dns: forwarding: T3804: bugfix DHCP name-servers used for recursion

Commit 2ecf7a9f9c ('name-server: T3804: merge "system name-servers-dhcp" into
"system name-server"') missed out an old dictionary key "system_name_server_dhcp"
and thus system nameservers configured via DHCP did not get used for the DNS
forwar recursor.

(cherry picked from commit 806ff50bf1a970d731c2227f9d2cd2342b8a1b4e)

1 files changed, 13 insertions, 12 deletions

diff --git a/src/conf_mode/dns_forwarding.py b/src/conf_mode/dns_forwarding.py
index 06366362a..bc3821f61 100755
--- a/src/conf_mode/dns_forwarding.py
+++ b/src/conf_mode/dns_forwarding.py
@@ -1,6 +1,6 @@
 #!/usr/bin/env python3
 #
-# Copyright (C) 2018-2020 VyOS maintainers and contributors
+# Copyright (C) 2018-2022 VyOS maintainers and contributors
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License version 2 or later as
@@ -16,6 +16,7 @@
 
 import os
 
+from netifaces import interfaces
 from sys import exit
 
 from vyos.config import Config
@@ -62,10 +63,6 @@ def get_config(config=None):
         if conf.exists(base_nameservers):
             dns.update({'system_name_server': conf.return_values(base_nameservers)})
 
-        base_nameservers_dhcp = ['system', 'name-servers-dhcp']
-        if conf.exists(base_nameservers_dhcp):
-            dns.update({'system_name_server_dhcp': conf.return_values(base_nameservers_dhcp)})
-
     return dns
 
 def verify(dns):
@@ -87,9 +84,8 @@ def verify(dns):
                 raise ConfigError(f'No server configured for domain {domain}!')
 
     if 'system' in dns:
-        if not ('system_name_server' in dns or 'system_name_server_dhcp' in dns):
-            print("Warning: No 'system name-server' or 'system " \
-                  "name-servers-dhcp' configured")
+        if not 'system_name_server' in dns:
+            print('Warning: No "system name-server" configured')
 
     return None
 
@@ -142,10 +138,15 @@ def apply(dns):
             hc.delete_name_server_tags_recursor(['system'])
 
         # add dhcp nameserver tags for configured interfaces
-        if 'system_name_server_dhcp' in dns:
-            for interface in dns['system_name_server_dhcp']:
-                hc.add_name_server_tags_recursor(['dhcp-' + interface,
-                                                  'dhcpv6-' + interface ])
+        if 'system_name_server' in dns:
+            for interface in dns['system_name_server']:
+                # system_name_server key contains both IP addresses and interface
+                # names (DHCP) to use DNS servers. We need to check if the
+                # value is an interface name - only if this is the case, add the
+                # interface based DNS forwarder.
+                if interface in interfaces():
+                    hc.add_name_server_tags_recursor(['dhcp-' + interface,
+                                                      'dhcpv6-' + interface ])
 
         # hostsd will generate the forward-zones file
         # the list and keys() are required as get returns a dict, not list