diff --git a/aclocal.m4 b/aclocal.m4
index 0b6f3d443..8f8528efe 100644
--- a/aclocal.m4
+++ b/aclocal.m4
@@ -1081,11 +1081,16 @@ _LT_EOF
       _lt_dar_allow_undefined='$wl-undefined ${wl}suppress' ;;
     darwin1.*)
       _lt_dar_allow_undefined='$wl-flat_namespace $wl-undefined ${wl}suppress' ;;
-    darwin*)
-      case ${MACOSX_DEPLOYMENT_TARGET},$host in
-	10.[[012]],*|,*powerpc*)
+    darwin*) # darwin 5.x on
+      # if running on 10.5 or later, the deployment target defaults
+      # to the OS version, if on x86, and 10.4, the deployment
+      # target defaults to 10.4. Don't you love it?
+      case ${MACOSX_DEPLOYMENT_TARGET-10.0},$host in
+	10.0,*86*-darwin8*|10.0,*-darwin[[91]]*)
+	  _lt_dar_allow_undefined='$wl-undefined ${wl}dynamic_lookup' ;;
+	10.[[012]][[,.]]*)
 	  _lt_dar_allow_undefined='$wl-flat_namespace $wl-undefined ${wl}suppress' ;;
-	*)
+	10.*)
 	  _lt_dar_allow_undefined='$wl-undefined ${wl}dynamic_lookup' ;;
       esac
     ;;
diff --git a/configure b/configure
index 098bce198..468276f36 100755
--- a/configure
+++ b/configure
@@ -7431,11 +7431,16 @@ $as_echo "$lt_cv_ld_force_load" >&6; }
       _lt_dar_allow_undefined='$wl-undefined ${wl}suppress' ;;
     darwin1.*)
       _lt_dar_allow_undefined='$wl-flat_namespace $wl-undefined ${wl}suppress' ;;
-    darwin*)
-      case ${MACOSX_DEPLOYMENT_TARGET},$host in
-	10.[012],*|,*powerpc*)
+    darwin*) # darwin 5.x on
+      # if running on 10.5 or later, the deployment target defaults
+      # to the OS version, if on x86, and 10.4, the deployment
+      # target defaults to 10.4. Don't you love it?
+      case ${MACOSX_DEPLOYMENT_TARGET-10.0},$host in
+	10.0,*86*-darwin8*|10.0,*-darwin[91]*)
+	  _lt_dar_allow_undefined='$wl-undefined ${wl}dynamic_lookup' ;;
+	10.[012][,.]*)
 	  _lt_dar_allow_undefined='$wl-flat_namespace $wl-undefined ${wl}suppress' ;;
-	*)
+	10.*)
 	  _lt_dar_allow_undefined='$wl-undefined ${wl}dynamic_lookup' ;;
       esac
     ;;
@@ -13976,6 +13981,45 @@ $as_echo "#define HAVE_SO_BINDTODEVICE 1" >>confdefs.h
 
 fi
 
+# Check for IP DF support
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking IP_MTU_DISCOVER or IP_DONTFRAG socket option" >&5
+$as_echo_n "checking IP_MTU_DISCOVER or IP_DONTFRAG socket option... " >&6; }
+if ${iperf3_cv_header_dontfragment+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <sys/socket.h>
+#include <netinet/ip.h>
+#include <netinet/in.h>
+#ifdef IP_MTU_DISCOVER
+  yes
+#endif
+#ifdef IP_DONTFRAG
+  yes
+#endif
+#ifdef IP_DONTFRAGMENT
+  yes
+#endif
+
+_ACEOF
+if (eval "$ac_cpp conftest.$ac_ext") 2>&5 |
+  $EGREP "yes" >/dev/null 2>&1; then :
+  iperf3_cv_header_dontfragment=yes
+else
+  iperf3_cv_header_dontfragment=no
+fi
+rm -f conftest*
+
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $iperf3_cv_header_dontfragment" >&5
+$as_echo "$iperf3_cv_header_dontfragment" >&6; }
+if test "x$iperf3_cv_header_dontfragment" = "xyes"; then
+
+$as_echo "#define HAVE_DONT_FRAGMENT 1" >>confdefs.h
+
+fi
+
 # Check if we need -lrt for clock_gettime
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for library containing clock_gettime" >&5
 $as_echo_n "checking for library containing clock_gettime... " >&6; }
diff --git a/configure.ac b/configure.ac
index 7e2b62fbb..596444aa7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1,4 +1,4 @@
-# iperf, Copyright (c) 2014-2020, The Regents of the University of
+# iperf, Copyright (c) 2014-2021, The Regents of the University of
 # California, through Lawrence Berkeley National Laboratory (subject
 # to receipt of any required approvals from the U.S. Dept. of
 # Energy).  All rights reserved.
@@ -237,6 +237,27 @@ if test "x$iperf3_cv_header_so_bindtodevice" = "xyes"; then
     AC_DEFINE([HAVE_SO_BINDTODEVICE], [1], [Have SO_BINDTODEVICE sockopt.])
 fi
 
+# Check for IP DF support
+AC_CACHE_CHECK([IP_MTU_DISCOVER or IP_DONTFRAG socket option],
+[iperf3_cv_header_dontfragment],
+AC_EGREP_CPP(yes,
+[#include <sys/socket.h>
+#include <netinet/ip.h>
+#include <netinet/in.h>
+#ifdef IP_MTU_DISCOVER
+  yes
+#endif
+#ifdef IP_DONTFRAG
+  yes
+#endif
+#ifdef IP_DONTFRAGMENT
+  yes
+#endif
+],iperf3_cv_header_dontfragment=yes,iperf3_cv_header_dontfragment=no))
+if test "x$iperf3_cv_header_dontfragment" = "xyes"; then
+    AC_DEFINE([HAVE_DONT_FRAGMENT], [1], [Have IP_MTU_DISCOVER/IP_DONTFRAG sockopt.])
+fi
+
 # Check if we need -lrt for clock_gettime
 AC_SEARCH_LIBS(clock_gettime, [rt posix4])
 # Check for clock_gettime support
diff --git a/src/iperf.h b/src/iperf.h
index c46001def..8a5427306 100644
--- a/src/iperf.h
+++ b/src/iperf.h
@@ -153,6 +153,7 @@ struct iperf_settings
     iperf_size_t blocks;            /* number of blocks (packets) to send */
     char      unit_format;          /* -f */
     int       num_ostreams;         /* SCTP initmsg settings */
+    int       dont_fragment;        /* Whether to set IP flag Do-Not_Fragment */
 #if defined(HAVE_SSL)
     char      *authtoken;           /* Authentication token */
     char      *client_username;
diff --git a/src/iperf3.1 b/src/iperf3.1
index aec3c0754..15b7640c7 100644
--- a/src/iperf3.1
+++ b/src/iperf3.1
@@ -1,4 +1,4 @@
-.TH IPERF3 1 "September 2020" ESnet "User Manuals"
+.TH IPERF3 1 "February 2021" ESnet "User Manuals"
 .SH NAME
 iperf3 \- perform network throughput tests
 .SH SYNOPSIS
@@ -397,6 +397,10 @@ It might help to test and reveal problems in networking gear with hardware
 compression (including some WiFi access points), where iperf2 and iperf3
 perform differently, just based on payload entropy.
 .TP
+.BR --dont-fragment
+Set the IPv4 Don't Fragment (DF) bit on outgoing packets.
+Only applicable to tests doing UDP over IPv4.
+.TP
 .BR --username " \fIusername\fR" 
 username to use for authentication to the iperf server (if built with
 OpenSSL support).
diff --git a/src/iperf_api.c b/src/iperf_api.c
index f9fca90ca..9bb0de3ae 100644
--- a/src/iperf_api.c
+++ b/src/iperf_api.c
@@ -394,6 +394,12 @@ iperf_get_test_idle_timeout(struct iperf_test *ipt)
     return ipt->settings->idle_timeout;
 }
 
+int
+iperf_get_dont_fragment(struct iperf_test *ipt)
+{
+    return ipt->settings->dont_fragment;
+}
+
 char*
 iperf_get_test_congestion_control(struct iperf_test* ipt)
 {
@@ -737,6 +743,12 @@ iperf_set_test_idle_timeout(struct iperf_test* ipt, int to)
     ipt->settings->idle_timeout = to;
 }
 
+void
+iperf_set_dont_fragment(struct iperf_test* ipt, int dnf)
+{
+    ipt->settings->dont_fragment = dnf;
+}
+
 void
 iperf_set_test_congestion_control(struct iperf_test* ipt, char* cc)
 {
@@ -968,6 +980,9 @@ iperf_parse_arguments(struct iperf_test *test, int argc, char **argv)
 	{"get-server-output", no_argument, NULL, OPT_GET_SERVER_OUTPUT},
 	{"udp-counters-64bit", no_argument, NULL, OPT_UDP_COUNTERS_64BIT},
  	{"no-fq-socket-pacing", no_argument, NULL, OPT_NO_FQ_SOCKET_PACING},
+#if defined(HAVE_DONT_FRAGMENT)
+	{"dont-fragment", no_argument, NULL, OPT_DONT_FRAGMENT},
+#endif /* HAVE_DONT_FRAGMENT */
 #if defined(HAVE_SSL)
     {"username", required_argument, NULL, OPT_CLIENT_USERNAME},
     {"rsa-public-key-path", required_argument, NULL, OPT_CLIENT_RSA_PUBLIC_KEY},
@@ -1381,6 +1396,12 @@ iperf_parse_arguments(struct iperf_test *test, int argc, char **argv)
 		return -1;
 #endif
 		break;
+#if defined(HAVE_DONT_FRAGMENT)
+        case OPT_DONT_FRAGMENT:
+            test->settings->dont_fragment = 1;
+            client_flag = 1;
+            break;
+#endif /* HAVE_DONT_FRAGMENT */
 #if defined(HAVE_SSL)
         case OPT_CLIENT_USERNAME:
             client_username = strdup(optarg);
@@ -1980,6 +2001,10 @@ send_parameters(struct iperf_test *test)
 	    cJSON_AddNumberToObject(j, "udp_counters_64bit", iperf_get_test_udp_counters_64bit(test));
 	if (test->repeating_payload)
 	    cJSON_AddNumberToObject(j, "repeating_payload", test->repeating_payload);
+#if defined(HAVE_DONT_FRAGMENT)
+	if (test->settings->dont_fragment)
+	    cJSON_AddNumberToObject(j, "dont_fragment", test->settings->dont_fragment);
+#endif /* HAVE_DONT_FRAGMENT */
 #if defined(HAVE_SSL)
 	/* Send authentication parameters */
 	if (test->settings->client_username && test->settings->client_password && test->settings->client_rsa_pubkey){
@@ -2088,6 +2113,10 @@ get_parameters(struct iperf_test *test)
 	    iperf_set_test_udp_counters_64bit(test, 1);
 	if ((j_p = cJSON_GetObjectItem(j, "repeating_payload")) != NULL)
 	    test->repeating_payload = 1;
+#if defined(HAVE_DONT_FRAGMENT)
+	if ((j_p = cJSON_GetObjectItem(j, "dont_fragment")) != NULL)
+	    test->settings->dont_fragment = j_p->valueint;
+#endif /* HAVE_DONT_FRAGMENT */
 #if defined(HAVE_SSL)
 	if ((j_p = cJSON_GetObjectItem(j, "authtoken")) != NULL)
         test->settings->authtoken = strdup(j_p->valuestring);
@@ -2883,6 +2912,7 @@ iperf_reset_test(struct iperf_test *test)
     test->settings->burst = 0;
     test->settings->mss = 0;
     test->settings->tos = 0;
+    test->settings->dont_fragment = 0;
 
 #if defined(HAVE_SSL)
     if (test->settings->authtoken) {
@@ -4063,6 +4093,45 @@ iperf_init_stream(struct iperf_stream *sp, struct iperf_test *test)
         }
     }
 
+#if defined(HAVE_DONT_FRAGMENT)
+    /* Set Don't Fragment (DF). Only applicable to IPv4/UDP tests. */
+    if (iperf_get_test_protocol_id(test) == Pudp &&
+        getsockdomain(sp->socket) == AF_INET &&
+        iperf_get_dont_fragment(test)) {
+
+        /*
+         * There are multiple implementations of this feature depending on the OS.
+         * We need to handle separately Linux, UNIX, and Windows, as well as
+         * the case that DF isn't supported at all (such as on macOS).
+         */
+#if defined(IP_MTU_DISCOVER) /* Linux version of IP_DONTFRAG */
+        opt = IP_PMTUDISC_DO;
+        if (setsockopt(sp->socket, IPPROTO_IP, IP_MTU_DISCOVER, &opt, sizeof(opt)) < 0) {
+            i_errno = IESETDONTFRAGMENT;
+            return -1;
+        }
+#else
+#if defined(IP_DONTFRAG) /* UNIX does IP_DONTFRAG */
+        opt = 1;
+        if (setsockopt(sp->socket, IPPROTO_IP, IP_DONTFRAG, &opt, sizeof(opt)) < 0) {
+            i_errno = IESETDONTFRAGMENT;
+            return -1;
+        }
+#else
+#if defined(IP_DONTFRAGMENT) /* Windows does IP_DONTFRAGMENT */
+        opt = 1;
+        if (setsockopt(sp->socket, IPPROTO_IP, IP_DONTFRAGMENT, &opt, sizeof(opt)) < 0) {
+            i_errno = IESETDONTFRAGMENT;
+            return -1;
+        }
+#else
+	i_errno = IESETDONTFRAGMENT;
+	return -1;
+#endif /* IP_DONTFRAGMENT */
+#endif /* IP_DONTFRAG */
+#endif /* IP_MTU_DISCOVER */
+    }
+#endif /* HAVE_DONT_FRAGMENT */
     return 0;
 }
 
diff --git a/src/iperf_api.h b/src/iperf_api.h
index c35f6eecd..432599183 100644
--- a/src/iperf_api.h
+++ b/src/iperf_api.h
@@ -82,6 +82,7 @@ typedef uint64_t iperf_size_t;
 #define OPT_SERVER_SKEW_THRESHOLD 23
 #define OPT_BIND_DEV 24
 #define OPT_IDLE_TIMEOUT 25
+#define OPT_DONT_FRAGMENT 26
 
 /* states */
 #define TEST_START 1
@@ -140,6 +141,7 @@ char*	iperf_get_extra_data( struct iperf_test* ipt );
 char*	iperf_get_iperf_version(void);
 int	iperf_get_test_no_delay( struct iperf_test* ipt );
 int	iperf_get_test_connect_timeout( struct iperf_test* ipt );
+int	iperf_get_dont_fragment( struct iperf_test* ipt );
 char*   iperf_get_test_congestion_control(struct iperf_test* ipt);
 
 /* Setter routines for some fields inside iperf_test. */
@@ -178,6 +180,7 @@ void    iperf_set_test_tos( struct iperf_test* ipt, int tos );
 void	iperf_set_test_extra_data( struct iperf_test* ipt, const char *dat );
 void    iperf_set_test_bidirectional( struct iperf_test* ipt, int bidirectional);
 void    iperf_set_test_no_delay( struct iperf_test* ipt, int no_delay);
+void    iperf_set_dont_fragment( struct iperf_test* ipt, int dont_fragment );
 void    iperf_set_test_congestion_control(struct iperf_test* ipt, char* cc);
 
 #if defined(HAVE_SSL)
@@ -415,6 +418,7 @@ enum {
     IEAUTHTEST = 142,       // Test authorization failed
     IEBINDDEV = 143,        // Unable to bind-to-device (check perror, maybe permissions?)
     IENOMSG = 144,          // No message was received for NO_MSG_RCVD_TIMEOUT time period
+    IESETDONTFRAGMENT = 145,    // Unable to set IP Do-Not-Fragment
     /* Stream errors */
     IECREATESTREAM = 200,   // Unable to create a new stream (check herror/perror)
     IEINITSTREAM = 201,     // Unable to initialize stream (check herror/perror)
diff --git a/src/iperf_config.h.in b/src/iperf_config.h.in
index a8aec642a..d78af669a 100644
--- a/src/iperf_config.h.in
+++ b/src/iperf_config.h.in
@@ -15,6 +15,9 @@
 /* Define to 1 if you have the <dlfcn.h> header file. */
 #undef HAVE_DLFCN_H
 
+/* Have IP_MTU_DISCOVER/IP_DONTFRAG sockopt. */
+#undef HAVE_DONT_FRAGMENT
+
 /* Define to 1 if you have the <endian.h> header file. */
 #undef HAVE_ENDIAN_H
 
diff --git a/src/iperf_error.c b/src/iperf_error.c
index 1769a7f54..0775ba2c3 100644
--- a/src/iperf_error.c
+++ b/src/iperf_error.c
@@ -433,6 +433,9 @@ iperf_strerror(int int_errno)
 	case IENOMSG:
 	    snprintf(errstr, len, "no message was received for %d seconds", NO_MSG_RCVD_TIMEOUT);
             break;
+    case IESETDONTFRAGMENT:
+	    snprintf(errstr, len, "unable to set IP Do-Not-Fragment flag");
+            break;
 	default:
 	    snprintf(errstr, len, "int_errno=%d", int_errno);
 	    perr = 1;
diff --git a/src/iperf_locale.c b/src/iperf_locale.c
index dce0f0b1c..9a74fc498 100644
--- a/src/iperf_locale.c
+++ b/src/iperf_locale.c
@@ -189,6 +189,9 @@ const char usage_longstr[] = "Usage: iperf3 [-s|-c host] [options]\n"
                            "  --udp-counters-64bit      use 64-bit counters in UDP test packets\n"
                            "  --repeating-payload       use repeating pattern in payload, instead of\n"
                            "                            randomized payload (like in iperf2)\n"
+#if defined(HAVE_DONT_FRAGMENT)
+                           "  --dont-fragment           set IPv4 Don't Fragment flag\n"
+#endif /* HAVE_DONT_FRAGMENT */
 #if defined(HAVE_SSL)
                            "  --username                username for authentication\n"
                            "  --rsa-public-key-path     path to the RSA public key used to encrypt\n"
diff --git a/src/iperf_util.c b/src/iperf_util.c
index d54cdf284..8155270ba 100644
--- a/src/iperf_util.c
+++ b/src/iperf_util.c
@@ -327,6 +327,16 @@ get_optional_features(void)
     numfeatures++;
 #endif /* HAVE_SO_BINDTODEVICE */
 
+#if defined(HAVE_DONT_FRAGMENT)
+    if (numfeatures > 0) {
+	strncat(features, ", ",
+		sizeof(features) - strlen(features) - 1);
+    }
+    strncat(features, "support IPv4 don't fragment",
+	sizeof(features) - strlen(features) - 1);
+    numfeatures++;
+#endif /* HAVE_DONT_FRAGMENT */
+
     if (numfeatures == 0) {
 	strncat(features, "None", 
 		sizeof(features) - strlen(features) - 1);