[monitoring-plugins] Improve handling of -4/-6

[William]

 Module: monitoring-plugins
 Branch: master
 Commit: 4acba2b3ecef7c1482b3cd25e35773947d80e2c6
 Author: William <william at blackhats.net.au>
   Date: Thu Mar 27 11:20:36 2025 +1000
    URL: https://www.monitoring-plugins.org/repositories/monitoring-plugins/commit/?id=4acba2b3

Improve handling of -4/-6

If fping is used with a target that has dual stack v4/v6, then due to
the logic during command construction, ipv4 will never be checked as v6
is preferred by fping.

This explicitly flags -4/-6 when it is requested by the user.

---

 plugins/check_fping.c | 33 ++++++++++++++++++++++-----------
 1 file changed, 22 insertions(+), 11 deletions(-)

diff --git a/plugins/check_fping.c b/plugins/check_fping.c
index ec7abb67..1c2b7689 100644
--- a/plugins/check_fping.c
+++ b/plugins/check_fping.c
@@ -79,7 +79,29 @@ int main(int argc, char **argv) {
 	server = strscpy(server, config.server_name);
 
 	char *option_string = "";
+	char *fping_prog = NULL;
+
 	/* compose the command */
+#ifdef PATH_TO_FPING6
+	if (address_family != AF_INET && is_inet6_addr(server)) {
+		fping_prog = strdup(PATH_TO_FPING6);
+	} else {
+		xasprintf(&option_string, "%s-4 ", option_string);
+		fping_prog = strdup(PATH_TO_FPING);
+	}
+#else
+	if (address_family != AF_INET) {
+		// -4 / -6 must be set explicitly as when a host has dual stack
+		// if we don't specify -4 then fping selects ipv6 which can mess
+		// with some checks.
+		xasprintf(&option_string, "%s-6 ", option_string);
+	} else {
+		xasprintf(&option_string, "%s-4 ", option_string);
+	}
+
+	fping_prog = strdup(PATH_TO_FPING);
+#endif
+
 	if (config.target_timeout) {
 		xasprintf(&option_string, "%s-t %d ", option_string, config.target_timeout);
 	}
@@ -99,17 +121,6 @@ int main(int argc, char **argv) {
 		xasprintf(&option_string, "%s-R ", option_string);
 	}
 
-	char *fping_prog = NULL;
-#ifdef PATH_TO_FPING6
-	if (address_family != AF_INET && is_inet6_addr(server)) {
-		fping_prog = strdup(PATH_TO_FPING6);
-	} else {
-		fping_prog = strdup(PATH_TO_FPING);
-	}
-#else
-	fping_prog = strdup(PATH_TO_FPING);
-#endif
-
 	char *command_line = NULL;
 	xasprintf(&command_line, "%s %s-b %d -c %d %s", fping_prog, option_string, config.packet_size, config.packet_count, server);