From 664ba39383e4f6ea79203e935b139bb2a058e9d5 Mon Sep 17 00:00:00 2001
From: Bob Lantz <rlantz@cs.stanford.edu>
Date: Thu, 4 Apr 2019 16:40:04 -0700
Subject: [PATCH] NAT warning if IPv4 loopback address in /etc/resolv.conf

---
 bin/mn | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/bin/mn b/bin/mn
index 4c5cf3d..ae55e4f 100755
--- a/bin/mn
+++ b/bin/mn
@@ -388,6 +388,10 @@ class MininetRunner( object ):
                   listenPort=opts.listenport )
 
         if opts.ensure_value( 'nat', False ):
+            with open( '/etc/resolv.conf' ) as f:
+                if 'nameserver 127.' in f.read():
+                    warn( '*** Warning: loopback address in /etc/resolv.conf '
+                          'may break host DNS over NAT\n')
             mn.addNAT( *opts.nat_args, **opts.nat_kwargs ).configDefault()
 
         # --custom files can set CLI or change mininet.cli.CLI