diff --git a/docs/interop/nbd.txt b/docs/interop/nbd.txt
index bdb0f2a41aca..6c99070b99c8 100644
--- a/docs/interop/nbd.txt
+++ b/docs/interop/nbd.txt
@@ -68,3 +68,4 @@ NBD_CMD_BLOCK_STATUS for "qemu:dirty-bitmap:", NBD_CMD_CACHE
 * 4.2: NBD_FLAG_CAN_MULTI_CONN for shareable read-only exports,
 NBD_CMD_FLAG_FAST_ZERO
 * 5.2: NBD_CMD_BLOCK_STATUS for "qemu:allocation-depth"
+* 7.0: NBD_FLAG_CAN_MULTI_CONN for shareable writable exports
diff --git a/docs/tools/qemu-nbd.rst b/docs/tools/qemu-nbd.rst
index 4c950f61998e..229414e3cdc7 100644
--- a/docs/tools/qemu-nbd.rst
+++ b/docs/tools/qemu-nbd.rst
@@ -139,8 +139,18 @@ driver options if :option:`--image-opts` is specified.
 .. option:: -e, --shared=NUM

   Allow up to *NUM* clients to share the device (default
-  ``1``), 0 for unlimited. Safe for readers, but for now,
-  consistency is not guaranteed between multiple writers.
+  ``1``), 0 for unlimited.
+
+.. option:: -m, --multi-conn=MODE
+
+  When multiple clients are allowed at once (via :option:`--shared`),
+  controls whether the NBD protocol MULTI_CONN bit is advertised to
+  clients.  This defaults to ``auto``, which advertises the bit for
+  readonly connections (:option:`--read-only`) but not for writeable
+  connections; it can also be set to ``on`` or ``off`` to override the
+  default.  Advertising this bit informs clients whether a flush from
+  one client has guaranteed consistency to subsequent access from a
+  parallel client.

 .. option:: -t, --persistent

diff --git a/qapi/block-export.json b/qapi/block-export.json
index f183522d0d2c..d9d38620156f 100644
--- a/qapi/block-export.json
+++ b/qapi/block-export.json
@@ -21,7 +21,9 @@
 #             recreated on the fly while the NBD server is active.
 #             If missing, it will default to denying access (since 4.0).
 # @max-connections: The maximum number of connections to allow at the same
-#                   time, 0 for unlimited. (since 5.2; default: 0)
+#                   time, 0 for unlimited. Setting this to 1 also stops
+#                   the server from advertising multiple client support
+#                   (since 5.2; default: 0)
 #
 # Since: 4.2
 ##
@@ -50,7 +52,9 @@
 #             recreated on the fly while the NBD server is active.
 #             If missing, it will default to denying access (since 4.0).
 # @max-connections: The maximum number of connections to allow at the same
-#                   time, 0 for unlimited. (since 5.2; default: 0)
+#                   time, 0 for unlimited. Setting this to 1 also stops
+#                   the server from advertising multiple client support
+#                   (since 5.2; default: 0).
 #
 # Returns: error if the server is already running.
 #
@@ -95,11 +99,20 @@
 #                    the metadata context name "qemu:allocation-depth" to
 #                    inspect allocation details. (since 5.2)
 #
+# @multi-conn: Controls whether multiple client support is advertised, if the
+#              server's max-connections is not 1. "auto" advertises support
+#              for read-only clients, but not for writeable (although this
+#              may be improved in the future for writeable devices that are
+#              known to be cache-consistent, such as the raw file driver),
+#              while "on" and "off" explicitly set the bit (default auto,
+#              since 7.0).
+#
 # Since: 5.2
 ##
 { 'struct': 'BlockExportOptionsNbd',
   'base': 'BlockExportOptionsNbdBase',
-  'data': { '*bitmaps': ['str'], '*allocation-depth': 'bool' } }
+  'data': { '*bitmaps': ['str'], '*allocation-depth': 'bool',
+            '*multi-conn': 'OnOffAuto' } }

 ##
 # @BlockExportOptionsVhostUserBlk:
diff --git a/include/block/nbd.h b/include/block/nbd.h
index c5a29ce1c61b..c74b7a9d2e6e 100644
--- a/include/block/nbd.h
+++ b/include/block/nbd.h
@@ -1,5 +1,5 @@
 /*
- *  Copyright (C) 2016-2020 Red Hat, Inc.
+ *  Copyright (C) 2016-2022 Red Hat, Inc.
  *  Copyright (C) 2005  Anthony Liguori <anthony@codemonkey.ws>
  *
  *  Network Block Device
@@ -346,6 +346,7 @@ void nbd_client_put(NBDClient *client);

 void nbd_server_is_qemu_nbd(int max_connections);
 bool nbd_server_is_running(void);
+int nbd_server_max_connections(void);
 void nbd_server_start(SocketAddress *addr, const char *tls_creds,
                       const char *tls_authz, uint32_t max_connections,
                       Error **errp);
diff --git a/blockdev-nbd.c b/blockdev-nbd.c
index add41a23af43..15ad7b8dbfd5 100644
--- a/blockdev-nbd.c
+++ b/blockdev-nbd.c
@@ -44,6 +44,11 @@ bool nbd_server_is_running(void)
     return nbd_server || qemu_nbd_connections >= 0;
 }

+int nbd_server_max_connections(void)
+{
+    return nbd_server ? nbd_server->max_connections : -1;
+}
+
 static void nbd_blockdev_client_closed(NBDClient *client, bool ignored)
 {
     nbd_client_put(client);
diff --git a/nbd/server.c b/nbd/server.c
index 5da884c2fc35..2cfdd56cda19 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -1,5 +1,5 @@
 /*
- *  Copyright (C) 2016-2021 Red Hat, Inc.
+ *  Copyright (C) 2016-2022 Red Hat, Inc.
  *  Copyright (C) 2005  Anthony Liguori <anthony@codemonkey.ws>
  *
  *  Network Block Device Server Side
@@ -1642,7 +1642,7 @@ static int nbd_export_create(BlockExport *blk_exp, BlockExportOptions *exp_args,
     int64_t size;
     uint64_t perm, shared_perm;
     bool readonly = !exp_args->writable;
-    bool shared = !exp_args->writable;
+    bool multi_conn;
     strList *bitmaps;
     size_t i;
     int ret;
@@ -1680,6 +1680,23 @@ static int nbd_export_create(BlockExport *blk_exp, BlockExportOptions *exp_args,
         return size;
     }

+    /*
+     * Determine whether to advertise multi-conn.  Default is auto,
+     * which resolves to on for read-only and off for writable.  But
+     * if the server has max-connections 1, that forces the flag off.
+     */
+    if (!arg->has_multi_conn) {
+        arg->multi_conn = ON_OFF_AUTO_AUTO;
+    }
+    if (nbd_server_max_connections() == 1) {
+        arg->multi_conn = ON_OFF_AUTO_OFF;
+    }
+    if (arg->multi_conn == ON_OFF_AUTO_AUTO) {
+        multi_conn = readonly;
+    } else {
+        multi_conn = arg->multi_conn == ON_OFF_AUTO_ON;
+    }
+
     /* Don't allow resize while the NBD server is running, otherwise we don't
      * care what happens with the node. */
     blk_get_perm(blk, &perm, &shared_perm);
@@ -1693,11 +1710,11 @@ static int nbd_export_create(BlockExport *blk_exp, BlockExportOptions *exp_args,
     exp->description = g_strdup(arg->description);
     exp->nbdflags = (NBD_FLAG_HAS_FLAGS | NBD_FLAG_SEND_FLUSH |
                      NBD_FLAG_SEND_FUA | NBD_FLAG_SEND_CACHE);
+    if (multi_conn) {
+        exp->nbdflags |= NBD_FLAG_CAN_MULTI_CONN;
+    }
     if (readonly) {
         exp->nbdflags |= NBD_FLAG_READ_ONLY;
-        if (shared) {
-            exp->nbdflags |= NBD_FLAG_CAN_MULTI_CONN;
-        }
     } else {
         exp->nbdflags |= (NBD_FLAG_SEND_TRIM | NBD_FLAG_SEND_WRITE_ZEROES |
                           NBD_FLAG_SEND_FAST_ZERO);
diff --git a/qemu-nbd.c b/qemu-nbd.c
index 8c25ae93df83..ddeb2fcf7f40 100644
--- a/qemu-nbd.c
+++ b/qemu-nbd.c
@@ -70,6 +70,7 @@
 #define QEMU_NBD_OPT_PID_FILE      265
 #define QEMU_NBD_OPT_SELINUX_LABEL 266
 #define QEMU_NBD_OPT_TLSHOSTNAME   267
+#define QEMU_NBD_OPT_MULTI_CONN    268

 #define MBR_SIZE 512

@@ -79,6 +80,7 @@ static SocketAddress *saddr;
 static int persistent = 0;
 static enum { RUNNING, TERMINATE, TERMINATED } state;
 static int shared = 1;
+static OnOffAuto multi_conn = ON_OFF_AUTO_AUTO;
 static int nb_fds;
 static QIONetListener *server;
 static QCryptoTLSCreds *tlscreds;
@@ -100,6 +102,8 @@ static void usage(const char *name)
 "  -k, --socket=PATH         path to the unix socket\n"
 "                            (default '"SOCKET_PATH"')\n"
 "  -e, --shared=NUM          device can be shared by NUM clients (default '1')\n"
+"  -m, --multi-conn=MODE     control multi-conn advertisement with -e, MODE is\n"
+"                            'on', 'off', or 'auto' (default based on -r)\n"
 "  -t, --persistent          don't exit on the last connection\n"
 "  -v, --verbose             display extra debugging information\n"
 "  -x, --export-name=NAME    expose export by name (default is empty string)\n"
@@ -513,7 +517,7 @@ int main(int argc, char **argv)
     char *device = NULL;
     QemuOpts *sn_opts = NULL;
     const char *sn_id_or_name = NULL;
-    const char *sopt = "hVb:o:p:rsnc:dvk:e:f:tl:x:T:D:AB:L";
+    const char *sopt = "hVb:o:p:rsnc:dvk:e:m:f:tl:x:T:D:AB:L";
     struct option lopt[] = {
         { "help", no_argument, NULL, 'h' },
         { "version", no_argument, NULL, 'V' },
@@ -536,6 +540,7 @@ int main(int argc, char **argv)
         { "detect-zeroes", required_argument, NULL,
           QEMU_NBD_OPT_DETECT_ZEROES },
         { "shared", required_argument, NULL, 'e' },
+        { "multi-conn", required_argument, NULL, 'm' },
         { "format", required_argument, NULL, 'f' },
         { "persistent", no_argument, NULL, 't' },
         { "verbose", no_argument, NULL, 'v' },
@@ -709,6 +714,17 @@ int main(int argc, char **argv)
                 exit(EXIT_FAILURE);
             }
             break;
+        case 'm':
+        {
+            Error *err = NULL;
+            multi_conn = qapi_enum_parse(&OnOffAuto_lookup, optarg,
+                                         ON_OFF_AUTO_AUTO, &err);
+            if (err) {
+                error_report_err(err);
+                exit(EXIT_FAILURE);
+            }
+            break;
+        }
         case 'f':
             fmt = optarg;
             break;
@@ -1107,6 +1123,8 @@ int main(int argc, char **argv)
             .bitmaps              = bitmaps,
             .has_allocation_depth = alloc_depth,
             .allocation_depth     = alloc_depth,
+            .has_multi_conn       = true,
+            .multi_conn           = multi_conn,
         },
     };
     blk_exp_add(export_opts, &error_fatal);
diff --git a/MAINTAINERS b/MAINTAINERS
index f2e9ce1da2ac..678c34e8077f 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3362,6 +3362,7 @@ F: qemu-nbd.*
 F: blockdev-nbd.c
 F: docs/interop/nbd.txt
 F: docs/tools/qemu-nbd.rst
+F: tests/qemu-iotests/tests/*nbd*
 T: git https://repo.or.cz/qemu/ericb.git nbd
 T: git https://src.openvz.org/scm/~vsementsov/qemu.git nbd

diff --git a/tests/qemu-iotests/tests/nbd-multiconn b/tests/qemu-iotests/tests/nbd-multiconn
new file mode 100755
index 000000000000..7d1179b33b05
--- /dev/null
+++ b/tests/qemu-iotests/tests/nbd-multiconn
@@ -0,0 +1,157 @@
+#!/usr/bin/env python3
+# group: rw auto quick
+#
+# Test cases for NBD multi-conn advertisement
+#
+# Copyright (C) 2022 Red Hat, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import os
+from contextlib import contextmanager
+import iotests
+from iotests import qemu_img_create, qemu_io_silent
+
+
+disk = os.path.join(iotests.test_dir, 'disk')
+size = '4M'
+nbd_sock = os.path.join(iotests.sock_dir, 'nbd_sock')
+nbd_uri = 'nbd+unix:///{}?socket=' + nbd_sock
+
+
+@contextmanager
+def open_nbd(export_name):
+    h = nbd.NBD()
+    try:
+        h.connect_uri(nbd_uri.format(export_name))
+        yield h
+    finally:
+        h.shutdown()
+
+class TestNbdMulticonn(iotests.QMPTestCase):
+    def setUp(self):
+        assert qemu_img_create('-f', iotests.imgfmt, disk, size) == 0
+        assert qemu_io_silent('-c', 'w -P 1 0 2M',
+                              '-c', 'w -P 2 2M 2M', disk) == 0
+
+        self.vm = iotests.VM()
+        self.vm.launch()
+        result = self.vm.qmp('blockdev-add', {
+            'driver': 'qcow2',
+            'node-name': 'n',
+            'file': {'driver': 'file', 'filename': disk}
+        })
+        self.assert_qmp(result, 'return', {})
+
+    def tearDown(self):
+        self.vm.shutdown()
+        os.remove(disk)
+        try:
+            os.remove(nbd_sock)
+        except OSError:
+            pass
+
+    @contextmanager
+    def run_server(self, max_connections=None):
+        args = {
+            'addr': {
+                'type': 'unix',
+                'data': {'path': nbd_sock}
+            }
+        }
+        if max_connections is not None:
+            args['max-connections'] = max_connections
+
+        result = self.vm.qmp('nbd-server-start', args)
+        self.assert_qmp(result, 'return', {})
+        yield
+
+        result = self.vm.qmp('nbd-server-stop')
+        self.assert_qmp(result, 'return', {})
+
+    def add_export(self, name, writable=None, multi_conn=None):
+        args = {
+            'type': 'nbd',
+            'id': name,
+            'node-name': 'n',
+            'name': name,
+        }
+        if writable is not None:
+            args['writable'] = writable
+        if multi_conn is not None:
+            args['multi-conn'] = multi_conn
+
+        result = self.vm.qmp('block-export-add', args)
+        self.assert_qmp(result, 'return', {})
+
+    def test_default_settings(self):
+        with self.run_server():
+            self.add_export('r')
+            self.add_export('w', writable=True)
+            with open_nbd('r') as h:
+                self.assertTrue(h.can_multi_conn())
+            with open_nbd('w') as h:
+                self.assertFalse(h.can_multi_conn())
+
+    def test_multiconn_option(self):
+        with self.run_server():
+            self.add_export('r', multi_conn='off')
+            self.add_export('w', writable=True, multi_conn='on')
+            with open_nbd('r') as h:
+                self.assertFalse(h.can_multi_conn())
+            with open_nbd('w') as h:
+                self.assertTrue(h.can_multi_conn())
+
+    def test_limited_connections(self):
+        with self.run_server(max_connections=1):
+            self.add_export('r', multi_conn='on')
+            self.add_export('w', writable=True, multi_conn='on')
+            with open_nbd('r') as h:
+                self.assertFalse(h.can_multi_conn())
+            with open_nbd('w') as h:
+                self.assertFalse(h.can_multi_conn())
+
+    def test_parallel_writes(self):
+        with self.run_server():
+            self.add_export('w', writable=True, multi_conn='on')
+
+            clients = [nbd.NBD() for _ in range(3)]
+            for c in clients:
+                c.connect_uri(nbd_uri.format('w'))
+                assert c.can_multi_conn()
+
+            initial_data = clients[0].pread(1024 * 1024, 0)
+            self.assertEqual(initial_data, b'\x01' * 1024 * 1024)
+
+            updated_data = b'\x03' * 1024 * 1024
+            clients[1].pwrite(updated_data, 0)
+            clients[2].flush()
+            current_data = clients[0].pread(1024 * 1024, 0)
+
+            self.assertEqual(updated_data, current_data)
+
+            for i in range(3):
+                clients[i].shutdown()
+
+
+if __name__ == '__main__':
+    try:
+        # Easier to use libnbd than to try and set up parallel
+        # 'qemu-nbd --list' or 'qemu-io' processes, but not all systems
+        # have libnbd installed.
+        import nbd  # type: ignore
+
+        iotests.main(supported_fmts=['qcow2'])
+    except ImportError:
+        iotests.notrun('libnbd not installed')
diff --git a/tests/qemu-iotests/tests/nbd-multiconn.out b/tests/qemu-iotests/tests/nbd-multiconn.out
new file mode 100644
index 000000000000..89968f35d783
--- /dev/null
+++ b/tests/qemu-iotests/tests/nbd-multiconn.out
@@ -0,0 +1,5 @@
+....
+----------------------------------------------------------------------
+Ran 4 tests
+
+OK
