Add --frag option to disable path MTU discovery

CHANGELOG.md

@@ -5,6 +5,7 @@ Next
 
 - New option --print-reply-dst to show the destination address of the received reply packet (#461, thanks @gsnw-sebast)
 - New option --oiface for outgoing interface (#463, thanks @gsnw-sebast)
+- Add --frag option to disable path MTU discovery (#476, thanks @gsnw-sebast)
 
 ## Bugfixes and other changes
 

ci/test-03-forbidden.pl

@@ -1,6 +1,7 @@
 #!/usr/bin/perl -w
 
-use Test::Command tests => 39;
+use Test::Command tests => 42;
+use Test::More;
 
 # fping -i 0
 my $cmd1 = Test::Command->new(cmd => "fping -i 0 -T10 -g 127.0.0.1/29");
@@ -54,6 +55,17 @@ END
 $cmd10->stdout_is_eq("");
 $cmd10->stderr_is_eq("fping: backoff factor 5.1 not valid, must be between 1.0 and 5.0\n");
 
+# fping -M --frag
+SKIP: {
+if($^O ne 'linux') {
+    skip '-M  option functionality is only tested on Linux', 3;
+}
+my $cmd11 = Test::Command->new(cmd => "fping -M --frag 127.0.0.1");
+$cmd11->exit_is_num(1);
+$cmd11->stdout_is_eq("");
+$cmd11->stderr_is_eq("fping: --dontfrag \(-M\) and --frag cannot be used together\n");
+}
+
 # non-negative only
 for my $arg (qw(i p Q t -seqmap-timeout)) {
     my $cmd = Test::Command->new(cmd => "fping -$arg -1");

ci/test-07-options-i-m.pl

@@ -1,6 +1,6 @@
 #!/usr/bin/perl -w
 
-use Test::Command tests => 31;
+use Test::Command tests => 34;
 use Test::More;
 
 #  -i n       interval between sending ping packets (in millisec) (default 25)
@@ -153,4 +153,15 @@
     $cmd->stderr_is_eq("");
 }
 
+# fping --frag
+SKIP: {
+    if($^O eq 'darwin') {
+        skip '--frag option not supported on macOS', 3;
+    }
+    my $cmd = Test::Command->new(cmd => "fping --frag 127.0.0.1");
+    $cmd->exit_is_num(0);
+    $cmd->stdout_is_eq("127.0.0.1 is alive\n");
+    $cmd->stderr_is_eq("");
+}
+
 # fping -m -> test-14-internet-hosts

doc/fping.pod

@@ -200,6 +200,10 @@ is recommended).
 
 Set the "Don't Fragment" bit in the IP header (used to determine/test the MTU).
 
+=item B<--frag>
+
+Set the "Fragment" bit in the IP header.
+
 =item B<-n>, B<--name>
 
 If targets are specified as IP addresses, do a reverse-DNS lookup on them

src/flags.c

@@ -50,4 +50,8 @@ int opt_oiface_on = 0;
 int opt_bindiface_on = 0;
 int opt_timestamp_on = 0;
 int opt_timestamp_format = 0;
-int opt_icmp_request_typ = 0;
+int opt_icmp_request_typ = 0;
+int opt_dontfrag_on = 0;
+int opt_frag_on = 0;
+int opt_pmtu = 0;
+int opt_pmtu_ipv6 = 0;

src/flags.h

@@ -55,5 +55,9 @@ extern int opt_bindiface_on;
 extern int opt_timestamp_on;
 extern int opt_timestamp_format;
 extern int opt_icmp_request_typ;
+extern int opt_dontfrag_on;
+extern int opt_frag_on;
+extern int opt_pmtu;
+extern int opt_pmtu_ipv6;
 
 #endif

src/fping.c

@@ -521,6 +521,7 @@ int main(int argc, char **argv)
         { "loop", 'l', OPTPARSE_NONE },
         { "all", 'm', OPTPARSE_NONE },
         { "dontfrag", 'M', OPTPARSE_NONE },
+        { "frag", 0, OPTPARSE_NONE },
         { "name", 'n', OPTPARSE_NONE },
         { "netdata", 'N', OPTPARSE_NONE },
         { "outage", 'o', OPTPARSE_NONE },
@@ -623,6 +624,17 @@ int main(int argc, char **argv)
 #else
                 fprintf(stderr, "%s: --oiface is not supported on this platform (IP_PKTINFO unavailable)\n", prog);
                 exit(3);
+#endif
+            } else if (strstr(optparse_state.optlongname, "frag") != NULL) {
+                opt_frag_on = 1;
+#ifdef IP_MTU_DISCOVER
+                opt_pmtu = IP_PMTUDISC_DONT;
+#ifdef IPV6
+                opt_pmtu_ipv6 = IPV6_PMTUDISC_DONT;
+#endif
+#else
+                fprintf(stderr, "%s: --frag option not supported on this platform\n", prog);
+                exit(1);
 #endif
             } else {
                 usage(1);
@@ -650,20 +662,11 @@ int main(int argc, char **argv)
 #endif
             break;
         case 'M':
+            opt_dontfrag_on = 1;
 #ifdef IP_MTU_DISCOVER
-            if (socket4 >= 0) {
-                int val = IP_PMTUDISC_DO;
-                if (setsockopt(socket4, IPPROTO_IP, IP_MTU_DISCOVER, &val, sizeof(val))) {
-                    perror("setsockopt IP_MTU_DISCOVER");
-                }
-            }
+            opt_pmtu = IP_PMTUDISC_DO;
 #ifdef IPV6
-            if (socket6 >= 0) {
-                int val = IPV6_PMTUDISC_DO;
-                if (setsockopt(socket6, IPPROTO_IPV6, IPV6_MTU_DISCOVER, &val, sizeof(val))) {
-                    perror("setsockopt IPV6_MTU_DISCOVER");
-                }
-            }
+            opt_pmtu_ipv6 = IPV6_PMTUDISC_DO;
 #endif
 #else
             fprintf(stderr, "%s, -M option not supported on this platform\n", prog);
@@ -1009,6 +1012,11 @@ int main(int argc, char **argv)
         exit(1);
     }
 
+    if (opt_dontfrag_on && opt_frag_on) {
+        fprintf(stderr, "%s: --dontfrag (-M) and --frag cannot be used together\n", prog);
+        exit(1);
+    }
+
     if (opt_count_on) {
         if (opt_verbose_on)
             opt_per_recv_on = 1;
@@ -1125,6 +1133,25 @@ int main(int argc, char **argv)
 #endif
     }
 
+#ifdef IP_MTU_DISCOVER
+    if (opt_pmtu != 0) {
+        if (socket4 >= 0) {
+            if (setsockopt(socket4, IPPROTO_IP, IP_MTU_DISCOVER, &opt_pmtu, sizeof(opt_pmtu))) {
+                perror("setsockopt IP_MTU_DISCOVER (pmtu)");
+            }
+        }
+    }
+#ifdef IPV6
+    if (opt_pmtu_ipv6 != 0) {
+        if (socket6 >= 0) {
+            if (setsockopt(socket6, IPPROTO_IPV6, IPV6_MTU_DISCOVER, &opt_pmtu_ipv6, sizeof(opt_pmtu_ipv6))) {
+                perror("setsockopt IPV6_MTU_DISCOVER (pmtu)");
+            }
+        }
+    }
+#endif
+#endif
+
 #if HAVE_SO_TIMESTAMPNS
     {
         int opt = 1;
@@ -3131,6 +3158,7 @@ void usage(int is_error)
     fprintf(out, "   -l, --loop         loop mode: send pings forever\n");
     fprintf(out, "   -m, --all          use all IPs of provided hostnames (e.g. IPv4 and IPv6), use with -A\n");
     fprintf(out, "   -M, --dontfrag     set the Don't Fragment flag\n");
+    fprintf(out, "       --frag         set the Fragment flag\n");
     fprintf(out, "   -O, --tos=N        set the type of service (tos) flag on the ICMP packets\n");
     fprintf(out, "   -p, --period=MSEC  interval between ping packets to one target (in ms)\n");
     fprintf(out, "                      (in loop and count modes, default: %.0f ms)\n", opt_perhost_interval / 1e6);