From c399d0dd03fb4df3456418bfbe15162edd16c7d1 Mon Sep 17 00:00:00 2001
From: Renato Botelho <renato@netgate.com>
Date: Tue, 29 Dec 2015 13:49:27 -0200
Subject: Restore default MTU when field is cleared. Fixes #3926

---
 src/etc/inc/interfaces.inc | 76 ++++++++++++++++++++++++----------------------
 1 file changed, 40 insertions(+), 36 deletions(-)

diff --git a/src/etc/inc/interfaces.inc b/src/etc/inc/interfaces.inc
index 8b6643b..7d937ba 100644
--- a/src/etc/inc/interfaces.inc
+++ b/src/etc/inc/interfaces.inc
@@ -3177,52 +3177,56 @@ function interface_configure($interface = "wan", $reloadall = false, $linkupeven
 		interfaces_bring_up($wancfg['if']);
 	}
 
-	if (!empty($wancfg['mtu'])) {
-		if (stristr($realif, "_vlan")) {
-			$assignedparent = convert_real_interface_to_friendly_interface_name($realhwif);
-			if (!empty($assignedparent) && !empty($config['interfaces'][$assignedparent]['mtu'])) {
-				$parentmtu = $config['interfaces'][$assignedparent]['mtu'];
-				if ($wancfg['mtu'] > $parentmtu) {
-					log_error("There is a conflict on MTU between parent {$realhwif} and VLAN({$realif})");
-				}
-			} else {
-				$parentmtu = 0;
+	if (isset($wancfg['mtu']) && is_numericint($wancfg['mtu'])) {
+		$mtu = $wancfg['mtu'];
+	} else {
+		$mtu = 1500; /* Default */
+	}
+
+	if (stristr($realif, "_vlan")) {
+		$assignedparent = convert_real_interface_to_friendly_interface_name($realhwif);
+		if (!empty($assignedparent) && !empty($config['interfaces'][$assignedparent]['mtu'])) {
+			$parentmtu = $config['interfaces'][$assignedparent]['mtu'];
+			if ($mtu > $parentmtu) {
+				log_error("There is a conflict on MTU between parent {$realhwif} and VLAN({$realif})");
 			}
+		} else {
+			$parentmtu = 0;
+		}
 
-			$parentmtu = interface_vlan_mtu_configured($realhwif, $parentmtu);
+		$parentmtu = interface_vlan_mtu_configured($realhwif, $parentmtu);
 
-			if (get_interface_mtu($realhwif) != $parentmtu) {
-				pfSense_interface_mtu($realhwif, $parentmtu);
-			}
+		if (get_interface_mtu($realhwif) != $parentmtu) {
+			pfSense_interface_mtu($realhwif, $parentmtu);
+		}
 
-			/* All vlans need to use the same mtu value as their parent. */
-			interface_vlan_adapt_mtu(link_interface_to_vlans($realhwif), $parentmtu);
-		} else if (substr($realif, 0, 4) == 'lagg') {
-			/* LAGG interface must be destroyed and re-created to change MTU */
-			if ($wancfg['mtu'] != get_interface_mtu($realif)) {
-				if (isset($config['laggs']['lagg']) && is_array($config['laggs']['lagg'])) {
-					foreach ($config['laggs']['lagg'] as $lagg) {
-						if ($lagg['laggif'] == $realif) {
-							interface_lagg_configure($lagg);
-							break;
-						}
+		/* All vlans need to use the same mtu value as their parent. */
+		interface_vlan_adapt_mtu(link_interface_to_vlans($realhwif), $parentmtu);
+	} else if (substr($realif, 0, 4) == 'lagg') {
+		/* LAGG interface must be destroyed and re-created to change MTU */
+		if ($mtu != get_interface_mtu($realif)) {
+			if (isset($config['laggs']['lagg']) && is_array($config['laggs']['lagg'])) {
+				foreach ($config['laggs']['lagg'] as $lagg) {
+					if ($lagg['laggif'] == $realif) {
+						interface_lagg_configure($lagg);
+						break;
 					}
 				}
 			}
-		} else {
-			if ($wancfg['mtu'] != get_interface_mtu($realif)) {
-				pfSense_interface_mtu($realif, $wancfg['mtu']);
-			}
+		}
+	} else {
+		if ($mtu != get_interface_mtu($realif)) {
+			pfSense_interface_mtu($realif, $mtu);
+		}
 
-			/* This case is needed when the parent of vlans is being configured */
-			$vlans = link_interface_to_vlans($realif);
-			if (is_array($vlans)) {
-				interface_vlan_adapt_mtu($vlans, $wancfg['mtu']);
-			}
-			unset($vlans);
+		/* This case is needed when the parent of vlans is being configured */
+		$vlans = link_interface_to_vlans($realif);
+		if (is_array($vlans)) {
+			interface_vlan_adapt_mtu($vlans, $mtu);
 		}
-		/* XXX: What about gre/gif/.. ? */
+		unset($vlans);
 	}
+	/* XXX: What about gre/gif/.. ? */
 
 	switch ($wancfg['ipaddr']) {
 		case 'dhcp':
-- 
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