firewall: fix _magic_ipv6_template()

circle/firewall/models.py

@@ -375,8 +375,7 @@ class Vlan(AclBase, models.Model):
                     'specification like %(a)02x to get the first byte as two '
                     'hexadecimal digits. Usual choices for mapping '
                     '198.51.100.0/24 to 2001:0DB8:1:1::/64 would be '
-                    '"2001:0DB8:51:1:1:%(d)d::" and '
-                    '"2001:0DB8:51:1:1:%(d)x00::".'),
+                    '"2001:db8:1:1:%(d)d::" and "2001:db8:1:1:%(d)02x00::".'),
         validators=[val_ipv6_template], verbose_name=_('ipv6 template'))
     dhcp_pool = models.TextField(blank=True, verbose_name=_('DHCP pool'),
                                  help_text=_(
@@ -406,31 +405,40 @@ class Vlan(AclBase, models.Model):
                         _("%(ip6)s (translated from %(ip4)s) is outside of "
                           "the IPv6 network.") % {"ip4": i, "ip6": i6})
         if not self.ipv6_template and self.network6:
-            # come up with some sensible default
             self.ipv6_template = self._magic_ipv6_template()
-        host4_bytes = int(ceil((32 - self.network4.prefixlen) / 8))
-        host6_bytes = int(ceil((128 - self.network6.prefixlen) / 8))
+        host4_bytes = self._host_bytes(self.network4.prefixlen, 4)
+        host6_bytes = self._host_bytes(self.network6.prefixlen, 16)
         if host4_bytes > host6_bytes:
             raise ValidationError(
                 _("IPv6 network is too small to map IPv4 addresses to it."))
 
-    def _magic_ipv6_template(self):
-            host4_bytes = int(ceil((32 - self.network4.prefixlen) / 8))
-            host6_bytes = int(ceil((128 - self.network6.prefixlen) / 8))
+    @staticmethod
+    def _host_bytes(prefixlen, maxbytes):
+        return int(ceil((maxbytes - prefixlen / 8.0)))
+
+    @classmethod
+    def _magic_ipv6_template(cls, network4, network6, verbose=None):
+        """Offer a sensible ipv6_template value."""
+        host4_bytes = cls._host_bytes(network4.prefixlen, 4)
+        host6_bytes = cls._host_bytes(network6.prefixlen, 16)
         letters = ascii_letters[4-host4_bytes:4]
         remove = host6_bytes // 2
-            ipstr = self.network6.network.format(ipv6_full)
+        ipstr = network6.network.format(ipv6_full)
         s = ipstr.split(":")[0:-remove]
-            if 2 * (host4_bytes + 1) < host6_bytes:  # use verbose format
+        if verbose is None:  # use verbose format if net6 much wider
+            verbose = 2 * (host4_bytes + 1) < host6_bytes
+        if verbose:
             for i in letters:
                 s.append("%({})d".format(i))
-            else:  # use short format
+        else:
             remain = host6_bytes
             for i in letters:
                 if remain % 2 == 1:  # can use last half word
+                    assert s[-1] == "0" or s[-1].endswith("00")
                     if s[-1].endswith("00"):
                         s[-1] = s[-1][:-2]
                     s[-1] += "%({})02x".format(i)
+                    s[-1].lstrip("0")
                 else:
                     s.append("%({})02x00".format(i))
                 remain -= 1