net: dhcpv4_server: dns server option

dhcv4 server was not providing the dns server details to the
client because dns option was handled. Added the dns server
option to be send to the client as response form dhcpv4 server

Signed-off-by: Muhammad Haziq <[email protected]>

Author: M-Haziq94

subsys/net/lib/dhcpv4/Kconfig

@@ -162,4 +162,15 @@ config NET_DHCPV4_SERVER_NAK_UNRECOGNIZED_REQUESTS
 	  will send a NAK, informing the client it should proceed with a full
 	  procedure.
 
+config NET_DHCPV4_SERVER_OPTION_DNS_ADDRESS
+	string "DNS Server address"
+	default DNS_SERVER1 if DNS_SERVER_IP_ADDRESSES
+	default ""
+	help
+	  This configuration option specifies the DNS server address that the DHCPv4
+	  server should provide to clients when they request an IP address. The
+	  DHCPv4 server adds this DNS server address in the DHCP offer and
+	  acknowledgment messages sent to the clients, allowing them to use the
+	  specified DNS server for name resolution.
+
 endif # NET_DHCPV4_SERVER

subsys/net/lib/dhcpv4/dhcpv4_server.c

@@ -30,6 +30,7 @@ LOG_MODULE_REGISTER(net_dhcpv4_server, CONFIG_NET_DHCPV4_SERVER_LOG_LEVEL);
 #define DHCPV4_OPTIONS_SERVER_ID_SIZE 6
 #define DHCPV4_OPTIONS_SUBNET_MASK_SIZE 6
 #define DHCPV4_OPTIONS_ROUTER_SIZE 6
+#define DHCPV4_OPTIONS_DNS_SERVER_SIZE 6
 #define DHCPV4_OPTIONS_CLIENT_ID_MIN_SIZE 2
 
 #define ADDRESS_RESERVED_TIMEOUT K_SECONDS(30)
@@ -398,6 +399,29 @@ static uint8_t *dhcpv4_encode_router_option(uint8_t *buf, size_t *buflen,
 	return buf + DHCPV4_OPTIONS_ROUTER_SIZE;
 }
 
+static uint8_t *dhcpv4_encode_dns_server_option(uint8_t *buf, size_t *buflen)
+{
+	struct in_addr dns_address;
+
+	if (buf == NULL || *buflen < DHCPV4_OPTIONS_DNS_SERVER_SIZE) {
+		return NULL;
+	}
+
+	if (net_addr_pton(AF_INET, CONFIG_NET_DHCPV4_SERVER_OPTION_DNS_ADDRESS, &dns_address)) {
+		LOG_ERR("Invalid DNS server address: %s",
+			CONFIG_NET_DHCPV4_SERVER_OPTION_DNS_ADDRESS);
+		return NULL;
+	}
+
+	buf[0] = DHCPV4_OPTIONS_DNS_SERVER;
+	buf[1] = sizeof(struct in_addr);
+	memcpy(&buf[2], dns_address.s4_addr, sizeof(struct in_addr));
+
+	*buflen -= DHCPV4_OPTIONS_DNS_SERVER_SIZE;
+
+	return buf + DHCPV4_OPTIONS_DNS_SERVER_SIZE;
+}
+
 static uint8_t *dhcpv4_encode_end_option(uint8_t *buf, size_t *buflen)
 {
 	if (buf == NULL || *buflen < 1) {
@@ -498,6 +522,13 @@ static uint8_t *dhcpv4_encode_requested_params(
 				goto out;
 			}
 			break;
+
+		case DHCPV4_OPTIONS_DNS_SERVER:
+			buf = dhcpv4_encode_dns_server_option(buf, buflen);
+			if (buf == NULL) {
+				goto out;
+			}
+			break;
 		/* Others - just ignore. */
 		default:
 			break;