From 7c833ea3d6ea78637446af5e788b06e9e6af35ce Mon Sep 17 00:00:00 2001
From: Daniil Baturin <daniil@baturin.org>
Date: Wed, 11 Jul 2018 04:13:55 +0700
Subject: [PATCH] Add a special case for /31 networks to
 openvpn_get_interface_ip() (another instance of #2529).

---
 src/etc/inc/plugins.inc.d/openvpn.inc | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/src/etc/inc/plugins.inc.d/openvpn.inc b/src/etc/inc/plugins.inc.d/openvpn.inc
index f16b9359f..1e089d6af 100644
--- a/src/etc/inc/plugins.inc.d/openvpn.inc
+++ b/src/etc/inc/plugins.inc.d/openvpn.inc
@@ -1485,9 +1485,21 @@ function openvpn_create_dirs()
 
 function openvpn_get_interface_ip($ip, $mask)
 {
-    $baselong = ip2long32($ip) & ip2long($mask);
-    $ip1 = long2ip32($baselong + 1);
-    $ip2 = long2ip32($baselong + 2);
+    $masklong = ip2long($mask);
+    $baselong = ip2long32($ip) & $masklong;
+
+    // Special case for /31 networks which lack network and broadcast addresses.
+    // As per RFC3021, both addresses should be treated as host addresses.
+    if ($masklong == 0xfffffffe)
+    {
+        $ip1 = long2ip32($baselong);
+        $ip2 = long2ip32($baselong + 1);
+    }
+    else
+    {
+        $ip1 = long2ip32($baselong + 1);
+        $ip2 = long2ip32($baselong + 2);
+    }
     return array($ip1, $ip2);
 }