diff -Nru iptables-persistent-1.0.15/debian/changelog iptables-persistent-1.0.15ubuntu1/debian/changelog
--- iptables-persistent-1.0.15/debian/changelog	2021-02-14 04:49:44.000000000 -0300
+++ iptables-persistent-1.0.15ubuntu1/debian/changelog	2021-12-10 09:49:39.000000000 -0300
@@ -1,3 +1,12 @@
+iptables-persistent (1.0.15ubuntu1) hirsute; urgency=medium
+
+  * Add configuration options IP[6]TABLES_RESTORE_NOFLUSH so that
+    existing rules are not flushed on start/load (LP: #1949643)
+    - debian/netfilter-persistent.default: Introduce options
+    - plugins/{15-ip4tables,25-ip6tables}: Check/apply options
+
+ -- Jorge Merlino <jorge.merlino@canonical.com>  Fri, 10 Dec 2021 09:49:39 -0300
+
 iptables-persistent (1.0.15) unstable; urgency=medium
 
   * [b89d12] Fix warning message on the ipset plugin
diff -Nru iptables-persistent-1.0.15/debian/netfilter-persistent.default iptables-persistent-1.0.15ubuntu1/debian/netfilter-persistent.default
--- iptables-persistent-1.0.15/debian/netfilter-persistent.default	2021-02-14 04:27:46.000000000 -0300
+++ iptables-persistent-1.0.15ubuntu1/debian/netfilter-persistent.default	2021-12-10 09:49:39.000000000 -0300
@@ -8,3 +8,9 @@
 # IPTABLES_SKIP_SAVE=yes
 # IP6TABLES_SKIP_SAVE=yes
 # IPSET_SKIP_SAVE=yes
+
+
+# Set to yes for not flushing existing ip[6]tables rules when netfilter-persistent
+# is called with the start parameter
+# IPTABLES_RESTORE_NOFLUSH=yes
+# IP6TABLES_RESTORE_NOFLUSH=yes
diff -Nru iptables-persistent-1.0.15/plugins/15-ip4tables iptables-persistent-1.0.15ubuntu1/plugins/15-ip4tables
--- iptables-persistent-1.0.15/plugins/15-ip4tables	2021-02-14 04:27:46.000000000 -0300
+++ iptables-persistent-1.0.15ubuntu1/plugins/15-ip4tables	2021-12-10 09:49:39.000000000 -0300
@@ -21,11 +21,17 @@
 
 load_rules()
 {
+    if [ "${IPTABLES_RESTORE_NOFLUSH}x" = "yesx" ]; then
+        NOFLUSH='--noflush'
+    else
+        NOFLUSH=''
+    fi
+
     #load IPv4 rules
     if [ ! -f /etc/iptables/rules.v4 ]; then
         echo "Warning: skipping IPv4 (no rules to load)"
     else
-        iptables-restore < /etc/iptables/rules.v4
+        iptables-restore $NOFLUSH < /etc/iptables/rules.v4
     fi
 }
 
diff -Nru iptables-persistent-1.0.15/plugins/25-ip6tables iptables-persistent-1.0.15ubuntu1/plugins/25-ip6tables
--- iptables-persistent-1.0.15/plugins/25-ip6tables	2021-02-14 04:27:46.000000000 -0300
+++ iptables-persistent-1.0.15ubuntu1/plugins/25-ip6tables	2021-12-10 09:49:39.000000000 -0300
@@ -17,19 +17,30 @@
 # Exit fast if IPv6 is disabled
 test -e /proc/sys/net/ipv6 || exit 0
 
+# Source configuration
+if [ -f "/etc/default/netfilter-persistent" ]; then
+    . /etc/default/netfilter-persistent
+fi
+
 load_rules()
 {
+    if [ "${IP6TABLES_RESTORE_NOFLUSH}x" = "yesx" ]; then
+        NOFLUSH='--noflush'
+    else
+        NOFLUSH=''
+    fi
+
     #load IPv6 rules
     if [ ! -f /etc/iptables/rules.v6 ]; then
         echo "Warning: skipping IPv6 (no rules to load)"
     else
-        ip6tables-restore < /etc/iptables/rules.v6
+        ip6tables-restore $NOFLUSH < /etc/iptables/rules.v6
     fi
 }
 
 save_rules()
 {
-    if [ ! "${IPTABLES_SKIP_SAVE}x" = "yesx" ]; then
+    if [ ! "${IP6TABLES_SKIP_SAVE}x" = "yesx" ]; then
         touch /etc/iptables/rules.v6
         ip6tables-save > /etc/iptables/rules.v6
         chmod 0640 /etc/iptables/rules.v6