networkd: send hostname to dhcp server

Send hostname (option 12) in DISCOVER and REQUEST messages so the
DHCP server could use it to register with dynamic DNS and such.

To opt-out of this behaviour set SendHostname to false in [DHCP]
section of .network file

[tomegun: rebased, made sure a failing set_hostname is a noop and moved
          config from DHCPv4 to DHCP]

src/network/networkd-link.c

@@ -21,6 +21,7 @@
 
 #include <netinet/ether.h>
 #include <linux/if.h>
+#include <unistd.h>
 
 #include "networkd.h"
 #include "libudev-private.h"
@@ -1927,6 +1928,18 @@ static int link_enter_enslave(Link *link) {
         return 0;
 }
 
+/* make sure the hostname is not "localhost" */
+static bool is_localhost(const char *hostname) {
+        assert(hostname);
+
+        return streq(hostname, "localhost") ||
+               streq(hostname, "localhost.") ||
+               endswith(hostname, ".localhost") ||
+               endswith(hostname, ".localhost.") ||
+               endswith(hostname, ".localdomain") ||
+               endswith(hostname, ".localdomain.");
+}
+
 static int link_configure(Link *link) {
         int r;
 
@@ -1992,6 +2005,7 @@ static int link_configure(Link *link) {
                         if (r < 0)
                                 return r;
                 }
+
                 if (link->network->dhcp_routes) {
                         r = sd_dhcp_client_set_request_option(link->dhcp_client, DHCP_OPTION_STATIC_ROUTE);
                         if (r < 0)
@@ -2000,6 +2014,18 @@ static int link_configure(Link *link) {
                         if (r < 0)
                                 return r;
                 }
+
+                if (link->network->dhcp_sendhost) {
+                        _cleanup_free_ char *hostname = gethostname_malloc();
+                        if (!hostname)
+                                return -ENOMEM;
+
+                        if (!is_localhost(hostname)) {
+                                r = sd_dhcp_client_set_hostname(link->dhcp_client, hostname);
+                                if (r < 0)
+                                        return r;
+                        }
+                }
         }
 
         if (link->network->dhcp_server) {

src/network/networkd-network-gperf.gperf

@@ -47,11 +47,12 @@ DHCP.UseDNS,                 config_parse_bool,                  0,
 DHCP.UseMTU,                 config_parse_bool,                  0,                             offsetof(Network, dhcp_mtu)
 DHCP.UseHostname,            config_parse_bool,                  0,                             offsetof(Network, dhcp_hostname)
 DHCP.UseDomainName,          config_parse_bool,                  0,                             offsetof(Network, dhcp_domainname)
+DHCP.UseRoutes,            config_parse_bool,                  0,                             offsetof(Network, dhcp_routes)
+DHCP.SendHostname,         config_parse_bool,                  0,                             offsetof(Network, dhcp_sendhost)
 DHCP.CriticalConnection,     config_parse_bool,                  0,                             offsetof(Network, dhcp_critical)
-/* backwards compatibility */
+/* backwards compatibility: do not add new entries to this section */
 DHCPv4.UseDNS,               config_parse_bool,                  0,                             offsetof(Network, dhcp_dns)
 DHCPv4.UseMTU,               config_parse_bool,                  0,                             offsetof(Network, dhcp_mtu)
 DHCPv4.UseHostname,          config_parse_bool,                  0,                             offsetof(Network, dhcp_hostname)
 DHCPv4.UseDomainName,        config_parse_bool,                  0,                             offsetof(Network, dhcp_domainname)
-DHCPv4.UseRoutes,            config_parse_bool,                  0,                             offsetof(Network, dhcp_routes)
 DHCPv4.CriticalConnection,   config_parse_bool,                  0,                             offsetof(Network, dhcp_critical)

src/network/networkd-network.c

@@ -90,6 +90,7 @@ static int network_load_one(Manager *manager, const char *filename) {
         network->dhcp_hostname = true;
         network->dhcp_domainname = true;
         network->dhcp_routes = true;
+        network->dhcp_sendhost = true;
 
         r = config_parse(NULL, filename, file, "Match\0Network\0Address\0Route\0DHCPv4\0", config_item_perf_lookup,
                         (void*) network_network_gperf_lookup, false, false, network);

src/network/networkd.h

@@ -169,6 +169,7 @@ struct Network {
         bool dhcp_mtu;
         bool dhcp_hostname;
         bool dhcp_domainname;
+        bool dhcp_sendhost;
         bool dhcp_critical;
         bool dhcp_routes;
         bool ipv4ll;