libsystemd-network: add support for "Client FQDN" DHCP option

This adds support for the Client Fully Qualified Domain Name (FQDN)
option [RFC 4702] to libsystemd-network. The option can be used to
exchange information about a DHCPv4 client's fully qualified domain
name and about responsibility for updating the DNS RR related to the
client's address assignment.

Other popular DHCP clients (dhclient, dhcpcd) support this option and
it would be useful to have it in networkd too.

2 files changed, 54 insertions, 9 deletions

diff --git a/src/libsystemd-network/dhcp-protocol.h b/src/libsystemd-network/dhcp-protocol.h
index 88a81d2866..5cf4e9e306 100644
--- a/src/libsystemd-network/dhcp-protocol.h
+++ b/src/libsystemd-network/dhcp-protocol.h
@@ -137,6 +137,7 @@ enum {
         DHCP_OPTION_REBINDING_T2_TIME           = 59,
         DHCP_OPTION_VENDOR_CLASS_IDENTIFIER     = 60,
         DHCP_OPTION_CLIENT_IDENTIFIER           = 61,
+        DHCP_OPTION_FQDN                        = 81,
         DHCP_OPTION_NEW_POSIX_TIMEZONE          = 100,
         DHCP_OPTION_NEW_TZDB_TIMEZONE           = 101,
         DHCP_OPTION_CLASSLESS_STATIC_ROUTE      = 121,
@@ -144,3 +145,12 @@ enum {
         DHCP_OPTION_PRIVATE_LAST                = 254,
         DHCP_OPTION_END                         = 255,
 };
+
+#define DHCP_MAX_FQDN_LENGTH 255
+
+enum {
+        DHCP_FQDN_FLAG_S = (1 << 0),
+        DHCP_FQDN_FLAG_O = (1 << 1),
+        DHCP_FQDN_FLAG_E = (1 << 2),
+        DHCP_FQDN_FLAG_N = (1 << 3),
+};
diff --git a/src/libsystemd-network/sd-dhcp-client.c b/src/libsystemd-network/sd-dhcp-client.c
index 137537253a..5ec0e661f7 100644
--- a/src/libsystemd-network/sd-dhcp-client.c
+++ b/src/libsystemd-network/sd-dhcp-client.c
@@ -34,6 +34,8 @@
 #include "dhcp-internal.h"
 #include "dhcp-lease-internal.h"
 #include "dhcp-protocol.h"
+#include "dns-domain.h"
+#include "hostname-util.h"
 #include "random-util.h"
 #include "string-util.h"
 #include "util.h"
@@ -298,6 +300,9 @@ int sd_dhcp_client_set_hostname(sd_dhcp_client *client,
 
         assert_return(client, -EINVAL);
 
+        if (!hostname_is_valid(hostname, false) && !dns_name_is_valid(hostname))
+                return -EINVAL;
+
         if (streq_ptr(client->hostname, hostname))
                 return 0;
 
@@ -539,6 +544,24 @@ static int client_message_init(sd_dhcp_client *client, DHCPPacket **ret,
         return 0;
 }
 
+static int client_append_fqdn_option(DHCPMessage *message, size_t optlen, size_t *optoffset,
+                                     const char *fqdn) {
+        uint8_t buffer[3 + DHCP_MAX_FQDN_LENGTH];
+        int r;
+
+        buffer[0] = DHCP_FQDN_FLAG_S | /* Request server to perform A RR DNS updates */
+                    DHCP_FQDN_FLAG_E;  /* Canonical wire format */
+        buffer[1] = 0;                 /* RCODE1 (deprecated) */
+        buffer[2] = 0;                 /* RCODE2 (deprecated) */
+
+        r = dns_name_to_wire_format(fqdn, buffer + 3, sizeof(buffer) - 3);
+        if (r > 0)
+                r = dhcp_option_append(message, optlen, optoffset, 0,
+                                       DHCP_OPTION_FQDN, 3 + r, buffer);
+
+        return r;
+}
+
 static int dhcp_client_send_raw(sd_dhcp_client *client, DHCPPacket *packet,
                                 size_t len) {
         dhcp_packet_append_ip_headers(packet, INADDR_ANY, DHCP_PORT_CLIENT,
@@ -576,13 +599,21 @@ static int client_send_discover(sd_dhcp_client *client) {
                         return r;
         }
 
-        /* it is unclear from RFC 2131 if client should send hostname in
-           DHCPDISCOVER but dhclient does and so we do as well
-        */
         if (client->hostname) {
-                r = dhcp_option_append(&discover->dhcp, optlen, &optoffset, 0,
-                                       DHCP_OPTION_HOST_NAME,
-                                       strlen(client->hostname), client->hostname);
+                /* According to RFC 4702 "clients that send the Client FQDN option in
+                   their messages MUST NOT also send the Host Name option". Just send
+                   one of the two depending on the hostname type.
+                */
+                if (dns_name_single_label(client->hostname)) {
+                        /* it is unclear from RFC 2131 if client should send hostname in
+                           DHCPDISCOVER but dhclient does and so we do as well
+                        */
+                        r = dhcp_option_append(&discover->dhcp, optlen, &optoffset, 0,
+                                               DHCP_OPTION_HOST_NAME,
+                                               strlen(client->hostname), client->hostname);
+                } else
+                        r = client_append_fqdn_option(&discover->dhcp, optlen, &optoffset,
+                                                      client->hostname);
                 if (r < 0)
                         return r;
         }
@@ -688,9 +719,13 @@ static int client_send_request(sd_dhcp_client *client) {
         }
 
         if (client->hostname) {
-                r = dhcp_option_append(&request->dhcp, optlen, &optoffset, 0,
-                                       DHCP_OPTION_HOST_NAME,
-                                       strlen(client->hostname), client->hostname);
+                if (dns_name_single_label(client->hostname))
+                        r = dhcp_option_append(&request->dhcp, optlen, &optoffset, 0,
+                                               DHCP_OPTION_HOST_NAME,
+                                               strlen(client->hostname), client->hostname);
+                else
+                        r = client_append_fqdn_option(&request->dhcp, optlen, &optoffset,
+                                                      client->hostname);
                 if (r < 0)
                         return r;
         }