config/forms: allow blacklisting email servers by IP

Adds a new config entry (EMAIL_SERVER_BLACKLIST, tuple of IPv4 addresses
as strings) and an email validator for registering, which will query all
the MX records for the domain and reject the registration if any of the
A records for any of the MX records are found in the blacklist.
If the query fails, the blacklist is ignored; the email is accepted.

config.example.py

@@ -59,6 +59,11 @@ EMAIL_BLACKLIST = (
     re.compile(r'(?i)@(msn\.com|passport\.(com|net))'),
     # '@dodgydomain.tk'
 )
+EMAIL_SERVER_BLACKLIST = (
+    # Bad mailserver IPs here (MX server.com -> A mail.server.com > 11.22.33.44)
+    # '1.2.3.4', '11.22.33.44'
+)
+
 
 
 # Recaptcha keys (https://www.google.com/recaptcha)

nyaa/forms.py

@@ -14,6 +14,9 @@ from wtforms.validators import (DataRequired, Email, EqualTo, Length, Optional,
 from wtforms.widgets import Select as SelectWidget  # For DisabledSelectField
 from wtforms.widgets import HTMLString, html_params  # For DisabledSelectField
 
+import dns.exception
+import dns.resolver
+
 from nyaa import bencode, models, utils
 from nyaa.extensions import config
 from nyaa.models import User
@@ -69,7 +72,7 @@ def upload_recaptcha_validator_shim(form, field):
         return True
 
 
-def register_email_validator(form, field):
+def register_email_blacklist_validator(form, field):
     email_blacklist = app.config.get('EMAIL_BLACKLIST', [])
     email = field.data.strip()
     validation_exception = StopValidation('Blacklisted email provider')
@@ -86,6 +89,42 @@ def register_email_validator(form, field):
     return True
 
 
+def register_email_server_validator(form, field):
+    server_blacklist = app.config.get('EMAIL_SERVER_BLACKLIST', [])
+    if not server_blacklist:
+        return True
+
+    validation_exception = StopValidation('Blacklisted email provider')
+    email = field.data.strip()
+    email_domain = email.split('@', 1)[-1]
+
+    try:
+        # Query domain MX records
+        mx_records = list(dns.resolver.query(email_domain, 'MX'))
+
+    except dns.exception.DNSException:
+        app.logger.error('Unable to query MX records for email: %s - ignoring',
+                         email, exc_info=False)
+        return True
+
+    for mx_record in mx_records:
+        try:
+            # Query mailserver A records
+            a_records = list(dns.resolver.query(mx_record.exchange))
+            for a_record in a_records:
+                # Check for address in blacklist
+                if a_record.address in server_blacklist:
+                    app.logger.warning('Rejected email %s due to blacklisted mailserver (%s, %s)',
+                                       email, a_record.address, mx_record.exchange)
+                    raise validation_exception
+
+        except dns.exception.DNSException:
+            app.logger.warning('Failed to query A records for mailserver: %s (%s) - ignoring',
+                               mx_record.exchange, email, exc_info=False)
+
+    return True
+
+
 _username_validator = Regexp(
     r'^[a-zA-Z0-9_\-]+$',
     message='Your username must only consist of alphanumerics and _- (a-zA-Z0-9_-)')
@@ -129,8 +168,9 @@ class RegisterForm(FlaskForm):
         Email(),
         DataRequired(),
         Length(min=5, max=128),
-        register_email_validator,
+        register_email_blacklist_validator,
-        Unique(User, User.email, 'Email already in use by another account')
+        Unique(User, User.email, 'Email already in use by another account'),
+        register_email_server_validator
     ])
 
     password = PasswordField('Password', [

requirements.txt

@@ -6,6 +6,7 @@ blinker==1.4
 cffi==1.10.0
 click==6.7
 dominate==2.3.1
+dnspython==1.15.0
 elasticsearch==5.3.0
 elasticsearch-dsl==5.2.0
 flake8==3.3.0