sd-dhcp-server: refuse invalid hostname in request

Currently, the received hostname is not used for assigning an address to
the host, or options in the subsequent reply message. But, the parsed
hostname is exposed through DBus, and possibly Varlink in the future.
Let's ignore spurious hostname.

src/libsystemd-network/dhcp-option.c

@@ -10,6 +10,8 @@
 #include "alloc-util.h"
 #include "dhcp-option.h"
 #include "dhcp-server-internal.h"
+#include "dns-domain.h"
+#include "hostname-util.h"
 #include "memory-util.h"
 #include "ordered-set.h"
 #include "strv.h"
@@ -420,6 +422,35 @@ int dhcp_option_parse_string(const uint8_t *option, size_t len, char **ret) {
         return 0;
 }
 
+int dhcp_option_parse_hostname(const uint8_t *option, size_t len, char **ret) {
+        _cleanup_free_ char *hostname = NULL;
+        int r;
+
+        assert(option);
+        assert(ret);
+
+        r = dhcp_option_parse_string(option, len, &hostname);
+        if (r < 0)
+                return r;
+
+        if (!hostname) {
+                *ret = NULL;
+                return 0;
+        }
+
+        if (!hostname_is_valid(hostname, 0))
+                return -EINVAL;
+
+        r = dns_name_is_valid(hostname);
+        if (r < 0)
+                return r;
+        if (r == 0)
+                return -EINVAL;
+
+        *ret = TAKE_PTR(hostname);
+        return 0;
+}
+
 static sd_dhcp_option* dhcp_option_free(sd_dhcp_option *i) {
         if (!i)
                 return NULL;

src/libsystemd-network/dhcp-option.h

@@ -44,3 +44,4 @@ int dhcp_option_parse(
                 char **ret_error_message);
 
 int dhcp_option_parse_string(const uint8_t *option, size_t len, char **ret);
+int dhcp_option_parse_hostname(const uint8_t *option, size_t len, char **ret);

src/libsystemd-network/sd-dhcp-server.c

@@ -746,14 +746,16 @@ static int parse_request(uint8_t code, uint8_t len, const void *option, void *us
                 req->agent_info_option = (uint8_t*)option - 2;
 
                 break;
-        case SD_DHCP_OPTION_HOST_NAME:
-                r = dhcp_option_parse_string(option, len, &req->hostname);
-                if (r < 0) {
-                        log_debug_errno(r, "Failed to parse hostname, ignoring: %m");
-                        return 0;
-                }
+        case SD_DHCP_OPTION_HOST_NAME: {
+                _cleanup_free_ char *p = NULL;
 
+                r = dhcp_option_parse_hostname(option, len, &p);
+                if (r < 0)
+                        log_debug_errno(r, "Failed to parse hostname, ignoring: %m");
+                else
+                        free_and_replace(req->hostname, p);
                 break;
+        }
         case SD_DHCP_OPTION_PARAMETER_REQUEST_LIST:
                 req->parameter_request_list = option;
                 req->parameter_request_list_len = len;