Compare mx domains with priority order and not priority value

app/dashboard/views/domain_detail.py

@@ -13,6 +13,7 @@ from app.dns_utils import (
     get_spf_domain,
     get_txt_record,
     get_cname_record,
+    is_mx_equivalent,
 )
 from app.log import LOG
 from app.models import (
@@ -77,7 +78,7 @@ def domain_detail_dns(custom_domain_id):
         elif request.form.get("form-name") == "check-mx":
             mx_domains = get_mx_domains(custom_domain.domain)
 
-            if sorted(mx_domains) != sorted(EMAIL_SERVERS_WITH_PRIORITY):
+            if is_mx_equivalent(mx_domains, EMAIL_SERVERS_WITH_PRIORITY):
                 flash("The MX record is not correctly set", "warning")
 
                 mx_ok = False

app/dns_utils.py

@@ -1,4 +1,4 @@
-from typing import Optional
+from typing import Optional, List, Tuple
 
 import dns.resolver
 
@@ -95,3 +95,28 @@ def get_txt_record(hostname) -> [str]:
             ret.append(record)
 
     return ret
+
+
+def is_mx_equivalent(
+    mx_domains: List[Tuple[int, str]], ref_mx_domains: List[Tuple[int, str]]
+) -> bool:
+    """
+    Compare mx_domains with ref_mx_domains to see if they are equivalent.
+    mx_domains and ref_mx_domains are list of (priority, domain)
+
+    The priority order is taken into account but not the priority number.
+    For example, [(1, domain1), (2, domain2)] is equivalent to [(10, domain1), (20, domain2)]
+    """
+    mx_domains = sorted(mx_domains, key=lambda priority_domain: priority_domain[0])
+    ref_mx_domains = sorted(
+        ref_mx_domains, key=lambda priority_domain: priority_domain[0]
+    )
+
+    if len(mx_domains) != len(ref_mx_domains):
+        return False
+
+    for i in range(0, len(mx_domains)):
+        if mx_domains[i][1] != ref_mx_domains[i][1]:
+            return False
+
+    return True

cron.py

@@ -23,7 +23,7 @@ from app.config import (
     HIBP_SCAN_INTERVAL_DAYS,
 )
 from app.db import Session
-from app.dns_utils import get_mx_domains
+from app.dns_utils import get_mx_domains, is_mx_equivalent
 from app.email_utils import (
     send_email,
     send_trial_end_soon_email,
@@ -695,7 +695,7 @@ def check_custom_domain():
     for custom_domain in CustomDomain.filter_by(verified=True):  # type: CustomDomain
         mx_domains = get_mx_domains(custom_domain.domain)
 
-        if sorted(mx_domains) != sorted(EMAIL_SERVERS_WITH_PRIORITY):
+        if is_mx_equivalent(mx_domains, EMAIL_SERVERS_WITH_PRIORITY):
             user = custom_domain.user
             LOG.w(
                 "The MX record is not correctly set for %s %s %s",

tests/test_dns_utils.py

@@ -1,4 +1,9 @@
-from app.dns_utils import get_mx_domains, get_spf_domain, get_txt_record
+from app.dns_utils import (
+    get_mx_domains,
+    get_spf_domain,
+    get_txt_record,
+    is_mx_equivalent,
+)
 
 # use our own domain for test
 _DOMAIN = "simplelogin.io"
@@ -22,3 +27,14 @@ def test_get_spf_domain():
 def test_get_txt_record():
     r = get_txt_record(_DOMAIN)
     assert len(r) > 0
+
+
+def test_is_mx_equivalent():
+    assert is_mx_equivalent([], [])
+    assert is_mx_equivalent([(1, "domain")], [(1, "domain")])
+    assert is_mx_equivalent(
+        [(10, "domain1"), (20, "domain2")], [(10, "domain1"), (20, "domain2")]
+    )
+    assert is_mx_equivalent(
+        [(5, "domain1"), (10, "domain2")], [(10, "domain1"), (20, "domain2")]
+    )