From ab5cbcd3ca9569eaa0e9b203d7fb65514b2f3bae Mon Sep 17 00:00:00 2001
From: Jason Crowley <65243090+jasonpcrowley@users.noreply.github.com>
Date: Thu, 14 Oct 2021 14:23:40 -0500
Subject: [PATCH] Updated guess_interface_from_ip to more accurately identify
 the interface using the subnet with the largest mask in the route table.
 (#5281)

---
 src/etc/inc/interfaces.inc | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/src/etc/inc/interfaces.inc b/src/etc/inc/interfaces.inc
index a63c29966..ef615a9ae 100644
--- a/src/etc/inc/interfaces.inc
+++ b/src/etc/inc/interfaces.inc
@@ -3773,14 +3773,25 @@ function guess_interface_from_ip($ipaddress)
 
     /* create a route table we can search */
     exec("/usr/bin/netstat -rnWf " . $family, $output, $ret);
+
+    /* search for the route with the largest subnet mask */
+    $largest_mask = 0;
+    $best_if = null;
     foreach ($output as $line) {
         $fields = preg_split("/\s+/", $line);
         if (is_subnet($fields[0])) {
             if (ip_in_subnet($ipaddress, $fields[0])) {
-                return $fields[5];
+                list($ip, $mask) = explode('/', $fields[0]);
+                if ($mask > $largest_mask) {
+                    $best_if = $fields[5];
+                    $largest_mask = $mask;
+                }
             }
         }
     }
+    if (isset($best_if)) {
+      return $best_if;
+    }
 
     $ret = exec_command("/sbin/route -n get {$ipaddress} | /usr/bin/awk '/interface/ { print \$2; };'");
     if (empty($ret)) {