Use DNS j2 for default DNS configuration (sonic-net#15901)

Why I did it
Support default DNS configuration

How I did it
Use j2 template to generate default DNS configuration.

How to verify it
Run sonic-config-engine unit test.

files/build_templates/dns.j2

@@ -0,0 +1,11 @@
+{# Please follow below example to add your DNS server
+{
+  "DNS_NAMESERVER": {
+      "6.6.6.6": {},
+      "2001:4860:4860::64": {}
+  }
+}
+#}
+{
+    "DNS_NAMESERVER": {}
+}

files/build_templates/sonic_debian_extension.j2

@@ -373,6 +373,9 @@ sudo cp $IMAGE_CONFIGS/ntp/ntp-systemd-wrapper $FILESYSTEM_ROOT/usr/lib/ntp/
 sudo cp $IMAGE_CONFIGS/ntp/ntp.service $FILESYSTEM_ROOT_USR_LIB_SYSTEMD_SYSTEM
 echo "ntp.service" | sudo tee -a $GENERATED_SERVICE_FILE
 
+# Copy DNS templates
+sudo cp $BUILD_TEMPLATES/dns.j2 $FILESYSTEM_ROOT_USR_SHARE_SONIC_TEMPLATES/
+
 # Copy warmboot-finalizer files
 sudo LANG=C cp $IMAGE_CONFIGS/warmboot-finalizer/finalize-warmboot.sh $FILESYSTEM_ROOT/usr/local/bin/finalize-warmboot.sh
 sudo LANG=C cp $IMAGE_CONFIGS/warmboot-finalizer/warmboot-finalizer.service $FILESYSTEM_ROOT_USR_LIB_SYSTEMD_SYSTEM

src/sonic-config-engine/minigraph.py

@@ -5,6 +5,7 @@
 import os
 import sys
 import json
+import jinja2
 import subprocess
 from collections import defaultdict
 
@@ -2010,6 +2011,27 @@ def parse_xml(filename, platform=None, port_config_file=None, asic_name=None, hw
     results['DHCP_SERVER'] = dict((item, {}) for item in dhcp_servers)
     results['DHCP_RELAY'] = dhcp_relay_table
     results['NTP_SERVER'] = dict((item, {}) for item in ntp_servers)
+    # Set default DNS nameserver from dns.j2
+    results['DNS_NAMESERVER'] = {}
+    if os.environ.get("CFGGEN_UNIT_TESTING", "0") == "2":
+        dns_conf = os.path.join(os.path.dirname(__file__), "tests/", "dns.j2")
+    else:
+        dns_conf = "/usr/share/sonic/templates/dns.j2"
+    if os.path.isfile(dns_conf):
+        text = ""
+        with open(dns_conf) as template_file:
+            # Semgrep does not allow to use jinja2 directly, but we do need jinja2 for SONiC
+            environment = jinja2.Environment(trim_blocks=True) # nosemgrep
+            dns_template = environment.from_string(template_file.read())
+            text = dns_template.render(results)
+        try:
+            dns_res = json.loads(text)
+        except ValueError as e:
+            sys.exit("Error: fail to load dns configuration, %s" % str(e))
+        else:
+            dns_nameservers = dns_res.get('DNS_NAMESERVER', {})
+            for k in dns_nameservers.keys():
+                results['DNS_NAMESERVER'][str(k)] = {}
     results['TACPLUS_SERVER'] = dict((item, {'priority': '1', 'tcp_port': '49'}) for item in tacacs_servers)
     if len(acl_table_types) > 0:
         results['ACL_TABLE_TYPE'] = acl_table_types

src/sonic-config-engine/tests/dns.j2

@@ -0,0 +1,9 @@
+{
+    "DNS_NAMESERVER": {
+{% if DEVICE_METADATA.localhost.cloudtype == "Public" %}
+        "6.6.6.6": {}
+{% else %}
+        "8.8.8.8": {}
+{% endif %}
+    }
+}

src/sonic-config-engine/tests/test_cfggen.py

@@ -696,6 +696,11 @@ def test_metadata_ntp(self):
         output = self.run_script(argument)
         self.assertEqual(utils.to_dict(output.strip()), utils.to_dict("{'10.0.10.1': {}, '10.0.10.2': {}}"))
 
+    def test_dns_nameserver(self):
+        argument = ['-m', self.sample_graph_metadata, '-p', self.port_config, '-v', "DNS_NAMESERVER"]
+        output = self.run_script(argument)
+        self.assertEqual(utils.to_dict(output.strip()), utils.to_dict("{'6.6.6.6': {}}"))
+
     def test_minigraph_vnet(self, **kwargs):
         graph_file = kwargs.get('graph_file', self.sample_graph_simple)
         argument = ['-m', graph_file, '-p', self.port_config, '-v', "VNET"]