powerdns: T1524: support setting allow-from network

Netmasks (both IPv4 and IPv6) that are allowed to use the server. The default
allows access only from RFC 1918 private IP addresses. Due to the aggressive
nature of the internet these days, it is highly recommended to not open up the
recursor for the entire internet. Questions from IP addresses not listed here
are ignored and do not get an answer.

https://docs.powerdns.com/recursor/settings.html#allow-from

Imagine an ISP network with non RFC1918 IP adresses - they can't make
use of PowerDNS recursor.

As of now VyOS hat allow-from set to 0.0.0.0/0 and ::/0 which created an open
resolver. If there is no allow-from statement a config-migrator will add
the appropriate nodes to the configuration, resulting in:

 service {
     dns {
         forwarding {
             allow-from 0.0.0.0/0
             allow-from ::/0
             cache-size 0
             ignore-hosts-file
             listen-address 192.0.2.1
         }
     }
 }

3 files changed, 79 insertions, 9 deletions

diff --git a/interface-definitions/dns-forwarding.xml b/interface-definitions/dns-forwarding.xml
index 56820608c..08a221ebe 100644
--- a/interface-definitions/dns-forwarding.xml
+++ b/interface-definitions/dns-forwarding.xml
@@ -97,6 +97,23 @@
                   <valueless/>
                 </properties>
               </leafNode>
+              <leafNode name="allow-from">
+                <properties>
+                  <help>Networks allowed to query this server</help>
+                  <valueHelp>
+                    <format>ipv4net</format>
+                    <description>IP address and prefix length</description>
+                  </valueHelp>
+                  <valueHelp>
+                    <format>ipv6net</format>
+                    <description>IPv6 address and prefix length</description>
+                  </valueHelp>
+                  <multi/>
+                  <constraint>
+                    <validator name="ip-prefix"/>
+                  </constraint>
+                </properties>
+              </leafNode>
               <leafNode name="listen-address">
                 <properties>
                   <help>Addresses to listen for DNS queries [REQUIRED]</help>
diff --git a/src/conf_mode/dns_forwarding.py b/src/conf_mode/dns_forwarding.py
index 3ca77adee..86683e72c 100755
--- a/src/conf_mode/dns_forwarding.py
+++ b/src/conf_mode/dns_forwarding.py
@@ -44,7 +44,7 @@ config_tmpl = """
 # Non-configurable defaults
 daemon=yes
 threads=1
-allow-from=0.0.0.0/0, ::/0
+allow-from={{ allow_from | join(',') }}
 log-common-errors=yes
 non-local-bind=yes
 query-local-address=0.0.0.0
@@ -83,6 +83,7 @@ dnssec={{ dnssec }}
 """
 
 default_config_data = {
+    'allow_from': [],
     'cache_size': 10000,
     'export_hosts_file': 'yes',
     'listen_on': [],
@@ -121,6 +122,9 @@ def get_config(arguments):
 
     conf.set_level('service dns forwarding')
 
+    if conf.exists('allow-from'):
+        dns['allow_from'] = conf.return_values('allow-from')
+
     if conf.exists('cache-size'):
         cache_size = conf.return_value('cache-size')
         dns['cache_size'] = cache_size
@@ -216,12 +220,10 @@ def get_config(arguments):
 
     return dns
 
-
 def bracketize_ipv6_addrs(addrs):
     """Wraps each IPv6 addr in addrs in [], leaving IPv4 addrs untouched."""
     return ['[{0}]'.format(a) if a.count(':') > 1 else a for a in addrs]
 
-
 def verify(dns):
     # bail out early - looks like removal from running config
     if dns is None:
@@ -231,6 +233,10 @@ def verify(dns):
         raise ConfigError(
             "Error: DNS forwarding requires either a listen-address (preferred) or a listen-on option")
 
+    if not dns['allow_from']:
+        raise ConfigError(
+                "Error: DNS forwarding requires an allow-from network")
+
     if dns['domains']:
         for domain in dns['domains']:
             if not domain['servers']:
@@ -239,7 +245,6 @@ def verify(dns):
 
     return None
 
-
 def generate(dns):
     # bail out early - looks like removal from running config
     if dns is None:
@@ -251,16 +256,14 @@ def generate(dns):
         f.write(config_text)
     return None
 
-
 def apply(dns):
-    if dns is not None:
-        os.system("systemctl restart pdns-recursor")
-    else:
+    if dns is None:
         # DNS forwarding is removed in the commit
         os.system("systemctl stop pdns-recursor")
         if os.path.isfile(config_file):
             os.unlink(config_file)
-
+    else:
+        os.system("systemctl restart pdns-recursor")
 
 if __name__ == '__main__':
     args = parser.parse_args()
diff --git a/src/migration-scripts/dns-forwarding/0-to-1 b/src/migration-scripts/dns-forwarding/0-to-1
new file mode 100755
index 000000000..6e8720eef
--- /dev/null
+++ b/src/migration-scripts/dns-forwarding/0-to-1
@@ -0,0 +1,50 @@
+#!/usr/bin/env python3
+#
+# Copyright (C) 2019 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
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+
+# This migration script will check if there is a allow-from directive configured
+# for the dns forwarding service - if not, the node will be created with the old
+# default values of 0.0.0.0/0 and ::/0
+
+import sys
+from vyos.configtree import ConfigTree
+
+if (len(sys.argv) < 1):
+    print("Must specify file name!")
+    sys.exit(1)
+
+file_name = sys.argv[1]
+
+with open(file_name, 'r') as f:
+    config_file = f.read()
+
+config = ConfigTree(config_file)
+
+base = ['service', 'dns', 'forwarding']
+if not config.exists(base):
+    # Nothing to do
+    sys.exit(0)
+else:
+    if not config.exists(base + ['allow-from']):
+        config.set(base + ['allow-from'], value='0.0.0.0/0', replace=False)
+        config.set(base + ['allow-from'], value='::/0', replace=False)
+
+    try:
+        with open(file_name, 'w') as f:
+            f.write(config.to_string())
+    except OSError as e:
+        print("Failed to save the modified config: {}".format(e))
+        sys.exit(1)