diff --git a/vpnsetup.sh b/vpnsetup.sh
index f3ab5f5..814b092 100755
--- a/vpnsetup.sh
+++ b/vpnsetup.sh
@@ -280,7 +280,7 @@ run_setup() {
       VPN_DNS_NAME="$VPN_DNS_NAME" VPN_CLIENT_NAME="$VPN_CLIENT_NAME" \
       VPN_PROTECT_CONFIG="$VPN_PROTECT_CONFIG" \
       VPN_CLIENT_VALIDITY="$VPN_CLIENT_VALIDITY" \
-      VPN_SKIP_IKEV2="$VPN_SKIP_IKEV2" \
+      VPN_SKIP_IKEV2="$VPN_SKIP_IKEV2" VPN_SWAN_VER="$VPN_SWAN_VER" \
       /bin/bash "$tmpdir/vpn.sh" || status=1
     else
       status=1
diff --git a/vpnsetup_alpine.sh b/vpnsetup_alpine.sh
index 9c52393..2185fbc 100755
--- a/vpnsetup_alpine.sh
+++ b/vpnsetup_alpine.sh
@@ -248,6 +248,18 @@ get_swan_ver() {
   if printf '%s' "$swan_ver_latest" | grep -Eq '^([3-9]|[1-9][0-9]{1,2})(\.([0-9]|[1-9][0-9]{1,2})){1,2}$'; then
     SWAN_VER="$swan_ver_latest"
   fi
+  if [ -n "$VPN_SWAN_VER" ]; then
+    if ! printf '%s\n%s' "4.15" "$VPN_SWAN_VER" | sort -C -V \
+      || ! printf '%s\n%s' "$VPN_SWAN_VER" "$SWAN_VER" | sort -C -V; then
+cat 1>&2 <<EOF
+Error: Libreswan version '$VPN_SWAN_VER' is not supported.
+       This script can install Libreswan 4.15+ or $SWAN_VER.
+EOF
+      exit 1
+    else
+      SWAN_VER="$VPN_SWAN_VER"
+    fi
+  fi
 }
 
 check_libreswan() {
diff --git a/vpnsetup_amzn.sh b/vpnsetup_amzn.sh
index 4def381..5403bc0 100755
--- a/vpnsetup_amzn.sh
+++ b/vpnsetup_amzn.sh
@@ -266,6 +266,18 @@ get_swan_ver() {
   if printf '%s' "$swan_ver_latest" | grep -Eq '^([3-9]|[1-9][0-9]{1,2})(\.([0-9]|[1-9][0-9]{1,2})){1,2}$'; then
     SWAN_VER="$swan_ver_latest"
   fi
+  if [ -n "$VPN_SWAN_VER" ]; then
+    if ! printf '%s\n%s' "4.15" "$VPN_SWAN_VER" | sort -C -V \
+      || ! printf '%s\n%s' "$VPN_SWAN_VER" "$SWAN_VER" | sort -C -V; then
+cat 1>&2 <<EOF
+Error: Libreswan version '$VPN_SWAN_VER' is not supported.
+       This script can install Libreswan 4.15+ or $SWAN_VER.
+EOF
+      exit 1
+    else
+      SWAN_VER="$VPN_SWAN_VER"
+    fi
+  fi
 }
 
 check_libreswan() {
diff --git a/vpnsetup_centos.sh b/vpnsetup_centos.sh
index dc2b5e5..87a81c7 100755
--- a/vpnsetup_centos.sh
+++ b/vpnsetup_centos.sh
@@ -368,6 +368,18 @@ get_swan_ver() {
   if printf '%s' "$swan_ver_latest" | grep -Eq '^([3-9]|[1-9][0-9]{1,2})(\.([0-9]|[1-9][0-9]{1,2})){1,2}$'; then
     SWAN_VER="$swan_ver_latest"
   fi
+  if [ -n "$VPN_SWAN_VER" ]; then
+    if ! printf '%s\n%s' "4.15" "$VPN_SWAN_VER" | sort -C -V \
+      || ! printf '%s\n%s' "$VPN_SWAN_VER" "$SWAN_VER" | sort -C -V; then
+cat 1>&2 <<EOF
+Error: Libreswan version '$VPN_SWAN_VER' is not supported.
+       This script can install Libreswan 4.15+ or $SWAN_VER.
+EOF
+      exit 1
+    else
+      SWAN_VER="$VPN_SWAN_VER"
+    fi
+  fi
 }
 
 check_libreswan() {
diff --git a/vpnsetup_ubuntu.sh b/vpnsetup_ubuntu.sh
index 85bf866..86899cf 100755
--- a/vpnsetup_ubuntu.sh
+++ b/vpnsetup_ubuntu.sh
@@ -321,6 +321,18 @@ get_swan_ver() {
   if printf '%s' "$swan_ver_latest" | grep -Eq '^([3-9]|[1-9][0-9]{1,2})(\.([0-9]|[1-9][0-9]{1,2})){1,2}$'; then
     SWAN_VER="$swan_ver_latest"
   fi
+  if [ -n "$VPN_SWAN_VER" ]; then
+    if ! printf '%s\n%s' "4.15" "$VPN_SWAN_VER" | sort -C -V \
+      || ! printf '%s\n%s' "$VPN_SWAN_VER" "$SWAN_VER" | sort -C -V; then
+cat 1>&2 <<EOF
+Error: Libreswan version '$VPN_SWAN_VER' is not supported.
+       This script can install Libreswan 4.15+ or $SWAN_VER.
+EOF
+      exit 1
+    else
+      SWAN_VER="$VPN_SWAN_VER"
+    fi
+  fi
 }
 
 check_libreswan() {