adding section for the alternative domain names to the details page of the domain

code goes through all of the alternative domain names and displays:
- name of the alternative domain
- DNS MX entry
- DNS SPF entry
- if main domain has DKIM key: DNS DKIM entry
- if main domain has DKIM key: DNS DMARC entry

file modified:   core/admin/mailu/models.py
file modified:   core/admin/mailu/ui/templates/domain/details.html

core/admin/mailu/models.py

@@ -355,6 +355,54 @@ class Alternative(Base):
     domain = db.relationship(Domain,
         backref=db.backref('alternatives', cascade='all, delete-orphan'))
 
+    @property
+    def dns_dkim(self):
+        """ return DKIM record for domain """
+        if self.domain.dkim_key:
+            selector = app.config['DKIM_SELECTOR']
+            return f'{selector}._domainkey.{self.name}. 600 IN TXT "v=DKIM1; k=rsa; p={self.domain.dkim_publickey}"'
+
+    @cached_property
+    def dns_dmarc(self):
+        """ return DMARC record for domain """
+        if self.domain.dkim_key:
+            domain = app.config['DOMAIN']
+            rua = app.config['DMARC_RUA']
+            rua = f' rua=mailto:{rua}@{domain};' if rua else ''
+            ruf = app.config['DMARC_RUF']
+            ruf = f' ruf=mailto:{ruf}@{domain};' if ruf else ''
+            return f'_dmarc.{self.name}. 600 IN TXT "v=DMARC1; p=reject;{rua}{ruf} adkim=s; aspf=s"'
+
+    @cached_property
+    def dns_dmarc_report(self):
+        """ return DMARC report record for mailu server """
+        if self.domain.dkim_key:
+            domain = app.config['DOMAIN']
+            return f'{self.name}._report._dmarc.{domain}. 600 IN TXT "v=DMARC1;"'
+
+    @cached_property
+    def dns_mx(self):
+        """ return MX record for domain """
+        hostname = app.config['HOSTNAME']
+        return f'{self.name}. 600 IN MX 10 {hostname}.'
+
+    @cached_property
+    def dns_spf(self):
+        """ return SPF record for domain """
+        hostname = app.config['HOSTNAME']
+        return f'{self.name}. 600 IN TXT "v=spf1 mx a:{hostname} ~all"'
+
+    def check_mx(self):
+        """ checks if MX record for domain points to mailu host """
+        try:
+            hostnames = set(app.config['HOSTNAMES'].split(','))
+            return any(
+                rset.exchange.to_text().rstrip('.') in hostnames
+                for rset in dns.resolver.resolve(self.name, 'MX')
+            )
+        except dns.exception.DNSException:
+            return False
+
 
 class Relay(Base):
     """ Relayed mail domain.

core/admin/mailu/ui/templates/domain/details.html

@@ -63,4 +63,36 @@
   </pre></td>
 </tr>
 {%- endcall %}
+
+{%- for alternative in domain.alternatives %}
+
+{%- call macros.table(datatable=False) %}
+<tr>
+  <th>{% trans %}Alternative Domain name{% endtrans %}</th>
+  <td>{{ alternative.name }}</td>
+</tr>
+<tr>
+  <th>{% trans %}DNS MX entry{% endtrans %} <i class="fa {{ 'fa-check-circle text-success' if alternative.check_mx() else 'fa-exclamation-circle text-danger' }}"></i></th>
+  <td>{{ macros.clip("dns_mx") }}<pre id="dns_mx" class="pre-config border bg-light">{{ alternative.dns_mx }}</pre></td>
+</tr>
+<tr>
+  <th>{% trans %}DNS SPF entries{% endtrans %}</th>
+  <td>{{ macros.clip("dns_spf") }}<pre id="dns_spf" class="pre-config border bg-light">{{ alternative.dns_spf }}</pre>
+  </td>
+</tr>
+{%- if alternative.domain.dkim_publickey %}
+<tr>
+  <th>{% trans %}DNS DKIM entry{% endtrans %}</th>
+  <td>{{ macros.clip("dns_dkim") }}<pre id="dns_dkim" class="pre-config border bg-light">{{ alternative.dns_dkim }}</pre></td>
+</tr>
+<tr>
+  <th>{% trans %}DNS DMARC entry{% endtrans %}</th>
+  <td>
+    {{ macros.clip("dns_dmarc") }}<pre id="dns_dmarc" class="pre-config border bg-light">{{ alternative.dns_dmarc }}</pre>
+    {{ macros.clip("dns_dmarc_report") }}<pre id="dns_dmarc_report" class="pre-config border bg-light">{{ alternative.dns_dmarc_report }}</pre>
+  </td>
+</tr>
+{%- endif %}
+{%- endcall %}
+{%- endfor %}
 {%- endblock %}