1 files changed, 41 insertions, 7 deletions

diff --git a/plugins/check_fping.c b/plugins/check_fping.c
index cf9c2d1c..9dadcec7 100644
--- a/plugins/check_fping.c
+++ b/plugins/check_fping.c
@@ -81,27 +81,61 @@ int main(int argc, char **argv) {
         char *option_string = "";
         char *fping_prog = NULL;
 
-        /* compose the command */
+        /* First determine if the target is dualstack or ipv6 only. */
+#ifdef USE_IPV6
+        bool server_is_inet6_addr = is_inet6_addr(server);
+#else
+        bool server_is_inet6_addr = false;
+#endif
+
+        /* PATH_TO_FPING6 implies USE_IPV6 */
 #ifdef PATH_TO_FPING6
-        if (address_family == AF_INET6 || (address_family == AF_UNSPEC && is_inet6_addr(server))) {
+        /*
+         * If the user requested -6 OR the user made no assertion and the address is v6 or dualstack
+         *   -> we use ipv6
+         * If the user requested -4 OR the user made no assertion and the address is v4 ONLY
+         *   -> we use ipv4
+         */
+        if (address_family == AF_INET6 || (address_family == AF_UNSPEC && server_is_inet6_addr)) {
                 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_INET6 || (address_family == AF_UNSPEC && is_inet6_addr(server))) {
-                // -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.
+#ifdef USE_IPV6
+        /*
+         * If the user requested -6 OR the user made no assertion and the address is v6 or dualstack
+         *   -> we use ipv6
+         * If the user requested -4 OR the user made no assertion and the address is v4 ONLY
+         *   -> we use ipv4
+         */
+        if (address_family == AF_INET6 || (address_family == AF_UNSPEC && server_is_inet6_addr)) {
                 xasprintf(&option_string, "%s-6 ", option_string);
         } else {
                 xasprintf(&option_string, "%s-4 ", option_string);
         }
-
+#else
+        /*
+         * If the user requested -6
+         *   -> warn that v6 is not available
+         * -> we use ipv4
+         */
+        if (address_family == AF_INET6) {
+                usage4(_("IPv6 support not available"));
+        }
+        /*
+         * Note here we still have to call with -4, else in the case of a dual stacked target
+         * we could potentially silently use ipv6 despite having just warned that it is not available
+         */
+        xasprintf(&option_string, "%s-4 ", option_string);
+        /* end USE_IPV6 */
+#endif
         fping_prog = strdup(PATH_TO_FPING);
+        /* end PATH_TO_FPING6 */
 #endif
 
+        /* compose the command */
         if (config.target_timeout) {
                 xasprintf(&option_string, "%s-t %d ", option_string, config.target_timeout);
         }