network: add DHCP server domain name option support (#39260)

Implements DHCP option 15 (Domain Name) for systemd-networkd's DHCP
server, allowing administrators to configure the DNS default domain that
clients should use.

This addresses the feature request in issue #37077, where users needed
to manually configure domain names using
SendOption=15:string:example.com as a workaround.

This adds two new configuration options to the [DHCPServer] section:
- EmitDomain= (boolean): whether to send domain name to clients
- Domain= (string): the domain name to send (e.g., "example.com")

Example configuration:
  [DHCPServer] EmitDomain=yes Domain=example.com

This eliminates the need for manual workarounds using
SendOption=15:string:...

Fixes #37077

src/network/networkd-dhcp-server.c

@@ -13,6 +13,7 @@
 #include "fd-util.h"
 #include "fileio.h"
 #include "hashmap.h"
+#include "hostname-setup.h"
 #include "network-common.h"
 #include "networkd-address.h"
 #include "networkd-dhcp-server.h"
@@ -31,6 +32,30 @@
 #include "string-util.h"
 #include "strv.h"
 
+static int get_hostname_domain(char **ret) {
+        _cleanup_free_ char *hostname = NULL;
+        const char *domain;
+        int r;
+
+        assert(ret);
+
+        /* Get the full hostname (FQDN if available) */
+        r = gethostname_full(GET_HOSTNAME_ALLOW_LOCALHOST | GET_HOSTNAME_FALLBACK_DEFAULT, &hostname);
+        if (r < 0)
+                return r;
+
+        /* Find the first dot to extract the domain part */
+        domain = strchr(hostname, '.');
+        if (!domain)
+                return -ENOENT;  /* No domain part in hostname */
+
+        domain++;  /* Skip the dot */
+        if (isempty(domain))
+                return -ENOENT;  /* Empty domain after dot */
+
+        return strdup_to(ret, domain);
+}
+
 static bool link_dhcp4_server_enabled(Link *link) {
         assert(link);
 
@@ -678,6 +703,29 @@ static int dhcp4_server_configure(Link *link) {
                 }
         }
 
+        if (link->network->dhcp_server_emit_domain) {
+                _cleanup_free_ char *buffer = NULL;
+                const char *domain = NULL;
+
+                if (link->network->dhcp_server_domain)
+                        domain = link->network->dhcp_server_domain;
+                else {
+                        r = get_hostname_domain(&buffer);
+                        if (r < 0)
+                                log_link_warning_errno(link, r, "Failed to determine domain name from host's hostname, will not send domain in DHCP leases: %m");
+                        else {
+                                domain = buffer;
+                                log_link_debug(link, "Using autodetected domain name '%s' for DHCP server.", domain);
+                        }
+                }
+
+                if (domain) {
+                        r = sd_dhcp_server_set_domain_name(link->dhcp_server, domain);
+                        if (r < 0)
+                                return log_link_error_errno(link, r, "Failed to set domain name for DHCP server: %m");
+                }
+        }
+
         ORDERED_HASHMAP_FOREACH(p, link->network->dhcp_server_send_options) {
                 r = sd_dhcp_server_add_option(link->dhcp_server, p);
                 if (r == -EEXIST)

src/network/networkd-network-gperf.gperf

@@ -381,6 +381,8 @@ DHCPServer.EmitRouter,                           config_parse_bool,
 DHCPServer.Router,                               config_parse_in_addr_non_null,                  AF_INET,                                offsetof(Network, dhcp_server_router)
 DHCPServer.EmitTimezone,                         config_parse_bool,                              0,                                      offsetof(Network, dhcp_server_emit_timezone)
 DHCPServer.Timezone,                             config_parse_timezone,                          0,                                      offsetof(Network, dhcp_server_timezone)
+DHCPServer.EmitDomain,                           config_parse_bool,                              0,                                      offsetof(Network, dhcp_server_emit_domain)
+DHCPServer.Domain,                               config_parse_dns_name,                          0,                                      offsetof(Network, dhcp_server_domain)
 DHCPServer.PoolOffset,                           config_parse_uint32,                            0,                                      offsetof(Network, dhcp_server_pool_offset)
 DHCPServer.PoolSize,                             config_parse_uint32,                            0,                                      offsetof(Network, dhcp_server_pool_size)
 DHCPServer.SendVendorOption,                     config_parse_dhcp_send_option,                  0,                                      offsetof(Network, dhcp_server_send_vendor_options)

src/network/networkd-network.c

@@ -755,6 +755,7 @@ static Network *network_free(Network *network) {
         free(network->dhcp_server_boot_server_name);
         free(network->dhcp_server_boot_filename);
         free(network->dhcp_server_timezone);
+        free(network->dhcp_server_domain);
         free(network->dhcp_server_uplink_name);
         for (sd_dhcp_lease_server_type_t t = 0; t < _SD_DHCP_LEASE_SERVER_TYPE_MAX; t++)
                 free(network->dhcp_server_emit[t].addresses);

src/network/networkd-network.h

@@ -220,6 +220,8 @@ typedef struct Network {
         struct in_addr dhcp_server_router;
         bool dhcp_server_emit_timezone;
         char *dhcp_server_timezone;
+        bool dhcp_server_emit_domain;
+        char *dhcp_server_domain;
         usec_t dhcp_server_default_lease_time_usec, dhcp_server_max_lease_time_usec;
         uint32_t dhcp_server_pool_offset;
         uint32_t dhcp_server_pool_size;