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

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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
This commit is contained in:
Govind Venugopal
2025-10-15 02:20:41 -07:00
committed by GitHub
parent 4cae0e9a78
commit 3eb7b881bd
9 changed files with 148 additions and 0 deletions
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28
man/systemd.network.xml
View File

@@ -3992,6 +3992,34 @@ ServerAddress=192.168.0.1/24</programlisting>
<xi:include href="version-info.xml" xpointer="v226"/></listitem>
</varlistentry>
<varlistentry>
<term><varname>EmitDomain=</varname></term>
<listitem><para>Takes a boolean. Configures whether the DHCP leases handed out
to clients shall contain domain name information (DHCP option 15). Defaults to
<literal>no</literal>.</para>
<xi:include href="version-info.xml" xpointer="v259"/></listitem>
</varlistentry>
<varlistentry>
<term><varname>Domain=</varname></term>
<listitem><para>Takes a domain name (such as <literal>example.com</literal>)
to pass to DHCP clients. This configures the DNS default domain for DHCP clients.
When set, DHCP clients will use this as their DNS search domain.</para>
<para>When <varname>EmitDomain=yes</varname> is set but <varname>Domain=</varname>
is not configured, the domain name will be automatically derived from the system's
fully qualified hostname. For example, if the system's hostname is
<literal>host.example.com</literal>, the domain <literal>example.com</literal>
will be sent to clients. If the system's hostname does not contain a domain part
(e.g., hostname is just <literal>host</literal>), no domain name will be sent to
DHCP clients. When empty or unset, defaults to no domain name.</para>
<xi:include href="version-info.xml" xpointer="v259"/></listitem>
</varlistentry>
<varlistentry>
<term><varname>BootServerAddress=</varname></term>

1
src/libsystemd-network/dhcp-server-internal.h
View File

@@ -46,6 +46,7 @@ typedef struct sd_dhcp_server {
uint32_t pool_size;
char *timezone;
char *domain_name;
DHCPServerData servers[_SD_DHCP_LEASE_SERVER_TYPE_MAX];
struct in_addr boot_server_address;

26
src/libsystemd-network/sd-dhcp-server.c
View File

@@ -128,6 +128,7 @@ static sd_dhcp_server *dhcp_server_free(sd_dhcp_server *server) {
free(server->boot_server_name);
free(server->boot_filename);
free(server->timezone);
free(server->domain_name);
for (sd_dhcp_lease_server_type_t i = 0; i < _SD_DHCP_LEASE_SERVER_TYPE_MAX; i++)
free(server->servers[i].addr);
@@ -625,6 +626,15 @@ static int server_send_offer_or_ack(
return r;
}
if (server->domain_name) {
r = dhcp_option_append(
&packet->dhcp, req->max_optlen, &offset, 0,
SD_DHCP_OPTION_DOMAIN_NAME,
strlen(server->domain_name), server->domain_name);
if (r < 0)
return r;
}
/* RFC 8925 section 3.3. DHCPv4 Server Behavior
* The server MUST NOT include the IPv6-Only Preferred option in the DHCPOFFER or DHCPACK message if
* the option was not present in the Parameter Request List sent by the client. */
@@ -1415,6 +1425,22 @@ int sd_dhcp_server_set_timezone(sd_dhcp_server *server, const char *tz) {
return 1;
}
int sd_dhcp_server_set_domain_name(sd_dhcp_server *server, const char *domain_name) {
int r;
assert_return(server, -EINVAL);
if (domain_name) {
r = dns_name_is_valid(domain_name);
if (r < 0)
return r;
if (r == 0)
return -EINVAL;
}
return free_and_strdup(&server->domain_name, domain_name);
}
int sd_dhcp_server_set_max_lease_time(sd_dhcp_server *server, uint64_t t) {
assert_return(server, -EINVAL);

39
src/libsystemd-network/test-dhcp-server.c
View File

@@ -316,6 +316,44 @@ static void test_static_lease(void) {
(uint8_t*) &(uint32_t) { 0x01020306 }, sizeof(uint32_t)));
}
static void test_domain_name(void) {
_cleanup_(sd_dhcp_server_unrefp) sd_dhcp_server *server = NULL;
log_debug("/* %s */", __func__);
ASSERT_OK(sd_dhcp_server_new(&server, 1));
/* Test setting domain name */
ASSERT_OK_POSITIVE(sd_dhcp_server_set_domain_name(server, "example.com"));
/* Test setting same domain name (should return 0 - no change) */
ASSERT_OK_ZERO(sd_dhcp_server_set_domain_name(server, "example.com"));
/* Test changing domain name */
ASSERT_OK_POSITIVE(sd_dhcp_server_set_domain_name(server, "test.local"));
/* Test clearing domain name */
ASSERT_OK_POSITIVE(sd_dhcp_server_set_domain_name(server, NULL));
/* Test clearing again (should return 0 - already cleared) */
ASSERT_OK_ZERO(sd_dhcp_server_set_domain_name(server, NULL));
/* Test invalid domain name */
ASSERT_ERROR(sd_dhcp_server_set_domain_name(server, "invalid..domain"), EINVAL);
/* Test empty string (treated differently from NULL) */
ASSERT_OK_POSITIVE(sd_dhcp_server_set_domain_name(server, ""));
/* Test clearing domain name with NULL */
ASSERT_OK_POSITIVE(sd_dhcp_server_set_domain_name(server, NULL));
/* Test valid domain with subdomain */
ASSERT_OK_POSITIVE(sd_dhcp_server_set_domain_name(server, "sub.example.com"));
/* Test single-label domain */
ASSERT_OK_POSITIVE(sd_dhcp_server_set_domain_name(server, "local"));
}
int main(int argc, char *argv[]) {
int r;
@@ -323,6 +361,7 @@ int main(int argc, char *argv[]) {
test_client_id_hash();
test_static_lease();
test_domain_name();
r = test_basic(true);
if (r < 0)

48
src/network/networkd-dhcp-server.c
View File

@@ -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)

2
src/network/networkd-network-gperf.gperf
View File

@@ -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)

1
src/network/networkd-network.c
View File

@@ -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);

2
src/network/networkd-network.h
View File

@@ -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;

1
src/systemd/sd-dhcp-server.h
View File

@@ -61,6 +61,7 @@ int sd_dhcp_server_set_boot_server_name(sd_dhcp_server *server, const char *name
int sd_dhcp_server_set_boot_filename(sd_dhcp_server *server, const char *filename);
int sd_dhcp_server_set_bind_to_interface(sd_dhcp_server *server, int enabled);
int sd_dhcp_server_set_timezone(sd_dhcp_server *server, const char *timezone);
int sd_dhcp_server_set_domain_name(sd_dhcp_server *server, const char *domain_name);
int sd_dhcp_server_set_router(sd_dhcp_server *server, const struct in_addr *address);
int sd_dhcp_server_set_servers(