,
+          the router accepts responsibility for routing packets to the "real" destination. (see <ulink
+          url="https://tools.ietf.org/html/rfc1027">RFC 1027</ulink>.
+          Defaults to unset.
+        </para></listitem>
+        </varlistentry>
         <varlistentry>
           <term><varname>Bridge=</varname></term>
           <listitem>

src/network/networkd-link.c

@@ -165,6 +165,21 @@ static bool link_ipv6_forward_enabled(Link *link) {
         return link->network->ip_forward & ADDRESS_FAMILY_IPV6;
 }
 
+static bool link_proxy_arp_enabled(Link *link) {
+        assert(link);
+
+        if (link->flags & IFF_LOOPBACK)
+                return false;
+
+        if (!link->network)
+                return false;
+
+        if (link->network->proxy_arp < 0)
+                return false;
+
+        return true;
+}
+
 static bool link_ipv6_accept_ra_enabled(Link *link) {
         assert(link);
 
@@ -1039,6 +1054,22 @@ static int link_set_bridge_fdb(Link *const link) {
         return r;
 }
 
+static int link_set_proxy_arp(Link *const link) {
+        const char *p = NULL;
+        int r;
+
+        if (!link_proxy_arp_enabled(link))
+                return 0;
+
+        p = strjoina("/proc/sys/net/ipv4/conf/", link->ifname, "/proxy_arp");
+
+        r = write_string_file(p, one_zero(link->network->proxy_arp), WRITE_STRING_FILE_VERIFY_ON_FAILURE);
+        if (r < 0)
+                log_link_warning_errno(link, r, "Cannot configure proxy ARP for interface: %m");
+
+        return 0;
+}
+
 static int link_set_handler(sd_netlink *rtnl, sd_netlink_message *m, void *userdata) {
         _cleanup_link_unref_ Link *link = userdata;
         int r;
@@ -2167,6 +2198,10 @@ static int link_configure(Link *link) {
         if (r < 0)
                 return r;
 
+        r = link_set_proxy_arp(link);
+        if (r < 0)
+               return r;
+
         r = link_set_ipv4_forward(link);
         if (r < 0)
                 return r;

src/network/networkd-network-gperf.gperf

@@ -61,6 +61,7 @@ Network.IPv6PrivacyExtensions,          config_parse_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.ProxyARP,                       config_parse_tristate,                          0,                             offsetof(Network, proxy_arp)
 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

@@ -132,6 +132,7 @@ static int network_load_one(Manager *manager, const char *filename) {
         network->ipv6_dad_transmits = -1;
         network->ipv6_hop_limit = -1;
         network->duid_type = _DUID_TYPE_INVALID;
+        network->proxy_arp = -1;
 
         r = config_parse(NULL, filename, file,
                          "Match\0"

src/network/networkd-network.h

@@ -139,6 +139,7 @@ struct Network {
         int ipv6_accept_ra;
         int ipv6_dad_transmits;
         int ipv6_hop_limit;
+        int proxy_arp;
 
         union in_addr_union ipv6_token;
         IPv6PrivacyExtensions ipv6_privacy_extensions;