Add self-check command

roles/matrix-server/tasks/self_check/self_check_dns.yml

@@ -0,0 +1,25 @@
+---
+
+- name: Check DNS SRV record
+  shell:
+    cmd: "dig -t srv {{ ('_matrix._tcp.' + hostname_identity + '.')|quote }}"
+  register: result_dig_srv
+  changed_when: false
+  ignore_errors: true
+
+- name: Fail if dig failed
+  fail:
+    msg: "Failed checking DNS SRV record. You likely don't have the `dig` program installed locally. Full error: {{ result_dig_srv }}"
+  when: "result_dig_srv.stderr != ''"
+
+# We expect an answer like this:
+# ;; ANSWER SECTION:
+# _matrix._tcp.DOMAIN. 10800	IN	SRV	10 0 8448 matrix.DOMAIN.
+- name: Fail if DNS SRV record incorrect
+  fail:
+    msg: "It appears the DNS SRV record for {{ hostname_identity }} is not set up correctly. See the 'Configuring DNS' documentation for this playbook. Full DNS answer was: {{ result_dig_srv.stdout }}"
+  when: "('8448 ' + hostname_matrix) not in result_dig_srv.stdout"
+
+- name: Report correct DNS SRV record
+  debug:
+    msg: "The DNS SRV record for {{ hostname_identity }} points to {{ hostname_matrix }}, as expected"