fix for https://github.com/opnsense/core/issues/1196

src/etc/inc/services.inc

@@ -109,8 +109,11 @@ function services_radvd_configure($blacklist = array())
 
         /* check if we need to listen on a CARP interface */
         $realif = get_real_interface($dhcpv6if, "inet6");
-        if (!empty($dhcpv6ifconf['rainterface'])) {
+        if (!empty($dhcpv6ifconf['rainterface']) && $dhcpv6ifconf['rainterface'] != 'interface') {
             $dhcpv6if = $dhcpv6ifconf['rainterface'];
+        } elseif (!empty($dhcpv6ifconf['rainterface'])) {
+            // backward compatibilty mode, unset invalid options
+            unset($dhcpv6ifconf['rainterface']);
         }
 
         $ifcfgipv6 = get_interface_ipv6($dhcpv6if);

src/www/services_router_advertisements.php

@@ -257,9 +257,9 @@ include("head.inc");
                     <td><a id="help_for_rainterface" href="#" class="showhelp"><i class="fa fa-info-circle"></i></a> <?=gettext("RA Interface");?></td>
                     <td>
                       <select name="rainterface" id="rainterface">
+                        <option value="" <?=empty($pconfig['rainterface'])  ? "selected=\"selected\"" : ""; ?> > <?=strtoupper($if); ?></option>
 <?php
                       foreach($carplistif as $ifname => $vip): ?>
-                        <option value="interface" <?php if ($pconfig['rainterface'] == "interface") echo "selected=\"selected\""; ?> > <?=strtoupper($if); ?></option>
                         <option value="<?=$ifname ?>" <?php if ($pconfig['rainterface'] == $ifname) echo "selected=\"selected\""; ?> > <?="$ifname - $vip"; ?></option>
 <?php
                       endforeach;?>