fix: Prefer legacy iptables backend for NAT (#1246)

src/network.sh

@@ -879,7 +879,11 @@ configureTables() {
   local tables_err="failed to configure IP tables!"
   local tables="the 'ip_tables' kernel module is not loaded. Try this command: sudo modprobe ip_tables iptable_nat"
 
-  clearTables
+  if ! clearTables; then
+    enabled "$ROOTLESS" && ! enabled "$DEBUG" && return 1
+    warn "failed to select a working IP tables backend!"
+    return 1
+  fi
 
   if [ -n "$exclude" ]; then
     if [[ "$exclude" != *","* ]]; then
@@ -960,10 +964,22 @@ configureTables() {
     warn "$tables_err" && return 1
   fi
 
-  # Allow forwarding from dev -> bridge
+  # Allow newly DNATed connections to the guest
   if ! iptables -A FORWARD \
     -i "$DEV" \
     -o "$BRIDGE" \
+    -d "$ip" \
+    -m conntrack --ctstate NEW --ctstatus DNAT \
+    -m comment --comment "$rule_tag" \
+    -j ACCEPT; then
+    warn "$tables_err" && return 1
+  fi
+
+  # Allow return and related traffic
+  if ! iptables -A FORWARD \
+    -i "$DEV" \
+    -o "$BRIDGE" \
+    -m conntrack --ctstate RELATED,ESTABLISHED \
     -m comment --comment "$rule_tag" \
     -j ACCEPT; then
     warn "$tables_err" && return 1
@@ -1048,20 +1064,34 @@ configureNAT() {
 #  Cleanup
 # ######################################
 
+selectTables() {
+
+  local backend=""
+
+  # Prefer the legacy backend when available, but fall back to nftables
+  # on systems where the legacy backend is unavailable.
+  if command -v iptables-legacy >/dev/null 2>&1 && iptables-legacy -V >/dev/null 2>&1; then
+    backend="legacy"
+  elif command -v iptables-nft >/dev/null 2>&1 && iptables-nft -V >/dev/null 2>&1; then
+    backend="nft"
+  else
+    return 1
+  fi
+
+  if ! update-alternatives --set iptables "$(command -v "iptables-$backend")" > /dev/null 2>&1; then
+    return 1
+  fi
+
+  return 0
+}
+
 clearTables() {
 
   local table="" line rules
   local rule_tag="remove"
   local re="--comment[[:space:]]+\"?$rule_tag\"?([[:space:]]|\$)"
 
-  # Choose between iptables or nftables
-  if command -v iptables-nft >/dev/null 2>&1 && iptables-nft -V >/dev/null 2>&1; then
-    update-alternatives --set iptables /usr/sbin/iptables-nft > /dev/null
-    update-alternatives --set ip6tables /usr/sbin/ip6tables-nft > /dev/null
-  else
-    update-alternatives --set iptables /usr/sbin/iptables-legacy > /dev/null
-    update-alternatives --set ip6tables /usr/sbin/ip6tables-legacy > /dev/null
-  fi
+  selectTables || return 1
 
   # Store the current iptables ruleset
   ! rules=$(iptables-save 2> /dev/null) && return 0
@@ -1097,7 +1127,7 @@ closeBridge() {
   ip link set "$BRIDGE" down &> /dev/null || :
   ip link delete "$BRIDGE" &> /dev/null || :
 
-  clearTables
+  clearTables || :
   return 0
 }