commit 5c54abe6e7f5f95d5b4ec0ba2201ee8759a3bc03
Author: Paul Crawford <paul@crawford-space.co.uk>
Date:   Mon Jun 8 17:39:17 2026 +0100

    Add delay to polling to avoid blocking behaviour
    
    - Many NTP servers will block/KOD respond if polled too quickly so delay
      typically 0.5 second between calls
    
    - Those servers might not permit querying either, so the check_ntp_peer
      cannot be used as an alternative

diff --git a/plugins/check_ntp_time.c b/plugins/check_ntp_time.c
index 984e86f0..9c61c26a 100644
--- a/plugins/check_ntp_time.c
+++ b/plugins/check_ntp_time.c
@@ -330,7 +330,7 @@ typedef struct {
 	mp_state_enum offset_result;
 	double offset;
 } offset_request_wrapper;
-static offset_request_wrapper offset_request(const char *host, const char *port, int time_offset) {
+static offset_request_wrapper offset_request(const char *host, const char *port, int time_offset, long poll_delay) {
 	/* setup hints to only return results from getaddrinfo that we'd like */
 	struct addrinfo hints;
 	memset(&hints, 0, sizeof(struct addrinfo));
@@ -475,7 +475,6 @@ static offset_request_wrapper offset_request(const char *host, const char *port,
 				if (verbose) {
 					printf("response from peer %zu: ", i);
 				}
-
 				read(ufds[i].fd, &req[i], sizeof(ntp_message));
 
 				struct timeval recv_time;
@@ -486,6 +485,7 @@ static offset_request_wrapper offset_request(const char *host, const char *port,
 				if (verbose) {
 					printf("offset %.10g\n", servers[i].offset[respnum]);
 				}
+				usleep(poll_delay);
 				servers[i].stratum = req[i].stratum;
 				servers[i].rtdisp = NTP32asDOUBLE(req[i].rtdisp);
 				servers[i].rtdelay = NTP32asDOUBLE(req[i].rtdelay);
@@ -560,6 +560,7 @@ static check_ntp_time_config_wrapper process_arguments(int argc, char **argv) {
 									   {"timeout", required_argument, 0, 't'},
 									   {"hostname", required_argument, 0, 'H'},
 									   {"port", required_argument, 0, 'p'},
+									   {"poll-delay", required_argument, 0, 'd'},
 									   {"output-format", required_argument, 0, output_format_index},
 									   {0, 0, 0, 0}};
 
@@ -574,7 +575,7 @@ static check_ntp_time_config_wrapper process_arguments(int argc, char **argv) {
 
 	while (true) {
 		int option = 0;
-		int option_char = getopt_long(argc, argv, "Vhv46qw:c:t:H:p:o:", longopts, &option);
+		int option_char = getopt_long(argc, argv, "Vhv46qw:c:t:H:p:o:d:", longopts, &option);
 		if (option_char == -1 || option_char == EOF || option_char == 1) {
 			break;
 		}
@@ -638,6 +639,12 @@ static check_ntp_time_config_wrapper process_arguments(int argc, char **argv) {
 		case 'o':
 			result.config.time_offset = atoi(optarg);
 			break;
+		case 'd':
+			result.config.poll_delay = (long)(0.5 + 1.0e6 * atof(optarg));
+			if (result.config.poll_delay < 0 || result.config.poll_delay > MAX_POLL) {
+				usage2(_("Invalid time"), optarg);
+			}
+			break;
 		case '4':
 			address_family = AF_INET;
 			break;
@@ -702,7 +709,7 @@ int main(int argc, char *argv[]) {
 
 	mp_subcheck sc_offset = mp_subcheck_init();
 	offset_request_wrapper offset_result =
-		offset_request(config.server_address, config.port, config.time_offset);
+		offset_request(config.server_address, config.port, config.time_offset, config.poll_delay);
 
 	if (offset_result.offset_result == STATE_UNKNOWN) {
 		sc_offset =
@@ -754,6 +761,8 @@ void print_help(void) {
 	printf("    %s\n", _("Offset to result in critical status (seconds)"));
 	printf(" %s\n", "-o, --time-offset=INTEGER");
 	printf("    %s\n", _("Expected offset of the ntp server relative to local server (seconds)"));
+	printf(" %s\n", "-d, --poll-delay=DELAY");
+	printf("    %s\n", _("Delay between polling to avoid KOD response (seconds, 0.0 to 5.0)"));
 	printf(UT_CONN_TIMEOUT, DEFAULT_SOCKET_TIMEOUT);
 	printf(UT_VERBOSE);
 	printf(UT_OUTPUT_FORMAT);
diff --git a/plugins/check_ntp_time.d/config.h b/plugins/check_ntp_time.d/config.h
index 9bbd82aa..13e3e658 100644
--- a/plugins/check_ntp_time.d/config.h
+++ b/plugins/check_ntp_time.d/config.h
@@ -5,6 +5,12 @@
 #include "thresholds.h"
 #include <stddef.h>
 
+/* Time in microseconds to delay between polling to avoid a blocking response. */
+#ifndef MAX_POLL
+#	define MAX_POLL (5000000L)
+#	define DEF_POLL (500000L)
+#endif
+
 typedef struct {
 	char *server_address;
 	char *port;
@@ -15,6 +21,7 @@ typedef struct {
 	mp_thresholds offset_thresholds;
 
 	bool output_format_is_set;
+	long poll_delay;
 	mp_output_format output_format;
 } check_ntp_time_config;
 
@@ -29,6 +36,7 @@ check_ntp_time_config check_ntp_time_config_init() {
 		.offset_thresholds = mp_thresholds_init(),
 
 		.output_format_is_set = false,
+		.poll_delay = DEF_POLL,
 	};
 
 	mp_range warning = mp_range_init();